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INTERGRANULAR CRACKING OF INOR-8 IN THE MSRE H. E . M c C o y B . McNabb

Printed in the United States of America. Available from National Technical Information Service US. Department of Commerce . 5285 Port Royal Road, Springfield, Virginia 22151 Price: Printed Copy $3.00; Microfiche $0.95 This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Atomic Energy Commission, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that i t s use would not infringe privately owned rights.

ORNL-4829 UC-25 -Metals, Ceramics, and Materials

. Contract No. W-7405-eng-26

METALS AND CERAMICS DIVISION

INTERGRANULAR CRACKING OF INOR-8 IN THE MSRE H. E. McCoy and B. McNabb

NOTICE IWS rpport contains information of a preliminary nature and was prepared primarily for internal at the orfginatlng 1nst.allation. It is subject to revision or correction and therefore does not rep=sent a final report. It is passed to the reciplent in confidence and should not be abstracted or further disclosed without the approval of the orlglnating installation or USAEC Technical Information Center. Oak Ridge, TN 37830

NOVEMBER 1972

OAK RIDGE NATIONAL LABORATORY 37830 operated by UNION CARBIDE CORPORATION for the U.S. ATOMIC ENERGY COMMISSION

. Oak Ridge, Tennessee

-i

CONTENTS r

............................ .......................... THE MSRE ITS OPERATION . . . . . . . . . . . . . . . . . . . Description . . . . . . . . . . . . . . . . . . . . . . . . History . . . . . . . . . . . . . . . . . . . . . . . . . . EXAMINATION OF MSRE SURVEILLANCE SPECLMENS . . . . . . . . . . . Specimens Exposed Before Power Operation . . . . . . . . . First Group of Surveillance Specimens . . . . . . . . . . . Second Group of Surveillance Specimens . . . . . . . . . . Third Group of Surveillance Specimens . . . . . . . . . . . Fourth Group of Surveillance Specimens . . . . . . . . . . Specimens Exposed to Cell Atmosphere . . . . . . . . . . . Studies Related to Modified Surface Microstructure . . . . Summary of Observations on Surveillance Specimens . . . . . EXAMINATION OF MSRE COMPONENTS . . . . . . . . . . . . . . . . . Control Rod Thimble . . . . . . . . . . . . . . . . . . . . Primary Heat Exchanger . . . . . . . . . . . . . . . . . . Pump Bowl Parts . . . . . . . . . . . . . . . . . . . . . . FreezeValve105 . . . . . . . . . . . . . . . . . . . . . EXAMINATION OF INOR-8 FROM IN-REACTOR LOOPS . . . . . . . . . . Pump Loops . . . . . . . . . . . . . . . . . . . . . . . . Thermal Convection Loop . . . . . . . . . . . . . . . . . . Summary of Observations on In-Reactor Loops . . . . . . . . CHEMICAL ANALYSES OF METAL REMOVED FROM THE IBRE . . . . . . . . ABSTRACT INTRODUCTION AM)

DISCUSSION OF OBSERVATIONS ON INOR-8 FROM THE MSRE AND IN-REACTOR LOOPS

.................. Summary of Observations . . . . . . . . . . . . . . . . . Possible Mechanisms . . . . . . . . . . . . . . . . . . . . POST-MSRE STUDIES . . . . . . . . . . . . . . . . . . . . . . . Corrosion Experiments . . . . . . . . . . . . . . . . . . . Diffusion of Te . . . . . . . . . . . . . . . . . . . . . . Experiments with an Applied Stress . . . . . . . . . . . . SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . ACKNOWLEDGMENT . . . . . . . . . . . . . . . . . . . . . . . . . REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . ;

iii

14 17 19 27

34 50 79 79 83 87

88 106 118 137 138 143 145 149

149 157 157 160

165 165 169 169 170 173

174

, w

INTERGRANULAR CRACKING OF INOR-8

I N THE MSRE

H. E. McCoy and B. McNabb

ABSTRACT

The INOR-8 s u r v e i l l a n c e specimens and components from t h e MSRE t h a t had been exposed t o f u e l s a l t formed shallow i n t e r g r a n u l a r cracks (2 t o 10 m i l s deep i n exposures t o g r e a t e r than 20,000 h r ) .

Some of t h e s e

cracks w e r e v i s i b l e i n polished s e c t i o n s of as-Yemoved materials, b u t many o t h e r s w e r e v i s i b l e a f t e r t h e samples had been deformed.

Consider-

a b l e evidence i n d i c a t e s t h a t t h e cracks w e r e due t o t h e inward d i f f u s i o n of f i s s i o n products. The f i s s i o n product cracking mechanism was f u r t h e r s u b s t a n t i a t e d by l a b o r a t o r y tests which c l e a r l y demonstrated t h a t t e l l u r i u m causes i n t e r g r a n u l a r cracking i n INOR-8.

These tests have included o t h e r

materials, and important v a r i a t i o n s e x i s t i n t h e i r r e s p e c t i v e s u s c e p t i b i l i t i e s t o cracking by t e l l u r i u m .

S e v e r a l materials, i n c l u d i n g types

300 and 400 s t a i n l e s s steels, nickel- and cobalt-base a l l o y s containing g r e a t e r than 15%C r y copper, monel, and INOR-8 containing 2% Nb, comp l e t e l y r e s i s t e d cracking i n t h e tests run thus f a r .

INTRODUCTION

The Molten-Salt Reactor Experiment w a s a unique f l u i d - f u e l r e a c t o r . It operated a t temperatures around 65OOC f o r more than 20,000 h r between

1965 and December, 1969.

The f u e l w a s a mixture of f l u o r i d e s a l t s , cir-

c u l a t e d through a c o r e of g r a p h i t e b a r s and an e x t e r n a l h e a t exchanger. Except f o r t h e g r a p h i t e , a l l p a r t s c o n t a c t i n g t h e s a l t w e r e of a nickelbase a l l o y known as INOR-8 and now a v a i l a b l e commercially under t h e t r a d e names of Hastelloy N and Allvac N.

This a l l o y , developed a t Oak

Ridge National Laboratory s p e c i f i c a l l y f o r use i n f l u o r i d e s a l t s a t high temperature ,2 . h a s t h e nominal composition of Ni-16% Mo-7% Cr-5% Fe-0.05% C. IIJOR-8 i n t h e MSRE behaved as expected w i t h regard t o c o r r o s i o n by t h e f l u o r i d e s a l t s and t h e containment atmosphere (very l i t t l e by e i t h e r ) . Two problems w i t h INOR-8 d i d appear, however.

The f i r s t w a s a d r a s t i c

reduction i n high-temperature creep-rupture l i f e and f r a c t u r e s t r a i n under creep conditions.

The second was t h e appearance of grain-boundary

cracks a t INOR-8 s u r f a c e s exposed t o t h e f u e l s a l t . Embrittlement phenomena have been s t u d i e d e x t e n s i v e l y f o r t h e p a s t s e v e r a l y e a r s i n connection w i t h various iron- and nickel-base a l l o y s and s p e c i f i c a l l y with regard t o INOR-8 by ORNL. INOR-8

The embrittlement of

i n t h e MSRE has been a t t r i b u t e d mainly t o t h e helium t h a t is

generated by thermal neutron i n t e r a c t i o n with l0B p r e s e n t i n t h e a l l o y

as an impurity.

We have found t h a t small changes i n chemical composition

are q u i t e e f f e c t i v e i n reducing t h e e f f e c t s of helium production, and our work is w e l l along toward developing a modified INOR-8 w i t h improved r e s i s t a n c e t o embrittlement by neutron i r r a d i a t i o n .

This work has been

reported extensively4-11 and w i l l be discussed i n t h i s document only i n s o f a r as i t relates t o t h e f i n d i n g and i n t e r p r e t a t i o n of evidence on t h e s u r f a c e cracking problem. The cause of t h e s u r f a c e cracking has not y e t been p r e c i s e l y d e f i n e d , nor can i t s very long-term behavior be p r e d i c t e d with confidence.

The

cause must be a s s o c i a t e d w i t h f u e l system'conditions a f t e r t h e beginning of power operation:

numerous cracks o r i n c i p i e n t cracks were found on

bd*

every INOR-8 s u r f a c e t h a t was examined a f t e r prolonged c o n t a c t w i t h t h e r a d i o a c t i v e f u e l s a l t ; few o r none could be found on INOR-8 s u r f a c e s (sometimes on t h e same piece) that had been exposed t o t h e f l u o r i d e s a l t

i n t h e c o o l a n t system o r t o t h e containment c e l l atmosphere.

The cracks

were observed t o open up when a f f e c t e d s u r f a c e s w e r e s t r a i n e d i n t e n s i o n , b u t some g r a i n boundary cracks were d e t e c t a b l e i n polished s e c t i o n s of u n s t r a i n e d specimens.

The depth of cracking was 2 t o 10 m i l s , and some

s e c t i o n e d specimens showed as many as 300 cracks p e r i n c h of edge.

g e n e r a l t r e n d t o more and deeper cracks w i t h i n c r e a s i n g exposure t i m e

w a s e v i d e n t , b u t t h e s t a t i s t i c a l s i g n i f i c a n c e of the changes a f t e r t h e f i r s t s e v e r a l thousand hours was poor.

Obviously t h e determination of

cause and long-term progression i s essential i n t h e development of molten-

s a l t r e a c t o r s t h a t must o p e r a t e r e l i a b l y f o r many y e a r s .

An i n t e n s i v e e f f o r t has been mounted w i t h i n t h e Molten-Salt Reactor Program t o i n v e s t i g a t e t h e INOR-8 cracking phenomenon and t o develop remedies or ways t o circumvent t h e problem. can be produced i n out-of-reactor

More o r less similar e f f e c t s

experiments.

FeF2 oxidant cause i n t e r - g r a n u l a r corrosion.

F l u o r i d e salts with added Tellurium deposited on

INOR-8 and allowed t o d i f f u s e a t high temperature produces b r i t t l e g r a i n boundaries.

Tellurium a l s o a f f e c t s n i c k e l and s t a i n l e s s steel, b u t t o

lesser degrees.

However, some of t h e o t h e r agents t h a t were suspected

of causing t h e cracking i n t h e MSRE f u e l system have given n e g a t i v e results.

This work i s c u r r e n t l y i n p r o g r e s s , and few f i r m conclusions

can y e t be.drawn. The p r e s e n t document has been prepared t o r e p o r t and summarize a l l t h e c u r r e n t l y known information obtained from t h e MSRE, which i s t h e s t a r t i n g p o i n t of t h e i n v e s t i g a t i o n .

It a l s o i n c l u d e s p e r t i n e n t observa-

t i o n s from o t h e r molten-salt systems, b r i e f accounts of c u r r e n t experiments, and some t e n t a t i v e conclusions.

4 I

A good, g e n e r a l d e s c r i p t i o n of the MSRE and an account of most o f ' i t s h i s t o r y appear i n r e f e r e n c e 1. Reference 12 is a d e t a i l e d d e s c r i p t i o n Because t h e s e r e f e r e n c e s are widely a v a i l -

a b l e , t h e d e s c r i p t i o n h e r e is confined t o those p o r t i o n s t h a t are germane

t o t h e d i s c u s s i o n of t h e INOR-8 cracking. Description The p a r t s of t h e MSRE with'which w e w i l l be concerned are included i n t h e s i m p l i f i e d flowsheet i n Fig. 1. The cracking phenomenon w a s observed on p i e c e s of INOR-8

from t h e r e a c t o r v e s s e l , t h e h e a t exchanger,

t h e f u e l pump and a f r e e z e valve near f u e l d r a i n tank No. 2.

INOR-8

specimens exposed t o t h e containment c e l l atmosphere o u t s i d e t h e r e a c t o r v e s s e l and s u r f a c e s exposed t o t h e coolant s a l t w e r e examined b u t d i d

not show t h e cracking. The f u e l s a l t composition w a s LiF-BeF2-ZrF4-UF4 t h e c o o l a n t , LiF-BeF2 (66-34 mole

X).

(65-30-5-<1

mole %>' ;

A t f u l l power t h e 1200-gpm f u e l

stream normally e n t e r e d t h e r e a c t o r v e s s e l a t 632OC and l e f t a t 654OC; t h e maximum o u t l e t temperature a t which t h e r e a c t o r operated f o r any s u b s t a n t i a l p e r i o d of t i m e w a s 663OC (1225OF).

When t h e r e a c t o r was a t

low power, t h e s a l t systems were u s u a l l y n e a r l y i s o t h e r m a l a t about 650OC. During extended shutdowns t h e s a l t w a s drained i n t o t a n k s , where i t w a s kept molten w h i l e t h e c i r c u l a t i n g loops were allowed t o cool.

Plugs of

s a l t f r o z e n i n f l a t t e n e d s e c t i o n s of p i p e ("freeze valves") w e r e used t o

i s o l a t e t h e d r a i n tanks from t h e loop.

The l i q u i d u s temperature of t h e

f u e l s a l t w a s about 44OOC and t h a t of t h e c o o l a n t s a l t w a s 459"C, so t h e loops w e r e heated t o 600-650째C w i t h e x t e r n a l electric h e a t e r s b e f o r e t h e

s a l t w a s t r a n s f e r r e d from t h e s t o r a g e tanks.

THE IlSRE AND ITS OPERATION

of a l l components and systems.

H e l i u m (sometimes argon)

was t h e cover gas over t h e f u e l and c o o l a n t s a l t s . During o p e r a t i o n , samples of f u e l s a l t were obtained by lowering

s m a l l copper buckets (capsules) i n t o t h e pool of s a l t i n t h e pump bowl.

The pump bowl served as t h e s u r g e space f o r t h e loop and a l s o f o r separ a t i o n of gaseous f i s s i o n products from a 50-gpm stream of s a l t sprayed o u t i n t o t h e gas space above t h e s a l t pool.

To p r o t e c t t h e sample bucket

from the s a l t spray i n t h e pump bowl, a s p i r a l b a f f l e of INOR-8 extended from t h e top of t h e bowl down i n t o t h e s a l t pool.

A cage of INOR-8 rods

i n s i d e t h e s p i r a l b a f f l e guided t h e sample capsule i n t h e pump bowl. The r e a c t o r c o r e w a s composed of vertical g r a p h i t e b a r s with flow passages between them.

I n t h e l a t t i c e near t h e c e n t e r of t h e c o r e t h r e e

thimbles of INOR-8 housed c o n t r o l rods.

Nearby, and a c c e s s i b l e during

shutdowns through a flanged nozzle, w a s an a r r a y of g r a p h i t e and m e t a l specimens, which w a s exposed t o t h e f u e l flowing up through t h e core. Throughout most of t h e MSRE operation t h e c o r e a r r a y w a s as shown i n Fig. 2.

This a r r a y w a s designed t o expose s u r v e i l l a n c e specimens of

g r a p h i t e and INOR-8 i d e n t i c a l t o t h e material used i n t h e MSRE c o r e and r e a c t o r vessel. ,Later, specimens of modified INOR-8 were included.

The

assembly was composed of t h r e e s e p a r a b l e s t r i n g e r s designated RL, RR, and RS.

Each s t r i n g e r included a column of g r a p h i t e specimens and two

rods of INOR-8.

( S t r i n g e r RS a l s o included a f l u x monitor tube.)

Not shown i n Fig. 1, but l o c a t e d i n t h e r e a c t o r b u i l d i n g , w a s a v e s s e l i n which specimen s t r i n g e r s i d e n t i c a l t o those i n t h e c o r e could be exposed t o f l u o r i d e s a l t having t h e same nominal composition as t h e f u e l salt.

(The s t r i n g e r s i n t h e c o n t r o l f a c i l i t y were designated CL,

CR, and CS.)

Electric h e a t e r s on t h e vessel were c o n t r o l l e d t o produce

a temperature p r o f i l e along t h e s t r i n g e r s l i k e t h e p r o f i l e i n t h e core.

The s a l t i n t h i s c o n t r o l f a c i l i t y d i d not c i r c u l a t e . The f u e l systemwas contained i n a c e l l i n which an atmosphere of n i t r o g e n containing from 2 t o 5%0 w a s maintained.

This containment

atmosphere w a s r e c i r c u l a t e d through a system t h a t provided c o o l i n g f o r t h e c o n t r o l rods and t h e f r e e z e valves.

Arrays of INOR-8 specimens

were exposed t o t h e c e l l atmosphere as shown i n Fig. 3.

Suspended j u s t

o u t s i d e t h e r e a c t o r vessel b u t i n s i d e the vessel furnace, t h e specimens

w e r e exposed t o p r a c t i c a l l y t h e same neutron f l u x and temperature as t h e vessel w a l l s .

I '

d v1

aJ rn

3 0

2 a

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d U

F 0)

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E9 W

B N

ORNL-DWG 68-8298

THERMAL SHIELD

REACTOR VESSELSURVEILLANCE STRINGEF FLOW DISTRIBUTOR--, (1 in.

81 in

5 in.

TOP OF LATTICE ELEVATION 8 2 8 f t h

t i&-in. LONG NOSE PIECE-

u INCHES

Fig. 3.

MSRE Surveillance Facility Outside the Reactor Vessel.

H i story

The h i s t o r y of t h e MSRE during t h e f o u r y e a r s i n which i t operated r'

a t s i g n i f i c a n t power is o u t l i n e d i n Fig. 4.

Construction had been f i n -

ished and s a l t charged i n t o t h e tanks l a t e i n 1964.

Prenuclear t e s t i n g ,

i n c l u d i n g 1100 h r of s a l t c i r c u l a t i o n , occupied January-May, 1965. During n u c l e a r s t a r t u p experiments i n May-July, 1965, f u e l s a l t w a s c i r c u l a t e d f o r 800 h r .

The s a l t was drained and f i n a l p r e p a r a t i o n s f o r

power operations were made i n t h e f a l l of 1965.

LowLpower experiments

i n December l e d i n t o t h e h i s t o r y covered i n Fig. 4.

MSRP semiannual progress r e p o r t s f o r more d e t a i l . )

(See r e f . 1 and About a y e a r a f t e r

t h e conclusion of o p e r a t i o n , a l i m i t e d program of examination w a s c a r r i e d out.

This included INOR-8 pieces from a c o n t r o l rod thimble, t h e h e a t

exchanger s h e l l and tubes, t h e pump bowl cage and b a f f l e , and a f r e e z e valve. The n u c l e a r f u e l w a s 33%-enriched 235U, t h e f u e l s a l t w a s 0 . 8 mole % u n t i l 1968.

Then t h e uranium w a s iemoved

by f l u o r i n a t i o n and 233UF4 was s u b s t i t u t e d . quired w i t h 233U was onl;

and t h e UF4 concentration i n The UF4 concentration re-

0.13 mole %. The composition of t h e f u e l s a l t

w a s observed by frequent sampling from t h e pump bowl.

Aside from t h e

233U loading and p e r i o d i c a d d i t i o n s of small increments of uranium o r

plutonium t o s u s t a i n t h e nuclear r e a c t i v i t y , t h e only o t h e r a d d i t i o n s t o t h e f u e l s a l t were more o r less r o u t i n e s m a l l ( ~ 1 0g) q u a n t i t i e s of beryllium, and, i n two o r three experiments, a f e w grams of zirconium

and FeF2.

The purpose of t h e s e a d d i t i o n s was t o a d j u s t t h e U ( I I I ) / U ( I V )

r a t i o , which a f f e c t s t h e c o r r o s i o n p o t e n t i a l and t h e ' o x i d a t i o n state of corrosion-product i r o n and n i c k e l and fission-product niobium. The primary corrosion mechanism i n t h e f u e l s a l t system w a s s e l e c t i v e removal of chromium by 2UF4

+ Cr(in

alloy)

2UF3

+ CrFp(in

salt)

and t h e c o n c e n t r a t i o n of chromium i n s a l t samples w a s t h e primary indic

c a t o r of corrosion.

Figure 5 shows chromium concentrations observed i n

UUU-M 6 9 - 7 W Z SALT N NELLOOP

SALT IN FUELLWP

POWER

DYNAMTS TESTS

YE" STRIPPING EXPERIMENTS

INVEsrmTE

a=FGAs PUlGwffi REPLACE VALVES AND FKTERS R A E POWER

REWJR SAMPLER ATTAN M L POWER OKCK CONTNNMENT

TEST AM W F Y FLUORINE CiSFUSX SYSTEM

R66 J

ML-POWER RUN MUN &OWER FALURE

PROCESS FLU%

SALT

REPLACE MAIN B L W R MELT SALT FRW GAS UNES REPLACE CORE SAMPLES TEST CONTAINMENT

PROCESS FUEL SALT

RVN WlTH ONE BU)WER

2% ZERO-POWER PHYSICS EXPERlMENTS

LDAD U1ANkW-233 REMOM LDAtWG E W E

INSTALL SECOND W E R

MIESTIGATE N E L SALT BWAV03

} %-&ZMkZ

3 0 - 9 RVN

LYES

REWlR SAMRER AM CONTRU Roo C f U M

AT FULL POWER

DYNAMICS TESTS

INVESTGATE GAS "ELLOOP

HM-POWER OPERATKJN To MEPSURE

ACE CORE SPMPLES TEST CONTAINMENT

REPLACE

CORE SAMPLES

REpPJRmCfUMs

I969

1967

CLEAR

OFFGPS LNES

REWlR SAMPLER

INMSTIGAE COKR

XENON, AND FISSION PROWCT BEHAVIOR

RRADIAE ENCAPSUATED U

PS)O PLUTONIUM

MAP EP. EPOS(TlDN WITH GAMMA SPECTROMETER MEASURE TRITIUM, SAMPLE N E L

N+ D

FUEL

FLUSH

0 2 4 6 8 1 0 POWERlMwl

REMDVE CORE ARRAY PUT REACTOR IN STANDBY

Fig. 4.

Outline of the Four Years of MSRE Power Operation.

t h e MSRE f u e l over t h e y e a r s of power operation.

The s t e p down i n chro-

mium concentration i n t h e s a l t i n 1968 w a s e f f e c t e d by processing t h e s a l t i n 1968 w a s e f f e c t e d by processing t h e s a l t a f t e r t h e 235U f l u o r i i

nation.

The t o t a l i n c r e a s e i n chromium i n t h e 4700-kg charge of f u e l

s a l t is e q u i v a l e n t t o leaching a l l of t h e chromium from t h e 852 f t 2 of INOR-8 exposed t o f u e l s a l t t o a depth of about 0.4 m i l . I

Throughout t h e operation of t h e MSRE a sample a r r a y of one kind o r another w a s p r e s e n t i n t h e core.

The a r r a y s t h a t were exposed between

September 1965 and June 1969 were of t h e design shown i n Fig. 2.

From

t h e t i m e of c o n s t r u c t i o n u n t i l August 1965 t h e specimen a r r a y i n t h e ( t o have t h e same

core contained s i m i l a r amounts of g r a p h i t e and INOR-8

nuclear r e a c t i v i t y e f f e c t ) b u t d i f f e r e d i n i n t e r n a l configuration.

During

t h e l a s t f i v e months of o p e r a t i o n , an a r r a y designed t o study t h e e f f e c t s of s a l t v e l o c i t y on f i s s i o n product deposition14 was exposed i n t h e core. Whenever a core specimen assembly of t h e type shown i n Fig. 2 was removed from t h e c o r e , i t w a s taken t o a h o t c e l l , t h e s t r i n g e r s were taken out of t h e b a s k e t , and a new assembly was prepared, u s u a l l y including one o r two of t h e previously exposed s t r i n g e r s . o l d basket w a s reused, sometimes not.

Sometimes t h e

The h i s t o r y of exposure of INOR-8

specimens i n t h i s core f a c i l i t y is o u t l i n e d i n Fig. 6.

The numbers in-

d i c a t e t h e h e a t s of INOR-8 from which t h e rods i n each s t r i n g e r w e r e made. Heats 5065, 5085, and 5081 were h e a t s of standard INOR-8 used i n f a b r i -

c a t i o n of t h e

The o t h e r h e a t s w e r e of modified composition

designed t o improve t h e resistance t o neutron embrittlement. Specimens exposed o u t s i d e t h e r e a c t o r v e s s e l were made of t h r e e of the h e a t s t h a t were a l s o exposed i n s a l t .

Specimens of h e a t s 5065 and

5085 w e r e exposed from August 1965 t o June 1967 and from August 1965 t o May 1968; specimens of modified h e a t 67-504 were exposed from June 1967 t o June 1969. Table 1 lists t h e chemical compositions of t h e h e a t s of standard INOR-8 t h a t were used i n t h e s u r v e i l l a n c e specimens and i n various items

t h a t were examined a f t e r exposure t o t h e f u e l s a l t .

The compositions of

*The r e a c t o r vessel s i d e s were of 5085; t h e heads, of 5065. w a s used f o r some of t h e v e s s e l i n t e r n a l s .

Heat

ORNL-WG 70-2!64

150

140

I30 I20 110

- f00 -g 90

g m $60 50 40 30

RUN Nc Fl

Corrosion of the MSRE Fuel Circuit in 235U and 233U Power Operations.

Fig. 5.

ORNL-DWG 72-7495

5085 5081

STRINGER L

5085 5065

STRINGER R

STRINGER S

67-551

67-504

7320

CORE TEMPERATURE HIGH SALT IN CORE

RUN NO.

1965

Fig. 6.

1966

1967

1968

Outline of MSRE Core Surveillance Program.

1969

. I

' c

Table 1. Heats of Standard INOR-8 Examined A f t e r Exposure in MSRE

Specimens Exposed

Heat

508Sa

Rods on L1, R l , ' L1, L2, R2,

5081a

Rods on L1. R1,

Content (wt X) P . cu

3.5

0.67

0.052

0.58

0,004

0.0043

7.1

3.5

0.65

0.059

0.52

0.002

16.7

7.3

16.0

0.01

0.15

0.02

0.20

<0.01

0.07

0.0038

0.012

0.01

0.10

0.05

0.20

<0.01

0.07

0.0040

xl. x2

s1 . 5065b

Rods on L2, R2. x1, x2

16.5

7.3

3.9

0.55

0.065

0.60

0.007

0.004

0.01

0.08

0.01

0.22

0.01

0.04

0.0024

5055

S t r a p s on R2, L2. R3. 54

16.1

7.5

3.8

0.43

0.07

0.64

0.008

0.003

0.03

0.11

0.08

0.24

0.02

0.26

0.001

5075

F o i l on R3. S4 and Sampler m i s t shield

16.4

6.64

4.0

0.46

0.07

0.58

0.006

0.003

0.01

0.06

0.02

0.26

0.02

0.09

0.001

5059

Sampler cage

16.9

6.62

3.9

0.35

0.07

0.59

0.003

0.001

0.07

0.01

0.21

0.01

0.04

Y-8487

Rod thimble

16.8

7.3

4.1

0.3

0.05

0.17

0.0075

0.004

0.03

0.1

0.16

0.52

0.006

0.001

0.01

0.07

0.01

0.28

0.25

0.007

0.01

0.005

5060

Rod' thimble sleeve

16.4

7.05

3.9

0.45

0.06

5068

Heat exchanger

16.5

6.45

4.0

0.45

0.05

0.58

0.008

0.03

0.02

0.1

0.01

0.27

0.01

16.4

6.9

3.9

0.45

0.06

0.60

0.009

0.001

0.01

0.1

0.01

0.33

0.01

Freeze v a l v e 105 16.3

7.1

3.8

0.52

0.07

0.76

0.007

0.001

0.01

0.08

0.02'

0.39

shell N2-5105 Heat exchanger tubes 5094

%ess than 0.002% Z r bLess than 0.1% Zr o r Hf 'A1 p l u s Ti

0.06

0.006

0.05

0.004

14 t h e modified h e a t s t h a t were exposed and then examined are l i s t e d i n Table 2.

These a l l o y s , b e s i d e s i n c l u d i n g a d d i t i o n s of T i , Hf, W, o r Z r ,

Lj e

had s u b s t a n t i a l l y less Mo, only a f r a c t i o n as much Fe, and less Mn, V, and S i than d i d t h e s t a n d a r d a l l o y s .

I n t h e c o r r e l a t i o n of the e f f e c t s of exposure f o r d i f f e r e n t periods of o p e r a t i o n , some common index of exposure would b e u s e f u l .

Possible

i n d i c e s are (1) t h e t i m e that t h e specimens w e r e exposed t o s a l t , (2) t h e t o t a l generation of n u c l e a r h e a t (and f i s s i o n products) during t h e t i m e t h e specimens w e r e exposed, (3) t h e t o t a l t i m e t h a t t h e specimens were

a t high temperature, and (4) t h e t i m e a t high temperature a f t e r exposure t o s a l t c o n t a i n i n g f r e s h f i s s i o n products.

As w i l l be discussed l a t e r ,

some r a t i o n a l e exists f o r using each of t h e s e , so a l l are included i n t h e s k a r y of exposures given i n Table 3.

EXAMINATION OF MSRE SURVEILLANCE SPECIMENS INOR-8 specimens removed from t h e c o r e and from t h e c o n t r o l f a c i l i t y w e r e s u b j e c t e d t o a v a r i e t y of examinations and tests.

F i r s t was a visual

i n s p e c t i o n f o r evidence of any d e p o s i t i o n o r c o r r o s i o n , p a r t i c u l a r l y any nonuniform o r l o c a l i z e d c o r r o s i o n .

Metallographic examination of

s e l e c t e d specimens w a s used t o g i v e more information:on t h e c o m p a t i b i l i t y of INOR-8 with the s a l t environment.

The major p a r t of t h i s e f f o r t w a s

mechanical property t e s t i n g i n connection w i t h t h e s t u d i e s on neutron embrittlement.

Creep tests t o determine creep rates, r u p t u r e l i f e , and

r u p t u r e s t r a i n were conducted a t 650째C and stress l e v e l s from 17,000 t o 55,000 p s i .

T e n s i l e tests g i v i n g y i e l d stress, u l t i m a t e stress, and

f r a c t u r e s t r a i n were made a t temperatures from 25 t o 850OC.

Selected

samples t h a t had been t e n s i l e t e s t e d a t 25 and a t 650째C w e r e examined metallographically.

S i m i l a r i n s p e c t i o n s and tests were given t h e s p e c i -

mens exposed t o t h e c e l l atmosphere.

A t e n s i l e specimen of INOR-8 being s t r a i n e d a t an e l e v a t e d tempert

a t u r e normally develops some f i s s u r e s b e f o r e t h e specimen f r a c t u r e s . The f i s s u r e s are i n t e r g r a n u l a r and t h e f r a c t u r e i s i n t e r g r a n u l a r

i s normal a t high temperatures.

this

A t room temperature, on t h e o t h e r hand,

t h e normal kind of f r a c t u r e is mostly t r a n s g r a n u l a r and f i s s u r e s away

Table 2.

Heat

Specimens Exposed

Heats of Modified INOR-8 Examined After Exposure t o MSRE Fuel S a l t

Content (wt X ) P cu

0.02

0.06

0.49

<0.01

0.10

0.01

0.0002

0.35

0.0002

21545

Rods on S2

12.0

7.18

0.034

0.29

0.05

0.015

<0..002

0.001

21554

Rods on S 2

12.4

7.4

0.097

0.16

0.065

0.01

C0.002

0.004

67-502

Rods

12.7

7.24

0.08

0.14

0.04

CO.01

0.004

0.003

0.04

0.02

0.12

0.06

0.53

<0.01

2.15

<0.01

0.0001

67-504

Rods on S 3

12.4

6.94

0.05

0.12

0.07

0.010

0.003

0.002

0.03

0.02

0.03

0.22

<0.02

0.50

0.03

0.01

0.0003

67-551

Rods on R 3 , S3 Rods on R3,S3

12.2

7.0

0.02

0.12

0.028

0.02

C0.002

0.0006

0.01

<0.05

12.0

7.2

<0.05

0.11

0.059

0.03

0.003

0.002

0.02

0.03 0.01

7320

0.04

0.02

0.03

0.15

0.003

<0.001 1.1 <0.02

0.65

0.001 <0.01 <O.b5 <0.05

0.0002 0.00002

Table 3.

Extent of Exposure of INOR-8 Specimens i n MSRE

Time a t high temperaturea (hr)

Date

Specimen

out

Total

With

Time . i n fuel

Fpb

(hr)

Reactor heat

neutrons/cm2)

Thermal Neutron Fluence'

x 1020

Pre-power c o r e a r r a y S t r i n g e r s RL1, RR1, RS1

8/65 7/28/66

3,306 5,550

2,550

2,813

5/15/67

5,554

5,220

4,112

S t r i n g e r RS3

9/16/66 5/31/67

4/2/68

6.379

6,300

S t r i n g e r RR2

9/16/66

4/2/68

11,933

11.600

S t r i n g e r RR3, RS4

4/18/68

6/6/69

7,203

3,310

4,868

S t r i n g e r RL2

9/16/66

6/6/69

19,136

18,800

Final core array

7/31/69

12/18/69

2,815

2,360

Components of f u e l loop

1964

1970

30,807

Stringer X1

8/24/65

6/5/67

S t r i n g e r X2

8/24/65

5/7/68

S t r i n g e r X4

6/7/67

6 1 I69

S t r i n g e r RS2

1964 9/8/65

1,090

0.0

7,980

1.3

25,120

4.1

5,877

32.990

5.3

9,989

58.110

9.4

18,720

5.1

14,857

76,830

14.6

2,280

11,870

3.2

24,500

21,040

96,680

0-19

11.104

33,100

0.13

17,483

66.100

0.26

13,582

51,700

0.25

'Time logged above 50OoC. A l l w a s a t 650 f 10째C w i t h t h e exception of 100 h r a t 76OoC i n October, 1965, about 750 h r a t 500-600째C i n February-March, 1966, and 500 h r a t 63OoC i n December, 1967-February, 1968. bTime above 500'C a f t e r exposure of specimen t o f u e l s a l t c o n t a i n i n g f r e s h f i s s i o n products.

'Fluence of n e u t r o n s w i t h Ec0.876 e V , based on f l u x monitor measurements made d u r i n g 235U o p e r a t i o n , with c a l c u l a t e d c o r r e c t i o n f o r h i g h e r f l u x d u r i n g 233U o p e r a t i o n .

17 from t h e f r a c t u r e are abnormal.

Thus a h i g h incidence of i n t e r g r a n u l a r

cracking i n samples deformed a t 25OC is a d e f i n i t e i n d i c boundary embrittlement o r weakness.

For t h i s reason, i n t h e d e s c r i p t i o n s

t h a t follow, a t t e n t i o n w i l l be focussed p r i m a r i l y on t h e metallography of u n s t r a i n e d specimens o r those s t r a i n e d a t 25째C.

The r e s u l t s of t h e me-

c h a n i c a l property t e s t i n g have been f u l l y r e p ~ r t e d ~ ' ~ and , only those r e s u l t s t h a t are r e l e v a n t t o t h e s u r f a c e cracking phenomenon w i l l be mentioned here. Specimens Exposed Before Power Operation The specimen a r r a y t h a t was i n t h e core during t h e prenuclear t e s t i n g qnd nuclear s t a r t u p experiments w a s t h e r e p r i m a r i l y as a neutronic standi n f o r t h e l a t e r s u r v e i l l a n c e assemblies.

Although i t contained similar

amounts of g r a p h i t e and metal, i t s design was d i f f e r e n t from t h a t of

l a t e r assemblies, and t h e INOR-8 used w a s from u n i d e n t i f i e d h e a t s of standard a l l o y .

The post-exposure examinations w e r e a l s o l i m i t e d .

How-

ever, t h e r e s u l t s are of some

terest, because t h e a r r a y had been a t

high temperature and exposed

s a l t f o r reasonably long t i m e s :

a t high

temperature f o r 3306 h r , exposed t o f l u s h s a l t f o r 990 h r , and exposed t o uranium-bearing s a l t f o r 1090 h r (see Table 3). The only v i s i b l e change i n t h e specimens as a r e s u l t of t h e i r exposure was t h a t t h e o r i g i n a l l y shiny specimens of INOR-8 had developed a b r i g h t gray-white matte surface.15

T h e microstructure of one of t h e

sp.ecimens a f t e r i t w a s fracpured a t 25OC is shown i n Fig. 7. boundary cracks are v i s i b l e .

No grain-

The 1 - m i l s u r f a c e l a y e r t h a t etched more

darkly caused some concern a t f i r s t .

However, as w i l l be brought o u t

l a t e r , t h e modified s u r f a c e l a y e r w a s 'present on c o n t r o l specimens as

w e l l as on those exposed i n t h e core and, on a series of specimens, showed no s y s t e m a t i c v a r i a t i o n w i t h exposure t i m e .

Further work showed

t h a t i t was l i k e l y cold-work from machining causing carbides t o p r e c i p i -

tate more r e a d i l y near t h e s u r f a c e . connection t o s u r f a c e cracking.)

(See l a t e r d i s c u s s i o n of p o s s i b l e

Q 2

. Fig. 7 . Microstructure of INOR-8 Sample Held Above 5OOOC for 3306 hr During the "Zero-power" Run of the MSRE. (Fuel was i n system 1090 hr.) (a) A s polished. (b) Etched with glyceria regia. The material i s cold worked, and the grains are not delineated by etching.

Li'

important p o i n t t

b e noted h e r e is t h a t INOR-8

pecimens expos d

t o f u e l s a l t f o r a considerable period of t i m e b e f o r e t h e generation of s u b s t a n t i a l nuclear power (and fission-product i n v e n t o r i e s ) showed no in-

i n t e r g r a n u l a r s u r f a c e cracks upon t e s t i n g a t 25OC. F i r s t Group of S u r v e i l l a n c e Specimens ( S t r i n g e r s RLl, RR1, and RS1)

When t h e f i r s t standard specimen a r r a y was taken out i n J u l y , 1966, p o r t i o n s w e r e found t o have been damaged.*

Because of t h e damage, none

of t h e t h r e e s t r i n g e r s could be put back i n t o t h e c o r e , but less t h a t one t h i r d of t h e INOR-8 specimens were a f f e c t e d , so t h e r e were p l e n t y of each h e a t (5085 and 5081) t h a t could be t e s t e d . mens from t h e c o n t r o l f a c i l i t y were a l s o t e s t e d .

Corresponding speciThe extensive r e s u l t s

are reported i n d e t a i l . 4

Corrosion of t h e core specimens appeared t o b e . v e r y minor.

v i s u a l i n s p e c t i o n t h e m e t a l s u r f a c e s w e r e a uniform d u l l gray, w i t h no s i g n of l o c a l i z e d corrosion.

Metallography a l s o revealed l i t t l e o r no

p e r c e p t i b l e corrosion. T e n s i l e tests showed a s m a l l decrease f n t h e y i e l d s t r e n g t h of t h e c o n t r o l specimens compared w i t h t h e unexposed specimens and those exposed i n the core.

U l t i m a t e t e n s i l e stresses decreased s i g n i f i c a n t l y from

t h e unexposed specimens t o t h e c o n t r o l specimens t o t h e core specimens, as shown i n Table 4.

T h e v a r i a t i o G i n t h e u l t i m a t e t e n s i l e stress is

a s s o c i a t e d with decreases i n f r a c t u r e s t r a i n .

(INOR-8 t e n s i l e specimens

t e s t e d a t 25OC continue t o strain-harden t o near f r a c t u r e , so any reduct i o n i n f r a c t u r e s t r a i n would cause f a i l u r e a t a lower u l t i m a t e stress.) Examination of t h e s e and l a t e r specimens i n d i c a t e d t h a t t h e reduction i n f r a c t u r e s t r a i n a t 25OC was n o t connected w i t h t h e surface-cracking phenomenon, but w a s due t o c a r b i d e p r e c i p i t a t i o n t h a t occurred upon aging and w a s enhanced by i r r a d i a t i o n . *The'damage occurred because, when t h e c o r e was drained, some s a l t c

was trapped between t h e specimens, where i t f r o z e and i n t e r f e r e d w i t h t h e d i f f e r e n t i a l c o n t r a c t i o n of t h e graphite-metal 'assembly during cooldown. l6 This problem w a s avoided i n subsequent assemblies by a s l i g h t design change.

Table 4. T e n s i l e P r o p e r t i e s o f S u r v e i l l a n c e Samples From F i r s t Group a t 25OC and a S t r a i n Rate of O.O5/min

Heat

Conditiona

Yield Stress (PS i)

U 1t i m a t e Tensile S t r e s s (Psi)

Unifoim E 1on ga t ion %

Total E lon ga t ion %

5081

Ann ea l e d

52,600

125,300

56.7

59.5

50.5

5081

Control

47,700

118,700

55.9

57.6

48.8

5081

Irradiated

51,100

105,500

38.5

38.7

31.3

50 81

Irradiated

54,100'

109,000

42.6

25.9

5085

Annealed

51,500

120,800

52.3

53.1

42.2

5085

Control

46,200

109,200

40.0

50 85

Control

45,500

111,200

46.8

31.5

5085

Irradiated

48,100

100,300

34.3

34.5

26.0

Reduction In Area %

28.6

a Annealed -Annealed 2 h r a t 900OC. C o n t r o l - Annealed 2 h r a t 900째C, annealed 4800 h r a t 65OOC i n s t a t i c s a l t . I r r a d i a t e d -Annealed 2 h r a t 900째C. i r r a d i a t e d t o a thermal f l u e n c e of 1 . 3 x 1020 neutrons/cm2 over 5550 h r a t 65OOC i n t h e *RE.

E f f e c t s o f Carbide P r e c i p i t a t i o n An observation t h a t connected t h e decrease i n 25OC f r a c t u r e s t r a i n

with c a r b i d e p r e c i p i t a t i o n w a s t h e following.

The f r a c t u r e s t r a i n a t

room temperature of i r r a d i a t e d specimens could be improved by an anneal of 8 h r a t 870째C (p. 1 7 , r e f . 5 ) .

This is a c a r b i d e agglomeration anneal,

and t h e recovery of d u c t i l i t y by such an anneal suggested t h a t t h e emb r i t t l e m e n t w a s due t o the p r e c i p i t a t i o n of copious amounts of carbide. The p r e c i p i t a t e was observed and i d e n t i f i e d as MgC, which i s b r i t t l e a t room temperature.

E x t r a c t i o n r e p l i c a s showed more p r e c i p i t a t e i n c o r e

specimens than i n c o n t r o l specimens.

Thus i t appeared t h a t , a t least i n

t h e s t a n d a r d a l l o y , i r r a d i a t i o n enhanced t h e n u c l e a t i o n and growth of t h e p r e c i p i t a t e t h a t occurs t o some e x t e n t a t high temperature without irradiation. Metallography of specimens broken i n t e n s i o n a t 25OC revealed d i f f e r ences i n t h e n a t u r e of t h e f r a c t u r e s i n c o r e and c o n t r o l specimens.

These

d i f f e r e n c e s are b e l i e v e d t o be another m a n i f e s t a t i o n of c a r b i d e p r e c i p i t a tion.

Figures 8 and 9 show s

and from t h e core.

imens of h e a t 5081 from t h e c o n t r o l

The f r a c t u r e i n t h e c o n t r o l specimen (Fig. 8 )

is t y p i c a l of t r a n s g r a n u l a r shear-type f a i l u r e (cup-cone appearance).

c o n t r a s t , t h e f r a c t u r e of t h e core specimen (Fig. 9) i s l a r g e l y i n t e r granular.

H e a t 5085 specimens are shown i n Figs. 10 and 11. The f r a c t u r e

i n t h e 5085 c o n t r o l specimen is mixed t r a n s g r a n u l a r and i n t e r g r a n u l a r , w i t h t h e elongated g r a i n s a t t e s t i n g t o t h e l a r g e amount of s t r a i n .

The f r a c -

t u r e i n t h e 5085 core specimen i s l a r g e l y i n t e r g r a n u l a r w i t h numerous i n t e r g r a n u l a r cracks i n t h e m i c r o s t r u c t u r e . Surf ace Cracking Specimens of both h e a t s exposed i n the c o n t r o l f a c i l i t y and t e s t e d i n t e n s i o n showed no cracks except very n e a r t h e f r a c t u r e s .

The c o r e

specimens of both h e a t s , on t h e o t h e r hand, showed s e v e r a l i n t e r g r a n u l a r

cracks along t h e gage l e n g t h s .

The d i f f e r e n c e i s c l e a r l y shown i n Fig.

1 2 , which is a composite of photomicrographs of l o n g i t u d i n a l s e c t i o n s of s t r a i n e d c o n t r o l and c o r e specimens of h e a t 5085.

P i c t u r e s of

t h e s e c t i o n s along t h e e n t i r e gage l e n g t h s w e r e examined t o determine

r7' zz

Fig. 9. Photomicrographs of Surveillance Specimen D-16 from Heat 5081 Tested at 25OC and at a Strain Rate of 0.05/min. (a) Fracture. (b) Edge of specimen about 1/4 in. from fracture. Etchant:'* glyceria regia.

. .

. I

I i i ~

I I ~

Fig. 10. Photomicrographs of Control Specimen DC-24 from Heat 5085 Tested at 25OC and at a Strain Rate of 0.05/min. (a) Fracture. (b) Edge of specimen about 1 / 4 in. from fracture. Etchant: glyceria regia.

ti . 8

Fig. 12. Photomicrographs of Heat 5085 strained a t 25OC After 5550 hr Above 5 0 0 O C . The top specimen was i n s t a t i c f u e l s a l t containing d e p l e t e d uranium, and the lower specimen was i n the MSRE core. \The true specimen diameters are 0.070 and 0.090 i n . , respectively.

frequency and depth of cracks.

I n the s e c t i o n of t h e c o n t r o l specimen,

only one s u r f a c e crack was found (a frequency of about 1 p e r i n c h ) . depth w a s 5.7 mils.

Its

The s e c t i o n of t h e core specimen showed 19 cracks

p e r inch with an average depth of 2.5 mils and a maximum depth of 8.8 mils.

The core specimen has more s u r f a c e cracks than t h e c o n t r o l speci-

men (and t h e pre-power core specimen). Second Group of Surveillance Specimens ( S t r i n g e r RS2) I n May 1967 s t r i n g e r RS2, w i t h spedmens of two modified h e a t s , w a s removed, and a s t r i n g e r containing specimens of two o t h e r modified h e a t s was installed.

A t t h e t i m e i t was removed, s t r i n g e r RS2 had been a t high

temperature f o r p r a c t i c a l l y t h e same length of t i m e as t h e f i r s t group, b u t had seen 3 t i m e s the neutron dose and f i s s i o n product concentration. The core specimens from RS2 and corresponding c o n t r o l specimens ( s t r i n g e r CS2) were i n t e n s i v e l y examined and t e s t e d . Visual i n s p e c t i o n showed t h e core specimens t o b e very s l i g h t l y d i s colored b u t otherwise apparently unaffected.

Photomicrographs of speci-

mens (unstrained) a f t e r exposure are shown i n Figs. 13 and 14.

(The

unusually f i n e g r a i n s i z e is due t o t h e f a b r i c a t i o n h i s t o r y of t h e specimens, which included 100-hr anneals a t 870째C.)

The as-polished

views of specimens of both heats show some evidence of g r a i n boundary modifications t o a depth of 1 t o 2 mils, and b o t h show some tendency t o e t c h more r e a d i l y near t h e s u r f a c e . Core specimens of both h e a t s t e s t e d t o f a i l u r e i n t e n s i o n developed numerous i n t e r g r a n u l a r cracks along t h e s u r f a c e s , while c o n t r o l specimens t e s t e d s i m i l a r l y showed few o r no s u r f a c e cracks. i l l u s t r a t e d i n Figs. 15-18.

The d i f f e r e n c e is

Figure 15 and 16 are c o n t r o l and core speci-

mens r e s p e c t i v e l y of h e a t 21554.

I n t h e core specimen s u r f a c e s n e a r l y

every g r a i n boundary is cracked t o a c o n s i s t e n t depth of about 2 mils. I n the c o n t r o l specimen t h e r e is no evidence of i n t e r g r a n u l a r edge cracking.

Figures 17 and 18 show t h e same t h i n g f o r h e a t 21545.

. Fig. 13. Photomicrographs of Zirconium-Modified INOR-8 (Heat 21554) Removed from the MSRE after 5554 hr above 500째C. Edge exposed to flowing salt. (a) As polished. 500x. (b) Etchant: aqua regia. 1OOx. (c) Etchant: aqua regia. 500x. Reduced 30%.

Fig. 14. Photomicrographs oâ&#x201A;Ź Titanium-Modified INOR-8 (Heat 21545) Removed from the MSRE After 5554 hr above 500OC. Edge exposed to flowing salt. (a) As polished. 500x. (b) Etchant: aqua regia. 1OOx. (c) Etchant: aqua regia. 500x. Reduced 33%.

Fig. 15.

Photomicrograph of the Fracture of a Zirconium-Modified

INOR-8 Sample (Heat 21554) Tested a t 25OC a t a Strain Rate of O.O5/min. Exposed t o a s t a t i c fluoride s a l t f o r 5554 hr above 5OO0C before testing. Note the shear fracture and the absence of edge cracking. 1OOx. Etchant. glyceria regia.

Fig. 16. Photomicrographs of a Zirconium-Modified INOR-8 S u r v e i l l a n c e Sample (Heat 21554) Tested a t 25OC a t a S t r a i n Rate of 0.05/min. Exposed i n t h e MSRE c o r e f o r 5554 h r above 5 O O O C t o a thermal f l u e n c e of 4.1 x 1020 neutrons/cm2. Etchant: aqua r e g i a . (a) Fracture. 1OOx. (b) Edge of sample about 1 / 2 i n . from f r a c t u r e . 1OOx. (c) Edge of sample showing edge cracking. 500x. Reduced 31.5%.

Fig. 1 7 . . Photomicrograph of the Fracture of a Titanium-Modified INOR-8 Surveillance Sample (Heat 21545) Tested a t 25OC a t a Strain Rate of O.OS/min. Exposed t o a s t a t i c fluoride s a l t for 5554 hr above 5OO0C before testing. Note the shear fracture and. the absence of edge cracking. 1OOx. Etchant: gly ceria regia.

Yield and u l t i m a t e s t r e n g t h s a t 25'C w e r e not appreciably d i f f e r e n t f o r t h e c o n t r o l and c o r e

specimen^.^

Furthermore, although t h e edges of

core specimens cracked i n t e r g r a n u l a r l y , the f r a c t u r e s were predominantly o r e n t i r e l y t r a n s g r a n u l a r , as seen i n Figs. 16 and 18. The f a c t t h a t a t

25OC t h e u l t i m a t e stresses were n o t diminished by exposure and t h a t t h e f r a c t u r e s were not i n t e r g r a n u l a r i n these modified a l l o y s ( i n c o n t r a s t

t o t h e observations on t h e s t a n d a r d h e a t s ) is a t t r i b u t e d t o t h e graini

boundary c a r b i d e p r e c i p i t a t i o n being less e m b r i t t l i n g i n the modified

alloy.

The most important observation r e l a t i v e t o t h e cracking phenomenon 1

i s t h a t the two modified a l l o y s (heats 21554 and 21545) w i t h much smaller

l I

g r a i n s i z e s e x h i b i t e d i n t e r g r a n u l a r cracking when deformed a f t e r exposure t o the f u e l salt.

Although t h e depths of t h e cracks are less i n t h e two

modified a l l o y s t h e same types of cracks are fornied i n both t h e standard l

and modified samples.

The a l l o y s involved had s i g n i f i c a n t v a r i a t i o n s i n

Fe (4% t o <0.1%), Mo (16.7% t'o 12.0%), S i (0.6% t o 0.1%), Z r (<0.1% t o

0.35%), and T i (<0.01% t o 0 . 4 9 % ) , and the f a c t t h a t a l l formed i n t e r g r a n ular cracks i n d i c a t e s t h a t these compositional v a r i a t i o n s are not impor,

t a n t i n t h e cracking process. Third Group of S u r v e i l l a n c e Specimens ( S t r i n g e r s RR2 and RS3) A t t h e conclusion of o p e r a t i o n w i t h 235U f u e l , t h e core a r r a y was

removed and specimens from two of t h e t h r e e s t r i n g e r s were t e s t e d .

The

remaining s t r i n g e r , containing specimens of v e s s e l h e a t s , was put back i n t o t h e c o r e f o r more exposure along with two new s t r i n g e r s c o n t a i n i n g !I

I I ~

specimens of modified h e a t s .

The s t r i n g e r s from t h e c o r e and t h e cor-

responding s t r i n g e r s from t h e c o n t r o l f a c i l i t y included two h e a t s (5065 and 5085) of standard INOR-8 and two d i f f e r e n t modified a l l o y s . Complete

I I

r e s u l t s of t e s t i n g of t h e s e spqcimens are reported. Visual examination showed t h e INOR-8 t o be s l i g h t l y d i s c o l o r e d b u t otherwise i n very good condition.17

u .

Standard Alloys S t r i n g e r s RR2, with t h e standard h e a t s , had been exposed 2 t o 4

t i m e s as long as t h e standard a l l o y specimens i n t h e f i r s t group. f a c t o r depends upon which exposure index i s used; see Table 3.)

(The Effects

of t h e longer exposure were evident i n metallographic examinations of u n s t r a i n e d specimens.

Figures 19 and 20 are of specimens of h e a t s 5065

and 5085 exposed on s t r i n g e r RR2 i n t h e core.

Some of t h e g r a i n bound-

aries near t h e s u r f a c e are v i s i b l e i n t h e as-polished condition, and a few appear t o b e opened as small cracks with a maximum v i s i b l e depth of about 1 m i l .

The etched views show t h e l a r g e amounts of carbide pre-

c i p i t a t e t h a t formed along t h e g r a i n boundaries and t h e modified s t r u c t u r e near t h e s u r f a c e , which is thought t o be due t o working from machining.

I n samples of t h e s e h e a t s exposed i n t h e c o n t r o l f a c i l i t y ,

g r a i n boundaries were not v i s i b l e i n as-polished samples. This is shown f f o r h e a t 5065 i n Fig. 21. As i n t h e core specimens, however, e t c h i n g brought out t h e c a r b i d e p r e c i p i t a t e and t h e modified s t r u c t u r e near t h e surface.

The m i c r o s t r u c t u r e of c o n t r o l specimens of h e a t 5085 w a s q u i t e

similar. Cracking a t t h e s u r f a c e of unstrained material w a s even more evident i n t h e examination of t h e s t r a p s t h a t bound s t r i n g e r RR2.

These

straps and those around t h e c o n t r o l s t r i n g e r CR2 were of standard INOR-8

h e a t 5055.

A s shown i n Fig. 22, t h e s t r a p exposed i n t h e core had cracks

t o a depth of about 1.5 mils.

Cracks were d i s t r i b u t e d uniformly, both

on s u r f a c e s exposed t o flowing s a l t and those f a c i n g t h e g r a p h i t e specimens.

I n c o n t r a s t , t h e s t r a p s exposed i n t h e c o n t r o l f a c i l i t y d i d not

show any cracks.

The s t r a p s a f t e r being formed, had been annealed f o r

1 h r a t 1 1 8 O O C and should n o t have been s t r e s s e d t h e r e a f t e r s i n c e they expanded more a t high temperature than d i d t h e g r a p h i t e they enclosed.

Samples of h e a t s 5065 and 5085 t h a t had been exposed i n t h e core and i n the c o n t r o l f a c i l i t y w e r e t e s t e d i n t e n s i o n a t 25OC, w i t h t h e r e s u l t s shown i n Table 5.

Some of t h e s e s t r a i n e d specimens w e r e s e c t i o n e d and

examined metallographically.

Fig. 19. Photomicrographs of INOR-8 (Heat 5065) Surveillance Specimens Exposed t o Fuel S a l t for 11,933 hr above 5OO0C, 500x. Shallow reaction layer i s seen near surface. (a) Unetched. (b) Etched (glyceria regia).

Fig. 20. Photomicrographs of INOR-8 (Heat 5085) Surveillance Specimens Expdsed t o Fuel Salt for 11,933 hr above 50OoC. 500x. Shallow reaction layer is seen near surface. (a) Unetched. (b) Etched (glyceria regia).

Fig. 21. Photomicrographs of INOR-8 (Heat 5065) Surveillance Control Specimens Exposed t o S t a t i c Barren Fuel Salt for 11,933 hr above 900OC. Note the shallow reaction layer near the surface. (a) Etched. 1OOx. (b) A s polished. 500x. (c) Etched, 500x. Etchant: glyceria regia.

liâ&#x20AC;&#x2122;

-20

-40

- -60

r -g

B x

-100

-120

-140

â&#x20AC;&#x2DC;W

Fig. 22. Photomicrographs of INOR-8 (Heat 5055) a f t e r Exposure to (a) the MSRE Core and (b) the MSRE Control F a c i l i t y for 11,933 hr above 500OC. This material was used for straps for the surveillance assembly. As polished. 500x.

~ _ _.

. _ .. ... __. . ......

.......

....

. .

Table 5. T e n s i l e P r o p e r t i e s of Surveillance Samples From Third Group a t 25OC and a S t r a i n Rate of 0.05/min

Heat

. Conditiona

Yield Stress (Psi)

U 1t imate Tensile S t r e s s (Psi)

Uniform Elongat ion %

Total Elongation %

Reduction I n Area, %

5085

Anneal e d

51,500

120,800

52.3

53.1

42.2

5085

Outside

46,500

99,100

32.8

24.5

50 85

Control

53,900

115,900

38.4

38.6

29.7

5085

Core

52,300

95,000

28.7

28.9

20.0

5065

Annealed

56,700

126,400

52.9

55.3

50.0

5065

Outside

49,000

118,800

57.8

59.7

38.4

5065

Control

60,900

126,700

46.5

47.4

39.3

5065

Core

51,700

109,300

41.4

41.5

34.1

a Annealed -Annealed 2 h r a t 900OC. Outside -Annealed, i r r a d i a t e d t o a thermal fluence of 2.6 x lOI9 neutrons/cm2 over a period of 17,483 h r a t 650OC. Control -Annealed, exposed t o depleted f u e l s a l t f o r 11,933 h r a t 650OC. Core - Annealed, i r r a d i a t e d t o a thermal f l u e n c e of 9.4 x 1020 neutrons/cm2 over a period of 11,933 h r a t 650OC.

. .. ..

. ..

41 Microstructures of s t r e s s e d samples of heat 5065 from t h e c o n t r o l f a c i l i t y and from t h e core are shown i n Figs. 23 and 24, r e s p e c t i v e l y . Numerous i n t e r g r a n u l a r edge cracks are found i n t h e sample from t h e c o r e (Fig. 24) and very few i n t h e sample from the c o n t r o l f a c i l i t y (Fig. 23). The f r a c t u r e s of both of these samples are l a r g e l y i n t e r g r a n u l a r a t a l l l o c a t i o n s and n o t j u s t near t h e edge, which is f u r t h e r evidence t h a t t h e reduction i n f r a c t u r e s t r a i n (Table 5) is due t o carbide p r e c i p i t a t i o n along t h e g r a i n boundaries and not r e l a t e d t o t h e i n t e r g r a n u l a r cracking near t h e s u r f a c e .

Tested samples of heat 5085 from t h e c o n t r o l f a c i l i t y

and t h e core are shown i n Figs. 25 and 26, r e s p e c t i v e l y . views i n the l a t t e r f i g u r e show some of t h e

The as-polished

arbide p a r t i c l e s t h a t

f r a c t u r e d during t e s t i n g . The samples t h a t were f r a c t u r e d a t 25OC

epolished a t a l a t e r

d a t e , one-half of each f r a c t u r e d sample w a s photographed. m i c r o s t r u c t u r e s f o r h e a t 5065 are shown i n Fig. 27.

The composite

There are obviously

more i n t e r g r a n u l a r cracks i n t h e sample exposed t o t h e core than i n t h e b

sample exposed i n t h e c o n t r o l f a c i l i t y .

There were 3 cracks p e r inch

with an average depth of 1.0 m i l i n t h e c o n t r o l sample and 230 cracks c

p e r inch with an average depth of 1.8 mils i n t h e sample from t h e core. A composite,photograph of t h e t e s t e d p o r t i o n s of t h e h e a t 5085 sample i s shown i n Fig. 28.

There were 134 cracks p e r inch along t h e edge of

t h e sample from t h e core w i t h an average depth of 1.9 mils.

None were

v i s i b l e along t h e edge of t h e c o n t r o l specimen. Modified Alloys S t r i n g e r RS3 w i t h t h e specimens of modified a l l o y had been exposed about h a l f as long as s t r i n g e r RR2 w i t h h e a t s 5065 and 5085.

The modified

a l l o y specimens deformed more b e f o r e f r a c t u r i n g than d i d t h e s t a n d a r d a l l o y specimens, b u t they a l s o developed numerous i n t e r g r a n u l a r edge cracks.

As w i t h t h e s t a n d a r d a l l o y s , t h e samples from t h e c o n t r o l fa-

c i l i t y d i d n o t show edge cracks. Figures 29 and 30 are photomicrographs of s t r a i n e d specimens of h e a t

67-502 (modified w i t h 0.49% T i and 2.15% W and containing 0.04% Fe). The specimen exposed i n t h e c o n t r o l f a c i l i t y (Fig. 29) has its edges

Fig. 23. Photomicrographs of a INOR-8 (Heat 5065) Sample Exposed t o S t a t i c Barren Fuel S a l t for 11,933 hr above.500'C and Then Tested at 25OC and a Strain Rate of 0.05/min. 1OOx. (a) Fracture, as polished. (b) Fracture, etched. (c) Edge of stressed portion, etched. Etchant: glyceria regia.

'a,

Fig. 24. Photomicrographs of INOR-8 (Heat 5065) Sample Fluoride S a l t i n the MSRE for 11,933 hr above 5OO0C and Then 25OC and a Strain Rate of 0.05/min. Thermal fluence was 9 . 4 trons/cm2. 1OOx. (a) Fracture, as polished. (b) Fracture, Edge of stressed portion. Etchant: aqua regia.

Exposed t o Tested a t x 1020 neuetched. (c)

Fig. 25. Photomicrographs of INOR-8 (Heat 5085) Specimen Exposed to Depleted S t a t i c Fuel S a l t for 11,933 hr a t 650' and Strained at 25OC. (a) Fracture, as polished, (b) typical edge of gage section, as polished, (c) typical unstressed edge, etched with glyceria regia.

Li'

R-47921

.. I

(4 .

. i

B.)

Fig. 26. Photomicrographs of INOR-8 (Heat 5085) Sample Exposed t o Fluoride S a l t in the ERE f o r 11,933 hr above 5OO0C and then Tested a t 25OC. Thermal fluence was 9 . 4 x 1020 neutrons/cm2. (a) Fracture, as polished. 1OOx. (b) Fracture, as polished. 500x. (c) Fracture, etched. 1OOx. (d) Edge of stressed portion, etched. 1OOx. Etchant: aqua regia.

F i g . 27. Sections of Heat 5065 Tested a t 25OC After Exposure t o Fuel S a l t for 11,933 hr above 500OC. Diameters of specimens are about 0 . 1 i n . The fractures are on the l e f t end.

c.,

Fig. 28. Photomicrographs of INOR-8 Specimens Strained t o Fracture After 11,933 hr above The upper sample was in the control f a c i l i t y and the lower sample was i n the MSRE core. 500째C. The sample diameter is about 0 . 1 i n .

Fig. 29. Photomicrographs of Alloy 67-502 (see Table 1) Exposed t o Depleted Fuel S a l t for 6379 hr Above 5OO0C and Tested a t 25OC. (a) Edge of unstressed portion, (b) Fracture, (c) Edge of stressed portion. Figs (a) and ( c ) etched l i g h t l y . Fig. (b) etched more heavily with glyceria regia.

Fig. 30. Photomicrographs of Alloy 67-502 Sample Exposed t o Fluoride S a l t i n the MSRE for 6379 hr Above 5OOOC and then T e s t e d a t 25OC. 1OOx. (a) Fracture, as polished. (b) Fracture, etched. (c) Edge o f stressed portion, etched. Etchant: aqua regia. Reduced 33%.

coated with small c r y s t a l s of almost pure i r o n .

(The c o n t r o l f a c i l i t y

w a s constructed of material containing 4 t o 5% Fe, so t h e t r a n s f e r of Fe t o an a l l o y t h a t contained only 0.04% Fe i s q u i t e reasonable.)

The sample 3

s t r a i n e d 55%b e f o r e f r a c t u r e , and t h e f r a c t u r e was mixed t and i n t e r g r a n u l a r .

The edge was uneven from t h e l a r g e def

t h e r e were very few i n t e r g r a n u l a r s e p a r a t i o n s .

The sample from t h e core

(Fig. 30) deformed almost as much (52%) b e f o r e f r a c t u r i n g .

I n contrast

t o t h e c o n t r o l specimen, edge cracking occurred a t almost every g r a i n boundary.

The cracks g e n e r a l l y extended t o a depth of about 5 mils,

with t h e maximum depth being about 7 mils.

. However, as shown i n t h e view

of t h e f r a c t u r e , t h i s material tended t o crack i n t e r g r a n u l a r l y and t h e s e i n t e r n a l cracks may have i n some cases l i n k e d t o g e t h e r w

h the surface

cracks t o make them extend deeper. Photomicrographs of samples of h e a t 67-504 t e s t e d a t 25째C a f t e r exposure i n the c o n t r o l f a c i l i t y and i n t h e c o r e are shown i n Figs. 31 and 32.

This h e a t modified w i t h 0.50% Hf, contained 0.07% Fe.

As shown

i n Fig. 31, i r o n deposited on t h e s u r f a c e of t h i s h e a t as i t d i d on 67-502.

This sample, from t h e c o n t r o l f a c i l i t y , deformed 55% b e f o r e

f a i l i n g w i t h a mixed i n t e r - and t r a n s g r a n u l a r f r a c t u r e . i n t e r g r a n u l a r edge cracks.

There were no

The sample exposed i n t h e c o r e deformed

The f r a c t u r e was p r i m a r i l y t r a n s g r a n u l a r b u t t h e r e

52% b e f o r e f r a c t u r e .

w e r e frequent edge cracks (Fig. 32).

Most boundaries were cracked, b u t

t h e cracks extended only t o a' depth of about 2 m i l s - i n t h e f i e l d s t h a t w e r e photographed.

Fourth Group of S u r v e i l l a n c e Specimens ( S t r i n g e r s RL2, RR3, and RS4) The last r e g u l a r core s u r v e i l l a n c e assembly w a s removed i n June, 1969 t o make way f o r a s p e c i a l experimental array.14 with specimens of standard INOR-8

Stringer m 2 ,

(heats 5065 and 5085), had been i n

t h e core a t high temperature almost 12,000 h r during t h e 235U'operation

* I

Fig. 31. Photomicrographs of Alloy 67-504 (see Table 1) Exposed t o Depleted Fuel Salt for 6379 hr Above 500째C and Tested a t 2 5 째 C . (a) Edge of unstressed portion, (b) Fracture, (c) Edge of stressed portion. Figs (a) and ( c ) as-polished and Fig. (b) etched with glyceria regia. Reduced 33.5%.

Fig. 32. Photomicrographs of INOR-8 (Heat 67-504) Sample Exposed t o Fluoride S a l t i n the MSRE for 6379 hr Above 500째C and Tested at 25OC. 1OOx. (a) Fracture, as polished. (b) Fracture, etched. (c) Edge of Stressed portion, etched. Etchant: aqua regia. Reduced 14%.

and 7200 h r during 233U operation.

The o t h e r s t r i n g e r s , w i t h two modi-

f i e d a l l o y s , had been exposed only during t h e 233U operation.

Results

of examinations of these specimens and corresponding specimens from t h e c o n t r o l f a c i l i t y are reported. Visual examination showed a l l of the INOR-8 specimens from t h e c o r e t o be n o t i c e a b l y more d i s c o l o r e d than those i n previous a r r a y s , w i t h surf a c e f i l m s t h i c k enough t o be seen i n c r o s s s e c t i o n by l i g h t microscopy. There w e r e a l s o s l i g h t changes i n the appearance of t h e graphite.18

Both

of t h e s e o b s e r v a t i o n s i n d i c a t e some d i f f e r e n c e i n the exposure c o n d i t i o n s during t h e most r e c e n t operation.

(From o t h e r o b s e r v a t i o n s i t was known

t h a t t h e f u e l s a l t was r e l a t i v e l y more o x i d i z i n g during a t least p a r t of See pp. 85-98 of r e f . 13.)

t h e 233U operation. Standard Alloys (RL2)

Photomicrographs of u n s t r a i n e d specimens of heats 5065 and 5085 from

RL2 revealed many g r a i n boundaries near t h e s u r f a c e s t h a t were v i s i b l e i n t h e as-polished condition.

The view of a specimen of heat 5065 i n t h e

as-polished c o n d i t i o n , (Fig. 33) shows a t h i n s u r f a c e l a y e r (believed t o f

be t h e l c a u s e of t h e v i s i b l e d i s c o l o r a t i o n ) and some g r a i n boundaries v i s i b l e t o a depth of about 2 mils.

Etching this p a r t i c u l a r sample re-

vealed t h e t y p i c a l c a r b i d e s t r u c t u r e p l u s a narrow band near t h e s u r f a c e t h a t seems t o have a high d e n s i t y of carbide.

Photomicrographs of h e a t , 5085 a f t e r removal from t h e c o r e are shown i n Fig. 34. Many of t h e g r a i n boundaries are v i s i b l e i n the as-polished Condition t o a depth of about 2 mils.

Etching r e v e a l s t h e t y p i c a l m i c r o s t r u c t u r e with very ex-

tensive carbide precipitation.

I n t h e as-polished view of t h e h e a t 5065

c o n t r o l specimen (Fig. 35), g r a i n boundaries are a l s o v i s i b l e , b u t i n t h i s case t h e appearance is uniform a c r o s s t h e sample and i s due t o p o l i s h i n g long enough that t h e d i f f e r e n t g r a i n s are a t d i f f e r e n t elevations.

Etching r e v e a l s t h e shallow s u r f a c e modification and t h e t y p i c a l

carbide s t r u c t u r e . I

The h e a t 5085 specimen from t h e c o n t r o l f a c i l i t y ,

shown i n Fig. 36, w a s a l s o s l i g h t l y overpolished so t h a t t h e g r a i n s are v i s i b l e i n t h e as-polished condition.

Etching revealed t h e t y p i c a l

m i c r o s t r u c t u r e and t h e shallow s u r f a c e modification.

Fig. 33. Typical Photomicrographs of INOR-8 (Heat 5065) Exposed t o the ISRE Core f o r 19,136 hr Above 500OC. (a). A s polished. (b) Etchant: aqua regia. 500x.

Fig. 3 4 . Typical Photomicrographs of INOR-8 (Heat' 5085) Exposed t o the MSRE Core for 19,136 hr Above 500OC. (a) As polished. (b) Etchant: glyceria regia. 500x.

Fig. 35. Typical Photomicrographs of'Heat 5065 After Exposure t o S t a t i c Unenriched Fuel S a l t for 19,136 hr Above 500OC. 500x. (a) A s polished. (b) Etchant: glyceria regia.

Fig. 36. Typical Photomicrographs of Heat 5085 After Exposure t o S t a t i c Unenriched Fuel S a l t for 19,136 hr above 500OC. 500x. (a) As polished. (b) Etchant: glyceria regia. .

As i n t h e previous set of c o r e specimens, t h e s t r a p s of h e a t 5055 showed more cracks i n t h e nominally u n s t r a i n e d c o n d i t i o n t h a n d i d t h e

A s e c t i o n of a s t r a p from

tensile specimens on t h e same s t r i n g e r .

s t r i n g e r RL2 t h a t had been exposed, along w i t h t h e specimens shown i n Figures 33 and 34, is shown i n Fig. 37.

The cracks i n t h i s as-polished

view are v i s i b l e t o a depth of about 3 mils, about twice as deep as i n t h e s t r a p exposed 12,000 hr during 235U o p e r a t i o n (Fig. 22).

Heat 5055

straps were a l s o used on the s t r i n g e r s exposed only during t h e 233U operation (7200 h r ) . Fig. 38.

A specimen of one of t h e s e s t r a p s i s shown i n

I n t h e as-polished condition, cracks w e r e v i s i b l e t o a depth

of about 1 m i l ; e t c h i n g made them v i s i b l e t o a depth of about 3 mils. The cracking w a s q u i t e uniform along a l l s u r f a c e s of t h e s t r a p s , i n d i c a t i n g t h a t t h e deformation of t h e 20-mil s t r a p s during removal w a s not

a f a c t o r i n t h e appearance of t h e cracks.

Examination of u n i r r a d i a t e d

c o n t r o l s t r a p s f a i l e d t o r e v e a l a s i m i l a r type of cracking. One o t h e r i n t e r e s t i n g specimen t h a t showed e x t e n s i v e cracking w a s

a p i e c e of t h i n INOR-8 s h e e t (heat 5075) that had been a t t a c h e d t o t h e

s t r a p shown i n Fig. 38.

0.5 h r a t 1180OC.

The s h e e t had been r o l l e d t o 4 mils and annealed

The edges of t h e f o i l were wrapped around t h e s t r a p

-L

t o hold t h e material i n place.

Thus t h e o u t s i d e s u r f a c e of t h e f o i l

would have been exposed t o flowing s a l t and t h e underside and t h e folded ends t o s a l t t h a t flowed less r a p i d l y .

The f o i l w a s extremely b r i t t l e

and broke while i t was being removed from t h e s t r a p .

The photomicrographs

i n Fig. 39 show t h a t t h e f o i l had e x t e n s i v e g r a i n boundary cracks, with many appearing t o extend throughout t h e 4 m i l thickness.

I n some areas

t h e cracks were more numerous on t h e o u t s i d e where the material was curved and w a s i n c o n t a c t w i t h r a p i d l y flowing s a l t .

Photomicrographs of a

s t r a p and f o i l t h a t were exposed t o s t a t i c s a l t i n t h e c o n t r o l f a c i l i t y

are shown i n Figs. 40 and 41.

The samples showed no evidence of i n t e r -

granular cracking. A f r a c t u r e s u r f a c e of t h e f o i l from t h e PlSRE was examined by t h e

scanning e l e c t r o n microscope (SEM) and Auger spectroscopy.

is shown i n Fig. 42 and i s i n t e r g r a n u l a r .

The f r a c t u r e

The SEM w i t h a d i s p e r s i v e x-ray

a n a l y t i c a l system i n d i c a t e d t h e presence only of t h e elements normally

ure to

. ...

. .

, ,

Fig. 38. Photomicrographs of INOR-8 (Heat 5055) Strap Exposed t o the MSRE Core 7203 hr above 50OoC. (a) &-polished view near cut (b) Etched view near cut (c) A s polished view of outside edge (d) Etched view of outside edge. Etchant: Lactic acid, "03, HC1. Reduced 32%.

Fig, 39. Photomicrographs of Two Specimens From a Thin,INOR-81(Heat 5075) F o i l that w a s Attached t o the Strap Shown i n the Previous Figure. The curvTture was caused by forming before being inserted into the MSRE. (a) As polished (b) Etched (c) As polished (d) Etched. Etchant: Lactic acid, "03, HC1. Reduced 32%.

Fig. 40. Photomicrographs of INOR-8 (Heat 5055) Straps Exposed t o S t a t i c Unenriched Fuel Salt 7203 hr Above 5OO0C. (a) &-Polished 33x, (b) As Polished, SOOX, cracked regions are carbide that fractured during i n i t i a l forming, ( c ) Etched with glyceria regia, 500x. Reduced 33%.

LJ-

Fig. 41. Photomicrographs of a Thin INOR-8 (Heat 5035) Foil that was attached t o the strap shown in the Previous Figure. (a) A s Polished, (b) Etched with glyceria regia. 500x.

Fig. 42. Scanning Electron Micrograph of INOR-8 (Heat 5075) Foil Exposed to the MSRE Core for 7203 hr Above 50OoC. (a) Topographical view of fracture showing the surfaces of the fractured grain boundaries. 500x. (b) 1OOOx.

found i n INOR-8.

Mo were concentrated i n t h e boundaries.

Auger e l e c t r o n spectroscopy i n d i c a t e d t h a t T e , S, and D e f i n i t e confirmation of t h e

presence of t h e s e elements w a s d i f f i c u l t because many of t h e s p e c t r a overlapped. Samples of h e a t s 5065 and 5085 t h a t were removed from t h e c o n t r o l and t h e s u r v e i l l a n c e f a c i l i t y a f t e r 19,000 h r were s u b j e c t e d t o numerous mechanical property tests, and some specimens w e r e examined metallog r a p h i c a l l y a f t e r having been s t r a i n e d t o f r a c t u r e . measured a t 25OC are summerized i n Table 6.

The t e n s i l e p r o p e r t i e s

The changes i n t h e y i e l d

stress are small and probably w i t h i n experimental accuracy.

The reduc-

t i o n s i n t h e u l t i m a t e t e n s i l e stress are due p r i m a r i l y t o t h e reduced f r a c t u r e s t r a i n , which, as explained b e f o r e , is believed t o be due t o c a r b i d e p r e c i p i t a t i o n t h a t is dependent upon t i m e a t temperature, i r r a d i a t i o n , and t h e p a r t i c u l a r h e a t involved. The c o n t r o l sample of h e a t 5065, whose m i c r o s t r u c t u r e is shown i n Fig. 43, deformed 50% b e f o r e f r a c t u r e .

The f r a c t u r e is mixed intergranu-

lar and t r a n s g r a n u l a r and no cracks formed along t h e edge away from t h e immediate v i c i n i t y of t h e f r a c t u r e . c

The m i c r o s t r u c t u r e of a h e a t 5065

sample from t h e core is shown i n Fig. 44. had a mixed f r a c t u r e .

This sample deformed 43% and

Numerous i n t e r g r a n u l a r cracks t o a depth of about

5 mils formed along t h e gage length.

Samples of h e a t 5085 from t h e con-

t r o l f a c i l i t y and t h e core are shown i n Figures 45 and 46. sample f a i l e d a f t e r 41% s t r a i n w i t h a mixed f r a c t u r e . cracks were v i s i b l e i n the f i e l d shown i n Fig. 45.

The c o n t r o l

No i n t e r g r a n u l a r

The sample of heat

5085 from t h e core t h a t w a s t e s t e d a t 25OC f a i l e d a f t e r much less s t r a i n (22%).

As shown i n Fig. 46, t h e fra'cture is mixed, and numerous i n t e r -

granular cracks formed along t h e s u r f a c e s t o ' a depth of 5 mils. Composite photographs of s e c t i o n s of t h e broken c o n t r o l and s u r v e i l l a n c e samples of h e a t s 5065 and 5085 were made f o r o v e r a l l viewing. These are Figures 47 and 48.

These p i c t u r e s show very c l e a r l y t h a t t h e

, i n t h e specimens i n t e r g r a n u l a r cracks are more frequent and deeper exposed i n t h e core than i n t h e c o n t r o l samples.

The photograph of t h e

h e a t 5085 core sample a l s o shows t h a t very s m a l l s t r a i n s are s u f f i c i e n t J

t o make t h e cracks v i s i b l e .

The sample had a 3/16-in.

radius at the

Table 6. T e n s i l e P r o p e r t i e s of S u r v e i l l a n c e Samples From Fourth Group a t 25OC and a S t r a i n Rate of 0.05 minâ&#x20AC;&#x2122;l

Condi t iona

Heat

Yield Stress (Psi)

5085 .

Annealed

51,500

5085

Control

48,100

U 1t imate T e n s i l e Stress (psi)

Uniform Elongation % -

120,800

52.3

53.1

42.2

,lo8,500

40.5

40.6

32.8.

89,000

22 .o

22.1

19.6

Total Elongation %

Redu ct i o n I n Area %

5085

- Irradiated

53,900

5065

Annealed

56,700

126,400

52.9

55.3

50.0

5065

Con t r o 1

61,200

126,500

48.5

49.8

36.8

5065

Irradiated

56,700

109,500

42.5

42.8

33.2

a Annealed - Annealed 2 h r a t 900OC. Control - Annealed, exposed t o depleted f u e l s a l t f o r 19,136 h r a t 650OC. I r r a d i a t e d -Annealed, i r r a d i a t e d i n c o r e t o a thermal f l u e n c e of 1 . 5 x lo2â&#x20AC;&#x2122; neutrons/cm2 over a period of 19,136 hu a t 650OC.

Fig. 43. Typical Photomicrographs of INOR-8 (Heat 5065) Sample Tested at 25OC After Being Exposed t o Static Unenriched Fuel Salt for 19,136 hr above 50OOC. Etchant: glyceria regia. (a) Fracture. 1OOx. (b) Edge near fracture. 1OOx. (c) Representative unstressed structure. 500x. Reduced 22%.

Fig. 44. Photomicrographs of INOR-8 (Heat 5065) Sample Tested a t 25OC A f t e r Being E t o t h e MSRE Core f o r 19,136 h r Above 500째C and I r r a d i a t e d t o a Thermal Fluence of 1.5 x 10 (a) F r a c t u r e , etched. 1OOx. (b) Edge, as polished. 1OOx. (c) Edge, as polished. neutrons/cm*. 5OOx. (d) Edge, etched. 1OOx. Etchant: g l y c e r i a r e g i a . Reduced 26%.

Tsed

. _

Fig. 45. Typical Photomicrographs of INOR 8 (Heat 5085) Sample Tested a t 25OC After Being Exposed to S t a t i c Unenriched Fuel Salt for 19,136 hr Above 50OoC. (a) Fracture, etched. 1OOx. (b) Edge near fracture, etched. 1OOx. (c) Representative unstressed structure, etched. 500x. Etchant: glyceria regia. Reduced 24.5%.

Fig. 46. Photomicrographs of INOR-8 (Heat 5085) Sample Tested a t 25OC After Being Exposed t o t h e MSRE Core f o r 19,136 h r Above 5 O O O C and I r r a d i a t e d t o a Thermal Fluence of 1.5 x 1021 neutrons/cm2. (a) F r a c t u r e , etched. 1OOx. (b) Edge, as polished. 1 0 0 ~ .(c) Edge, as polished. 500x. (d) Edge, etched. 1OOx. Etchant: g l y c e r i a r e g i a . Reduced 27%.

i g . 47. Pho crographs of INOR-8 (Heat 5065) Strained t o Fracture at 2 5 O C . Top specimen was exposed t o s t a t i c unenriched f u e l s a l t for 19,136 hr above 5 O O O C and the lower specimen was exposed to the MSRE core for the same time. The diameters are about 0 . 1 in.

Fig. 48. Photomicrographs of INOR-8 (Heat 5085) Strained t o Fracture a t 25OC. The top specimen was exposed to s t a t i c unenriched f u e l s a l t f o r 19,136 hr above 5OOOC and the lower specimen was exposed to the %RE core for the same t i m e . The diameters are about 0.1 in.

end of t h e gage s e c t i o n and t h e diameter increased r a p i d l y from 1 / 8 i n .

t o 1/4 in.

Thus t h e stress, and hence t h e s t r a i n , decreased r a p i d l y

along t h i s r a d i u s .

Note i n Fig. 48 t h a t t h e cracks continue a l a r g e

d i s t a n c e along t h e r a d i u s , i n t o t h e region of low s t r a i n .

An a d d i t i o n a l experiment w a s made with a c o r e sample of h e a t 5085 t h a t had been c u t too s h o r t f o r mechanical property t e s t i n g .

The re-

maining segment was bent about 30' i n a v i s e , then w a s s e c t i o n e d and examined metallographically. i n Fig. 49.

The r e s u l t i n g photomicrographs are shown

The t e n s i o n s i d e had i n t e r g r a n u l a r cracks t o a depth of

about 4 mils, b u t no cracks were e v i d e n t on t h e compression s i d e .

Both

t h e t e n s i o n and compression s i d e s etched abnormally near t h e s u r f a c e because of t h e modification thought t o b e due t o cold working. Modified Alloys (RR3 and RS4) The specimens on s t r i n g e r s RR3 and RS4 w e r e of two d i f f e r e n t h e a t s of titanium-modified a l l o y :

h e a t 7320, containing 0.65% T i , and h e a t

67-551, containing 1.1% Ti.

The 7200-hr exposure of t h e s e specimens had

been e n t i r e l y during t h e 233U o p e r a t i o n , i n which t h e i n i t i a l o x i d a t i o n

state w a s r e l a t i v e l y high.

The specimens were discolored and had much

t h e same appearance as t h e standard h e a t s . t o various mechanical property tests ically.

The samples were s u b j e c t e d

some w e r e examined metallograph-

The m i c r o s t r u c t u r e of h e a t 7

a f t e r exposure i n t h e c o n t r o l

f a c i l i t y and t e s t i n g a t 25OC is shown

Fig. 50.

50%before fracturing i n a mixed mode

o edge cracks w e r e v i s i b l e .

The sample deformed A

similar sample t h a t w a s exposed t o t h e f u e l s a l t is shown i n Fig. 51. It f a i l e d a f t e r deforming 45%.

Edge cracks were p r e s e n t along almost

every g r a i n boundary t o a depth of 3 . t o 5 mils.

Note t h a t many of t h e s e

cracks proceed t o where t h e cracking g r a i n boundary i n t e r s e c t s another boundary.

Note a l s o t h a t t h e crack t i p s are q u i t e b l u n t i n d i c a t i n g t h a t

they formed e a r l y during deformation and w e r e simply spread by f u r t h e r

deformation and d i d not propagate.

were made on h e a t 67-551.

S i m i l a r metallographic observations

Control and core specimens deformed almost

equal amounts b e f o r e f r a c t u r i n g (52% and 51%). No edge cracks were observed i n t h e sample from t h e c o n t r o l f a c i l i t y (Fig. 52), but numerous i n t e r g r a n u l a r cracks were e v i d e n t i n t h e sample from t h e core (Fig. 53).

Fig. 49. Typical Photomicrographs of INOR-8 (Heat Core for 19,136 hr Above 500OC. The sample was bent in s i d e . 1OOx. (b) A s polished, tension s i d e . . 500x. (c) Etched, compression s i d e . 500x. Etchant: aqua regia.

'1'

5085) Sample Exposed t o the MSRE a vise. (a) A s polished, tension Etched, tension s i d e . 5 0 0 ~ . (d) Reduced 27%.

. W

Fig. 50. Photomicrographs of a Modified INOR-8 (Heat 7320) Sample Tested a t 25OC After Being Exposed t o S t a t i c Unenriched Fuel S a l t for 7203 hr Above 50OoC. (a) Fracture. 1OOx. (b) Edge. 1OOx. (c) Typical unstressed microstructure. 500x. Etchant: glyceria regia. Reduced 22.5%.

Fig. 51. Photomicrographs of a Modified INOR-8 (Heat 7320) Sample Tested a t 25OC After Being Exposed to the MSRE Core for 7203 hr Above 5OOOC and Irradiated t o a Fluence of 5 . 1 x lo2' neutrons/cm2!. (a) Fracture, etched, 1OOx. (b) Edge, as polished. 1OOx. (c) Edge, Etched. 1OOx. Etchant: glyceria regia. Reduced 22%.

Fig. 52. Photomicrographs of a aodified INOR-8 (Heat 67-551) Sample Tested at 25OC After Being Exposed t o S t a t i c Unenriched Fuel S a l t f o r 7203 hr Above 50OoC. (a) Fracture. 1OOx. (b) Edge. 1OOx. (c) S p i c a 1 unstressed microstructure. 500x. Etchant: glyceria regia. Reduced 20.5%.

Fig. 53. ' Photomicrographs of a Modified INOR-8 (Heat 67-551) Sample Tested a t 25OC After Being Exposed to the MSRE Core for 7203 hr Above 5OOOC and Irradiated t o a Fluence of 5 . 1 x 1020 neutrons/cm2. (a) Fracture, etched. 1OOx. (b) Edge, as polished. 1OOx. (c) Edge, etched. 1OOx. Etchant: Glyceria regia. Reduced 17%.

Specimens Exposed t o C e l l Atmosphere

A l l of t h e INOR-8 specimens exposed t o t h e c e l l atmosphere o u t s i d e *

t h e r e a c t o r v e s s e l (N2 with 2-5% 02) developed a dark gray-green, tenacious s u r f a c e film.

Examination i n d i c a t e d t h a t t h e f i l m w a s oxide, and

t h e r e w a s i n d i c a t i o n of n i t r i d i n g . Figure 54 is a photomicrograph of one of t h e c e l l specimens exposed t h e l o n g e s t (17,483 h r a t high temperature).

It shows a very t h i n , uni-

form l a y e r of oxide on t h e s u r f a c e , w i t h i n t e r n a l o x i d a t i o n extending t o a depth of 1 t o 2 mils.

Also evident i s t h e u s u a l %C-type

carbide

formed during t h e primary working and t h e long thermal aging.

Results of 25OC tensile tests on t h e standard INOR-8 specimens (heats 5065 and 5085) showed reductions (relative t o unexposed m a t e r i a l ) i n ultimate stress and f r a c t u r e s t r a i n c o n s i s t e n t w i t h expectations from t h e r e s u l t s on c o n t r o l and core specimens.

(See Table 5.)

Figure 55 shows a sample of h e a t 5065 exposed t o t h e c e l l environment I

and f r a c t u r e d a t 25OC.

The elongated g r a i n s a t t e s t t o t h e l a r g e f r a c t u r e

s t r a i n (59%), and t h e f r a c t u r e is mixed t r a n s g r a n u l a r and i n t e r g r a n u l a r . A h e a t 5085 specimen exposed t h e same l e n g t h of t i m e f a i l e d w i t h only 33%

f r a c t u r e s t r a i n , i n a p r i m a r i l y i n t e r g r a n u l a r mode as shown i n Fig. 56. Both specimens showed a few shallow s u r f a c e cracks, b u t t h e oxide l a y e r d i d not appear t o a f f e c t t h e deformation of t h e specimens. Studies Related to Modified Surface Microstructure

Reference was made i n several i n s t a n c e s t o a modified m i c r o s t r u c t u r e .

Metals q u i t e o f t e n have d i f f e r e n t m i c r o s t r u c t u r e s near t h e s u r f a c e than deeper i n t h e piece.

Processing methods can account f o r t h e s e v a r i a t i o n s

on a s - f a b r i c a t e d s u r f a c e s .

Hot working is o f t e n accompanied by o x i d a t i o n

o r d e c a r b u r i z a t i o n t h a t can produce e i t h e r l a r g e r o r smaller g r a i n s near the surface.

Tubing is o f t e n made by a l t e r n a t e l y drawing cold and an-

n e a l i n g t o s o f t e n t h e metal.

The l u b r i c a n t s used i n drawing are d i f f i c u l t

t o c l e a n from t h e i n s i d e s u r f a c e , and r e s i d u e s can d i f f u s e i n t o t h e tubing

Fig. 5 4 . Photomicrographs of Hastelloy N (Heat 5065) Surveillance Specimens Exposed to the C e l l Environment of N2 + 2 to 5%02 f o r 20,789 hr a t 650OC. 500x. (a) Unetched showing surface oxidation (b) Etched (glyceria regia) showing shallow modification of microstructure due t o reaction with c e l l environment.

Fig. 55. Photomicrographs o f INOR-8 (Heat 5065) Sample Tested a t 25OC a t a S t r a i n Rate of 0.05/min. Sample had been i r r a d i a t e d t o a thermal f l u e n c e of 2.6 x 1019 neutrons/cm2 while b e i n g exposed t o t h e c e l l environment above 5OO0C f o r 17,483 h r . 1OOx. ( a ) F r a c t u r e , as polished. (b) F r a c t u r e , etched. (c) Edge of s t r e s s e d p o r t i o n . Etchant: aqua r e g i a .

Fig. 56. Photomicrographs of INOR-8 (Heat 5085) Sample Tested a t 25OC Sample had been i r r a d i a t e d t o a thermal f l u e n c e a t S t r a i n Rate of O.O5/min. of 2.6 x 1019 neutrons/cm2 w h i l e being exposed t o t h e c e l l environment above 5OOOC f o r 17,483'hr. 1OOx. ( a ) F r a c t u r e , as polished. (b) F r a c t u r e , etched. (c) Edge of s t r e s s e d portion. Etchant: aqua r e g i a .

during annealing, The lubricants are usually high in silicon or carbon and these elements can cause a layer of small grains to form near the affected surface, Our surveillance samples were machined from larger pieces of material, so none of the explanations that we have given can apply.

The 1/4-in.-diam surveillance rods were fabricated by several methods, all

involving some cold working, The rods were annealed for 1 hr at 1180째C (standard mill anneal) in argon and then centerless ground to the final dimensfon of about 0.245 in, The 1/8-in.-dfam gage section was machined. The photomicrographs in Fig. 57 show typical surface modifications noted on surveillance and control specimens after long annealing times at 650OC. The modification is generally less 'than 1 mil deep and is not detectably influenced by irradiation. Microprobe examination of the first control sample indicated that the modified region was slightly enriched in carbon and that none of the other alloying elements varied. Thus the modification appears to be a region of fine carbides that are dispersed along lines. We postulated that these lines were slip lines that resulted from the surface working during the final stages of fabrication. These slip lines would provide preferred sites for carbide precipitation when the alloy was held at 650째C. We were able to duplicate the surface modification by solution annealing a rod, centerless grinding, and annealing for a long period at 650째C. The resulting microstructure is shown in Fig. 58. It is quite similar to the microstructures in Fig. 57 and confirms that the modification is due to surface working, Thus, this modification is

unrelated to the intergranular cracking phenomenon. Summary of Observations on Surveillance Specimens This section is intended as a convenient summary of the observations

on the surveillance specimens that we believe are pertinent to the intergranular surface cracking phenomenon.

Discussion and interpretations will

come later, after the descriptions of the observations on MSRE components.

. Fig. 57. Photomicrographs of Unstressed INOR-8 (Heat 5085) Control and Irradiated Samples.

Fig. 58. Photomicrograph of INOR-8 (Heat 5085) Rod that was Annealed 1 h r a t 1 1 7 7 O C , Centerless Ground 5 . 2 mils, and Annealed 4370 hr a t 65OOC i n Argon. 500x. Etchant: glyceria regia.

INOR-8 specimens exposed to fuel salt for more than 1000 hr before the beginning of power operation showed no intergranular surface cracking. All specimens exposed thereafter-inthe core for periods ranging from 2500 to 19,000 hr did show intergranular surface cracks after being strained in tension. No surface cracks were visible i n unstrained specimens from the first set of core surveillance specimens (exposed for 2500 hr after the beginning of power operation), but specimens from all arrays removed later did show some cracks o r incipient cracks in the unstrained, as-polished condition. In contrast to the core specimens,â&#x20AC;?specimens exposed to salt in the control facility for equal times at high temperature and then tested in tension showed only a few surface cracks. Specimens exposed to the cell atmosphere developed an oxidized layer, which cracked upon being strained, but few or no cracks extended deeper than the oxide layer. The severity of surface cracking in core and control specimens of MSRE vessel heats exposed during various periods of time was measured as of follows. Photomicrographs were made of polished longitudinal sections , â&#x20AC;&#x2DC; specimens of heats 5065 and 5085 that had been fractured in tension. Cracks visible along the edges of the gage portions of the counted, and the average and maximum depths were determin gives the observed crack frequencies (number per inch of gage length) and depths , We saw vary evident differences between different heats in the degree of surface cracking, Strained specimens of heat 5065 had more cracks than did specimens of heat 5085 exposed simultaneously, but average depths were not very different, (See Table 7.) Statistics were not gathered on specimens of modified heats, but the photomicrographs of strained specimens show crack frequencies and depths comparable to those in specimens of heat 5065. Straps and a foil of different standard heats showed more cracks than were visible in unstrained specimens of heats 5065 and 5085. The surfaces of specimens exposed during the first part of the 233U

operation were noticeably more discolored than the surfaces of specimens exposed only during the 235U operation. Presumably this was evidence of

Table 7.

Sample Description

Crack Formation i n Hastelloy N Surveillance Samples Strained t o Failure a t 25OC

T i m e a t High Temperaturea (hr) Tot a 1 With ITb

Crack Count Counted (in. )1'

Depth ( m i l s ) Av Max

Control, Heat 5085 Heat 5085

5,550 5,550

2,550 2,550

1 24

1 19

5.7 2.5

5.7 8.8

Control, h e a t 5085 Heat 5085

11,933 11,933

11,600 11,600

0 178

134

1.9

6.3

Control, Heat 5065 Heat 5065

11,933 11,933

11,600 11,600

3 277

3 230

1.0 1.8

2.0 3.8

Control, Heat 5085 Heat 5085 Heat 5085

19,136 19,136 19,136

18,800 18,800 18,800

4 213 140

3 176 146

1.5 5.0 3.8

2.8 7.OC 8.8

Control, Heat 5065 Heat 5065

19,136 19,136

18,800 18,800

3 2 40

3 229

2.5 5.0

4.0 7.5

T i m e logged above 500OC. A l l was a t 650 f 10째C with t h e exception of 100 h r a t 76OOC i n 10/65, about 750 h r a t 500-600째C i n 2-3/66 and 500 h r a t 630째C i n 12/67-2/68. '

bTime above 5OOOC a f t e r exposure of specimen t o f u e l s a l t containing f r e s h f i s s i o n prod-

ucts. C

One crack t h a t may n o t be s u r f a c e connected and probably had a d i f f e r e n t cause was 15 mils deep, next l a r g e s t w a s 7 mils.

03 QI

t h e r e l a t i v e l y more o x i d i z i n g c o n d i t i o n of t h e s a l t following t h e processing.

It a l s o i n d i c a t e s t h e importance of recognizing t h a t corrosion

(and p o s s i b l y t h e s u r f a c e cracking) w a s not p r o p o r t i o n a l t o t i m e .

The

f l u c t u a t i o n i n t h e concentration of chromium i n t h e f u e l s a l t (Fig. 5)

i s f u r t h e r evidence of t h i s f a c t . Aging a t high temperature caused %C-type throughout specimens of h e a t s 5065 and 5085. process.

carbide t o p r e c i p i t a t e I r r a d i a t i o n enhanced t h i s

The i n c r e a s e d amounts of i n t e r g r a n u l a r M g C (which is b r i t t l e

a t room temperature) were attended by reductions i n t h e f r a c t u r e s t r a i n s and u l t i m a t e stresses a t 25OC and a s h i f t i n t h e n a t u r e of t h e f r a c t u r e s

a t 25OC from l a r g e l y t r a n s g r a n u l a r t o more n e a r l y i n t e r g r a n u l a r .

These

changes were observed i n t h e s u r v e i l l a n c e specimens from t h e c o r e and t h e

c e l l and i n t h e c o n t r o l specimens, although t h e changes were g r e a t e r i n t h e s u r v e i l l a n c e specimens.

L i t t l e o r no coarse MgC p r e c i p i t a t e formed

i n specimens of modified compositions, s i n c e t h e modifications w e r e t a i l o r e d t o produce f i n e c a r b i d e of t h e MC type.

A l l aged INOR-8 specimens showed near t h e s u r f a c e a l a y e r t h a t etched more darkly.

The amount of t h i s modified l a y e r d i d not vary systemati-

tally w i t h time o r temperature.

Experiments produced e-ridence t h a t t h e

e f f e c t w a s l i k e l y due t o cold work from machining, causing c a r b i d e t o p r e c i p i t a t e more r e a d i l y near t h e s u r f a c e .

EXAMINATION OF MSRE COMPONENTS

r obje tive of t h e p o s t o p e r a t i o n examinatia

of t h e MSRE1's20

was t o supplement t h e f i n d i n g s on t h e INOR-8 s u r v e i l l a n c e specimens by examining p i e c e s of INOR-8

from d i f f e r e n t p a r t s of t h e f u e l system.

A1.l

p a r t s had been a t high temperature f o r 31,000 h r and i n c o n t a c t w i t h f u e l

s a l t f o r 21,000 h r (about 1.5 t i m e s as long as any s u r v e i l l a n c e specimen). Otherwise t h e conditions of exposure were q u i t e v a r i e d .

1. A c o n t r o l rod thimble (which had s e e n t h e h i g h e s t neutron fluence of any INOR-8 ever examined) o f f e r e d t h r e e kinds of s u r f a c e exposure on t h e same'piece:

t h e i n s i d e exposed t o N2

+ 2% 02

a t 650째C, o u t s i d e sur-

face exposed t o flowing f u e l s a l t , and o u t s i d e s u r f a c e t h a t had been under a l o o s e - f i t t i n g INOR-8 s l e e v e .

Heat exchanger tubes had been exposed t o f u e l s a l t on t h e out-

s i d e and coolant s a l t on t h e i n s i d e .

The h e a t exchanger s h e l l had been exposed t o f u e l s a l t on one

s i d e , c e l l atmosphere on t h e o t h e r .

4. The sampler cage and mist s h i e l d i n the pump bowl extended from beneath the s u r f a c e of t h e s a l t pool, through t h e s u r f a c e (where c e r t a i n

classes of f i s s i o n products tended t o concentrate) i n t o a gas space. The major p a r t of t h e examination e f f o r t was on determinations of t h e amount and n a t u r e of d e p o s i t s and metallography, t o r e v e a l evidence Although t h e p i e c e s were g e n e r a l l y

of corrosion and s u r f a c e cracking.

of awkward shapes, some t e n s i l e and bend tests were run, and t h e e x d n a t i o n of t h e s t r a i n e d p i e c e s proved t o be most i n t e r e s t i n g . Control Rod Thimble I

The MSRE used t h r e e c o n t r o l rodsp1 f a b r i c a t e d of GdpOg and A1203 canned i n Inconel 600. made of 2-in.-OD

The c o n t r o l rods operated i n s i d e INOR-8 thimbles

x 0.065-in.-wall

i n t h e MSRE i s shown i n Fig. 59.

tubing.

The assembly b e f o r e i n s e r t i o n

A f t e r being i n service f o r several

y e a r s (above 5OOOC f o r 30,807 h r ) , t h e lower p o r t i o n of c o n t r o l rod thimble 3 was severed by electric arc c u t t i n g and moved t o t h e h o t cells f o r examination.

Figure 60 shows the electric arc c u t a t t h e l e f t (about

at t h e midpoint of t h e c o r e ) , t h e s p a c e r s l e e v e s , and t h e end c l o s u r e ( l o c a t e d a t t h e bottom of t h e c o r e ) .

The thimble was made of h e a t Y-8487,

and t h e s p a c e r s l e e v e s were made of h e a t 5060 (see Table 1 f o r chemical

compositions). '

The f i r s t c u t w a s made through t h e s l e e v e and thimble n e a r e s t t h e

e l e c t r i c arc c u t .

The s p a c e r s l e e v e had been machined w i t h r i b s 0.100

in. high and 0.125 i n . wide t o p o s i t i o n i t relative t o t h e g r a p h i t e moderator.

The sleeve had f o u r d r i l l e d h o l e s through which weld beads

were depsoited on t h e thimble t o hold t h e s l e e v e i n place.

According

t o t h e shop drawings, t h e minimum and maximum d i a m e t r a l c l e a r a n c e s between t h e thimble and t h e s l e e v e were 0.000 and 0.015 i n . ,

respectively.

Thus salt would l i k e l y e n t e r t h i s annular region and be i n c o n t a c t with most of t h e m e t a l s u r f a c e s .

Fig. 59.

Control Rod Assembly before Insertion i n the MSRE.

Fig. 60. Portion of Control Rod Thimble that was Examined After Operation of MSRE was Terminated. The thimble i s made of 2-in-OD x 0.065The e l e c t r i c arc cut on the l e f t in.-wall INOR-8 tubing (Heat Y-8487). was near the center l i n e of the core.

Undeformed Samples Samples of t h e c o n t r o l rod thimble and t h e s p a c e r s l e e v e were c u t and examined t o determine t h e i r c o n d i t i o n a t t h e end of s e r v i c e .

Typical

photomicrographs of t h e i n s i d e of the thimble tube are shown i n Fig. 61.

This s u r f a c e w a s oxidized t o a depth of about 2 mils by t h e c e l l environment of n i t r o g e n containing 2 t o 5% 02.

The o x i d a t i o n process modified

t h e m i c r o s t r u c t u r e t o a depth of 4 mils, l i k e l y due t o t h e s e l e c t i v e removal of chromium. Photomicrographs of one of the weld beads are shown i n Fig. 62.

A l l of t h e s u r f a c e shown w a s exposed t o flowing f u e l s a l t .

Some d i s -

lodged g r a i n s are near t h e s u r f a c e , and g r a i n boundaries are v i s i b l e i n t h e as-polished condition t o a depth of 1 t o 1.5 mils. Photomicrographs involving t h e i n t e r f a c e between t h e thimble and

sleeve are shown i n Fig. 63.

Figure 63(a) shows t h e annular region w i t h

a s e p a r a t i o n of about 7 mils and some s a l t p r e s e n t .

Few s u r f a c e i r r e g u -

larities are v i s i b l e a t a magnification of lOOx, i n d i c a t i n g t h a t they are considerably below 1 - m i l .

Figure 63(b) is a 500x v i e w of t h e thimble The o u t s i d e of t h e

and shows some s u r f a c e cracks t o a depth of 0.3 m i l .

s l e e v e i s shown i n Fig. 63(c); a few g r a i n boundaries t o a depth of about

1 m i l are v i s i b l e . i n Fig. 64.

Additional photomicrographs of t h e s l e e v e are shown

The s l e e v e material does n o t appear t o have received much

working s i n c e t h e c a r b i d e is very inhomogeneously d i s t r i b u t e d . s i z e is l a r g e r than u s u a l away from t h e s t r i n g e r s . s u r f a c e s both have modified s t r u c t u r e s .

The g r a i n

The i n n e r and o u t e r

A h i g h e r magnification view of

t h e i n n e r s u r f a c e [Fig. 64(b)] shows t h a t much of t h e m o d i f i c a t i o n i s a high d e n s i t y of primary carbide.

The o u t e r s u r f a c e [Fig. 64(c)] has a

shallow l a y e r of small g r a i n s , l i k e l y due t o a working operation.

A second c u t w a s made away from t h e s l e e v e , and t y p i c a l photomicrographs are shown i n Fig. 65. flowing s a l t . ~

This p a r t of t h e thimble w a s exposed t o

Some of t h e g r a i n boundaries are v i s i b l e i n t h e as-polished

condition t o a depth of about 4 m i l s and t h e r e i s some s u r f a c e modificat i o n [Fig. 65(a)].

Etching [Fig. 65(b)] d e l i n e a t e s more of t h e g r a i n

s t r u c t u r e and shows t h e shallow s u r f a c e modification.

Fig. 61. Photomicrographs of the Inner Surface of the Control Thimble i n the As-removed Condition. This surface was exposed t o c e l l environment of N2 containing 2 t o 5% 02. (a) A s polished. (b) Etched. (c) Etched, typical microstructure. Etchant: Aqua regia. Reduced 30.5%.

' I

(b)1

Fig. 63. Photomicrographs of As-polished Surfaces o f ' C o n t r o 1 Rod Thimble and Sleeve. ( a ) Annulus between thimble and sleeve. Thimble s u r f a c e is i n upper p a r t of p i c t u r e . (b) Outslde s u r f a c e of c o n t r o l r o d thimble, showing presence of some s a l t and shallow s u r f a c e cracking. (c) Outside s u r f a c e of s l e e v e . Surface exposed t o flowing f u e l salt. '

Fig. 64. Photomicrographs o f Control Rod Sleeve. (a) Etched v i e w showing general microstructure. Inner surface i s on l e f t and outer surface is on right. (b) Inner surface of sleeve. (c) Outer surface of sleeve. Etchant: Aqua regia. Reduced 30.5%

W *

Fig. 65. Photomicrographs of Control Rod Thimble where i t was Exposed Directly t o Flowing S a l t . (a) A s polished. (b) Etched: Aqua regia.

96 Microprobe scans were run on t h e thimble samples t h a t w e r e taken from under t h e sleeve and o u t s i d e t h e s l e e v e .

I n t h e f i r s t case t h e

thimble w a l l was exposed t o almost s t a t i c f u e l s a l t , a n d , n o g r a d i e n t s i n i r o n and chromium concentration could be d e t e c t e d w i t h i n t h e 3-pm region of u n c e r t a i n t y near t h e s u r f a c e .

The sample o u t s i d e t h e s l e e v e t h a t w a s

exposed t o flowing f u e l s a l t w a s depleted i n chromium t o a depth of almost 20 pm and i n i r o n t o a depth of 10 pm (Fig. 66).

Thus t h e amount of

corrosion t h a t occurred v a r i e d considerably i n t h e two regions.

The

measurements were n o t a c t u a l l y made, b u t a similar r e s u l t l i k e l y occurred f o r the spacer sleeve.

The i n s i d e s u r f a c e was exposed t o almost s t a t i c

s a l t and w a s l i k e l y not corroded d e t e c t a b l y .

The o u t s i d e s u r f a c e w a s

exposed t o flowing s a l t and l i k e l y showed i r o n and chromium d e p l e t i o n . Deformed Samples

The next step w a s t o deform some of the thimble and the sleeve t o determine whether s u r f a c e cracks were formed similar t o those noted i n t h e s u r v e i l l a n c e samples.

Since t h e product was t u b u l a r , w e used a r i n g

test t h a t is r e l a t i v e l y quick and cheap.

The f i x t u r e shown i n Fig. 67

w a s made by ( l ) . c u t t i n g 518 i n . through a 1-in.-thick w i t h a Z-in.-diam

carbon s t e e l p l a t e

h o l e saw, (2) c u t t i n g out t h e p a r t i a l l y c u t region w i t h

ample c l e a r a n c e around t h e h o l e , (3) c u t t i n g t h e p l a t e i n two along t h e diameter and removing 1/8 i n . of material on each s i d e of t h e c u t , and (4) tapping a 3/4 i n . diam t h r e a d i n t o t h e two pieces. i n . wide w e r e c u t from t h e thimble and t h e s l e e v e .

Then r i n g s 1 / 4

They f i t i n t o t h e

groove and w e r e p u l l e d t o f a i l u r e w i t h t h e r e s u l t a n t geometry shown i n Fig. 6 7 .

The i n i t i a l loading curves i n c l u d e s t r a i n a s s o c i a t e d w i t h t h e

r i n g conforming t o t h e geometry of t h e g r i p , so w e cannot t e l l p r e c i s e l y how much t h e sample is deforming.

Thus t h e y i e l d s t r e n g t h and t h e elonga-

t i o n are only relative numbers, b u t t h e u l t i m a t e t e n s i l e s t r e n g t h and reduction i n area obtained from t h e s e tests are t r u e values.

Obviously,

t h i s type of test is d e f i c i e n t i n g i v i n g good mechanical property d a t a b u t is s u f f i c i e n t f o r deforming t h e material and observing t h e incidence

of s u r f ace cracking.

R-55m

I I I

I I

I I I I

I Edge exposed to flowing salt

â&#x20AC;&#x2DC; I

I MO

I I

f l

5 1

E l

E a

a 1

0 0

TRAVERSE

DISTANCE-

MICRONS

F i g . 6 6 . Electron Hcroprobe Scan of Sample from Control Rod Thimble. The thimble had been exposed t o the f u e l s a l t for 21,040 hr and had been above 5OOOC f o r 30,807 hr.

Fig. 6 7 . Fixture for Testing Rings of Control Rod Thimble and Spacer Sleeve. A t e s t e d sample i s shown on the l e f t .

The t e n s i l e p r o p e r t i e s of t h e r i n g s are shown i n Table 8.

The test

r e s u l t s show t h e following important f a c t s :

1. A l l of t h e unannealed specimens of h e a t Y-8487 t e s t e d a t 25째C

have a "yield stress" of 52,000 t o 61,000 p s i , w i t h t h e s e values appearing t o be random i n t h e v a r i a b l e s involved.

A t 25째C t h e crosshead displacement w a s about 1.2 i n . f o r t h e

u n i r r a d i a t e d tubing and 0.4 t o 0.5 i n . f o r t h e i r r a d i a t e d tubing.

Again

t h e v a r i a t i o n s of 0.4 t o 0.5 i n . appeared random.

The y i e l d stresses a t 650째C w e r e about e q u i v a l e n t f o r i r r a d i a t e d

and u n i r r a d i a t e d tubing.

However, t h e crosshead displacement b e f o r e

f r a c t u r e decreased from about 0.4 i n . t o 0.1 i n . a f t e r i r r a d i a t i o n . was due t o embrittlement from helium formed from t h e 10B(n,a)7Li

This

transmu-

t a t ion.

No material i s a v a i l a b l e of h e a t 5060 ( s l e e v e m a t e r i a l ) f o r un-

i r r a d i a t e d tests, b u t t h e vendor's c e r t i f i c a t i o n s h e e t showed a y i e l d stress of 46,300 p s i , an u l t i m a t e tensile stress of 117,000 p s i , and a

f r a c t u r e s t r a i n of 52%. The values t h a t we obtained show h i g h e r s t r e n g t h s and lower d u c t i l i t y . Several of t h e specimens t h a t had been s t r a i n e d were examined m e t a l l o g r a p h i c a l l y t o determine t h e e x t e n t of cracking during s t r a i n i n g . Several photomicrographs of t h e c o n t r o l rod thimble t h a t w a s exposed t o flowing s a l t are shown i n Fig. 68.

The i n s i d e s u r f a c e w a s oxidized, and

t h e oxide cracked as t h e specimen w a s s t r a i n e d .

but i t i s important t h a t t h e s e cracks d i d n o t p e n e t r a t e t h e

brittle, metal.

The oxide should b e

The s i d e exposed t o t h e s a l t e x h i b i t e d profuse i n t e r g r a n u l a r

cracking.

Almost every g r a i n boundary cracked, and t h e cracks g e n e r a l l y

propagated t o a depth of one g r a i n , although t h e r e are several i n s t a n c e s where t h e crack depth exceeded a depth of one g r a i n .

S e v e r a l photomicro-

graphs were combined and rephotographed t o o b t a i n Fig. 69.

This sample

had 192 cracks p e r i n c h w i t h average and maximum depths of 5.0 and 8.0 mils, r e s p e c t i v e l y .

Many of t h e cracks shown i n Fig. 68 and 69 have

b l u n t t i p s , i n d i c a t i n g t h a t they d i d not propagate i n t o t h e metal as t h e specimen was s t r a i n e d .

Table 8.

Specimen

Condition

Postirradiation Anneal

Unirradiated Unirradiated Un i r r adi a ted Unirradiated Unirradiated Irradiated

.2 3 4

5 6 7 8

Irradiated Irradiated Irradiated Irradiated Irradiated

10 11

Irradiated b I rr adia t ed

13 14

Irradiated'

Tensile Data on Rings From Control Rod Thimble 3 (Heat Y-8487)

Test

Temperature ("0

None

None None None None 8 hr a t 871째C 141 h r a t 871째C None None

25 25 25 650 650 25 25 25 650 650 25 25 25 25

Crosshead Speed (in. /min) 0.05 0.05 0.05 0.05 0.002 0.05 0.05 0.05 0.05 0.002 0.05 0.05 0.05 0.05

aBased on 0.002 in. o f f s e t of crosshead travel. bLocated under spacer sleeve. C Spacer sleeve, Heat 5060.

Yielda Stress

Ultimate Stress

(psi)

52,000 56,900 58,000 39,100 40,700 54,400 53,300 60,500 38,300 34,200 49,300 48,700 51,700 42,400

114,400 117,500 124,300 76,700 62,300 105,100 102,500 110,800 51,200 38,200 100,500 104,900 98,600 123,000

Crosshead Travel (in.) 1.20 1.17 1.18 0.37 0.20 0.54 0.51 0.42 0.099 0.061 0.36 0.39 0.45 0.20

Reduction in Area

(%I 44.5 46.0 43.0 28.7 20.6 23.2 29.7 28.5 12.1 9.7 34.9 34.4 32.9 18.0

0 0

101

Fig. 68. Photomicrographs of Control Rod Thimble Specimen Exposed t o Flowinn - S a l t and Tested a t 25OC. (a) F r a c t u r e - dark edge is oxide on i n n e r s u r f a c e of tube, and cracks formed on t h e o u t e r s u r f a c e ; etched. (b) Cracks (c) Cracks i n o u t e r s u r f a c e of i n o u t e r s u r f a c e of tubing as polished. tubing etched. Etchant: Aqua r e g i a . Reduced 30.5%.

Fig. 69. Photomicrographs of a Deformed Section of the Control Rod Thimble. The fracture i s on the l e f t . The upper surface was i n contact with flowing s a l t and the lower surface w a s exposed t o the c e l l environment of N2 plus 2 t o 5% 02. The sample is about 0.06 in. thick. â&#x20AC;&#x2122;

c,:

103

A s i m i l a r sample was c u t from t h e thimble under t h e sleeve, where t h e s a l t access was r e s t r i c t e d .

a t 25OC are shown i n Fig. 70.

Photomicrographs of a specimen t e s t e d The cracks are q u i t e s i m i l a r t o those i n

Fig. 68 f o r a sample t h a t w a s exposed t o flowing s a l t .

The composite

photograph i n Fig. 71 is q u i t e similar t o Fig. 69 of t h e sample exposed t o flowing s a l t .

The sample i n Fig. 71 had 257 cracks p e r i n c h , w i t h

average and m a x i m u m depths of 4.0 and 8.0 mils, r e s p e c t i v e l y .

The accu-

racy of our s t a t i s t i c s and p o s s i b l e sampling inhomogenities l e a d us t o conclude t h a t t h e s e v e r i t y of cracking i s e q u i v a l e n t i n t h e samples exposed t o flowing and almost s t a t i c s a l t .

Thus flow rate over t h e

range represented by t h e s e two sample l o c a t i o n s does not appear t o b e an important v a r i a b l e . The s p a c e r sleeve gave another opportunity t o examine whether a r e l a t i o n s h i p e x i s t e d between s a l t v e l o c i t y and cracking.

The photomicro-

graph i n Fig. 72 shows cracks on both s i d e s , with more being p r e s e n t i n t h e f i e l d photographed on t h e s i d e where t h e s a l t w a s r a p i d l y flowing. However, t h e composite photograph i n Fig. 73 shows t h a t t h e cracking is about e q u i v a l e n t on both s i d e s .

Cracking s t a t i s t i c s on t h e s i d e exposed

t o r a p i d l y flowing s a l t revealed 178 cracks p e r i n c h w i t h a m a x i m u m depth of 7.0 mils.

The s i d e exposed t o r e s t r i c t e d s a l t flow had 202 cracks

p e r inch, w i t h a maximum depth of 5.0 mils.

mils on both s i d e s .

The average depth w a s 3.0

These observations again show.that s a l t flow rate

is not a s i g n i f i c a n t f a c t o r i n i n t e r g r a n u l a r cracking. Important Observations The examination of t h e c o n t r o l rod thimble and t h e thimble s p a c e r revealed two important c h a r a c t e r i s t i c s of t h e cracking. (1) I r r a d i a t i o n of t h e metal is not r e s p o n s i b l e f o r t h e cracking.

The thimble was i r r a d -

i a t e d t o a peak thermal f l u e n c e of 1.9 x 1021 neutrons/cm2.

The f l u x

a t t e n u a t i o n a c r o s s t h e 0.065 i n . wall of t h e thimble would have been very s l i g h t , b u t cracks only formed i n t h e metal on t h e f i e l d s i d e (Fig. 6 8 ) . Thus, i r r a d i a t i o n alone does n o t cause t h e cracking, b u t c o n t a c t w i t h t h e f u e l s a l t is required. (2) The s e v e r i t y of cracking is not s e n s i t i v e t o

s a l t flow rate over t h e range experienced i n a f u e l channel and under a s p a c e r w i t h r e s t r i c t e d flow.

104

*Fig. 70. Photomicrographs of Control Rod Thimble Exposed t o Fuel Salt Under a Spacer Sleeve and Tested a t 25OC. (a) As-polished view of fracture. 40x. (b) As-polished view of cracks. (c) Etched v i e w of cracks. Etchant: Aqua regia. Reduced 30.5%

105

R- 56472

Fig. 71. Composite Photograph Showing the Density of Cracking Along the Edge of a P o r t i o n of t h e Deformed Control Rod Thimble. 9 . 5 ~ . The sample is 0.58 i n . long. The f r a c t u r e i s on the l e f t , t h e ' u p p e r s u r f a c e was exposed t o f u e l s a l t , and t h e lower s u r f a c e was exposed t o t h e c e l l environment of N2 plus 2 t o 5% 02.

1 -

25OC. & polished. .The upper s u r f a c e w a s exposed t o almost s t a t i c f u e l s a l t and t h e lower s u r f a c e w a s exposed t o r a p i d l y flowing f u e l s a l t .

106

Primary Heat Exchanger

The primary h e a t exchanger w a s a t temperature f o r 30,807 h r and opera t e d with f u e l s a l t on t h e s h e l l s i d e and coolant s a l t i n t h e tubes.

The

s h e l l w a s 16-3/4 i n . OD x 1 / 2 i n . w a l l and constructed of h e a t 5068.

The

tubing was 1 / 2 i n . OD x 0.042 i n . w a l l from h e a t N2-5101 (see T a b l e 1 f o r

An o v a l p i e c e of t h e s h e l l 10 x 13 i n . w a s c u t by plasma t o r c h near the o u t l e t end of t h e heat exchanger. This same c u t severed a p i e c e of one tube, and pieces of f i v e o t h e r s were c u t by an chemical composition).

Photographs of these p a r t s are shown i n Figs. 74 and 75.

a b r a s i v e wheel.

The o u t s i d e of t h e s h e l l had a dark adherent oxide; t h e i n s i d e s u r f a c e s

w e r e d i s c o l o r e d s l i g h t l y and had a t h i n bluish-gray c o a t i n g probably caused by t h e plasma c u t t i n g operation.

Some of t h i s powder w a s brushed

o f f and found by gamma scanning t o c o n t a i n lo6Ru and 125Sb. Undeformed Samples Photomicrographs of a c r o s s s e c t i o n through t h e o u t e r s u r f a c e of t h e s h e l l are shown i n Fig. 76.

The oxide on t h e o u t s i d e s u r f a c e i s t y p i c a l

of t h a t observed on INOR-8 a t t h i s temperature and extends t o a depth of about 5 mils.

Etching a t t a c k s the m e t a l i n t h e oxidized l a y e r and e t c h e s

t h e g r a i n boundaries n e a r t h e s u r f a c e more r a p i d l y than those i n t h e i n terior.

The i n s i d e s u r f a c e of t h e s h e l l (Pig. 77) shows some s t r u c t u r a l â&#x20AC;&#x2122;

modifications t o a, depth of about 1 mil.

No samples of t h e s h e l l were

deformed. Photomicrographs of a t y p i c a l c r o s s s e c t i o n of t h e h e a t exchanger tubing are shown i n Fig. 78.

The o v e r a l l view shows t h a t both s i d e s e t c h

more r a p i d l y than t h e c e n t e r t o a depth of about 7 mils.

This is not un-

usual f o r a t u b u l a r product and is o f t e n a t t r i b u t e d t o i m p u r i t i e s ( p r i t

â&#x20AC;&#x2DC; m a r i l y l u b r i c a n t s ) t h a t are worked i n t o t h e metal s u r f a c e during repeated s t e p s of deformation and annealing.

The h i g h e r magnification v i e w s i n

t h e as-polished condition show t h a t g r a i n boundary a t t a c k (or cracking) occurred on t h e s u r f a c e exposed t o f u e l s a l t (outer) b u t n o t on t h e coolant s i d e (inner).

The temperatures under normal o p e r a t i n g conditions

were such t h a t t h e f u e l s a l t s i d e (OD) would have been i n compression.

107

R 06471

Fig. 73. Composite Photograph Comparing t h e Density of Cracking of Sides of Spacer Sleeve Exposed t o Almost S t a t i c S a l t (bottom) and t o flowing s a l t (top). llx. The sample is 0.52 i n . long.

Fig. 74. S e c t i o n 10 x 13 i n . Cut from t h e Primary H e a t Exchanger S h e l l . The p i e c e w a s c u t w i t h a plasma t o r c h , and t h e c e n t e r s t u d w a s used f o r guiding t h e torch.

108

Fig. 75. The 1/2-in.-OD Hastelloy N Tubes from the Primary Heat Exchanger. The different shades arise from a dark f i l m that i s thought t o have been deposited when the s h e l l was being cut. The f i l m was deposited on the s i d e of tlie tubes facing the s h e l l .

109

Fig. 76. Photomicrographs Showing the Outside Edge of the Primary Heat Exchanger Shell. This surface was exposed t o 2 to 5% 02 in N2. (a) A s polished, showing the s e l e c t i v e oxidation that occurred. (b) Etched view showing that the metal in the oxidized layer was completely removed. Etched with aqua regia.

110

Fig. 7 7 . Photomicrographs o f the Inside Surface of the Primary Heat Exchanger Shell. The surface was exposed t o f u e l s a l t , and the modified structure to a depth of about 1 m i l i s apparent. (a) As polished. (b) Etched with aqua regia.

F i g . 78. Photomicrographs of a Cross Section of anâ&#x20AC;&#x2DC; INOR-8 Heat Exchanger Tube. (a) Etched with glyceria regia. (b) Inside surface i n the as-polished condition a f t e r exposure t o coolant salt. (c) Outs i d e surface i n the as-polished condition a f t e r exposure t o f u e l s a l t . Reduced 34%.

112

Similar photomicrographs of a l o n g i t u d i n a l s e c t i o n of tubing are shown i n Fig. 79. f o r Fig. 78.

The f e a t u r e s are q u i t e similar t o those discussed

Both f i g u r e s show some metallic d e p o s i t s on t h e o u t s i d e

s u r f a c e , which w a s exposed'to f u e l s a l t .

These d e p o s i t s are extremely

small and could not be analyzed w i t h much accuracy w i t h t h e i n - c e l l microprobe.

However, they seemed t o b e predominately i r o n .

Scanning

a c r o s s t h e tubing d e t e c t e d no concentration g r a d i e n t s of t h e major

a l l o y i n g elements i n INOR-8. Deformed Samples T e n s i l e tests w e r e run on t h r e e of t h e tubes, and t h e r e s u l t s are compared i n Table 9 w i t h those f o r as-received tubing.

The tubes were

p u l l e d i n tension, and we assumed t h a t t h e entire s e c t i o n between t h e g r i p s was deforming.

The g r i p s o f f e r some end r e s t r a i n t , so our assump-

tion is obviously i n error, and the errors are such that our yield s t r e s s e s

are high and our f r a c t u r e s t r a i n s are low.

The u l t i m a t e stresses and re-

ductions i n area are unaffected by thi's assumption.

The as-received and

postoperation tests were run by t h e same technique, so t h e r e s u l t s should be comparable.

The l a r g e s t changes during service were reductions i n t h e

y i e l d stress, f r a c t u r e s t r a i n , and reduction i n area.

The tubing w a s

bought i n t h e "cold drawn and annealed" c o n d i t i o n , b u t such tubing normally is cold worked some during a f i n a l s t r a i g h t e n i n g o p e r a t i o n .

The

reduction i n y i e l d s t r e n g t h during service may have been due t o annealing out t h e e f f e c t s of t h i s working.

The reductions i n t h e d u c t i l i t y parame-

ters w e r e l i k e l y due t o t h e p r e c i p i t a t i o n of c a r b i d e s along t h e g r a i n boundaries. Photomicrographs of one of t h e tubes deformed a t 25OC are shown i n Fig. 80.

Profuse i n t e r g r a n u l a r cracks were formed t o a depth of about

5 mils on t h e s i d e exposed t o f u e l s a l t , but none were formed on t h e

coolant salt s i d e .

Etching again revealed t h e abnormal e t c h i n g charac-

teristics near t h e s u r f a c e , b u t whatever caused t h i s e t c h i n g c h a r a c t e r i s t i c d i d not r e s u l t i n g r a i n boundary embrittlement. Unstressed and s t r e s s e d s e c t i o n s of tubing were photographed over r e l a t i v e l y l a r g e d i s t a n c e s t o g e t more accurate s t a t i s t i c s on cracks

Table 9 .

Tensile Data on Heat Exchanger Tubes (Heat N2-5101)

Crosshead Speed (in /min)

Temperature ("0

Yield Stress (psi)

U1t imat e Tensile Stress ( p s i )

Fracture Strain (2)

Reduction in Area ( X )

Spe c imen

Condition

7 6 5 4 2

From heat exchanger

0.05

650

44,300

67,400

21.3

22.4

From heat exchanger

I .O

66,300

118,000

37.0

20.5

From heat exchanger

0.05

64,800

122,000

39.0

29.0

As received As received

0.05

650

53,900

74,600

40.0

14.5

0105

73,000

127,400

51.1

42.6

As received

0.05

70,900

126,200

50.1

40.0

Vendor c e r t i f i c a t i o n

58,900

120,700

47.0

114

I Fig. 79. Photomicrographs of a Longitudinal Section of an INOR-8 Heat Egchanger Tube. (a) Etched with glyceria regia. (b) Inside surface i n the as-polished condition after exposure t o coolant s a l t . (c) Outside surface i n the as-polished condition after exposure t o fuel s a l t . Reduced 31%.

115

u D

Fig. 80. Photomicrographs of a Heat Exchanger Tube Deformed t o Fracture a t 25OC. (a) A s polished. (b) Etched with aqua regia.

116

numbers and depths. i n Fig. 81.

A composite photograph of t h e two samples is shown

The cracks are r e a d i l y v i s i b l e i n t h e stres

may not be apparent i n t h e unstressed sample.

Li *

A higher

p r i n t used i n making t h e composite is shown i n Fig. 82; t h e cracks can be more e a s i l y seen.

The s t r e s s e d sample had 262 cracks p e r inch with

an average depth of 5.0 mils. sample.

Two counts w e r e made on the unstressed

I n t h e f i r s t count only those f e a t u r e s t h a t unequivocally were

cracks were counted.

This gave a crack count of 228 p e r inch with an

average depth of 2.5 mils.

I n another count, w e included some f e a t u r e s

t h a t were possibly cracks and t h i s gave a crack frequency of 308 p e r Thus t h e number of cracks present before s t r a i n i n g was about

inch.

equivalent t o t h a t noted a f t e r s t r a i n i n g .

visibze depth.

S t r a i n i n g d i d i n c r e a s e the

Some i n d i r e c t evidence t h a t t h e cracks were present

i n i t i a l l y and only spread open f u r t h e r during t h e t e n s i l e test w a s obtained from examination of t h e sample i n Fig. 81.

The sample length

shown deformed 39% (Table 9), and t h e combined widths of t h e cracks . account f o r 35% s t r a i n .

Thus, t h e cracks widths very c l o s e l y account

f o r t h e t o t a l s t r a i n and i n d i c a t e t h a t t h e cracks formed . . with l i t t l e o r no deformation and spread open as t h e sample was defornied. Important Observations The examination of the heat exchanger tubes revealed t h r e e very important c h a r a c t e r i s t i c s of t h e cracking.

(1) The neutron fluence

rFceived by t h e tubing w a s extremely low, b u t i n t e r g r a n u l a r cracks w e r e formed.

The c o n t r o l rod thimble showed t h a t i r r a d i a t i o n of t h e m e t a l

alone w a s not s u f f i c i e n t t o cause cracking s i n c e only t h e s i d e of t h e thimble exposed t o f u e l s a l t cracked.

Examination of t h e h e a t exchanger

tubing suggests t h e f u r t h e r conclusion t h a t i r r a d i a t i o n of t h e m e t a l is not a f a c t o r , s i n c e t h e heat exchanger w a s exposed t o a n e g l i g i b l e f l u ence but s t i l l cracked.

(2) Exposure t o f u e l s a l t is a necessary condi-

t i o n f o r cracking t o occur.

Only t h e o u t s i d e of t h e tubing, which w a s

exposed t o f u e l s e l t , cracked, and t h e i n s i d e , which w a s exposed t o coola n t s a l t , d i d not crack.

(3) The cracks were present i n t h e as-polished

tubing as i t w a s removed from t h e MSRE.

S t r a i n i n g caused t h e cracks t o

open wider without much p e n e t r a t i o n i n length.

8.I

117

Fig. 81. Composite Photographs of Heat Exchanger Tubing Before and A f t e r Stressing. The width of the unstressed sample i s 0.042 i n .

Fig. 82. Photomicrograph of Heat Exchanger Tubing i n t h e Unstressed Condition. The s i d e with t h e small cracks w a s exposed t o f u e l s a l t and t h e o t h e r w a s exposed t o coolant s a l t .

118

Pump Bowl P a r t s

A schematic v i e w of 'the pump i s shown i n Fig. 83. examined w e r e t h e mist s h i e l d and the sampler cage.

The components

The sampler w a s low-

ered by a windlass arrangement i n t o the sampler cage and w a s used primari l y f o r taking s a l t samples f o r chemical analysis.

The mist s h i e l d was

provided t o mimimize t h e amount of s a l t spray t h a t would reach t h e sampler. The vertical sampler cage rods were 1 / 4 i n . d i m and made of h e a t 5059. The mist s h i e l d w a s made of 1/8-in. chemical a n a l y s i s ) .

s h e e t of h e a t 5057 (see Table 1 f o r

The mist s h i e l d w a s a s p i r a l with an i n s i d e diameter

of about 2 i n . and o u t s i d e diameter of about 3

in.

The s p i r a l w a s about

1 1 / 4 t u r n s , and t h e o u t s i d e w a s exposed t o a g i t a t e d s a l t and t h e i n s i d e t o s a l t flowing much slower.

The o u t s i d e was exposed t o s a l t up t o t h e

normal l i q u i d l e v e l and t o s a l t spray above t h i s level.

The i n s i d e was

exposed t o s a l t up t o t h e normal salt l e v e l and primarily t o gas above t h i s level. The general appearance of the sampler cage is shown i n Fig. 84.

The

s a l t normally stayed a t a l e v e l near t h e c e n t e r of t h e cage, b u t t h e l e v e l fluctuated slightly.

The amount of material deposited on t h e s u r f a c e i s

much higher below t h e l i q u i d i n t e r f a c e than above.

A gross gamma scan

had a similar p r o f i l e with a maximum near t h e l i q u i d i n t e r f a c e . carbonaceous d e p o s i t w a s present a t t h e top of t h e sampler.

The mist

s h i e l d had some d e p o s i t s , and t h e s e were heavier below t h e l i q u i d l e v e l and on the o u t e r s u r f a c e of t h e s h i e l d . Sampler Cage One of t h e sampler cage rods was examined metallograpically.

Photo-

micrographs of samples from t h e vapor and l i q u i d regions are shown i n Fig. 85 and 8 6 , respectively. v i s i b l e a t . t h i s magnification. from t h e l i q u i d region.

There is no i n t e r g r a n u l a r p e n e t r a t i o n

A heavy s u r f a c e deposit i s on t h e sample

This deposit w a s not examined by t h e e l e c t r o n

microprobe analyzer, b u t a similar deposit on a copper capsule taken from

119 i

'L,

. Fig. 83. Section of MSRE Fuel Pump with Details o f Several Areas. The mist s h i e l d and the sampler cage that were examined are shown i n the lower l e f t insert.

Fig. 84. Sampler Cage. The assembly is 8.5 i n . high. The salt-vapor interface corresponds with the region of highest material deposition.

120

Fig. 8 5 . Photomicrographs of an INOR-8 Rod from the Sampler Assembly Located Above the Normal Salt Level. (a) As-polished. (b) Etched with aqua regia.

hpi

121

Fig. 86. Photomicrographs of an INOR-8 Rod from tIie Sampler Assembly. Normally located below the s a l t level. (a) A s polished. (b) Etched with aqua regia.

122

the sampler region was examined.

The deposit contained some s a l t , b u t had

regions t h a t w e r e high i n t h e a l l o y i n g elements i n INOR-8:

N i , C r , Fe,

the sampler cage rods was pulled i n tension a t 25OC.

The

o p e r t i e s of this material were a y i e l d stress of 5%,200 p s i ,

an u l t i m a t e tensile stress of'115,300, and an elongation of 51%. The measured p r o p e r t i e s of the sampler cage rod w e r e a y i e l d stress of 42,500 p s i , an u l t i m a t e t e n s i l e stress of 93,400 psi, and an elongation of 35.7%. The property changes are not l a r g e and are l i k e l y due t o some stress rel i e f t h a t decreased t h e y i e l d and u l t i m a t e stress values and carbide prec i p i t a t i o n , which reduced t h e d u c t i l i t y .

A composite of t h e t e s t e d sample

i n Fig. 87 shows t h a t t h e f r a c t u r e occurred below t h e c e n t e r near t h e average liquid-gas i n t e r f a c e .

One metallographic sample w a s taken on t h e

l i q u i d s i d e of t h e f r a c t u r e .

A composite showing t h e badly cracked edges

is shown i n Fig. 8 8 .

Note t h a t t h e cracks extended t o a depth of 12 mils.

A higher magnification v i e w of an area near t h e f r a c t u r e shows t h a t t h e

cracks e x t e n d ' i n excess of 3 g r a i n s deep and t h a t s e v e r a l g r a i n s have f a l l e n out (Fig. 89). above t h e f r a c t u r e .

Another metallographic sample was taken 3/4 i n . A composite of photographs along t h e edge shows t h a t

the depth of cracking decreases with increasing d i s t a n c e i n t o t h e vapor region (Fig. 90). of the rod.

The s e v e r i t y of cracking is d i f f e r e n t on t h e two edges

Unfortunately, we do not know t h e o r i e n t a t i o n of t h e rod

r e l a t i v e t o t h e sampler and t h e mist s h i e l d . Mist Shield Figure 9 1 shows t h e mist s h i e l d a f t e r removal from t h e MSRE. s h i e l d has been s p l i t t o reveal t h e i n s i d e surface.

The

Four specimens

approximately 1 i n . ( v e r t i c a l ) x 1 / 2 i n . (circumferential) were c u t from t h e mist s h i e l d .

Their l o c a t i o n s were (1) o u t s i d e t h e s p i r a l and immersed

i n s a l t , (2) o u t s i d e t h e s p i r a l and exposed t o s a l t spray; (3) i n s i d e t h e s p i r a l and immersed i n s a l t , and (4) i n s i d e t h e s p i r a l and exposed primarily 'to gas.

Bend tests w e r e performed on these specimens a t 25OC

using a t h r e e p o i n t bend f i x t u r e . 2 2

They were bent about a l i n e p a r a l l e l

123 ~

R-55722

Fig. 87. (a) Tensile-tested Sampler Cage Rod from Pump Bowl. Rupture occurred n e a r the average liquid-vapor i n t e r f a c e . (b) Photograph of r u p t u r e area showing extensive s u r f a c e cracking. Rod diameter is 1 / 4 i n .

Fig. 88. Section of Sample Cage Rod t h a t w a s Deformed a t 25OC. 9 . 2 ~ . The f r a c t u r e ( l e f t ) occurred a t t h e salt-vapor i n t e r f a c e . The rod is immersed f u r t h e r i n t h e l i q u i d from l e f t t o r i g h t . The length of t h e sample is 0.63 i n .

Fig. 89. Photomicrograph of Deformed Sampler Cage Rod Near t h e F r a c t u r e . As-polished.

i L

. . ~ . ,. . .

Fig. 90. Composite of Photomicrographs of Sampler Cage Rod t h a t w a s S t r a i n e d at 25'C. 8 . 2 ~ . L e f t end of sample w a s 3/4 i n . from f r a c t u r e . The rod progressed f u r t h e r i n t o t h e vapor region from l e f t t o r i g h t . The sample l e n g t h is 0.72 i n .

125

Ld *

F i g . 91. Interior of Mist Shield. lapped left'part of segment on left.

Right part of right segment over-

126

t o the 1 / 2 i n . dimension and so that the o u t e r s u r f a c e w a s i n tension.

The information obtained from such a test is a load-deflection curve. The equations normally used t o convert t h i s t o a s t r e s s - s t r a i n curve do not take i n t o account t h e p l a s t i c deformation of t h e p a r t , and t h e stresses obtained by t h i s method become progressively i n e r r o r (too high) as t h e deformation progresses. Table 10 shows the r e s u l t s of bend tests on t h e specimens from t h e mist s h i e l d .

Note t h a t t h e ' y i e l d and u l t i m a t e stresses are about double

those reported f o r u n i a x i a l tension. of primary i n t e r e s t .

The f r a c t u r e s t r a i n is t h e parameter

The test of unexposed INOR-8 (heat N3-5106) d i d not

f a i l a f t e r 40.5% s t r a i n i n t h e o u t e r f i b e r s .

(No material w a s a v a i l a b l e

of h e a t 5075, the material used i n f a b r i c a t i n g t h e mist s h i e l d . )

Although

a l l of t h e samples s t r a i n e d more than 10%b e f o r e f a i l u r e , t h e two samples from t h e o u t s i d e of t h e s p i r a l were more b r i t t l e than t h e o t h e r samples. T h e bend samples of t h e mist s h i e l d w e r e examined metallographically.

Photomicrographs of t h e sample from the i n s i d e vapor region are shown i n

Fig. 9 2 , and a composite of s e v e r a l photomicrographs is shown i n Fig. 9 3 . The edge cracks were i n t e r g r a n u l a r and about 1 m i l deep.

The photomicro-

graphs of t h e sample from t h e l i q u i d region (Fig. 9 4 ) show t h a t t h e cracks extended t o a depth of about 8 mils i n t h e l i q u i d region.

The composite

photograph i n Fig. 95 a l s o shows t h e increased frequency and depth of cracking i n t h e l i q u i d region. The sample from t h e o u t s i d e l i q u i d region was q u i t e similar metallographically t o t h a t from t h e i n s i d e l i q u i d region.

Photomicrographs

of t h e f r a c t u r e and a t y p i c a l region of t h e tension near t h e f r a c t u r e

are shown i n Figs. 96 and 9 7 , respectively. photomicrographs is shown i n Fig. 9 8 .

A composite of s e v e r a l

Photomicrographs of t h e o u t s i d e

vapor (salt spray) bend specimen a t t h e f r a c t u r e and on t h e tension s i d e near t h e f r a c t u r e are shown i n Figs. 99 and 100, respectively.

The com-

p o s i t e of s e v e r a l photomicrographs i n Fig. 101 shows t h a t t h e cracks are not much deeper nor more frequent than i n t h e l i q u i d regions (compare Figs. 9 5 , 9 8 , and 101), b u t t h e tendency f o r g r a i n s t o f a l l o u t is much greater.

This is even mre s i g n i f i c a n t when one notes t h a t t h e s t r a i n

w a s t h e least i n t h e sample from t h e o u t s i d e vapor region (Table 10).

L4 I

Table 1 0 .

Bend Tests a t 25OC on P a r t s of t h e MSRE Mist S h i e l d a

Specimen

Yield Stress (psi)

Maximum' Tensile Stress (PS i1

x 103

Strain

(XI

Environment

S-52

M i s t s h i e l d top i n s i d e

269

46 .gd

Vapor r e g i o n , s h i e l d e d

S-62

Mist s h i e l d t o p o u t s i d e

155

224

10.7d

Vapor r e g i o n , s a l t s p r a y

S-60

M i s t s h i e l d bottom i n s i d e

161

292

31.6d

Liquid salt Liquid salt

M i s t s h i e l d bottom o u t s i d e

S-68 N3-5106

Unirradiated control' test, 1/8 i n . thick

187

17.7

128

238

40.5

region, shielded, flow region, rapid flow

a The m i s t s h i e l d w a s f a b r i c a t e d of h e a t 5075 w i t h c e r t i f i e d room-temperature p r o p e r t i e s of 53,000 p s i y i e l d stress, 116,000 p s i u l t i m a t e stress, and 49% e l o n g a t i o n .

bBased on 0.002 i n , o f f s e t of crosshead travel. 'Maximum t e n s i l e stress was c o n t r o l l e d by f r a c t u r e o f sample o r by s t r a i n l i m i t a t i o n of test f i x t u r e . dSpecimen broke.

c1 h,

128

Fig. 92. Photomicrographs of Bend Specimen of Mist Shield from Inside Vapor Region. (a) Fracture and tension s i d e , as polished. (b) Fracture and tension s i d e , etched. ( c ) Tension s i d e , etched. Etchant: Aqua regia. Reduced 3l%.

130

Fig. 94. Photomicrographs of Bend Specimen of Mist Shield from Inside Liquid Region. (a) Fracture and tension s i d e , as polished. (b) Fracture and tension s i d e , etched. (c) Fracture and tension s i d e , etched. Etchant: Aqua Regia.

132

F i g . 96. Photomicrographs of.the Fracture of a Bend Specimen from the Outside Liquid Region of the Mist Shield. (a) 1OOx. (b) 500x. Etchant: Lactic acid, "03, HC1.

133

Fig. 97. Photomicrographs of the Tension Side Near the Fracture of a Bend Specimen from the Outside Liquid Region of the Mist Shield. (a) 1OOx. (b) 500x. Etchant: lactic acid, HNO3, HC1.

Fig. 98. Composite of Several Photomicrographs of a Bend Specimen from the Outside Liquid Region of the M i s t Shield. Top portion shows the tension s i d e and the lower portion shows the compression s i d e . 24x.

Fig. 99. Photomicrographs of the Fracture of a Bend Specimen from the Outside Vapor Portion of the Mist Shield. (a) 1OOx. (b) 500x. HC1. Etchant: Lactic acid, "03,

136

Fig. 100. Photomicrographs of the Edge Near the Fracture of a Bend Specimen from the Outside Vapor Portion of the Mist Shield. (a) 1OOx. (b) 500x. Etchant: Lactic acid, "03, HC1.

137

The reduced d u c t i l i t y of these samples remains unexplained.

The

maximum crack depth i n t h e l i q u i d region was only 8 mils, and i t is q u i t e

unlikely t h a t an 8 m i l reduction i n a sample thickness of 125 mils would cause much embrittlement;

The embrittlement may have been p a r t i a l l y due

t o t h e extensive carbide p r e c i p i t a t i o n shown i n Figs. 93, 95, 96, 98, and 99.

However, the outside and i n s i d e of t h e s h i e l d received i d e n t i c a l

thermal treatments, b u t the samples from the o u t s i d e had lower d u c t i l i t i e s .

Thus, a t t r i b u t i n g the embrittlement s o l e l y t o carbide formation does n o t seem appropriate. Important Observations The only f r e e s u r f a c e i n t h e primary c i r c u i t was i n t h e pump bowl, and t h e r e appears t o have been a c o l l e c t i o n of material a t t h e surface. Visual observation, gamma scanning, and t h e e l e c t r o n microprobe show an accumulation of s a l t , f i s s i o n products, and corrosion products a t t h i s surface.

Besides having a f r e e s u r f a c e , t h e region around t h e sampler

w a s o f t e n q u i t e reducing because of t h e a d d i t i o n of beryllium m e t a l a t this location.

Fluorides of t h e s t r u c t u r a l metals would have been reduced

t o t h e metallic form and accumulated a t t h e f r e e s u r f a c e o r deposited on nearby metal surfaces.

Some of t h e less s t a b l e f i s s i o n product f l u o r i d e s

would have l i k e l y reacted i n a similar way.

Thus, i t i s not s u r p r i s i n g

t h a t t h e i n t e r g r a n u l a r cracks are most severe near t h e f r e e surface.

The

most important observation relative t o t h e cracking is t h a t i t s s e v e r i t y w a s much less i n material exposed primarily t o gas.

This allows t h e con-

c l u s i o n t h a t exposure t o l i q u i d s a l t is necessary f o r t h e cracking t o occur. Freeze Valve 105 Freeze valve 105 f a i l e d during t h e f i n a l thermal cycle involved with terminating operation of t h e MSRE.

The f a i l u r e w a s a t t r i b u t e d t o f a t i g u e

valve consisted of a s e c t i o n of 1 1/2-in. from a m ~ d i f i c a t i o n . ~This ~ sched. 40 INOR-8 pipe (heat 5 0 9 4 ) with a j a c k e t f o r a i r cooling.'

It w a s

used t o i s o l a t e t h e d r a i n tanks and was frozen only when s a l t w a s i n t h e

138 d r a i n tanks.

The l i n e was f i l l e d with s a l t from t h e d r a i n tanks, so t h e

f i s s i o n product concentration would have been r e l a t i v e l y low.

w a s f i l l e d & t h s a l t and above 5OOOC f o r about 21,000 h r . a l s o s t a t i c except when t h e system was being f i l l e d ;

The l i n e

The s a l t was

Thus, t h i s p a r t i c -

u l a r component was subjected t o a unique set of conditions. Rings 3/16 i n . wide were c u t , away from t h e f l a t t e n e d p o r t i o n of t h e pipe. They were subjected t o t h e r i n g test described previously. A bend specimen and a specimen f o r chemical a n a l y s i s w e r e c u t from adjacent regions.

The r e s u l t s of the mechanical property tests are given i n

Table 11. As shown i n Fig. 102 both t h e bend specimen and t h e r i n g specimens showed some s u r f a c e cracking when viewed a t low magnification.

A metallographic s e c t i o n of one of t h e r i n g specimens is shown i n Figs. 103 and 104.

The f r a c t u r e and both edges are v i s i b l e i n Fig. 103.

The oxide is on t h e o u t s i d e s u r f a c e t h a t was exposed t o a i r (top) and shallow cracks formed on t h e s a l t s i d e . are shown i n Fig. 104.

Typical edges near t h e f r a c t u r e

The cracks on t h e a i r s i d e followed t h e oxide and The cracks on t h e s a l t s i d e were i n t e r g r a n u l a r

did not p e n e t r a t e f u r t h e r . and penetrated about 1 m i l .

The most important observation on t h i s component is t h a t t h e cracking I

was less severe under conditions where t h e f i s s i o n product concentration

w a s less.

However, i n t e r g r a n u l a r cracking occurred on t h e s u r f a c e s t h a t

were exposed t o f u e l s a l t .

The corrosion ( s e l e c t i v e removal of chromium)

t h a t occurred under these conditions should have been extremely small s i n c e the s a l t was s t a t i c most of the t i m e .

Thus, corrosion does not

seem t o be a requirement f o r i n t e r g r a n u l a r cracking although i t may acc e l e r a t e t h e process.

EXAMINATION OF INOR-8 FROM IN-REACTOR LOOPS The observation of i n t e r g r a n u l a r cracking i n t h e MSRE caused us t o reexamine the a v a i l a b l e r e s u l t s on t h r e e in-reactor

loops.

Two pumped

loops were run by Trauger and C ~ n l i n i~n ~1959, , ~ but ~ the material from these loops had been discarded and only the r e p o r t s and photomicrographs remained f o r examination.

A more recent thermal convection loop w a s run

i *

' C

Table 11. Results of Mechanical Property Tests on Specimens From Freeze Valve 105 (Heat 5094 a t 25OC and a Deformation R a t e of 0.05 in./min ~~~

Type of T e s t

Yield Stress (Psi)

U 1t i m a t e Tensile S t r e s s (Psi)

Crosshead Travel (in.)

Reduction i n Area

45,800

106,800

Ring , tens il e

45,800

89 700

0.72

Ring , t e n s i l e

48,900

90,100

0.59

Ring, t e n s i l e

41,900

90 300

0.73

Wall segment, bend

71,300

0.41

33a

Vendor's,

tensile

52.6

%laximum strain i n o u t e r f i b e r s .

140.

Fig. 101. Composite of Several Photomicrographs of a Bend Specimen From t h e Outside Vapor Region of t h e M i s t Shield. Top p o r t i o n is t h e tension s i d e and t h e lower p a r t is the compression s i d e . 15x.

. Fig. 102. The Surfaces Exposed t o S a l t i n Freeze Valve 105 After Deformation a t 25OC. (a) Fracture of r i n g specimen pulled i n tension. Note s u r f a c e cracks near t h e f r a c t u r e . 4x (b) Surface of bend specimen. Note some cracks on s u r f a c e and edge cracks. 7x. Reduced 18.5%.

141

Fig. 103. Photomicrographs of the Fracture of a Ring Specimen from Freeze Valve'l05 that was Deformed a t 25OC. (a) As polished. (b) Etchant: Lactic acid, "03, HC1. 40x. Reduced 29.5%.

142

i 3

Fig. 104. Photomicrographs Near the Fracture of a Ring Sample from Freeze Valve 105 Deformed at 25OC. (a) Edge exposed to air, as polished. (b) Edge exposed to salt,, as polished. (c) Edge exposed to salt, etched with lactic acid, â&#x201A;Źâ&#x201A;ŹNO$, HC1. Reduced 30%.

143

by Compere e t a1.,26 and some of t h e pieces were s t i l l a v a i l a b l e and t h e i r l o c a t i o n i n t h e loop i d e n t i f i e d .

Some samples of t h e s e pieces were de-

formed and examined metallographically. Pump Loops Trauger and Conlin ran two INOR-8 pump loops i n t h e MTR.24925

These

operated a t a peak temperature of 704OC a t an average power density of 66 W/cm3.

The loops both contained s a l t of composition7 LiF-BeF2-UF4

(62-37-1 mole X ) .

The f i r s t loop ran a t a peak Reynolds number of 4100

and operated f o r 638 h r .

The second loop had a peak Reynolds number

of 3100 and operated f o r 766 h r .

The operation of both loops was termin-

a t e d by leaks i n t h e heat exchanger. a t i o n w a s made of the f i r s t

A d e t a i l e d metallographic examinThe loop had a nose cone t h a t was

c l o s e s t t o t h e r e a c t o r , and i t was here t h a t the type of a t t a c k shown i n Fig. 105 w a s noted.

The a t t a c k i s a c t u a l l y a s u r f a c e roughening i n

which g r a i n s w e r e removed from t h e i n s i d e of t h e INOR-8 tubing. second loop d i d not show t h i s type of attack.28

The

The l e a k i n t h e heat

exchanger was not l o c a t e d i n e i t

as 66 W/cm3, with s a l t volumes

The f i s s i o n density i n t h e s

cm2 each.

of 135 cm3 and s u r f a c e areas of

One of t h e f i s s i o n

products t h a t w i l l be discusse

t h e r is tellurium, and a comparison

of t h e amount produced i n t h e s

ps with t h a t produced i n ' t h e MSRE is

useful.

About 1 . 4 x 1017 a t o

metal s u r f a c e area i n t h e MS

Tellurium w e r e produced p e r u n i t of

only 0.2 x 1017 atoms/cm2 i n t h e s e

The t i m e a t temperatur

as a l s o much smaller f o r t h e s e loops than

f o r even t h e f i r s t group of su

i l l a n c e specimens from t h e MSRE, i n which

loops.

cracking w a s hardly d e t e c t a b l e (Fig. 12). The uneven i n n e r s u r f a c

e loop tubing may or may not have been

due t o t h e same phenomenon t

sed t h e i n t e r g r a n u l a r cracking i n t h e

MSRE.

t e r i a l w a s removed i n increments of

Note i n Fig. 105 t h a t

s i n g l e grains.

However, t h e r

no evidence of p a r t i a l l y cracked g r a i n s

that had not been completely removed.

The explanation of c a v i t a t i o n is

a l s o not very acceptable s i n c e t h e peak v e l o c i t y was only about 1 f t / s e c .

144

Fig. 105. Inside Tube Wall of INOR-8 from In-pile Coop MTR 44-1. The coating is nickel plating used for edge preservation.

145 Being a b l e t o deform a piece of t h e tubing and then examine it f o r cracks should be enlightening, but no material from e i t h e r loop remains.

Thus,

t h e observations on these loops are not very h e l p f u l i n t h e present analysis. Thermal Convection Loop

A more recent in-reactor thermal convection loop w a s run by Compere et a1.26

A schematic of t h e loop is shown i n Fig. 106, and t h e p e r t i n e n t

operating s t a t i s t i c s are given i n Table 12.

The loop w a s constructed of

INOR-8, contained s a l t of compost i i o n 7LiF-BeF2-ZrF~-UF~(65.3-28.2-4.8-1.7 mole X ) , and had a graphite region where power d e n s i t i e s of 150 W/cm3 of

s a l t were a t t a i n e d .

The loop had maximum and minimum operating tempera-

t u r e s of 720 and 545"C, respectively.

The loop f a i l e d a t t h e h o t t e s t

p o r t i o n a f t e r 1366 h r of nuclear operation.

The f a i l u r e (Fig. 107) i s

q u i t e t y p i c a l of high-temperature f a i l u r e s noted previously i n t h i s mat h a t t h e i n s i d e s u r f a c e is f r e e of t e r i a l a f t e r i r r a d i a t i ~ n . ~ ' Note ~ cracks except very near t h e p o i n t of f a i l u r e . Two pieces of t h e loop were r e t r i e v e d and t e s t e d .

One sample w a s

from t h e f l a t s h e e t used t o f a b r i c a t e t h e top of t h e "core section" where t h e g r a p h i t e was located.

A small s h e e t was c u t , bent so t h a t the sur-

face exposed t o t h e f u e l s a l t was i n tension, and examined metallographically.

A t y p i c a l photomicrograph i s shown i n Fig. 108.

Surface cracks

w e r e very infrequent and extended t o a maximum depth of 0.5 m i l .

A piece of tubing w a s a v a i l a b l e from t h e c o l d e s t s e c t i o n , and i t

was deformed by crushing i n a t e n s i l e machine. i n s i d e s u r f a c e is shown i n Fig. 109.

A t y p i c a l view of t h e

Cracks occurred along almost every

g r a i n boundary, b u t they only extended t o a maximum depth of 1 m i l . The amount of t e l l u r i u m produced i n t h i s loop p e r u n i t area of m e t a l surface'was 4.2 x 10l6 atoms/cm2 (reference 2 9 ) , compared with 1 . 4 x 1017 atoms/cm2 i n t h e MSRE.

Thus, t h e amount of t e l l u r i u m was one-fourth t h a t

i n t h e test loop, but the system w a s above 400째C only 937 h r when f i s s i o n products were present.

..._."..I.

Table 12.

....

. ".

. ...

...

.~ -...

.. ....

Summary of Operating P e r i o d s f o r In-Reactor Molten-Salt Loop 2

Operating P e r i o d ( h r ) Total

Irrahiat i o n

F u l l Power Dose Equivalent

Out-of -Reactor Flush

77.8

Solvent S a l t

171.9

In-Reac t o r Preirradiation

73.7

Solvent S a l t

343.8

339.5

136.0

Fueled s a l t

1101.9

937.4

547.0

435.0

428.3

11.2

2204.1

1705.2

694.2

Retracted-fuel removala Total

% f a i n t a b e d a t 350 t o 4OOOC (frozen) e x c e p t during salt-removal o p e r a t i o n s and f i s s i o n product 1 e a k . i n v e s t i g a t i o n s .

c ,

147

SALT SAMPLE LINE/

Fig. 107. Photomicrograph of a Cross Section Showing the Inner Surface and Crack i n Core Outlet P i p e of In-Reactor Loop 2 . Located on top s i d e of tubing. Etchant: Aqua regia.

148

Fig. 108. Bend Specimen from Section of In-Reactor Loop Which Operated at 720OC. The tension side was exposed to the fuel salt.

Fig. 109. Tension Side of Crushed Tubing from Coldest Part of In-Reactor Loop, Which Operated at 545OC.

149

Summary of Observations on In-Reactor Loops The loops j u s t discussed w e r e exposed t o f i s s i o n product concentrat i o n s much lower than those observed i n t h e MSRE.

The t i m e a t tempera-

t u r e a f t e r f i s s i o n products were produced was a l s o much'less than f o r t h e MS'RE.

The loop t i m e s w e r e a l l less than 1000 h r , and t h e f i r s t group

of MSRE s u r v e i l l a n c e samples was removed a f t e r 2550 h r exposure (Table

3) t o f i s s i o n products a t elevated temperatures.

The number of cracks i n

t h i s f i r s t group of s u r v e i l l a n c e samples w a s q u i t e s m a l l (Fig. 12, Table

7). Thus, i t i s questionable whether these loops had adequate exposure t o cause d e t e c t a b l e cracking. The loops run by Trauger and Conlin had some regions from which g r a i n s w e r e removed, b u t no i n t e r g r a n u l a r cracks were v i s i b l e .

Strained

samples of Compere's loop had shallow i n t e r g r a n u l a r cracks similar t o those noted i n samples from t h e E R E , p a r t i c u l a r l y the sample exposed

a t 545째C (Fig. 109). C'fIEMICAL ANALYSES OF METAL REMOVED FROM THE MSRE

W e found t h e e l e c t r o n microprobe analyzer t o be u s e f u l f o r measuring chromium gradients i n INOR-8, but w e were not s u c c e s s f u l i n l o c a t i n g any f i s s i o n products.

We then used t h e technique of e l e c t r o l y t i c a l l y removing

s u r f a c e l a y e r s and analyzing t h e s o l u t i o n s .

S p e c i f i c a l l y , t h e method

involved a methanol-30% HNO3 s o l u t i o n a t -15 t o -2OoC, and the specimen as the anode.

a platinum cathode,

The e l e c t r i c p o t e n t i a l was 6 V, a level

t h a t had been shown by laboratory experiments t o p o l i s h r a t h e r than e t c h . The s o l u t i o n s w e r e analyzed by two methods.

The concentrations of t h e

s t a b l e elements were obtained by evaporating 2 cc of t h e s o l u t i o n i n t o a mass spectrometer and then analyzing t h e vapor by mass numbers.

Numerous experimental d i f f i c u l t i e s w e r e encountered i n taking t h e samples.

Surface deposits (formed i n t h e pump bowl) and t h i n oxide films

(found on many p a r t s from t h e core) a c t e d as i n h i b i t o r s and made t h e

methanol-30% HNO3 s o l u t i o n a t t a c k nonuniformly o r not a t a l l .

The voltage

usually had t o be increased t o a t t a c k these s u r f a c e b a r r i e r s .

Thus t h e

150

removal was n o t uniform on most samples u n t i l a few mils had been removed.

A second problem was i n sample p r e p a r a t i o n .

The round geometry of t h e

s u r v e i l l a n c e sample and t h e h e a t exchanger tube was d e s i r a b l e , b u t s t r i p s had t o be cut from components such as t h e c o n t r o l rod thimble and t h e

mist s h i e l d .

W e were n o t s u c c e s s f u l i n masking t h e s u r f a c e s n o t exposed

t o f u e l s a l t , so t h e r a t i o s of f i s s i o n product c o n c e n t r a t i o n t o t h a t of n i c k e l are lower than would have been obtained had a l l t h e s u r f a c e s been exposed t o f i s s i o n products.

Two of t h e b e t t e r p r o f i l e s are shown i n Fig. 110 and 111. The sample i n Fig. 110 was a segment of h e a t exchanger tubing.

Epoxy w a s cast in-

s i d e t o mask t h e s u r f a c e t h a t w a s exposed t o c o o l a n t salt. Fig. 111 w a s a s u r v e i l l a n c e sample.

The sample i n

We made d i a m e t r a l measurements w i t h

a micrometer, b u t t h e s e were n o t very a c c u r a t e .

W e a c t u a l l y obtained t h e

depth measurements by c a l c u l a t i o n from t h e measured n i c k e l c o n c e n t r a t i o n s using t h e supplemental knowledge t h a t the a l l o y w a s 70% Ni and

t h a t the

volume of t h e s o l u t i o n w a s 140 cc. Compere compared t h e amounts of f i s s i o n products t h a t we a c t u a l l y found w i t h the t o t a l inventory i n t h e MSRE.

H e assumed t h a t t h e f i s s i o n

products were deposited uniformly on t h e t o t a l system metal area of

7.9 x

lo5

cm2.

( I n c l u s i o n of t h e g r a p h i t e s u r f a c e area i n c r e a s e s the

d e p o s i t i o n area t o 2.3 x

lo6

cm2.)

The r e s u l t s of t h i s type of a n a l y s i s

are summarized i n Table 13 f o r t h e most h i g h l y concentrated f i s s i o n products.

The v a r i o u s samples are l i s t e d i n t h e o r d e r t h a t they would occur

around t h e primary c i r c u i t , beginning at t h e . b o t t o m of t h e c o r e and prog r e s s i n g up through t h e c o r e , through t h e pump, i n t o t h e h e a t exchanger, and back t o t h e core.

The values f o r t h e second sample should b e q u i t e

low because of t h e shallow sampling, and t h e samples from t h e s u r v e i l l a n c e specimen and t h e h e a t exchanger tube should y i e l d t h e most a c c u r a t e d a t e . However, t h e r e seem t o b e many i n c o n s i s t e n c i e s and no s y s t e m a t i c v a r i a t i o n of the elements around t h e c i r c u i t .

One of t h e most i n t e r e s t i n g sets of chemica1,results w a s obtained from a s u r v e i l l a n c e sample of h e a t 5058 from t h e f o u r t h group.

The

sample was oxidized f o r a few hours i n a i r a t 65OOC b e f o r e b e i n g s t r a i n e d .

A very t h i n oxide f i l m w a s formed and should have a c t e d as a b a r r i e r t o

151

ORNL- DWG 71 7106

HEAT EXCHANGER TUBE 1 2 3

10-

-W

kJ k!

10-5

40-6

a Iz

g 0

10-* -

10-'O

1 2 3 DEPTH (mils)

Fig. 110. Concentration Profiles from the Fuel Side of a MSRE Heat Exchanger Tube. Arrows indicate that analytical values were reported as being less than the indicated point.

152 ORNL- DWG 12 -2934

Id'

lppb

I---

!be

.---*---. 2 DEPTH (mils)

Fig. 111. Concentration Profiles from a Surveillance Sample Exposed in the E R E for 19,136 hr. Arrows indicate that analytical values were reported as being less than the indicated point.

' d

Table 13.

Concentrations of Several Fission Products on the Surfaces of Hastelloy N, Compared with the Total Inventory

Sample Location

Depth of Penetration (mils)

Concentrat ion of Nuclide Compared with Inventory

1 2 7 ~ ~1 3 4 ~ ~

125Sb

l o3Ru

lo6Ru

9 5Nb

99Tc P

Control rod thimble (bottom)

2.4

0.43

0.84

0.85

0.40

0.13

0.37

0.32

Control rod thimble (middle)

0.1

0.14

0.24

0.35

0.15

0.11

0.26

0.19

Su r v e i 1lance specimen

3.5

0.001

0.35

1.04

0.006

0.087

0.006

0.30

Mist s h i e l d outside, l i q u i d

6.0

0.23

0.035

0.74

0.069

0.10

0.067

0.19

Heat exchanger s h e l l

4.2

0.35

0.017

0.68

0.027

0.05

0.085

0.27

Heat exchanger tube

4.3

0.67

0.006

1.13

0.028

0.14

0.070

0.31

4.4

154 ,-

chemical d i s s o l u t i o n .

The sample was deformed about halfway t o f a i l u r e ,

and two e l e c t r o l y t i c d i s s o l u t i o n s were made.

The material should have

been s e l e c t i v e l y removed from t h e regions t h a t were f r e s h l y cracked s i n c e these surfaces were not oxidized.

The s o l u t i o n s were analyzed, and t h e

r e s u l t s divided by the n i c k e l content. found i n T e , C e , Sb, S r , and C s .

S i g n i f i c a n t enrichments were

These s o l u t i o n s were analyzed f o r sev-

eral o t h e r elements t h a t had not been determined previously. was present a t a concentration of 37 x

Uranium-235

i n t h e , second layer.

concentrations correspond t o 26 and 7 ppm, respectively.

These

Determinations

were made of A1 (0.23), B (0.01), Co (0.23), K ( 0 . 0 5 ) , N a (1.2), P (0.23),

V (0.23), and S (1.2) with t h e amounts shown i n parenthesis being t h e concentrations i n percent found i n t h e o u t e r l a y e r .

The i n n e r l a y e r

Samples of d i s t i l l e d water and unused meth-

showed very l i t t l e change.

anol-30% HNO3 had extremely l o w levels of t h e elements, and w e know of no source of contamination i n t h e sampling operation. The high concentrations of s u l f u r and phosphorus are indeed reasons f o r concern, s i n c e these elements are knqwn t o e m b r i t t l e nickel-base a l l o y s . 30

Haubenreich c a l c u l a t e d t h a t t h e t o t a l s u l f u r introduced by

pump o i l inleakage w a s 27 g and t h a t t h e i n i t i a l f u e l charge contained

15 ppm S o r a maximum of 24 g.

Thus, s u l f u r w a s i n t h e system and i t

may have concentrated along t h e g r a i n boundaries.

The s u l f u r (and t h e

o t h e r elements) a l s o could have been i m p u r i t i e s i n t h e INOR-8 and segregated i n t h e g r a i n boundaries during t h e long exposure a t 650OC. Another group of specimens w a s c o l l e c t e d , and a d i f f e r e n t approach

was taken t o t h e a n a l y s i s of t h e concentration of f i s s i o n products.

These

samples were f i r s t exposed t o a s o l u t i o n of Versene, b o r i c a c i d , and

c i t r i c acid.

As shown i n Table 14 t h i s s o l u t i o n is not very aggressive

toward the metal and very s m a l l weight changes were noted.

Thus, t h i s

s o l u t i o n should be enriched i n t h e material t h a t was on o r very near the s u r f a c e of the metal. a separate solution.

The sample w a s then completely dissolved i n

Both s o l u t i o n s were counted f o r various f i s s i o n

products, and t h e r e s u l t s are given i n Table 14. can be made from t h e s e data.

Several observations

Table 14.

Sample Description

Spacer s l e e v e , smooth Space s l e e v e , with r i b Thimble under s p a c e r

Concentrations of Fission Products Found on Several Samples From the MSREaSb

Surface Weight Weight Area Before After (cm2) Leaching Leaching ( g) ( 9) 12.2 8.4 6.6

2.45489 5,36317 3.80443

2.44200 5.33873 3.78641

Thimble under spacer

6.3

3.65780

3.64322

Thimble, b a r e

4.5

2.59565

2.59272

Thimble, b a r e

6.1

3.55622

3.55265

Surface Concentration (atomslcm2) lZ5Sb

127%

3.5 9.6 2.2 2.7 3.1 5.9 3.2 4.2 3.6 1.8 1.5

x 10l1 x ioI2 x 10l1

x 1013 x 10l1

1012

x 10l1 x io12 x 10l1

x 1013 x 10l1

1.6 x 1013

F o i l on f o u r t h group surveillance

1.2

0.02047

0.02012

9.3 x 10l1 1.6 x l O I 3

S t r a p on f o u r t h group surveillance

1.3

0.11191

0.11178

1.6 x 10" 4.9 x l o l l

Freeze valve 105

6.2

Concentration a t end of operation

10.2255

1.5 x 4.6 x 4.0 x ~ 1 . 5x 1.8 x <3 x 1.4 x <9 x 4 x C1.5 x 3.7 x <2 x 5.6 x <7 x

1013 1014 1013 1015 1013 1014 1014 1014 1013 1015 1013 1015 1013

loi4

2.0 x

lo1'

2.1 x 3.3 x 5.8 x 3.6 x 1.8 x 3.6 x 1.7 x 7.5 x 1.4 x 5.9 x 1.1 x 1.1 x 2.8 x

1014 1014 1014

lo1'

1014 1014 1014 1013 1014 1013 1014 1013 1013

137cs

5.5 <4.8 5.0 <3.5 7.5 <2

x 1013 x 1014 x 1013

134cs

x 1014 x 1013 x 1015 3.0 x 1013 < 3 x 1ols 3.9 x 1013 <1x 1015 4.0 x 1013 <8 x 2.6 x 1013 2.6 x 1015

6.6 x 10l1 <5 1013 1.8 x 10l1 <1x 1013 2 . 1 x 10l1 <2 x 1013 1.5 x lo1' x 1014 7.8 x 10l1 <6 x 1013 4.8 x 10l1 <1 x <5 x 10l1 <1 x 1013

1.6 x 1013 2.2 x 1014

6.4 x lo1' 2.6 x 1013

4.0 x 10l2 3 1014

<2 x 10l1 <2 1013

6.0 x lo9 7.5 x 2.0 x 10'1 9.1 x 1012 8.9 x 1 0 1 ~ 1.4 x i014

2.6 x 1013 1.0 x 1012 2.5 x 1017

4.1 x 1013 5.9 x 1012 1.8 1017

<9 x 1O1O <5 x 1010

14"ce

lo6Ru

8.9 x 10l1 1.8 x 10l2 <5 1013 1.7 x 1 O l 2 x 1014 1.3 x i 0 l 2 x 101-4 <5.9 x 10l1 2.7 x 10l1 r8 x 1015

1013 6 x

lo1'

6 x lolo

6.6 x 1 0 l 6

99 Tc

3.1 x 1014 C

6.7 1014 8.6 x 10l2 <2 x 1014 C

<3 6.9 <3 6.4 <5 4.9 2.7

x 1014 x

lo1'

x 1014

x 10l2 x 1014 x 1013

x 1014

1.6 x 10l6 9.4 x 1 0 1 ~ 2.3 x 10l6 2.4 1017 1.7 x 10l6 2.2 x 1017 1.0 x 1016 1.9 1017 4.2 x 1015 1.1 1017 2 x 1015 9.4 1 x 1016 1 x 10l6

1.1 x 1013 9.4 x 1013

4 x 1015 2.4 1016

8.6 x lo1' 7.9 x lo1' 4.7 x 1015

6 x 1014 5 x 1014 2.7 x lo1'

asample* counted about 2 years a f t e r end of MSRE operation. The f o i l and s t r a p samples were removed 6 months before operation was terminated. The h a l f - l i v e s were 109 days f o r 12*e, 2.7 years f o r lZ5Sb, 28 years f o r 90Sr, 30 y e a r s f o r 137Cs, 2.3 years f o r 1 3 4 C s , 284 days f o r 141cCe, 1 y e a r f o r 108Ru. 5 x lo5 years f o r 99Tc. bThe samples were f i r s t leached i n a s o l u t i o n of Versene, b o r i c acid, and c i t r i c acid. This s o l u t i o n was analyzed, and the f i r s t number under each i s o t o p e i s t h e r e s u l t . The remainder of t h e sample was dissolved and the second number under each isotope is the a n a l y t i c a l r e s u l t on the dissolved sample. 'Present

i n , p a r t i c u l a t e form, but n o t i n s o l u t i o n .

156

1. Generally t h e concentration of f i s s i o n products per u n i t s u r f a c e

area exposed t o f u e l s a l t is g r e a t e r i n the metal than near t h e surface. However, the d i f f e r e n c e v a r i e s considerably f o r d i f f e r e n t isotopes.

For

example, the 127Te concentration is about two orders of magnitude higher i n the metal than on t h e s u r f a c e , whereas 90Sr i s only s l i g h t l y concent r a t e d beneath the surface. 2.

The thimble samples t h a t were exposed t o flowing s a l t (desig-

nated "bare" i n Table 13) and those t h a t were covered by a spacer allow some comparison of t h e e f f e c t s of flow rate on t h e deposition of f i s s i o n products.

The sample exposed t o r e s t r i c t e d s a l t flow c o n s i s t e n t l y has

a lower concentration of f i s s i o n products, but only by a f a c t o r of 4 o r less. 3. products.

Freeze valve 105 (FV 105) had a lower concentration of f i s s i o n This was expected because of i t s operating conditions (p. 30b)

and is c o n s i s t e n t with t h e observation t h a t i n t e r g r a n u l a r cracking w a s

less severe i n t h i s component (Table 7 ) . 4.

With due consideration of t h e h a l f l i v e s , appreciable f r a c t i o n s

of the t o t a l inventory of 127Te, 125Sb, lo6Ru, and 99Tc are present on t h e metal surfaces.

This generally agrees with t h e i n d i c a t i o n s of

the d a t a i n Table 1 3 except f o r t h e behavior of 1 3 k s .

Some of t h e

incremental d i s s o l u t i o n d a t a i n Table 13 i n d i c a t e t h a t l a r g e amounts of 1 3 k s were p r e s e n t , whereas t h e d a t a i n Table 14 do not support t h i s observation. These r e s u l t s are i n t e r e s t i n g but must be viewed with reservations. The technique of e l e c t r o l y t i c a l l y removing s e c t i o n s i n t h e hot cells has not been used previously ( t o our knowledge) a t ORNL, and numerous experimental d i f f i c u l t i e s arose.

Uneven removal of material and d e t e r i -

o r a t i o n of c o n t a c t s , l e a d s , e t c . were some of t h e main problems.

The

r e s u l t s f o r r a d i o a c t i v e s p e c i e s w e r e obtained by proven methods.

However,

the l e v e l of gamma r a d i a t i o n from 6oCo o f t e n masked t h e a c t i v i t y from elements of i n t e r e s t .

The technique of evaporating 2 m l of t h e s o l u t i o n

i n t o t h e mass spectrograph is not new, but t h e complexity of t h e s p e c t r a presented caused many problems i n i n t e r p r e t a t i o n .

The d i f f e r e n t s p e c i e s

are i d e n t i f i e d only by mass number by t h i s method, and t h e presence of

157

the alloying elements in INOR-8, numerous fission products, constituents of the salt, and possible compounds between these elements and the electrolytic solution made it difficult to interpret the patterns. The method used to obtain the data in Table 14 involved only radiochemistry and dissolution techniques that were better established. However, the chemical reactivity of the first solution (leach) with the surface material leaves an uncertainity whether the materials removed were simply salt residues or small amounts of the metal.

The surface

areas were uncertain in most cases because of the complex geometry. With the qualifications that have been made the chemical analyses indicate several important points.

1. Several of the fission products penetrated the metal to depths of a few mils (Figs. 110 and 111). 2.

The fission products Te, Ce, Sb, Sr, and Cs were concentrated

in the cracked regions of a strained surveillance specimen.

Sulfur and

phosphorous were also concentrated in these same regions.

It is possible that the segregation of sulfur and phosphorus to the grain boundaries in this alloy is a normal phenomenon. 3. Significant fractions of the total amounts of Te, Sb, Ru, and Tc were deposited on the metal surfaces. i

These experiments were good introductions to the types of studies that could be performed. More work was needed to develop confidence in them, but termination of operation of the MSRE stopped our best source

of experimental material.

The full significance of the cracking problem

was only fully realized in the postoperation examination of the MSRE. DISCUSSION OF OBSERVATIONS ON INOR-8 FROM THE MSRE AND IN-REACTOR LOOPS Summary of Observations Observations have been presented on three basic types of samples. The first samples were the surveillance samples that were present in the MSRE primarily for the purpose of following the corrosion and radiation

â&#x20AC;&#x2122;

158

damage t o t h e metal.

The second set of samples consisted of several

components from the MSRE t h a t were removed f o r examination a f t e r termination of i t s operation.

The t h i r d set of samples came from in-reactor

pump and thermal convection loops. made and these w i l l be discussed.

Several important observations were

These observations w i l l then be used

t o propose some mechanisms that may be responsible f o r t h e cracking. The s u r v e i l l a n c e samples included two h e a t s of material t h a t w e r e c a r r i e d throughout t h e program and removed p e r i o d i c a l l y f o r examination The samples were usually s l i g h t l y discolored, but t h e r e

and t e s t i n g .

w a s no evidence of corrosion beyond t h e slow rate of chromium removal, i n d i c a t e d by t h e a n a l y s i s of t h e s a l t t o be equivalent t o removing a l l the chromium t o a depth of only 0.4 m i l o r by t h e microprobe t o be a concentration gradient i n the thimble extending t o a depth of 20 pm

(0.8 m i l ) .

When t h e samples were deformed a t 25*C, i n t e r g r a n u l a r cracks

formed to depths of a few mils.

The frequency of cracking increased with

t i m e , b u t the maximum depth d i d not i n c r e a s e detectably (Table 7). Examination of s e v e r a l components showed t h a t shallow i n t e r g r a n u l a r cracks w e r e present i n a l l materials t h a t had been exposed t o f u e l s a l t . The s t a t i s t i c s on t h e number and depth of cracks are given i n Table 15 along with t h e tellumium concentrations based on t h e chemical analyses

i n Table 14.

Very t h i n s u r f a c e cracks were o f t e n noted when t h e various

components w e r e removed from t h e MSRE. heat exchanger tubing.

This w a s p a r t i c u l a r l y t r u e of t h e

Samples from t h e pump bowl where t h e r e w a s a

l i q u i d i n t e r f a c e gave an opportunity t o observe the decrease i n crack s e v e r i t y i n t r a v e r s i n g from t h e l i q u i d t o t h e gas region.

A l l of t h e

s u r f a c e s t h a t were exposed t o f u e l s a l t were coated with f i s s i o n products. E l e c t r o l y t i c s e c t i o n i n g showed t h a t ' t h e f i s s i o n products penetrated a few mils i n t o t h e metal, b u t i t is not d e f i n i t e whether they had d i f f u s e d

into t h e metal o r whether they had p l a t e d on t h i n i n t e r g r a n u l a r cracks. Chemical analyses on a sample t h a t was oxidized t o p a s s i v a t e t h e s u r f a c e and s t r a i n e d t o expose t h e reactive cracked regions showed t h a t t h e cracked regions were enriched i n T e , Ce, Sb, S r , Cs, S, and P.

The only

sample t h a t showed a d e f i n i t e dependence of crack s e v e r i t y on f i s s i o n

- -_

Table 15. Crack Formation i n Various Samples from t h e MSRE S t r a i n e d a t 25OC (>5OO0C f o r 30,807 h r and Exposed t o F i s s i o n P r o d u c t s 24,500 h r )

Sample D e s c r i p t i o n

Cracks Counted Per Inch

Exposed thimble

Depth ( m i l s ) Av Max

Atoms/cm2

T o t a l Teb Atoms/c$

1279,a

192

5.0

8.0

1.8,1.8

2.9,2.9

148

25 7

4.0

8.0

0.59,O. 46

0.95 ,O. 74

Thimble s p a c e r , Outer S u r f a c e

178

3.0

Thiinble s p a c e r , I n n e r S u r f a c e

106

202

3.0

5.0 7*0

1.0,2.8

1.6.4.5

192

1.o

2 .o

M i s t shield, inside liquid

150

4.0

6.5

M i s t s h i e l d , o u t s i d e vapor

363

4.0

5.0

M i s t shield, outside liquid

300

3.0

Sampler cage rod, vapor

100

143

2.5

5.0

Sampler cage r o d , vapor

170

237

. 3.2

10.0

Sampler cage r o d , l i q u i d

102

165

3.7

10.0

Sampler cage rod, l i q u i d

131

238

7.5

12.5

F r e e z e valve 105

131

2 40

0.75

1.5

Thimble under s p a c e r sleeve

I d , I n s i d e vapor

5.0'

0.55

0.89

0.04

0.06

%easured. b C a l c u l a t e d from measured 127mTe, r e l a t i v e i s o t o p i c r a t i o s , and h a l f l i f e of 127mTe. C '

One c r a c k was 12 m i l s deep, n e x t l a r g e s t w a s 5 mils.

160 product concentration w a s the one from FV 105.

The f i s s i o n product con-

c e n t r a t i o n s were generally less than on t h e o t h e r samples by an o r d e r of

L; I

magnitude (Table 14) and the cracks w e r e very shallow (Fig. 104).

. P o s s i b l e Mechanisms These observations raise s e v e r a l important questions including t h e cause of t h e cracking and how r a p i d l y these cracks propagate.

Unfortu-

n a t e l y , t h e information obtained from t h e MSRE and t h e in-reactor loops is only s u f f i c i e n t t o allow s p e c u l a t i o n on these questions, and f u r t h e r

experiments such as those summarized i n t h e next s e c t i o n w i l l be required f o r complete evaluation. The f i r s t mechanism t h a t must be considered is t h a t t h e cracking is due t o some form of corrosion.

The most l i k e l y form of corrosion under

t h e MSRE operating conditions would be t h e s e l e c t i v e removal of chromium

by t h e reaction.

2UFq(salt)

+ Cr(meta1)

e CrF2(salt)

+ 2UF3

(salt)

Some impurities i n t h e s a l t , p a r t i c u l a r l y a t t h e beginning of operation with 233U, would have removed chromium from t h e metal by r e a c t i o n s such

as FeF2(salt) NiF2(sait)

+ Cr(meta1) CrFn(sa1t) + Fe(deposited) + Cr(meta1) F= C r F ~ ( s a 1 t )+ Ni(deposited)

The net r e s u l t of a l l of t h e corrosion r e a c t i o n s is t h a t chromium is s e l e c t i v e l y removed from t h e metal.

This process is c o n t r o l l e d by

the d i f f u s i o n of chromium t o t h e s u r f a c e of t h e metal, where i t is availa b l e f o r reaction. i n INOR-8,

DeVan31 measured t h e rate of d i f f u s i o n of chromium

and h i s measurements can be used t o c a l c u l a t e t h e depth t o which

*It is unlikely t h a t t h e s a l t w a s ever oxidizing enough t o form f l u o r i d e s of N i and Mo. Even i f t h i s had occurred, metal would be uniformly removed. As t h e s a l t became less oxidizing, t h e less s t a b l e f l u o r i d e s of N i , Mo, and Fe would react with C r i n t h e a l l o y . It is l i k e l y t h a t t h e C r would r e s i d e i n ' t h e s a l t as CrF2 and t h a t t h e o t h e r metals would be deposited i n t h e metallic form.)

, I

161

c-/

chromium could have been removed. The most extreme case would occur when t h e chromium concentration a t t h e s u r f a c e was maintained a t zero so t h a t t h e d r i v i n g force f o r chromium d i f f u s i o n toward t h e s u r f a c e would b e a

maximum.

The c a l c u l a t e d chromium concentration p r o f i l e s based on t h e

assumption of zero s u r f a c e concentration and t h e measured d i f f u s i o n coe f f i c i e n t of 1.5 x

cm2/sec are shown i n Fig. 112 f o r various t i m e s .

The measured chromium p r o f i l e f o r t h e thimble (Fig. 6 6 ) showed d e p l e t i o n t o a depth of only 0 . 8 m i l (0.0008 i n . ) which is somewhat less than t h e c a l c u l a t e d d e p l e t i o n depth shown i n Fig. 112.

However, t h e assumption

of zero concentration of chromium used i n c a l c u l a t i n g t h e curves i n Fig. 112 is too severe, and i t i s reasonable t h a t t h e measured chromium deplet i o n p r o f i l e s would be less than those calculated.

The rate of d i f f u s i o n

along g r a i n boundaries, as w e w i l l discuss more i n d e t a i l l a t e r , is more r a p i d than through t h e grains.

Thus, t h e depth of chromium depletion

along t h e g r a i n boundaries could be much g r e a t e r than t h e 0.8 m i l measured 8

w i t h i n t h e grains. The evidence t h a t has j u s t been examined shows t h a t t h e chromium could be depleted along t h e g r a i n boundaries t o depths approaching those

of t h e observed cracks.

However, two s i g n i f i c a n t pieces of evidence sug-

g e s t t h a t chromium depletion alone does not cause t h e cracking.

First,

thousands of hours of loop corrosion tests were run involving s e v e r a l f l u o r i d e s a l t s and INOR-8, with no i n t e r g r a n u l a r cracks being observed. 32 s 3 3 , 3 4 The second and most convincing evidence i s t h a t chromium depletion could not b e detected i n samples from the MSRE heat exchanger

and i n t h e s e c t i o n of t h e c o n t r o l rod thimble under a spacer sleeve. However, t h e s e samples were cracked as severely as those (e.g.,

the bare

c o n t r o l rod thimble) i n which chromium d e p l e t i o n w a s d e t e c t a b l e (Table 15). Thus, i t seems unlikely t h a t chromium depletion alone can account f o r t h e observed cracking. Another mechanism t o b e considered is t h e d i f f u s i o n of some element(s) i n t o the material, p r e f e r e n t i a l l y along t h e g r a i n boundaries.

A reaction

of t h i s type could cause (1) t h e formation of a compound t h a t . i s very b r i t t l e , o r (2) a change i n composition along t h e g r a i n boundaries so t h a t they are l i q u i d , o r s o l i d b u t very weak.

Some deformation would

l i k e l y be needed t o form t h e cracks i n a l l cases.

162

ORNL-DWG 72-8704

0.002

0.004

0.008 0.040 DISTANCE FROM SURFACE ( in.)

0.006

0.012

0.044

0.016

Fig. 112. Calculated Chromium Profiles at 65OOC in INOR-8 Assuming Zero Surface Concentration of Chromium.

1 .

163

It is extremely important t h a t t h e element(s) responsible f o r t h e cracking be i d e n t i f i e d and t h e mechanism determined.

Examination of t h e

a n a l y t i c a l d a t a i n Tables 13 and 1 4 shows t h a t a l l of t h e f i s s i o n products with s u f f i c i e n t h a l f - l i v e s t o be d e t e c t a b l e a f t e r 2 years were present. The d a t a on t h e sample t h a t was preoxidized show enrichments i n s e v e r a l f i s s i o n products as w e l l as s u l f e r and phosphorus.

These d a t a o f f e r some

i n d i c a t i o n of t h e elements t h a t are p r e s e n t , b u t t h e d e t e c t i o n limits on t h e nonradioactive elements were not s u f f i c i e n t l y l o w t o ensure t h a t some of t h e s t a b l e f i s s i o n products w e r e not present i n even l a r g e r conc e n t r a t i o n s than t h e r a d i o a c t i v e elements. Thus, i t seemed p r o f i t a b l e t o look a t a l l of t h e elements i n t h e f i s s i o n spectrum with s u f f i c i e n t h a l f - l i f e t o d i f f u s e i n t o t h e m e t a l . Possible e f f e c t s of these elements on INOR-8 are l i s t e d i n Table 16. Many f a c t o r s may be important, b u t only t h e information t h a t w e f e l t t o be most r e l e v a n t has been included.

Bieber and Decker35 have summa-

r i z e d t h e observations on pure N i and Wood and Cook36 have examined t h e e f f e c t s of several elements on r e l a t i v e l y complex niobium-base a l l o y s . The thermodynamic p r o p e r t i e s and t h e behavior i n t h e s a l t have been accumulated by W. R. Grimes et a137 from t h e l i t e r a t u r e , research, and s t u d i e s of the MSRE.

Several pieces of information are a v a i l a b l e from

our c u r r e n t research and w i l l be summarized i n t h e next s e c t i o n . The elements s u l f u r and selenium have detrimental e f f e c t s under some tests conditions, but T e has had a more pronounced e f f e c t i n a l l types of tests run t o date.

These t h r e e elements form r e l a t i v e l y un-

s t a b l e f l u o r i d e s and would l i k e l y b e deposited on t h e metal and g r a p h i t e surfaces.

Also As, Sb, and Sr would b e deposited, b u t no d e l e t e r i o u s

e f f e c t s of t h e s e elements on t h e mechanical p r o p e r t i e s of n i c k e l a l l o y s have been noted.

Zinc and cadmium may be deposited o r present i n t h e

s a l t , depending on t h e oxidation s t a t e of t h e s a l t .

Both of t h e s e ele-

ments are reported t o be i n s o l u b l e i n n i c k e l and w e have not observed any d e l e t e r i o u s e f f e c t s i n our test.

Although Ru, T c , Mo, and Rh should

be deposited, w e have seen no d e l e t e r i o u s e f f e c t s i n our tests.

Since

Z r , S r , C s , and C e form very s t a b l e f l u o r i d e s , they should remain i n t h e

salt.

We have no evidence, p o s i t i v e o r negative, on t h e e f f e c t s of stron-

tium and cesium on t h e mechanical behavior, b u t w e p r e s e n t l y f e e l t h a t

-. --.. .

.... ...._-_.__.....----.---.--.----------------.I---

Table 16.

Element

Melting Point ("C)

119

217

450

017

Concentrated Vapor and Near Cracks E l e c t r o p l a t e d Specimen&

.-.

~...I

+ +

232

Zn Cd

420 321

Insoluble Insoluble

2500

+ + +

1052

2610

760

29 004

1966

+ + + + +

.- .

+ + +

2130 2468

. ...

+ +

630

... ... ... . ...

....

Free Energy E f f e c t on E f f e c t on E f f e c t on E f f e c t on o f Formation Expected T e n s i l e P r o p e r t i e s Creep P r o p e r t i e s Tensile P r o p e r t i e s Creep P r o p e r t i e s o f F l u o r i d e Location O v e r a l l of N i c k e l C on N i c k e l Alloyd of H a s t e l l o y Nb of H a s t e l l o y Nb a t lOOO'K of Rating (kcal/mole Fle Element

+ +

P o s s i b l e E f f e c t s of S e v e r a l Elements on t h e Cracking o f H a s t e l l o y Na

+ + + + ,+ + + +

Insoluble

aThe symbols used i n t h i s t a b l e should be i n t e r p r e t e d i n the following way:

+ +

+ A

"+"

r e f e r s t o good behavior and a

-34 -2 7

Deposit edf Deposited

-39

Deposited

-62

Deposited

-55

Deposited

-60

Deposited

-60 -64 -51

Deposited

-46

Deposited

-70

. g

-99

Salt

-57

Deposited

-125

Salt

-106

Salt

-120

Salt

-42

Deposited

"-" i n d i c a t e s

detrimental effects.

b R e s u l t s of c u r r e n t r e s e a r c h . 'C.

G. Bieber and R. F. Decker, "The Melting of Malleable Nickel and Nickel Alloys," Tmns. AI!&

221,

629 (1961).

dD. R. Wood and R. M. Cook, " E f f e c t s of Trace Contents of Impurity Elements on the Creep-Rupture P r o p e r t i e s of Nickel-Base Elements," Metallurgia -7, 109, (1963). e P r i v a t e c o m u n i c a t i o n , W. R. G r i m e s , ORNL. fMay appear as H2S i f HF c o n c e n t r a t i o d of m e l t is a p p r e c i a b l e . %fay appear i n s a l t i f s a l t mixture is s u f f i c i e n t l y o x i d i z i n g .

165

t h e s e elements w i l l s t a y i n t h e s a l t and not e n t e r t h e metal. and cerium do not have adverse e f f e c t s when added t o INOR-8.

Zirconium Niobium

can be i n t h e s a l t o r deposited, but i t has very favorable e f f e c t s on t h e mechanical p r o p e r t i e s .

Thus, although some exploratory work remains, i t

appears t h a t the cracking could be caused by t h e inward d i f f u s i o n of elements of the S, Se, T e family, with t e l l u r i u m having t h e most adverse e f f e c t s i n t h e experiments run t o date.

Our s t u d i e s have concentrated

on T e on t h e b a s i s t h a t T e has had t h e most adverse e f f e c t s .

Because

these elements a l l behave s i m i l a r l y , an understanding of how t e l l u r i u m causes cracking should lead t o an understanding of t h e r o l e of t h e o t h e r elements i n the cracking process as w e l l . POST-ERE STUDIES Since t h e s u r v e i l l a n c e samples and p a r t s of the E R E were examined, c

numerous laboratory experiments have been conducted i n an e f f o r t t o b e t t e r understand the cause of t h e cracking and i t s e f f e c t s on t h e operation of

a reactor.

Most of our work has concentrated on t h e f i s s i o n product

tellurium, and t h e r a t i o n a l e f o r t h i s choice w a s discussed i n t h e previous

section.

i n s a l t , (2) exposure t o several f i s s i o n products t o compare t h e tendencies

t o produce cracks-, (3) d i f f u s i o n of T e , and (4) experiments with an applied

stress.

The experiments f a l l i n t o t h e general categories of (1) corrosion

These experiments have involved materials o t h e r than INOR-8 i n

an e f f o r t t o b e t t e r understand t h e cracking phenomenon and t o f i n d a mate-

r i a l t h a t i s more r e s i s t a n t t o cracking.

These experiments are continuing

and only t h e most important findings w i l l be summarized i n t h i s r e p o r t . I

Corrosion Experiments Arguments have already been presented t h a t i n d i c a t e our b e l i e f t h a t the i n t e r g r a n u l a r cracking i n t h e MSRE could not have been due s o l e l y t o e

chromium depletion.

1 ~

W e d i d run one f u r t h e r experiment.

A thermal convec-

t i o n loop containing a f u e l s a l t had operated f o r about 30,000 h r with a

W ~

very low corrosion rate.

The oxidation p o t e n t i a l of t h e s a l t w a s in-

creased by two a d d i t i o n s of 500 ppm FeF2, and t h e specimens were examined

166 perio,dically.

The corrosion rate increased and some selective g r a i n

boundary a t t a c k occurred, b u t t h e g r a i n boundaries d i d not crack when t h e corrosion samples from t h e loop were deformed. I

Thus, we conclude t h a t corrosion alone cannot account f o r t h e ob-

served cracking.

However, chromium depletion by corrosion could make t h e

material more s u s c 6 p t i b l e t o cracking by t h e inward d i f f u s i o n of f i s s i o n products, and t h i s w i l l be discussed later i n more d e t a i l . I

Exposure t o Fission Products Three types of experiments have been run i n which samples are exposed t o f i s s i o n products: (1) exposure of mechanical property samples t o vapors of f i s s i o n products, (2) e l e c t r o p l a t i n g t e l l u r i u m on mechanical property samples, and ( 3 ) studying t h e mechanical p r o p e r t i e s of a l l o y s t h a t contain

small a l l o y i n g a d d i t i o n s of t h e f i s s i o n products.

S u l f u r has been included

i n t h i s work even though i t is not a f i s s i o n product, since it is i n t r o duced i n a r e a c t o r by l u b r i c a n t inleakage and i s reputed t o be detrimental t o nickel-base a l l o y s .

The experiments with f i s s i o n product vapors have included S, Se, T e , 12, As, Sb, and Cd. These materials have s u f f i c i e n t vapor pressure a t 65OOC t o t r a n s p o r t t o mechanical property samples.

The samples were

annealed i n a q u a r t z capsule w i t h each f i s s i o n product f o r various t i m e s a t 65OoC, deformed a t 25OC, and sectioned f o r metallographic examination. ,

Examination of the samples exposed i n t h i s way has revealed several important f a c t s . 1. Of t h e samples of INOR-8,

type 304L stainless steel, and nickel-

200 exposed t o a l l seven elements f o r 2000 h r a t 650째C, only INOR-8 and n i c k e l 200 exposed t o t e l l u r i u m had i n t e r g r a n u l a r cracks a f t e r deformation

at 25OC. 2.

The cracks produced i n INOR-8 by exposure t o t e l l u r i u m look q u i t e

s i m i l a r t o those formed i n t h e material from t h e MSRE (Fig. 113). 3.

Exposure t o t e l l u r i u m over t h e temperature range of 550 t o 7OOOC f

and concentration range of two orders of magnitude showed t h a t t h e s e v e r i t y of cracking i n INOR-8 and n i c k e l 200 increased with i n c r e a s i n g temperature and t e l l u r i u m concentration.

Type 304L s t a i n l e s s steel d i d not crack

under any of t h e conditions i n v e s t i g a t e d .

167

MSRE Thimble

1.4

- 31,000 hr in Fuel Salt at 650OC x

1017atoms/cm2 of Te

Vapor Plated with 5.4 x 10l8atoms/cm2 of Te Annealed 1000 hr at 65OOC

. W

Fig. 113. Samples of INOR-8 Strained to Failure at 25OC. The upper photograph was made of a piece of the E R E thimble. The upper side was exposed to fuel salt and cracked when strained. The lower side was exposed to the cell environment and the oxide that formed cracked during straining. The lower specimen was exposed to tellurium vapor on both sides and deformed. The cracks are similar to those formed on the surface of the upper sample that had been exposed to fuel salt.

168

Inconel 600 and a h e a t of INOR-8 modified with 2% T i showed less

LJ t

severe cracking than INOR-8 under similar exposure conditions. About 60 commercial a l l o y s have been e l e c t r o p l a t e d w i t h t e l l u r i u m t o compare t h e r e l a t i v e tendencies t o form i n t e r g r a n u l a r cracks.

One s e t of

samples w a s annealed f o r 1000 h r a t 65OOC and t h e o t h e r set w a s annealed 200 h r a t 700OC.

The materials included consisted of (1) i r o n and sever-

a l s t a i n l e s s steels, (2) n i c k e l and s e v e r a l nickel-base a l l o y s , (3) copper

and monel, (4) two cobalt-base a l l o y s , and (5) s e v e r a l h e a t s of INOR-8 with modified chemical compositions.

The samples were e l e c t r o p l a t e d ,

welded i n a metal capsule, evacuated and b a c k f i l l e d with argon, and The environment w a s impure enough t h a t some oxides w e r e

welded closed.

detected by x r a y s , and t h e r e s u l t s may be influenced tiy t h e presence of the oxide.

The samples were deformed a t 25OC and examined metal-

lographically t o determine whether cracks were present. The r e s u l t s of t h e two experiments were q u i t e c o n s i s t e n t and several important observations were made.

Cracks did not form i n any of t h e iron-base a l l o y s .

Nickel, Hastelloy B (1% Maximum C r )

INOR-8

, Hastelloy

W (5% C r )

, and

(7% C r ) formed cracks, b u t s e v e r a l a l l o y s containing 15% o r more

C r did not crack (e.g.,

Inconel 600, Hastelloy C, and Hastelloy X).

Copper and Monel did not crack.

The two cobalt-base a l l o y s contained about 20% C r and d i d not

crack. 5.

Several of t h e modified h e a t s of INOR-8 cracked l e s s s e v e r l y

than the standard a l l o y .

The b e t t e r a l l o y s seemed t o contain more niobium,

although t h e r e were usually o t h e r a d d i t i o n s a l s o ,

Two a l l o y s containing

2% Nb d i d not have any cracks. The r e s u l t s of t h e two types of experiments, one exposing t h e materi a l t o f i s s i o n product vapors and t h e o t h e r t o e l e c t r o p l a t e d t e l l u r i u m , generally agree.

Not a l l of t h e a l l o y s have been t e s t e d by both methods. f

Small melts have been made containing nominal a d d i t i o n s of 0.01% each of S,'Se, T e , S r , T c , Ru, Sn, Sb, and As.

Tests on t h e s e a l l o y s

show no measurable e f f e c t s on t h e i r mechanical p r o p e r t i e s a t 25OC.

169

creep tests a t 6500C only S, Se, and T e ..ave ,--eterious

effects.

elements reduce t h e rupture l i f e and t h e f r a c t u r e s t r a i n .

These

Alloys con-

t a i n i n g s u l f u r and t e l l u r i u m have been made with and without chromium. The e f f e c t s of s u l f u r and t e l l u r i u m are more d e l e t e r i o u s when chromium

is not present.

This suggests a p o s s i b l e t i e between corrosion by se-

lective- chromium removal and i n t e r g r a n u l a r cracking due t o tellurium. The regions depleted i n chromium should be much less t o l e r a n t of tellurium than those containing chromium. Diffusion of T e

304L s t a i n l e s s steel has been measured.

The p e n e t r a t i o n depths w e r e q u i t e

s m a l l i n some samples, b u t t h e d a t a generally give t h e bulk and g r a i n boundary d i f f u s i o n c o e f f i c i e n t s a t 65OOC and 760째C accurately enough t o make p r e d i c t i o n s of t h e depth of p e n e t r a t i o n i n s e r v i c e .

The depth w a s

very shallow i n type 304L s t a i n l e s s s t e e l , a t least twice as deep i n

INOR-8,

and many times g r e a t e r i n n i c k e l 200.

The Fisher model f o r g r a i n boundary d i f f u s i o n relates t h e depth of p e n e t r a t i o n t o t h e d i f f u s i o n t i m e t o the one-fourth power.38

Thus, t h e

i n c r e a s e i n the depth of p e n e t r a t i o n f o r a system a t temperature f o r 30 years (an MSBR) i s only 1 . 8 t i m e s as g r e a t as t h a t f o r a system a t temp e r a t u r e s f o r t h r e e years ( t h e MSRE).

However, t h e s e c a l c u l a t e d values

are lower limits and make no allowance f o r t h e d i f f u s i o n f r o n t advancing as i n t e r g r a n u l a r cracks form and propagate. Experiments with an Applied S t r e s s Several types of experiments w i t h an applied stress have been run. One is a standard creep test with t h e sample i n an environment of argon and tellurium.

The t e l l u r i u m i s preplaced i n a quartz v i a l a t a l o c a t i o n

where i t w i l l have a vapor pressure of aobut 0.5 t o r r . badly a t 65OOC a t r e l a t i v e l y high stress levels.

INOR-8 cracks very

The cracks extend 35

mils, compared with about 5 mils i n t h e tests t h a t w e r e s t r e s s e d a f t e r t h e

170

exposure t o tellurium.

T e s t s near reasonable design stresses have been i n

progress several thousand hours.

Type 304L s t a i n l e s s steel, nickel-200,

and Inconel 600 have been i n test several thousand hours, b u t have not been examined metallographically. The r e a c t i o n products of t e l l u r i u m with

W I

nickel-200 and INOR-8 are b a s i c a l l y n i c k e l t e l l u r i d e s , b u t t h e only detectable product on s t a i n l e s s s t e e l is an oxide of t h e Feg04 type. Tube b u r s t tests of INOR-8 were run with t h e o u t s i d e of t h e tubes i n helium o r s a l t environments.

Some of t h e tubes were e l e c t r o p l a t e d

with t e l l u r i u m before t e s t i n g .

The test environment had no d e t e c t a d l e

e f f e c t , but the tubes p l a t e d with t e l l u r i u m f a i l e d i n s h o r t e r t i m e s than those not p l a t e d .

Metallographic examination revealed i n t e r g r a n u l a r

cracks about 10 mils deep along almost every g r a i n boundary of t h e p l a t e d specimens.

Some of t h e s t a i n l e s s steel tubes are s t i l l i n t e s t , b u t t h e

rupture lives of those t h a t have f a i l e d are equivalent f o r p l a t e d and unplated specimens. The o t h e r type of s t r e s s e d test t h a t has been run is one with controlled strain l i m i t s .

Thermal stresses commonly occur i n components

such as h e a t exchangers where high h e a t fluxes develop and t r a n s i e n t s are frequent. exchangers.

Thermal s t r a i n s of +0.3% are a n t i c i p a t e d f o r MSBR h e a t

Our f i r s t test has u t i l i z e d a Te-plated Hastelloy N speci-

men s t r a i n e d between limits of +-0.16%. The rupture l i f e w a s s h o r t e r than a n t i c i p a t e d on t h e b a s i s of tests with t e l l u r i u m , and numerour i n t e r granular cracks were present on t h e o u t s i d e where t h e t e l l u r i u m w a s plated. SUMMARY

These tests show t h e t e l l u r i u m causes t h e formation of i n t e r g r a n u l a r cracks i n INOR-8 and t h a t t h e s e cracks are deeper i f t h e material is s t r e s s e d and exposed t o t e l l u r i u m simultaneously than they would be i f the material were exposed t o t e l l u r i u m and then s t r e s s e d .

Although t h e

d i f f u s i o n rate of t e l l u r i u m i n INOR-8 has been measured, t h e r o l e of t

stress i n a c c e l e r a t i n g crack propagation makes t h e d i f f w i o n measurements of questionable value i n estimating t h e e x t e n t of cracking i n service. Only elements of t h e S, Se, and T e family were found t o cause i n t e r granular cracking inâ&#x20AC;&#x2122;INOR-8.

171

Exploratory experiments i n d i c a t e t h a t s e v e r a l materials are more r e s i s t a n t t o i n t e r g r a n u l a r cracking than INOR-8.

copper, and Monel s e e m completely r e s i s t a n t .

Iron-base a l l o y s ,

Nickel- o r cobalt-base

a l l o y s with about 20% C r seem r e s i s t a n t , b u t t h e r e s u l t s f o r a l l o y s such as Inconel 600 with 15%C r are inconclusive.

Some modified compositions

of INOR-8 have exhibited improved r e s i s t a n c e t o cracking. The INOR-8 from the MSRE, which had been exposed t o f u e l s a l t , w a s noted t o contain i n t e r g r a n u l a r cracks t o depths of -a few m i l s .

These

cracks w e r e v i s i b l e i n some materials i n as-polished metallographic sect i o n s as they w e r e removed from t h e MSRE.

When t h e samples were deformed

a t room temperature, t h e cracks became more v i s i b l e but s t i l l reached a l i m i t i n g depth of about 10 mils.

The s e v e r i t y of cracking could not be

r e l a t e d with t h e amount of chromium removal but w a s most severe a t t h e l i q u i d i n t e r f a c e i n t h e pump bowl and least severe i n regions of t h e pump bowl exposed t o gas. 1c

Samples were sectioned electrochemically, and

some f i s s i o n products were found t o depths of s e v e r a l m i l s , although i t

was not clear whether t h e f i s s i o n products had d i f f u s e d through t h e m e t a l t o these depths o r simply p l a t e d on t h e s u r f a c e s of cracks t h a t already existed

Not being a b l e t o relate t h e i n t e r g r a n u l a r cracking t o corrosion (chromium leaching) i n e i t h e r t h e MSRE o r i n laboratory experiments,

w e i n v e s t i g a t e d the p o s s i b l e e f f e c t s of f i s s i o n products on t h e material. The MSRE had been s h u t down and t h e work was continued i n laboratory experiments.

This work included the exposure of INOR-8

to small amounts

of vapor o r e l e c t r o p l a t e d f i s s i o n products, t h e study of s e v e r a l a l l o y s with f i s s i o n products added, experiments with t e l l u r i u m and an applied

stress present simultaneously, and t h e measurement of t h e d i f f u s i o n of t e l l u r i u m i n INOR-8.

These experiments have shown c l e a r l y t h a t of t h e

many elements t e s t e d , only t e l l u r i u m caused i n t e r g r a n u l a r cracking s i m -

i l a r t o t h a t noted i n samples from t h e MSRE. Several o t h e r materials were a l s o included i n these experiements t o determine whether they might be more r e s i s t a n t t o embrittlement by t e l l u rium.

Several a l l o y s , including 300 and 400 series s t a i n l e s s steels,

cobalt- and nickel-base a l l o y s containing more than 15%C e , copper, Monel,

172

and some.modified compositions of INOR-8 are resistant to cracking in the tests run to date.

Further work will be necessary to show unequivocally

that these materials resist cracking in nuclear environments, including

in-reactor capsule tests.

173 ACKNOWLEDGMENT

The observations i n t h i s r e p o r t cover several years and include contributions from many individuals.

W. H. Cook and A. Taboda designed

the s u r v e i l l a n c e f i x t u r e , and W. H. Cook was responsible f o r i t s assembly and disassembly.

The MSRE operations s t a f f , headed by P. N. Haubenreich,

exercised extreme care i n handling t h e s u r v e i l l a n c e f i x t u r e and i n removing t h e various components f o r 'examination.

The Hot C e l l Operation

S t a f f , headed by E. M. King, developed s e v e r a l s p e c i a l t o o l s and techniques f o r various examinations and tests of materials from t h e MSRE.

The metal-

lography w a s performed by H. R. Tinch, E. L e e , N. M. Atchley, and E. R. Boyd.

The microprobe s c a n s w e r e made by T. J. Henson and R. S. Crouse.

The technique f o r e l e c t r o l y t i c a l l y dissolving samples i n t h e hot cells

was developed by R. E. Gehlbach and S. W. Cook.

The mechanical property

tests w e r e performed by B. C. W i l l i a m , H. W. Kline, J. W. Chumley, L. G.

Rardon, and J. C. F e t t n e r .

The chemical analyses w e r e performed under

t h e supervision of W. R. Laing, E. I. Wyatt, and J. Carter.

Assistance

was received from s e v e r a l members of t h e Reactor Chemistry Division i n s e v e r a l phases of t h i s work.

J. V. Cathcart, J. R. DiStefano, P. N.

Haubenreich, and J. R. Weir reviewed t h e manuscript of t h i s r e p o r t and made many h e l p f u l suggestions.

Kathy Gardner made t h e o r i g i n a l d r a f t s

of t h i s r e p o r t , and t h e drawings were prepared by t h e Graphic A r t s Department.

174

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P L'

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