ORNL-TM-0497 by Jon Morrow

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OAK RIDGE NATIONAL'

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UNION CARBIDE CORPORATION NUCLEAR DIVISION for the U.S. ATOMIC ENERGY COMMISSION

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ORNL- TM-497 COPY NO, DATE

HELP

August 16, 1966

ANALYSIS OF FILLING ACCIDENTS I N MSRE J. R. Engel, P. N. Haubenreich, and S. J . B a l l

msrr

PRJCTC

-*-YL-bi

iCC c

Whenever the MSRE i s shut down, the f u e l s a l t i s drained from the core. Then, during a normal startup, the graphite and the f u e l are preheated and the c o n t r o l rods are positioned so t h a t t h e r e a c t o r remains s u b c r i t i c a l while it i s being f i l l e d . Certain abnormal circumstances could r e s u l t i n c r i t i c a l i t y and a power excursion i n the p a r t i a l l y f i l l e d core. Various postulated incidents were surveyed and the worst case was analyzed i n d e t a i l . This case involved s e l e c t i v e freezing i n the d r a i n tanks t o concentrate the uranium i n the molten salt f r a c t i o n . Physical r e s t r i c t i o n s on the f i l l rate and s a f e t y a c t i o n s of c o n t r o l rods and gas c o n t r o l valves l i m i t e d the calculated power and temperature excursions so t h a t any damage t o the r e a c t o r would be prevented.

NOTICE This document contains informotion of a preliminary nature and was prepared primarily for internal use a t the Oak Ridge National Laboratory. It is subject to revision or correction and therefore does not represent a final report.

LEGAL NOTICE

This nport was proparod as an account of Govorntmnt sponsorod work. Noithor tho United Staahs, nor tho Commission, nor any person acting on b h a l f of tho Commission: A. Makos any warranty or npnsontatian, oxprossod or impliod, with r o s p c t to tho accumcy, complotonoss, or usofulmss of rho informotion contained i n this roport, or that tho us. of any information, apparatus, mthod, w procoss disclosod in this roport moy not infringe privatoly a n o d tights; or B. Assumes any liabilitios with n s p c t us* of, or for darnages nsultins h a m tho us* of any information, apparatus, lethod, or procosa diaclosod i n this report.

As used in tho obovo, "person acting on behalf of tho Commission" includes any omployoo or contractor of tho Commission, or on~ployooof such contmctor, to tho oxmnt that such omployoo

or cantractor of tho Commission, w omployoo of such contractor pnpons, diasominates, er pavidos access to, any information pursuant to his omploytmnt or contract with tho Commission, OT his omploymont with such conhactor.

PREFACE The a n a l y s i s described here was done i n written early i n

1963 and

t h e r e p o r t was

1964. Although t h e r e has been an i n o r d i n a t e delay i n

publishing, t h e r e p o r t i s being issued as w r i t t e n because a s i g n i f i c a n t p a r t of t h e MSRE s a f e t y c i r c u i t r y was based analysis.

t h e r e s u l t s of t h i s

Well over two years have now passed, and now we a r e b e t t e r

a b l e t o assess t h e c r e d i b i l i t y of some of t h e assumptions involved.

plan t o do t h i s as p a r t of a c r i t i c a l review of t h e s a f e t y system.

COrnNTS Page

. 2.

.......................... 1 Mechanics of F i l l i n g . . . . . . . . . . . . . . . . . . . . . . 2 2 . 1 General Description . . . . . . . . . . . . . . . . . . . . 2 2.2 Systemvolumes . . . . . . . . . . . . . . . . . . . . . . 4 2.3 Drain Tank Pressure During a F i l l . . . . . . . . . . . . . 4 2.4 Amount of Gas i n Tank During a F i l l . . . . . . . . . . . . 4 2.5 Gas Supply . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.6 Course of a Normal F i l l . . . . . . . . . . . . . . . . . 9 2.7 Coast-Up of Fuel Level . . . . . . . . . . . . . . . . . . 11 3 . Nuclear Considerations . . . . . . . . . . . . . . . . . . . . 11 3.1 E f f e c t of Fuel Level on R e a c t i v i t y . . . . . . . . . . . . 13 3.2 ControlRods . . . . . . . . . . . . . . . . . . . . . . . 13 3.3 Temperature C o e f f i c i e n t s of R e a c t i v i t y . . . . . . . . . . 16 4 . Survey of Possible F i l l i n g Accidents . . . . . . . . . . . . . 18 4.1 F i l l i n g with Control Rods Withdrawn . . . . . . . . . . . 18 4.2 F i l l i n g with Fuel a t Low Temperature . . . . . . . . . . . 19 4.3 F i l l i n g with Concentrated Fuel . . . . . . . . . . . . . . 20 5 . Analysis of Maximum F i l l i n g Accident . . . . . . . . . . . . . 22 5.1 S p e c i f i c a t i o n of Accident . . . . . . . . . . . . . . . . . 22 5.2 Preliminary D i g i t a l Calculations . . . . . . . . . . . . . 23 1

Introduction

5.3

D e t a i l e d Analog Simulation

5.4

Discussion

Conclusions

. . . . . . . . . . . . . . . . 23

........................ ..........................

3k 32

7 LIST OF' FIGURES Fig. No.

Title

System Used i n F i l l i n g Fuel Loop

Calculated Volume C a l i b r a t i o n of

3 4

Calculated Volume C a l i b r a t i o n of

5 6 7

Page

. ..... .... Fuel Loop . . . . . . . Fuel Drain Tank . . . . . e

5 6

Drain Tank Pressure vs Liquid Level i n Fuel Loop a t Various S a l t F i l l Rates , .

. . . . . . . . . . . . . '7 Amount of G a s Required i n Drain Tank vs S a l t Level i n Loop a t S e v e r a l F i l l Rates . . . . . . . . . . . . . . 8 S a l t Level i n Fuel Loop During Normal F i l l . . . . . . . . .10 Coast-Up A f t e r Gas Addition i s Stopped vs F i l l Rate a t S e v e r a l I n i t i a l Fuel Levels . . . . . . . . . . . . 12 E f f e c t of Fuel Level on R e a c t i v i t y . . . . . . . . . . 14 Control Rod Worth vs P o s i t i o n . . . . . . . . . . . . . . 15 Liquid Composition Resulting from S e l e c t i v e Freezing of Fuel S a l t "A" i n Drain Tank . . . . . . . . . . . 21 Block Diagram of Mathematical Model f o r F i l l Accident Simulation . . . . . . . . . . . . . . . . . 25 Cross S e c t i o n of Typical Graphite S t r i n g e r with Adjacent Fuel S t r i n g e r s . . . . . . . . .. . . . . . . 26 e

a 9 10

11 12

. 29 . . 30

Net R e a c t i v i t y I n s e r t e d During Maximum F i l l i n g Accident.

Power and Temperature During Maximum F i l l i n g Accident.

9 ANALYSIS OF FILLING ACCIDENTS I N MSFB

J . R . Engel, P. N. Haubenreich, and S . J . Ball

1. IfJTROIxTC T I O N One of t h e f e a t u r e s of t h e MSFG3 (and f l u i d - f u e l r e a c t o r s i n g e n e r a l )

i s t h a t it can be p o s i t i v e l y shut down by d r a i n i n g t h e f u e l o u t of t h e The c o n t r o l rods provide a small shutdown margin t o t a k e t h e re-

core.

a c t o r s u b c r i t i c a l whenever desired, 5ut for any shutdown i n which the r e a c t o r i s t o be cooled down, t h e f u e l must be drained.

Draining and r e -

f i l l i n g t h e core i s t h e r e f o r e an o p e r a t i o c which w i l l probably be done many times. The normal procedure f o r a s t a r t u p r e q u i r e s t h a t t h e r e a c t o r and the f u e l be heated by e l e c t r i c h e a t e r s t o near o p e r a t i n g temperature

before t h e f u e l i s t r a n s f e r r e d from t h e d r a i n tank t o t h e c o r e .

The

rods normally a r e p a r t i a l l y i n s e r t e d s o t h a t t h e r e a c t o r i s j u s t subc r i t i c a l a t t h e f i l l temperature when t h e f u e l f i l l s t h e core.

Criti-

c a l i t y i s a t t a i n e d by withdrawing t h e rods a f t e r t h e f u e l and coolant loops are f i l l e d and c i r c u l a t i o n has been s t a r t e d . It i s conceivable t h a t c r i t i c a l i t y could be a t t a i n e d during a f i l l

before t h e core i s completely full. This could r e s u l t from one or more of t h e following abnormal c o n d i t i o n s :

1) t h e c o n t r o l rods are withdrawn

t o o far, 2 ) t h e temperature of t h e f u e l and/or t h e core g r a p h i t e i s t o o l o w , a n d 3) t h e uranium concentration of the f u e l was increased (or t h e

poison concentration w a s decreased) while t h e f u e l s a l t w a s i n t h e &rain tank. If c r i t i c a l i t y i s reached prematurely, t h e n u c l e a r power w i l l rise,

p o s s i b l y causing damaging temperatures i n t h e p a r t i a l l y f i l l e d core.

soon as t h e o n s e t of such an umlesirable s i t u a t i o n i s detected, t h e rcds a r e i n s e r t e d , t h e f i l l i s stopped and t h e f u e l r e t u r n e d t o t h e d r a i n tank. Supplementing t h e e f f e c t s of t h e s e a c t i o n s w i l l be t h e r e a c t i v i t y feedback from any changes i n t h e f u e l and g r a p h i t e temperatures. I n o r d e r t o e v a l u a t e t h e s e v e r i t y and consequences of various pcstul a t e d f i l l i n g accider.ts it i s necessary t o have c e r t a i n q u a n t i t a t i v e b f o r mation.

This includes:

1) t h e f i l l i n g r a t e (fuel l e v e l vs time),

2) t h e r e l a t i o n of &ff t o f u e l l e v e l f o r t h e p a r t i c u l a r abnormal s i t u a t i o n being considered, 3) rod worth au?d t h e speed with which they can a c t i n t h e p a r t i a l l y f i l l e d core, 4) how r a p i d l y t h e f i l l can be stopped ( l e v e l vs time a f t e r a c t i o n i s taken t o s t o p t h e fill),and 5) temperature coeff i c i e n t s of r e a c t i v i t y a p p r o p r i z t e f o r t h i s abnormal s i t u a t i o n .

This

information has been developed f o r a v a r i e t y of cases and i s presented i n Sections 2 and 3. The r e l a t i v e s e v e r i t y of a number of p o s t u l a t e d a c c i d e n t s i s surveyed i n Sectiozl

I n Section 5, t h e most severe of t h e p o s t u l a t e d

a c c i d e n t s i s analyzed i n considerable d e t a i l .

Conclusions a r e summarized

i n Section 6. 2.

2.1

MECHANICS OF FILLING General Description

Figure 1 i s a s i m p l i f i e d flowsheet of t h e r e a c t o r fill, drain, and vent systems showing only those f e a t u r e s which a r e e s s e n t i a l t o a des c r i p t i o n of t h e normal fill and d r a i n procedures.

All valves a r e shown

i n t h e normal p o s i t i o n s f o r f i l l i n g t h e r e a c t o r from f u e l d r a i n tank NO.

1 (FD-1) The r e a c t o r i s f i l l e d by admitting heliu?;, from a supply a t 40 psig,

t o t h e d r a i n tanks t o f o r c e t h e f u e l s a l t up through t h e r e a c t o r d r a i n l i n e i n t o t h e primary loop.

The fill r a t e i s l i m i t e d by a r e s t r i c t i o n

i n t h e gas supply l i n e and t h e maximum l e v e l i n t h e loop i s s e t by l i m i t i n g t h e p r e s s u r e with PIC-517.

Helium displaced from t h e loop by t h e

incoming f u e l i s vented from t h e pump bowl, a t t h e high p o i c t i n t h e loop, through t h e a u x i l i a r y charcoal bed t o t h e s t a c k .

(This vent r o u t e by-

passes t h e main charcoal beds t o avoid t h e e l u t i o n and r e l e a s e of xenon and krypton which may be i n those beds.) When t h e fill i s complete, s a l t i s frozen i n t h e d r a i n l i n e a t FV-103.

The pump-bowl vent i s switched t o t h e main charcoal beds and,

a f t e r t h e drain-tank pressure has been vented throzlgh t h e a x c i l i a r y charc o a l bed, t h e pump-bowl and drain-tank gas spaces a r e connected through

HCV-544.

The system i s now i n readiness f o r operation, with t h e ocly

a c t i o n required t o drain being t o thaw FV-103.

ORNL-DWG 63-7320

TO FILTER, FAN, AND STACK

SUPPLY HELIUM

F i g . 1. System Used i n F i l l i n g F u e l Loop.

2.2

System Volumes

The r e l a t i o n between t h e volume of s a l t t r a n s f e r r e d from t h e d r a i n tank and t h e l i q u i d l e v e l i n t h e Fdel loop i s shown i n Fig. 2.

The

datum l e v e l i s t h e bottom of t h e r e a c t o r v e s s e l (elevation 826.92 f t ) . The midplane of t h e 65-in.-high g r a p h i t e matrix i n t h e core i s a t 3.75 f t and t h e operating l e v e l i n t h e pump bowl i s a t 13.4 f t .

The f i r s t 1 . 5 f t 3

of s a l t t r a n s f e r r e d from t h e d r a i n tank i s r e q u i r e d t o f i l l t h e d r a i n l i n e . The l i q u i d volume-level r e l a t i o n i n a f u e l d r a i n tank i s shown Fig. 3.

The volume of t h e tank above t h e s a l t l e v e l i s t h e d i f f e r e n c e

between 80.5 f t 3 ( t h e t o t a l v o l m e of t h e tank) and t h e l i q u i d volume. The datum plane f o r Fig. 3 i s 1 2 . 1 f t below t h a t f o r Fig. 2 (814.82 P t ) .

2.3

Drain Tank Pressure During a F i l l

The p r e s s u r e d i f f e r e n c e between t h e d r a i n tank and t h e p m p bowl i s

determined p r i m a r i l y by t h e f u e l - s a l t d e n s i t y and t h e d i f f e r e n c e ic t h e Pressure drop i n t h e f i l l l i n e adds

l e v e l s i n t h e loop and i n t h e tank.

t o t h i s p r e s s u r e difference when f u e l i s flowing.

The p r e s s u r e i n t h e

pump bowl i s normally above atmospheric p r e s s u r e because of p r e s s u r e drop i n t h e gas vent from t h e pump bowl t o t h e s t a c k .

(The p r e s s u r e approaches

1 p s i g a t zero flow because of a check valve i n t h e vent l i n e . )

Figure

shows t h e r e l a t i o n between t h e a c t u a l p r e s s u r e i n t h e d r a i n tank and t h e l e v e l i n t h e f u e l loop during a f i l l .

This f i g u r e i s based on a t o t a l

s a l t inventory of 73.2 f t " and a s a l t d e n s i t y of 130 l b / f t 3 .

The increased

p r e s s u r e s a t s a l t f i l l r a t e s of 1 and 2 ft3/min r e f i e c t t h e p r e s s u r e drcjp i n t h e f i l l and vent l i n e s .

2.4

Amount of Gas i n %mk Luring a F i l l

The amount of gas i n t h e d r a i n tank i s a f u n c t i o n of t h e tempera-hre, t h e p r e s s u r e and t h e volume i n t h e tank above t h e l i q u i d . t h e amount of gas i n t h e tank (voluqe of gas a t 3;ioF,

14.7 p s i a )

function of loop l i q u i d l e v e l f o r varicus s a l t f i l l r a t e s . tank was assumed t o be a t 1200째F.

Figure 5 shcms

as a

(The gas i;; t h e

The p r e s s u r e and t h e a c t u a l voPume

were obtained from t h e information a l r e a d y discussed.)

Fig.

4. Drain Tank Pressure vs Liquid Level i n Fuel Loop a t Various

Salt F i l l Rates.

F i g . 5 . Amount of Gas Required i n Drain Tank vs S a l t Level in Loop a t Several F i l l Rates.

17 2.5

Gas Supply

The gas a d d i t i o s r a t e , and t h e r e f o r e t h e s a l t fill r a t e , i s Limited

This system was designed t o avoid over-

by the gas a d d i t i o n system.

f i l l i n g t h e f u e l loop and t o l i m i t t h e s e v e r i t y of p o s s i b l e f i l l i n g a c c i dents, t h e l a t t e r by r e s t r i c t i n g t h e fill rate. The f i r s t o b j e c t i v e

was achieved by l i m i t i n g , with PIC-517, t h e pressure which ca3 be put on t h e d r a i n tank t o a value j u s t sufficilent t o a t t a i n t h e d e s i r e d l e v e l i n t h e pump bowl a t t h e end of t h e fill. The f i l l - r a t e l i m i t a t i o n

a t t a i n e d by i n s t a l l i n g a c a p i l l a r y r e s t r i c t o r i n t h e supply l i n e and (Details of t h i s l i m i t a t i o n l i m i t i n g t h e primary gas supply pressure. are described l a t e r i n t h i s r e p o r t . ) During a normal fill, t h e c o n t r o l l e r PIC-517 w f l l be s e t t o give

drain-tank pressure no g r e a t e r than t h a t required t o j u s t b r i n g t h e s a l t t o t h e d e s i r e d l e v e l i n t h e p u p bowl.

Throughcjut most of t h e f i l l i r g

operation, FCV-517 i s wide open, because t h e tank pressure i s w e l l below t h e s e t p o i n t pressure, and t h e c o n t r o l l i n g r e s i s t a n c e i s t h e c a p i i l a r y . Only when t h e drain-tank pressure approaches t h e s e t p o i n t does PCU-517 f u n c t i o n t o s t o p t h e gas flow.

2.6

Course of a Noma1 F i l l

The fill r a t e , or l e v e l vs time, i s determined 5y t h e combilati,on of t h e r e l a t i o n s among l e v e l , fill rate, and amomt of gas i n t h e d r a i n tar.k and t h e c h a r a c t e r i s t i c s of t h e gas supply system (gas a d d i t7on l r a t e vs d r a i n tank p r e s s u r e ) . Figure 6 shows t h e p r e d i c t e d l e v e l i n t h e f u e l loop as a f u n c t i o n of t i m e during a normal f i l l .

For t h i s p r e d i c t i o n , done with t h e a i d cf

an analog computer, t h e gas supply was assumed t o be a t 40 p i g .

The

c a p i l l a r y r e s t r i c t o r was s i z e d t o give a s a l t fill r a t e of 0.5 ft3/min when t h e l e v e l i s a t t h e core midplane with a gas Suppiy pressure of

50 p s i g . The time required t o complete t n e e n t i r e fill was 3-3/4 hrs. (The small delay a t t h e Seginning i s t h e t i m e required t o fill t h e drafi? lice.

2.7 Coast-Up of Fuel Level A f i l l i n g operation can be i n t e r r u p t e d a t any time by any one of

three actions:

venting t h e d r a i n tank through t h e a u x i l i a r y charcoal

bed, e q u a l i z i n g drain-tank and loop pressures through l i n e 521., and s h u t t i n g off t h e gas a d d i t i o n t o t h e d r a i n tanks.

I n an emergency a i l

t h r e e of these would be done or attempted simultaneously.

E i t h e r of t h e

f i r s t two a c t i o n s would n o t only s t o p t h e f i l l b u t would a l l o w t h e s a l t i n t h e loop t o d r a i n back t o t h e tank.

The t h i r d a c t i o n , stopping gas

a d d i t i o n , would be used i f it were d e s i r e d t o hold up t h e f i l l a t axy point Simply stopping gas a d d i t i o n does n o t immediately s t o p t h e s a l t flow i n t o t h e loop.

This has important implications, f o r i f t h e gas a d d i t i o n

i s stopped while t h e l e v e l i s r i s i n g through t h e a c t i v e part of t h e core, t h e l e v e l and t h e r e a c t i v i t y w i l l continue t o i n c r e a s e f o r some time a f t e r t h e gas i s s h u t o f f . A s can be seen from Fig. 5, t h e amount of gas i n t h e tank a t a

given loop l e v e l and an appreciable flow r a t e i s capable of supporting t h e s a l t a t a considerably higher l e v e l when t h e f i l l r a t e has gone t o zero.

The amount of l e v e l increase i n t h e loop a f t e r gas a d d i t i o n i s

stopped (coast-up) i s a f u n c t i o n of t h e i n i t i a l l e v e l and flow r a t e . Figure 7 shows t h i s r e l a t i o n s h i p for s e v e r a l i n i t i a l s a l t l e v e l s i n t h e core.

Analog c a l c u l a t i o n s of t h e coast-up t r a n s i e n t i n d i c a t e d t h a t t h e

level approaches the final value with a time constant of about

1.4 =in.

NUCLEAR CONSIDERATIONS

The occurrence of a f i l l a c c i d e n t presupposes t h e e x i s t e n c e of an abnormal condition which causes t h e r e a c t o r t o be c r i t i c a l before t h e core i s completely f i l l e d w i t h f u e l s a l t .

The b a s i c nuclear c h a r a c t e r -

i s t i c s which must be considered are q u a l i t a t i v e l y similar for all of t h e a c c i d e n t s examined.

20 e

ORNLDWG66-7772

2.0

(ft) 1.6

1.2

0.8

0.4

Fill Fig. 7. Coast-Up After' Several Initial Fuel Levels.

Rate (cf'm)

Gas Addition

is StopFed vs Fill

Rate at

3.1 E f f e c t of Fuel Level on R e a c t i v i t y I n a l l of t h e f i l l i n g accidents, the system m u l t i p l i c a t i o n constant

i s increased above u n i t y by the continued flow of f u e l i n t o the c r i t i c a l core.

The shape of t h e r e a c t i v i t y curve as a f u n c t i o n of f u e l l e v e l ,

along w i t h t h e r a t e of f u e l addition, determines t h e r a t e of r e a c t i v i t y increase. The e f f e c t of fuel l e v e l on m u l t i p l i c a t i o n was c a l c u l a t e d f o r a s i m p l i f i e d model of t h e r e a c t o r which considered only t h e uniformchannelled p o r t i o n of t h e graphite-moderated region.

Figure 8 shows

t h e e f f e c t i v e m u l t i p l i c a t i o n constant as a function of t h e f r a c t i o n (H/L) of t h i s region f i l l e d with f u e l .

0.997 f o r t h e

The curve was normalized t o a k of

f u l l region; t h i s i s t h e t a r g e t m u l t i p l i c a t i o n constant i n

t h e MSRE f o r a fill under normal circumstances.

The f r a c t i o n a l l e v e l s ,

0 and 1 i n t h i s model correspond t o a c t u a l f u e l l e v e l s of 1.33 and

6.50

f t , r e s p e c t i v e l y , i n t h e MSRE.

(See Fig. 2)

Since t h e m u l t i p l i -

c a t i o n constant i n t h e MSRE rises above zero before t h e f u e l l e v e l reaches t h e channelled region and continues t o r i s e u n t i l t h e e n t i r e v e s s e l i s f u l l , the curve i n Fig. 8 i n d i c a t e s a g r e a t e r d i f f e r e n t i a l change i n rea c t i v i t y w i t h l e v e l than a c t u a l l y e x i s t s .

Thus, use of t h i s curve shape

i n t h e accident analyses l e a d s t o s l i g h t overestimates of t h e i r s e v e r i t y . The f l a t t e n i n g of t h e curve i n Fig. 8 i s due t o t h e f a c t t h a t t h e g r a p h i t e i n t h e core i s s t a t i o n a r y .

A t low f u e l l e v e l s , t h e g r a p h i t e above t h e

f u e l a c t s as a r e f l e c t o r which a f f e c t s t h e m u l t i p l i c a t i o n s u b s t a n t i a l l y ; t h i s added e f f e c t diminishes as t h e core f i l l s .

3.2

Control Rods

The t h r e e c o n t r o l rods i n t h e MSRE a r e c l u s t e r e d near t h e a x f s of t h e r e a c t o r and e n t e r t h e core from above.

Since t h e core f i l l s w i t h

f u e l from t h e bottom, t h e r e a c t i v i t y worth of a p a r t i a l l y i n s e r t e d rod depends on t h e f u e l l e v e l .

Figure

shows t h e c a l c u l a t e d f r a c t f o n a l

worth of a rod as a function of the f r a c t i o n of t o t a l i n s e r t i o n f o r a f u l l core and f o r a core w i t h only 0.72 of t h e channelled region f i l l e d w i t h fuel.

The t o t a l worth of t h e f u l l y i n s e r t e d rod w a s e s s e n t e a l l y t h e

ORNL DWG 66-7773

1.0

0.9

0.8

0.7 0.4

(

0.8

H/L Fig. 8. E f f e c t of F u e l Level on R e a c t i v i t y .

1.0

ORNL-IX-DWG.

Fig. 9. C o n t r o l Rod Worth vs Position.

'70056

same f o r both c a s e s ,

The rod wortks used i c analjrzin;: t h e f i l l i n g a c c i -

dents were 2.9% 6k/k f o r a sir-gle, f u l l y i n s e r t e d rod a t a l l fue; l e v e l s

6.7% f o r

and

a l l t h r e e rods.

.Tke f r a c t i o n a l worth of a p a r t l y i n s e r t e d

rod a t a given p o s i t t o r was ase-mcd t o vary E r e & r l y with %he f r a c t i o n of t h e core f i l l e d with f u e l . During a r.omal fill, t h e t h r e e c o n t r o l rod3 w i l l be withdrawn e q a a l amounts and plssitioned s x h t h a t t h e r e a c t o r i s j n s t s u b c r i t i c a l

0.997) wher, completely P i l i e

( P r o t e c t i v e i n t e r l o c k s w i l l rec@ire

t h a t the rods be wittdrawr- a n i 3 i ~ md i s t a x c t o

iYisidr2

t h a t negative r e -

a c t i v i t y can be i n s e r t e d i n the evect of premature c r i t i c a l i t y . )

The

rods, positioned i n t h i s way, w i l l c o n t r o l 4.3% 6k/k i n t h e cleen, f u l l r e a c t o r w i t h t h e operating concentration of uranium i r , t h e f u e l .

,They

w i l l , however, c o n t r o l sx7Dstantialiy less a t t h e same p o s i t i o n i n a

p a r t l y f i l l e d reacfJor and c a i , therefore, i n s e r t correspondingly more negative r e a c t i v i t y duriag a f i l l a c c i d e n t . Each control-rod d r i v e i s equipped with a magnetic c i u t c h so t h a t the rcd can be dropped i n t o t h e r e a c t o r if necessary.

A 0.1-sec r e l e a s e time

and an a c c e l e r a t i n g f o r c e of 0.5 g were assumed f o r t h e rods ir, t h e accident analyses.

3.3

Temperature C o e f f i c l e n t s of Reac,tivity

The values of t h e f u e l and g r a p h i t e temperature c o e f f i c i e n t s of r e a c t i v i t y depend on the f u e l compositioc.

Tabie 1 l i s t s t h e nominal compc-

s i t i o n s a n d the d ? n s i t i e s of three fuels Seing comiderefi fcr use i n t h e ERE.

The temperatme c o e f f l c i s z t s of r e a c t i v i t y shown & r e f o r t h e P i l i

core. Temperature c o e f f i c i e n t s of r e a c t i v i t y i n t h e p a r t i a l l y f i l l e d eG.re a r e q u i t e d l f f e r e i t from the c o e f f l c f e n t s f o r a f u l l core.

ThEs i s due

i n p a r t t o t h e d i f f e r e x e i n e f f e e t l v e size and shape of t h e c o r e ,

importantly, i n the p a r t i a l l y f i l l e d si)r- a n f x r e a s e i n f u ~ temperature, l

*Recent

c a l c u l a t i o n s i n d i c a t e the worth of t h r e e f u l l y i n s e r t e d rods ranges from 5.6 t o 7.676, depexding m t h e f u e l composltion. The usable worth i s fron 5 - 2 t o 7.2%.

25. Table 1 MSRE Fuel S a l t s Considered i n F i l l i n g Accident Allalyses

Fuel m e S a l t Composition: (mole 8)

LiFa BF2 ZrF4 m 4

uF4

U Composition (atom %)

(approx.)

u23 4

u235 TT236 -

u23 8

Density a t 1200째F ( l b / f t 3 )

66.8

23-7

29.2

1 0.3

0 0.2

5 144.5

0.8 (3.3

35 0.3

64.4 i42.7

134.5

Temp. Coeff. of R e a c t i v i t y ("F-l)

-3.0 x 10'~ -5.0 x -3.4 x 10-5 -4.9 x

Fue 1 Graphite

-3.3 x

-3.7

io+

99.9926% ~i~

with i t s a t t e n d a n t decrease i n f u e l density, r a i s e s t h e f u e l l e v e l and i n c r e a s e s t h e e f f e c t i v e height of t h e core.

This i s i n c o n t r z s t to a

full. core i n w h i c h a reduction i n density expels m e 1 withoat changing

t h e core s i z e .

The c o e f f i c i e n t s a r e a l s o a f f e c t e d by any diff;.re-nce in

f u e l coccentrations between t h e normal and abnormal s i t u a t i o r s . The f u e l temperature c o e f f i c i e n t of r e a c t i v i t y for t k e p a r t i a l l y f i l l e d core was evaluated by comparing keff c a l c u l a t e d f o r two cases, each with Fuel B concentrated by s e l e c t i v e f r e e z i n g of 39$ of t k e s a l t .

* **

I n t h e f i r s t case, H/L was 0.60 and t h e f u e l d e n s i t y was proper for 1200째F'.

*See pp 20 - 21 for discussion **T h i s choice of s a l t type and

of s e l e c t i v e f r e e z i n g . R/L w i l l be e x p l nined l later.

26 I n the second, H/L -ms 0.61 a-cd tne f b e l d e n s i t y w a s r e d x e d t o keep the same t o t a l mass of f u e l ii the core. Neutron microscopic cross s e c t i o n s were evaulated a t 1200째F i n S e t h cases and t h e g r a p h i t e d e n s i t y was changed.

.,..

UTI-

These two cases simulated a r i s e i n f u e l temperature from 1200째F

t o 1341'F i n a time s o s h o r t t h a t t h e graphfte temperature does not r i s e appreciably.

Use of constant microscopic cross s e c t i o n s implies t h a t the

f u e l temperature has no e f f e c t on t h e thermal nezltron energy d i s t r i b u t i o n when, i n f a c t , it does.

Results of these c a l c u l a t i o n s gave a 6k/k of

0.061$, equivalent t o a f u e l tercperature c o e f f i c i e n t of -0.43 x

SURVEY OF FILLIFJG ACCIDENTS

The r e l a t i v e s e v e r i t y of f i l l i n g a c c i d e n t s can be described, q u a i i t a t i v e l y , i n terms of t h e amount of excess r e a c t i v i t y a v a i l a 5 l e f o r add i t i o n t o t h e core and t h e rate a t which it can be added, p m t i c u l a r l y ir the v i c i n i t y of

hff =

1. The amount of excess r e a c t i v i t y a v a i l a b l e

depends p r i m a r i l y on the circumstances p o s t u l a t e d f o r t h e a c c i d e n t and t h e composition of t h e f u e l mixture.

Three s e t s of circumstances which

can produce f i l l i n g accidents have been considered; t h e s e a r e discussed under separate headings below.

The influence of f u e l composition was

examined for each type of accident. The r a t e of r e a c t i v i t y a d d i t i o n involves, i n a d d i t i o n t o t h e f a c t o r s mentioned above, t h e r a t e of f u e l a d d i t i o n .

I n order t o r e s t r i c t t h e

most severe accident t o a t o l e r a b l e l e v e l , it was necessary t o l i m i t t h e

s a l t a d d i t i o n r a t e under normal circumstances t o 0.4 ft3/min.

The normal

h e l i m supply pressure t o t h e d r a i n tanks i s 40 p s i g with as? u l t i m a t e

l i m i t a t 50 p s i g imposed by a rupture d i s c .

The p h y s i c a l r e s t r i c t i o n s

which e s t a b l i s h t h e normal f i l l r a t e l i m i t t h e maximum rate t c 0.5 f.t3/min with t h e s a l t l e v e l i n t h e main p o r t i o n of t h e core.

A l l of thz f i l l i n g

accidents were examiried on the b a s i s of t h e 0.5 f t 3 / m i n r a t e .

4.1

F i l l i n g With Control Rods Withdrawn

The amount of excess r e a c t i v i t y t h a t can be added i n t h e MSRE by f i l l i n g the core with the c o n t r o l rods withdrawn i s l i m i t e d t o the amoilnt required i n the f u e l f o r normal, full-power operation.

Although t h i s

requirement v a r i e s somewhat with the f u e l mixture i t i s not expected t o exceed

4% i n any cage and a d m i n i s t r a t i v e c o n t r o l w i l l be exercised t o keep

t h e r e a c t i v i t y a t or below t h i s value. s u f f i c i e n t uranium for

4% excess

If t h e f u e l were loaded w i t h

r e a c t i v i t y and a l l t h r e e rods were f u l l y

withdrawn, the core would be c r i t i c a l a t 74% of f u l l .

A t t h i s level,

t h e s a l t a d d i t i o n r a t e of 0.5 ft3/min corresponds t o a r e a c t i v i t y ramp of

0.01% Sk/k per sec.

Dropping t h e c o n t r o l rods a f t e r t h e power reaches t h e

normal scram l e v e l (150% of f u l l power or 15 Mw) checks t h e excursion produced by such a ramp w i t h no s i g n i f i c a n t r i s e i n f u e l temperature.

Even

if only two c o n t r o l rods a r e dropped, s u f f i c i e n t negative r e a c t i v i t y i s

i n s e r t e d t o prevent c r i t i c a l i t y from being a t t a i n e d again i f t h e core i s completely f i l l e d . 4.2

F i l l i n g With Fuel a t Low Temperature

I n t h i s accident i t i s p o s t u l a t e d t h a t t h e g r a p h i t e has been preheated t o t h e normal s t a r t u p temperature of 1200째F and f u e l s a l t i s added a t a s i g n i f i c a n t l y lower temperature.

The amount of excess reac-

t i v i t y a v a i l a b l e depends on t h e temperature c o e f f i c i e n t of r e a c t i v i t y of t h e f u e l i n t h e full r e a c t o r (see Table 1) and t h e degree of subcooling of the s a l t .

The h e a t c a p a c i t y of t h e g r a p h i t e i n t h e core i s 3.53 Mw-

sec/"F while t h a t of t h e s a l t i n t h e graphite-bearing regions i s only

1.45 Mw-sec/"F.

Therefore, if t h e f u e l and g r a p h i t e a r e allowed t o come

t o thermal equilibrium, t h e temperature r i s e of t h e s a l t i s

decrease i n g r a p h i t e temperature.

2.4 t i m e s

the

Since t h e r a t i o of the g r a p h i t e t o t h e

f u e l temperature c o e f f i c i e n t i s less than 2.4, h e a t t r a n s f e r from t h e g r a p h i t e t o t h e s a l t reduces t h e excess r e a c t i v i t y . The l i q u i d u s temperature of Fuel B, t h e s a l t w i t h t h e l a r g e s t negat i v e temperature c o e f f i c i e n t of r e a c t i v i t y , i s about

810"~.If s a l t a t

t h i s temperature were added t o t h e r e a c t o r and h e a t t r a n s f e r from t h e

g r a p h i t e were neglected, t h e maximum amount of excess r e a c t i v i t y would be

1.9%.

This i s well below t h e 3.2% shutdown margin provided by t h e con-

t r o l rods.

4.3

F i l l i n g with Concentrated Fuel

The c r y s t a l l i z a t i o n paths of a l l t h r e e s a l t mixtures being considered f o r use as MSFE f u e l are such t h a t l a r g e q u a n t i t i e s of s a l t can be s o l i d i f i e d , under equilibrium conditions, before any uranium (or thorium) appears i n t h e s o l i d phase.

S e l e c t i v e freezing, t h e r e f o r e , provides one

means by which t h e uranium concentration i n t h e l i q u i d s a l t can be i n creased s i g n i f i c a n t l y while t h e s a l t i s i n t h e d r a i n tank.

Since t h e r e -

a c t o r v e s s e l i s t h e f i r s t major component of t h e f u e l loop t h a t f i l l s on

s a l t addition, approximately

40% of

t h e s a l t mixture can be frozen i n t h e

d r a i n tank before it becomes impossible t o completely f i l l t h e core. The changes i n l i q u i d composition as s e l e c t i v e f r e e z i n g proceeds depend upon t h e i n i t i a l cornpositron and t h e conditions of f r e e z i n g . Figure 10 shows t h e composition of t h e remaining m e l t f o r Fuel A as a function of t h e f r a c t i o n of s a l t frozen.

The curves a r e based on t h e

assumption t h a t only t h e equilibrium primary s o l i d phase,

6 LiF*BeFz-ZrF4,

appears. The e f f e c t on premature c r i t i c a l i t y was evaluated f o r each of t h e t h r e e s a l t s with 3976, by weight, frozen i n t h e d r a i n tank as

6 LiF*BeF2'ZrF,..' had about

4% excess

Under t h e s e conditions t h e f u l l r e a c t o r a t 1200째F r e a c t i v i t y f o r Fuels A and C and 15% f o r Fuel B.

Fuels A and C contain s i g n i f i c a n t amounts of thorium and

2z,%,

respec-

t i v e l y , which remain i n t h e melt with t h e 235U during s e l e c t i v e f r e e z i n g . The poisoning e f f e c t of t h e s e s p e c i e s g r e a t l y reduces t h e s e v e r i t y of t h e f i l l i n g a c c i d e n t when they a r e p r e s e n t .

The excess r e a c t i v i t i e s i n t h i s

a c c i d e n t exceed t h e shutdown margin of t h e c o n t r o l rods so i t i s necessary t o s t o p t h e f i l l i n g process t o prevent a second r e a c t i v i t y excursion a f t e r t h e rods have been dropped.

The a c c i d e n t involving Fuel B determines t h e

speed with which t h e f i l l must be stopped because t h e r e a c t i v i t y a d d i t i o n r a t e f o r t h i s case i s 0.025% Gk/k/sec a t k = 1 vs O.Ol%/sec f o r Fuels A and C .

*The

composition of t h e s o l i d phase has l i t t l e e f f e c t on t h e nuclear c a l c u l a t i o n s as long as it does not include f i s s i l e or f e r t i l e m a t e r i a l .

CD CJ

0 Iu

0 0 I-'

0 0

Mole Fraction

0 wl

0 -4

It i s c l e a r from t h e preceding s e c t i c n t h a t t h e most severe of the f i l l i n g accidents considered occurs when the mixtnre whicn remains a f t e r s e l e c t i v e f r e e z i n g of 33% of F ~ f 3l f u e l loop.

If it car, be show:-

i,r,

t h e d r a i n tank i s f o r c e d i g t o the

t h a t t h i s accident i s t o l e r a b l e , then a l l

of the other a c c i d e c t s a r e a l s o t o l e r a b l e .

5.1

S p e c i f i c a t i o n of Aeciderit

The accident which -xis analyzed i n d e t a i l included a number of abnormal conditions i n a d d i t i o n t o f i l l i n g t h e r e a c t o r with highly concentrated fuei.

The conditions of t h e a c c i d e i t and eqilipment performance,

both normal and abnormal, ar? described b - l ow. F i r s t , it was assumed t h a t t h e gas a d d i t i o n system had failed t c t h e e x t e n t t h a t t h e gas supply pressure t o t h e d r a i n tank was 50 psig, t h e

l i m i t imposed by t'ne rupture disc, r a t h e r than t h e normal 40 p s i g . gave a s a l t a d d i t i o x r a t e of 0.5 f t 3 / m i a

This

a t t h e time t h e r e a c t o r f i r s t

became c r i t i c a l ( a t 55% f u l l ) and r e s a l t e d i n a r e a c t i v i t y ramp of The f i r s t c o r r e c t i v e a c t i o n was an automatic rod

0.025% 6k/k p e r sec.

drop i n i t i a t e d when t h e neut-on fl-ax reached 150% of design power (15 Nw). A r e l e a s e time of 0 . 1 see was assumed and t h e rods were allowed t o f a l l

with an a c c e l e r a t i o n of 0.5 times t h e a c c e l e r a t l o n ~f g r a v i t y . I t was assumed t h a t only two of the t h r e e rods a c t u a l i y drcpped.

Ec,vJever, Acrlc:

t o s t o p t h e f i l l and i i l t i a t z a d r a i n was assume4 t o occur a t t h e s2x-e tfme as the rod drop.

This a c t i o n i n v c i v t d a t o m 6 . t i c openizg of t h e

t q u a l i z i n g valve, HCV-544,

a i d t h e drai;?

tacK

c l o s i n g of t h e gas a d d i t i o e va17e, HCV-77%.

vent ?Jalve, X8-573, an9

,t vas assomed t h a t o i l y one

of these valves, HCV-572, acktialiy functioned ar_d 1 sec t h e valve t o c l o s e .

* was

alloved f o r

This actfCiE., coupled with t h e i n i t i a l s a l t fill r a t e ,

*This time 5,s not e r i t ~ i e a i . Calculations u s ixg a 5-see d o s i n g time did n o t produce detectably d i f f e r e n t r e s u l t s .

31 gave a l e v e l coast-up of 0.2 f t which added a t o t a l of 0.52s r e a c t i v i t y

i n excess of t h a t compensated by t h e two i n s e r t e d c o n t r o l rods.

(If

e i t h e r of t h e o t h e r two gas valves had functioned, t h e l e v e l would have dropped and t h e r e would have been no second excursion.)

5.2

Preliminary D i g i t a l Calculations

?"ne major p o r t i o n of t h e t r a n s i e n t s a s s o c i a t e d with t h i s a c c i d e n t

was c a l c u l a t e d with t h e a i d of the ORPJL analog computer.

However, s t a r t u p

accidents t y y i c a l l y begin a t very low powers and t h e power v a r i e s over s e v e r a l orders of magnitude.

Since the u s e f u l range of t h e analog com-

p u t e r covers only about two orders of magnitude f o r any v a r i a b l e , exc e s s i v e range switching would be required t o simulate t h e e n t i r e t r a m i e y t on t h e analog f a c i l i t y .

To circumvent t h i s problem, t h e i n i t i a l p a r t of

t h e power t r a n s i e n t , from source power t o a power which began t o a f f e c t t h e f u e l temperature, was c a l c u l a t e d with t h e a i d of a d i g i t a l program, MURGATR0YD.l

MURGATROYD i s a p o i n t model, nuclear k i n e t i c s program,

using s i x groups of delayed neutrons, with provisions f o r adding react i v i t y i n t h e form of s t e p s and/or ramps.

The d i g i t a l program was

s t a r t e d a t keff = 1 and a power of 1 watt and was used t o c a l c u l a t e t h e v a r i a t i o n of power with time up t o 10 kw. consumed

24 sec

This p o r t i o n of t h e t r a n s i e n t

of r e a c t o r time and r a i s e d t h e f u e l temperature 0.01"F.

The r e a c t o r p e r i o d a t 10 kw was about 0.7 sec.

The r e s u l t s of t h i s d i g i -

t a l c a l c u l a t i o n were used as i n p u t t o s t a r t t h e analog simulation.

5.3

Detailed Analog Simulation

Description of Model I n order t o p r e d i c t t h e excursions of power and temperature r e s u l t i n g from t h e p o s t u l a t e d fill accident, a mathematical model was constructed

IC. W. Nestor, Jr., MURGATROYD- An Iâ&#x201A;ŹM 7090 Program f o r t h e Analysis of the Kinetics of t h e 1vIsRE, USAEC Report ORXL-TM-203, Oak Ridge National Laboratory, A p r i l 6, 1962

which described t h e heat t r a n s f e r , t h e nuclear k i n e t i c s , and t h e e x t e r n a l inputs t o t h e system.

Figure 11 shows a block diagram of t h e model used

as a b a s i s f o r t h e simulation. It may be noted t h a t except f o r t h e heat t r a n s f e r equations, a l l of t h e compJtations a r e r e l a t i v e l y straightforward f o r analog s o l u t i o n s i n c e they involve ordinary d i f f e r e n t i a l equations.

I r i s p i t e of t h e preliminary

d i g i t a l c a l c u l a t i o n s , t h e power excursioi? exceeded t h e u s e f u l range of t h e analog computer and provisions f o r r e s c a l i n g t h e power v a r i a b l e during the s o l u t i o n were required. A s shown i n Fig. 11, however, t h e f u e l and g r a p h i t e temperatures a r e

functions of both p o s i t i o n and time, and thus a r e represented by p a r t i a l d i f f e r e n t i a l equations.

To solve t h e s e equations on an arialog computer,

one must use f i n i t e difference techniques.

Since t h e accuracy of t h i s

s o l u t i o n t u r n s o u t t o be a very important p a r t of t h e simulation, i t sill be discussed i n d e t a i l .

Simulation of Local Fuel and Graphite Temperatures Since the core c o n s i s t s of a l a r g e number of i d e n t i c a l f u e l charxels and v e r t i c a l g r a p h i t e s t r i n g e r s , l e t us f i r s t consider t h e temperature i n a t y p i c a l s t r i n g e r cross-section, show, i n Fig. 1 2 .

Due t o t h e symaetry,

we can consider a b a s i c h e a t t r a n s f e r "element" as h a l f of a f u e l ehanr?el c r o s s - s e c t i o n and one-fourth of a s t r i n g e r as shown shaded i n Fig. 12* I n i t i a l l y , considering the f u e l and g r a p h i t e as s i n g l e regions having

mean temperatures T

and TG, h e a t would be t r a n s f e r r e d from t h e f u e l t o

g r a p h i t e a t a r a t e Q, where

4 = K ( TF

- TG)

where K may r e a d i l y be seen t o be a function of t h e c o n d u c t i v i t i e s , the geometry, and t h e conductance of a f i l m a t t h e i n t e r f a c e .

It i s importact

t o note however, t h e K a l s o depends on t h e rate of change of t h e temperatures.

I n general, the higher t h e frequency of t h e perturbation,

the

g r e a t e r t h e e r r o r i n t h e computed h e a t t r a n s f e r r a t e betveen t h e two materials, where t h e approximate t r a n s f e r r a t e i s always lower than t h e

ORNL DWG 66-7775

P ( r ? Z?.t)

Til

KdC LEAR

AVERAGE TEMPERATURES

NUCLEAR

POWER P

NlSCT;EIIR

KINETICS EQUATIONS (6 DELAYED-NEUTRON GROUPS)

.-.I i 1

F?EACTITJITY COMRJTATIOX --A i

FLSEL I;EV'EL COWTATXG!T

! i

Fig. 11 Block Diagram of Mathematical Model for Fill Accident Simulation

34 ORNL DWG 66-7776

2 I'

SHAmD PORTION

SHOWS TYPICAL "MSIC " HEAT TRANSFER ELEMENT-

Fig. 12. Cross-Section of Typical Graphite Stringer With Adjacent Fuel Channels

actual value.2

Thus it is advantageous to use as fine an approximation

as possible in this case, since heat transfer from the hot fuel to the graphite and the subsequent rise in graphite temperature is the major mechanism for curbing the power excursion (because of the small temperature coefficient of the fuel in the partly f u l l core). In the simulation, the temperature distribution for a basic element was approximated by five regions for the fuel and

15 regions for the

graphite, and using slab geometry wlth a corrected surface-to-volume ratio.

First-order central difference equations were used.

%ne accuracy

of this simulation (assuming a negligible error in the geometry approximation) was found from a comparison of the frequency response characteristics of a distributed slab with those of a lumped-parameter approximation.

2S. J. Bll, Approximate Models for Distributed-Parameter Heat Transfer Systems, Preprints of Technical Papers, Fourth Joint Automatic Conference, University of Minnesota, Minneapolis, Minnesota, June 19-21, 1963, AIChE, New York, 1963.

This comparison was made by means of a d i g i t a l computer code which can be used f o r general slab-geometry c a l c u l a t i o n s of t h i s type.

The c a l c u l a t i o n

showed t h a t f o r t h e approximation used, t h e simulated heat t r a n s f e r r a t e

t o t h e g r a p h i t e would be within 10% of t h e exact s o l u t i o n values f o r p e r t u r b a t i o n frequencies of up t o 25 cycles p e r second.

For t h e tempera-

t u r e changes involved i n t h e i n c i d e n t being considered, t h e approximation

was more than adequate.

Two o t h e r simplifying assumptions were made: 1)

Since, during a f i l l , t h e MSRE: i s a " s t a t i o n a r y f u e l " r e a c t o r ,

t h e only means f o r a x i a l heat t r a n s f e r i s by conduction, and t h i s was found t o be n e g l i g i b l e .

Radial conduction (between b a s i c elements) was a l s o assumed

zero. Temperature Averaging The e n t i r e r e a c t o r was divided i n t o four major regions of fuel and graphite.

Average nuclear importances of temperature changes ana f r a c -

t i o n s of t o t a l power generation were assigned t o t h e components of each region.

These assignments were based on the s p a t i a l v a r i a t i o n of nuclear

importance and power d e n s i t y i n t h e p a r t l y full core.

Since t h e model

f o r c a l c u l a t i n g temperatures i n a b a s i c h e a t - t r a n s f e r element was assumed l i n e a r , the temperature changes i n one region were p r o p o r t i o n a l t o those i n any other, and were d i r e c t l y r e l a t e d t o t h e f r a c t i o n of t h e power generated i n t h e region and i n v e r s e l y r e l a t e d t o t h e volume of t h e region. Thus, f r o m t h e simulation of a s i n g l e basic h e a t - t r a n s f e r elemeut, a d i r e c t computation was made of t h e mean f u e l and g r a p h i t e temperatures of each region.

The region temperatures were weighted with t h e i r r e s p e c t i v e

importances and summed t o o b t a i n t h e nuclear average f u e l and g r a p h i t e temperatures f o r t h e r e a c t o r .

These temperatures were used with t h e i r

r e s p e c t i v e c o e f f i c i e n t s of r e a c t i v i t y t o compute t h e i n t e r n a l c o n t r i b u t i o n t o t h e k i n e t i c behavior of t h e r e a c t o r . Other Aspects The r e a c t i v i t y e f f e c t s of t h e f u e l and g r a p h i t e temperatures were combined with the o t h e r r e a c t i v i t y inputs t o c a l c u l a t e t h e power t r a n s i e n t s . The o t h e r r e a c t i v i t y i n p u t s (see Fig. 11) included 1) t h e i n i t i a l ramp a s s o c i a t e d with t h e fill of t h e r e a c t o r , 2) a s e r i e s of ramps t o simulate

the dropping control rods, and 3) the decaying ramp associated with an internal computation of the fuel-level coast-up after the gas-addition valve was closed.

The computer was set up to automatically insert the

appropriate reactivity term with the appropriate time delay during the transient.

The net reactivity was fed to a set of kinetic equations using

six groups of delayed neutrons for the actual power calculation. Provisions were included for stopping the calculation at any time to permit the necessary changes in the range of the power calculation. Facilities were also available for recording any of the computed parameters. Results of Simulation The results of the fill-accident simulation are shown graphically in Figs. 1 3 and

14. Figure 1 3 shows the externally imposed reactivity

transient exclusive of temperature compensation effects.

The essential

features are the initial, almost-linear rise which produced the first power excursion as fuel flowed into the core, the sharp decrease as the rods were dropped, and the final slow rise as the fuel coasted up to its equilibrium level. Figure temperatures.

14 shows the power transient and some pertinent

The fuel and graphite auclear average temperatures are the

quantities which ultimately compensated for the excess reactivity introduced by the fuel coast-up. The maximum fuel temperature refers to the temperature at the center of the hottest portion of the hottest fuel channel.

The initial power excursion reached 24 Mw before being checked

by the dropping control rods which were tripped at 15 Mw. This excursion is not particularly important since it did not result in much of a elel temperature rise. After the initial excursion, the power dropped to about 10 kw and some of the heat that had been produced in the fuel was transferred to the graphite.

The resultant increase in the graphite nuclear

average temperature helped to limit the severity of the second power excursion. Reactivity was added slowly enough by the fuel coast-up that the rising graphite temperature was able to limit the second power excursfon to only 2.5 Mw.

The maximum temperature attained, 135k째F, is well within

the range that can be tolerated.

150

?IK)

250

TIME ( s e c )

Fig. 13.

Net R e a c t i v i t y During Maximum F i l l i n g Accident.

350

38 ORNL DWG 66-7778

!rIME (SEC)

Fig. 14. Power and 'Tenpcratures During Plaximum E'1lli.ng Accident.

5.4

Discussion

The above a n a l y s i s i n d i c a t e s t h a t t h e r e a c t o r system i s not l i k e l y t o be damaged by any of t h e f i l l i n g accidents considered.

The accident

t h a t was s t u d i e d i n d e t a i l would probably be l e s s severe than t h e calcul a t i o n s i n d i c a t e because of the conservatism i n estimating t h e e f f e c t of f u e l l e v e l on r e a c t i v i t y .

The degree of fuel concentration by s e l e c t i v e

f r e e z i n g used i n t h i s study was chosen a r b i t r a r i l y as t h a t amount which l e f t j u s t enough s a l t i n l i q u i d form t o completely f i l l t h e r e a c t o r .

i s p h y s i c a l l y possible, under i d e a l conditions, t o achieve g r e a t e r concent r a t i o n s of uranium than t h a t assumed.

There i s no v a l i d b a s i s f o r as-

s e s s i n g t h e amount of s e l e c t i v e f r e e z i n g t h a t could occur i n t h e d r a i n tank without being detected.

There i s a l s o no assurance t h a t f r e e z i n g

i n t h e d r a i n tank would, i n f a c t , leave a l l of t h e uranium i n t h e remaining melt. I n a s s e s s i n g t h e c r e d i b i l i t y of t h i s accident, it should 5e recogc

nized t h a t s e v e r a l eqilipment f a i l u r e s were p o s t u l a t e d which compounded t h e e f f e c t s of f i l l i n g t h e core w i t h excessively concentrated f u e l .

These

a r e 1) t h e f a i l u r e i n t h e gas-addition system which allowed f u e l t o flow i n t o t h e r e a c t o r a t an excessively high r a t e , 2) f a i l u r e of one of t h e t h r e e c o n t r o l rods t o drop, and 3) f a i l u r e of a p a r t i c u l a r two of t h r e e valves t o f u n c t i o n t o s t o p t h e f i l l .

Elimination of any one of t h e s e

p o s t u l a t e d f a i l u r e s would prevent tlne second r e a c t i v i t y excursion and r e duce t h e e f f e c t s of t h e accident t o t r i v i a l proportions.

It has been pointed out t h a t t h e s e v e r i t y of t h e f i l l i n g a c c i d e n t could be f u r t h e r compounded by p o s t u l a t i n g t h a t t h e f u e l loop vent valve,

HCV-533, i s closed a t t h e s t a r t of t h e f i l l and i s manually opened j u s t as t h e i n i t i a l c r i t i c a l i t y i s achieved. F i l l i n g with HCV-533 closed would allow t h e p r e s s u r e i n t h e primary loop t o r i s e t o about 5 psig, t h e normal s e t p o i n t of t h e loop p r e s s u r e c o n t r o l l e r , PCV-522.

Analog c a l c u l a t i o n s

of t h e f u e l l i q u i d l e v e l showed t h a t , if HCV-533 were opened a t tlnis press u r e and a l l of t h e previously p o s t u l a t e d f a i l u r e s occurred, t h e l e v e l i n t h e core would r i s e

1.5

f t , producing an excursion w i t h temperatures t h a t

would damage t h e r e a c t o r v e s s e l .

However, i f e i t h e r

HCV-544 o r HCV-573

40 opens on demand, t h e gas flow from t h e d r a i n tank i n t o t h e primary loop

or o u t through t h e vent l i n e l i m i t s t h e pressure decrease i n t h e loop and keeps the accident within t o l e r a b l e l i m i t s . It does not seem reasonable t o add two more improbable events, f a i l u r e t o have HCV-533 open a t the

s t a r t and then opening the valve a t t h e c r u c i a l moment, t o t h e l i s t of conditions a l r e a d y postulated.

If these two events a r e postulated, i t i s

probably reasonable t o p o s t u l a t e t h a t only one valve ( r a t h e r than two)

f a i l s t o function on demand.

This accident can be f u r t h e r m i t i g a t e d by

reducing the s e t p o i n t of PCV-522 during t h e f i l l i n g operation.

A setting

of 1 . 5 t o 2 p s i g would a s s u r e t h a t t h e displaced gas from t h e f u e l loop goes t o t h e a u x i l i a r y charcoal bed during a normal fill b u t would l i m i t t h e pressure i n t h e f i e 1 loop during a f i l l w i t h HCV-533 closed.

Such

a c t i o n might increase t h e p o s s i b i l i t y of an a c t i v i t y r e l e a s e from t h e normal charcoal beds b u t t h e consequences of t h i s are minor, p a r t i c u l a r l y in view o f the low probability of occurrence.

CONCIXJSIONS

None of t h e f i l l i n g a c c i d e n t s t h a t were s t u d i e d i n d e t a i l poses any t h r e a t t o t h e r e a c t o r system.

On t h e o t h e r hand, it i s p o s s i b l e t o con-

ceive of a s e t of circumstances, however unlikely, t h a t could produce temperatures high enough t o breach t h e primary containment.

Such a breach

would probably occur as a melting of t h e c o n t r o l rod thimbles.

Even ther,,

t h e a c t i v i t y and/or s a l t r e l e a s e would be confined within t h e secondary containment and t h e r e l e a s e would not approach t h e maximum c r e d i b l e a c c i dent f o r which t h e secondary containment i s designed.

Therefore, even t h e

worst conceivable f i l l i n g accident does n o t r e p r e s e n t a hazard t o t h e operating personnel or t h e environment.

*Since

the r e a c t o r loop i s l e s s than h a l f - f u l l of s a l t during a f i l l i n g accident, it i s not p o s s i b l e for a nuclear i n c i d e n t t o generate pressures of t h e magnitude required f o r a c a t a s t r o p h i c r u p t u r e of t h e reactor vessel.

Internal

...

1. MSRP D i r e c t o r ' s Off i c e Rm. 219, 9204-1 2. G. M. Adamson 3. R. G. A f f e l 4. S . J . B a l l 5. S . E. Beall 6. E. S. B e t t i s 7. F. F . Blankenship 8. E . G. Bohlmann 9. W. H. Cook 10. J . L. Crowley 11. F . L. C u l l e r 12. S. J . D i t t o 13-17. J . R. Engel 18. E . P. Epler 1-9 A. P. Fraas 20. C H. Gabbard 21. W. R. Grimes 22. R . H. Guymon 23 P. H. Harley 24. P. N. Haubenreich 25 V. D. Holt 26. T. L. Hudson 27 P. R. Kasten 28. A. I. Krakoviak 29. R. B. Lindauer 30 M. I. Lundin

Distribution R . N. Lyon

32. H. G. MacPherson 33. R . E . MacPherson 34 H. C . McCurdy 35 H. 3'. McDuffie 36. A . J . M i l l e r 37 R . L . Moore 38 H. R. Payne 39 A. M. Perry 40. H. B. P i p e r 41. B. E . Prince 42. 9.C . R o l l e r 43 A. W. Savolainen 44. D. S c o t t 45 M. J . Skinner 46. I. Spiewak 47. R . C . S t e f f y 48. J . R. Tallackson 49 D. 13. Trauger 9

R . E . Thoma

W. C , U l r i c h M. E . Whatley G. D. Wnitman

52.

53. 54-55. 56-57 58-60. 61.

C e n t r a l Research Library (CRL) Y - 1 2 Document Reference S e c t i o n Laboratory Records Department Laboratory Records Department (RC)

External D i s t r i b u t i o n

62-63. 64. 65.

66. 67. 68-82.

D. F. Cope, Reactor Division AEC-OR0 C . B. Deering, Reactor Division, AEC-OR0 A. Giambusso, AEC, Washington H. M. Roth, Division of Research and Development, AEC-OR0 W. L. Smalley, Reactor Division, AEC-OR0 Division of Technical Iiformation Extension (D'I'IE)