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DESIGN STUDIES OF A MOLTEN-SALT REACTOR D EM0 NSTRATI 0 N PLANT E. S. Bettis 1. G. A l e x a n d e r H. L. W a t t s

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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-TM-3832

C o n t r a c t N o . W-7405-eng-26

Reactor Division

DESIGN STUDIES OF A MOLTEN-SALT REACTOR DEMONSTRATION PLANT

E . S. B e t t i s , L . G. A l e x a n d e r , H. L. Watts

Molten-Salt R e a c t o r Program

JIJ N E 1972

N Q'1 I C E This report was prepded as an account of work sponsored by the United States Government. Neither the United States nor yhe United States Atomic Energy Commission, nor any, of their employees, nor M y of their contractors, rdcontractors, or their employees, makes any warranty: express Dr implied, or assumes any legal Libility or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights.

NOTICE This document contains information of a preliminary pature and was prepared primarily for internal use at the Oak Ridge National Laboratory. It i s subiect to revision or correction and therefore does not represent a f i n d report.

OAK RIDGE NATIONAL LABORATORY O a k R i d g e , Tennessee 37830 operated by UNION CARBIDE CORPORATION for t h e U. S. ATOMIC ENERGY COMMISSION

iii

CONTENTS Page ---------------*----------------------------------

List

of Figures

List

of Tables ---------------------~-----------------------------

Abstract

---------c----c-----3--------------------------------------

summary -------------------------c-------c-------------------------------------_------------------------------------------

Introduction

--------____-_--------------------------------

General Description Primary

System -------------_-_------------------------"----------

iv V 1 1

3 5 10

Reactor

-----------------------------------------------------

Primary

Heat Exchanger -c---c--------------------------------

PrimaryPumps

-----------------------------------------------

Secondary Circuits

------c-------c--------------------------------

Tertiary

Salt

Circuit

--------------------------------------------

Steam System -----------__-c--------------------------------------

-------------------------------------------------

Reactor Building Cell Heating

and Cooling

----_-----_-___--------------------------

Drain Tank System -----_--------------____________________--------

Off-Gas System ---------------------------------------------------

Rods ------c---c------------------------------------------

Control

Instrumentation

Maintenance

----------~-"------e-----rr------------------------. -----cc--------c---------c------------------------------

Performance

---------------c--------------------------------------

-.-.

-.A

LIST OF FIGURES a Fig. 1.

Simplified Flowsheet for 300&N(e) Molten-Salt Demonstration Reactor ---c-l---------------~~~~~~~~~~~-~~

Reactor Vessel Elevation -----c-"-----l------L-----------

Fig. 4.

Axial Reflector Myunting ------c---c----c--"----I--------

Fig. 5.

Reflector Attachment -I-c----c-----~------~~~~~~~~~~~~~~~

Fig. 6.

Reactor Core and Reflector

Fig.

Fig. 3.

Plan -------------------------

Fig. 7. Fig. 8.

Core Peripheral Cell Plan -0-0-0-0-0-0-c--0-0------------

Fig. 9.

Bottom Graphite Grid ----c-e-----c------3----------------

Fig. 10.

Graphite Collar Le-------C---~-------~~~~~~~~~~~~~~~~~~~

Fig. 11.

Core Orificing

Fig. 12.

Control Rod Cell --L"~-r-r--r-------------------3r------

Fig. 13.

Primary Heat Exchanger -c--1--c------3------~~~"~~~~~~~~

Fig. 14.

Primary Heat Exchanger Head Closure Detail -------------

and

Tie

Plates -"c---c---"L-------cc-----

Fig. 15. Reactor Building Elevation "cc-t-------3---3-----------Fig. 16.

Reactor Building Plan -----c-----c--~-3----~~~~~~~~~~~~~

40 41

Fig. 17. Reactor Cell and Heat Exchanger Cell Elevation ---------

Fig. 18. Drain Tank Cell mevation --"---~~--"-~""-L----ec------e

Fig. lg.

Primary Heat Exchanger Support

-------------------------

Mg. 20. Drain Tank Plan -------LLI-"-"--I------------------C--3-

Fig. 21. Drain Tank Elevation rr-rrr~-i~---------r-~~~~~~~-

Fig. 22.

Drain Tank Heat Sink I~~-----c"----"-l"--"~~~"~~~~~~"~~~

fig.

Drain Tank Jet PumpAssembly ----~--~----------c--~~~~~~

24.

vi rLd

Fig. 25. 26.

Fig.

Processing and Storage T@k,Plan -----------------------

pocessing and Storage Tank Elevation ------------------

LIST OF TABIJZS Table,l.

Ax$al Reflector Hydrau$iq.Data --------T----------------

Table .2.

Table ,3.

Physical Properties of the Fuel and Coolant -0-0-0-0-1--0-0-0-30-----------Salts Used in the MSDR-'

MSDRSecondary Heat Exchanger Design Data --------------

Table 4.

Primary Heat Exchanger Design, Data -------,---------

Table 5.' &eam'Gen&ating Data -------------------------I-------'II .,.. Table 6. Heater Design Data f&I&DR Containment Cells -i ----- .i, Table 7.

Cell Cooling System -----------------c---~~~~~~~~~~~~~~~

46 47 1

Table 8.." DumpTank Heai Sink Data ---------------------~~~~~~~~~~

Table 9. ',Lifetime Averaged Performance of a 750~MW(S) Molten-Salt Demonstration Reactor -------------------c-

â&#x20AC;&#x2DC;.,

-4 &

DESIGN STUDIES OF A MOLTEN-SALT REACTOR

DEMONSTRATION PLANT E: S. Bettis, 2 . G, Alexander, H. L. Watts

ABSTRACT The MSDR, a 350-MW(e) Molten-Salt Reactor Demonstration Reactor, i s based on technology much of which was demons t r a t e d by the MSRE. The cylindrical vessel (26 f t diam by 26 f't high) houses a matrix of graphite slabs forming s a l t passages having 8 volume fraction i n the core of lo$. The flow of f u e l s a l t i s distributed s o t h a t the temperature r i s e along any p a t h : i s t h e same from 1050 t o 1250'F. I n t h e primary exchanger, heat i s transferred t o barren c a r r i e r s a l t ( i n g t WOOF, out a t l l W 째 F ) , I n the secondary exchanger, heat i s transferred t o a stream of Hitec s a l t ( i n a t 800"F, out a t 1000'F). The Hitec oxidizes tritium t o t r i t i a t e d water which i s removed and disposed o f , The Hitec generates steam a t WOOF, 2400 p s i i n a b o i l e r , superheater, and reheater. E l e c t r i c i t y i s produced a t an overall efficiency of 36.e. Soluble f i s s i o n products a r e removed by discarding t h e c a r r i e r s a l t every 8 years a f t e r recovery of the, uranium by fluorinat$on. Volatile f i s s i o n products a r e removed by sparging the f u e l s a l t with helium bubbles i n t h e reactor primary system. The f u e l cycle cost was estimated t o 0.7 mill/kWhr f o r inventory, 0.3 mill/kWhr Tor replacement, and 0 .I mill/kWhr f o r processing, giving a t o t a l of 1.1mills/kWhr.

The purpose o f t h i s study was t o describe a semi-commercial-scale molten-salt reactor and power p l t h a t would be based on the technology, much of which was demonstrated by t h e Molten-Salt Reactor Experiment. The p l a a t was desagned t o produce 350 electriaalmegawatts. The nuclear conversion r a t i o i s about O,9, and the s p e c i f i c power i s about 0.5 MW(e) per kilogram f i s s i l e . The reactor consists of a c y l i n d r i c a l vessel about 26 f t i n diameter and about

f t high f i l l e d w i t h a matrix of graphite slabs forming f l o w

passages 0.142 i n . t h i c k by 9-3/8 i n . wide and with a volume f r a c t i o n i n

The reflectors consist of graphite slabs cooled the core of about 1 the core and reflectors is by a small flow of temperature rise of the fuel, with regulated by orifices so minor exceptions, along any flow path is approximately the same. The fuel salt consists of a mixture of the fluorides of 7Ld, berylSalt leaving the reactor lium, thorium, and uranium (initially '"U). at 1250째F flows through the tubeside of a shell-and-tube exchanger where heat is transferred to a secondary salt stream composed of barren carrier salt. The fuel salt exits at 10 F and is recirculated to the,reactor. Secondary salt enters the exchanger at gOO째F and leaves at 1100'F. It flows to a secondary exchanger, also shell-and-tube,where the heat is transferred to a tertiary salt stream, a mixture of KN03, NaN03, and N d O a known as Hitec. The purpose of the Hitec loop is to trap tritium formed in the reactor and which diff'uses through the exchangers in the direction of the steam system. .Tritiumis oxidized by the Hitec to tritiated water, which is readily removed for safe disposal. The Hitec enters the exchanger at 8000~ and leaves at 1OOOOF. The heat exchangers are arranged so that, after the removal of shield plugs, the heads may be removed and leaky tubes may be plugged off by remotely manipulated equipment. Heat is transferred from the Hitec to water in the steam generator which consists of a boiler, a superheater, and a reheater, all shelland-tube types. Steam at W 0 F and 2400 psi is produced which, after being expanded through high, intermediate, and low-pressure turbines, generates electricity with an overall efficiency of 3 6 . q . In the event that there is an interruption in the generation of power, the reactor is drained through a freeze valve into a drain tank provided with an NaK cooling system. The NaK system dumps heat to a free-flowing water stream by thermal convection. Hence, the system is reliable even when all power fails. Xenon and other noble gases are removed from the fuel stream by sparging it with helium in a bubble generator located in a bypass loop from pump discharge to pump inlet. After contacting the salt to absorb noble gases, the bubbles are removed in a centrif'ugal gas separator. Following a holdup of about 6 hours in the drain tank, the gases pass

b ?

through a p a r t i c l e t r a p for removal of solids.

recycled t o t h e bubble generator.

About half the gas i s The other h a l f i s routed t o a cleanup

system consisting of charcoal beds where t h e effective holdup time i s about 90 days, allowing f o r almost complete decay of radioactivity. The effluent i s recycled t o pump shaft seals and other purge points. Removal of f i s s i o n products from t h e f i e 1 stream i s effected by discarding c a r r i e r salt every 8 years a f t e r fluorination t o recover uranium.

The spent salt i s stored for future recovery when a complete

molten-salt processing plant i s available, Although t h e main control of t h e reactor consists i n adjusting t h e concentration of f i s s i l e uranium i n the f'uel salt, auxiliary control i s achieved by means of 6 cruciform control rods loaded with B4C and clad with Hastelloy N, Fuel Cycle Costs f

Inventory Fis s i l e s Salt Replacement Fi s s i l e s

Mills/kWhr 0.62 0.07

0.18 0.13

Salt

1.o

Total

0.1 -

Processing (estimated) Total f i e 1 cycle cost

1.1

INTRODUCTION

j e c t i v e of t h e Molt

S a l t Reactor Program a t ORNL i s

t o develop a high-performance thermal breeder reactor t h a t u t i l i z e s a molten salt f'uel and breeds on the t h o r i u ~ ~ ' ~f'uel ~ U cycle. Conceptual designs f o r such reactors have been studied f o r several years. A r e f erence design f o r a lOOQ-MW(e) plant and the uncertainties t h a t must be

resolved t o achieve a commercial thermal breeder plant were described

4 eport Om-4541 and *inNuclear Applications and Technology.'

It Reactor Eqeriment a 7.5-MW(t) reactor was operated from December 1964 to December 1969 to demonstrate the feasibility and investigate some aspects he chemistw', engineering, and operation of molten-salt reactors. Although successful operation of the MSRF, was a notable achievement, the power density wgs low, the heat was rejected to air, and the experiment lacked many other complexities breeder plant. The next step in the Program plan for developing the breeder is the construction of a Molten-Salt Breeder Experiment ould be a l5O-MW(t) reactor that would have all the technical features of a high-performance breeder. "he maximum temperature (1300째F) and peak power density (114W/cc) would be as high or higher than in the reference breeder design. Supercritical steam would be generated in the reactor steam supply system, and the plant would have the fuel reprocessing facilities required for a breeder. The purpose of the MSBE would be to demonstrate on an intermediate scale the solutions to all the technical problems of a high-performance Molten-Salt Breeder Reactor (MSBR) An alternative approach to the development of a commercial MSBR has also evoked interest. This approach emphasizes more rapid attainment of commercial size but more gradual attainment of high performance. The step beyond the MSRE is construction of a 3OO-MW(e) Molten-Salt Demonstration Reactor (MSDR). The purpose of the MSDR would be to

'Molten-Salt Reactor Program Staff, Roy C. Robertson, ed., Design Study of a Single-Fluid Molten-Salt Breeder Reactor, (June 1971).

aE. S. Bettis and Roy C. Robertson, "The Design and Performance Features of a Single-Fluid Molten-Salt Breeder Reactor," Nucl. Appl. Tech., 8, 190 (1970). 'P. N. Haubenreich and J. R. Engel, "Experience with the Molten-Salt Reactor Experiment," Nucl. Appl. Tech., 8, 118 (1970). 4J. R. McWherter, Molten Salt Breeder Qeriment TM-3177 (Nov. 1970).

Design Bases, ORNL-

L t

h( I

demonstrate the molten-salt reactor concept on a semi-commercial scale while requiring l i t t l e development of basic technology beyond t h a t demons t r a t e d i n the MSRE. The objective of t h e study reported here was t o prepare a conceptual design of an MSDR plant. The overall engineering design of the plant and the d e t a i l s of some aspects of the design a r e described i n t h i s report. Basic information on chemistry, materials, neutron physics, and f u e l reprocessing was reported recently i n ORNL-4541 and i s not repeated here. The problem t o which t h i s study was addressed concerns design of a first-of-a-kind reactor p l a n t which could be b u i l t with a minimum of development and from which higher performance breeder plants could evolve, Concepts which could not be used i n future breeder plants were t o be avoided. Two major simplifications were made i n the design of t h i s demons t r a t i o n p l a n t as compared t o the design of the breeaer plants.

First,

t h e MSDR has only such chemical processing as was demonstrated i n t h e MSRE and has no provision f o r removing f i s s i o n product poisons on a short time cycle. This r e s u l t s i n a much l e s s complicated chemical processing plant, although it means t h a t the reactor has a breeding r a t i o l e s s than one and i s therefore a converter. The second major simplification i s t h a t t h e power density was made low enough f o r the graphite core t o l a s t the 30-year design lifetime of the plant, thus simplifying the reactor vessel and eliminating the equipment f o r replacing the core. Other areas of design were a l s o simplified by t h e very low power density of t h e reactor as w i l l be noted t h e description of t h e plant t h a t follows. We believe t h a t t h e des described here represents a molten-salt reactor p l a n t whi amount of e l e c t r i

i s feasible t

power, and w family of breeder reactors.

-. t

i l d , w i l l produce a significant be a major step toward a useful

GENERAL DESCRIPTION

This plant i s designed t o produce 750 thermal megawatts i n a singlef l u i d molten-salt reactor.

Thorium in the primary, or fuel, salt i s

converted t o f i s s i l e uranium. Because of the simplified salt processing, the conversion r a t i o i s of the order of 0.9. The heat generated i n the

6 reactor is transported to the primary heat exchangers as the salt is circulated through the reactor and heat exchangers by the primary pumps. Figure 1 is a s-lified flow diagram of the system. In the primary heat exchangers the primary salt gives up its heat to the secondary salt which is circulated between the primary and secondary heat exchangers by the secondary salt pumps. "he secondary salt has the same composition (7LiF-BeFa) as the primary carrier salt. Since it contains no fissile or fertile material, it is very much less radioactive than the primary salt. The secondary salt is cooled in the secondary heat exchanger by a third molten salt which circulates between the secondary heat exchangers and the steam generating equipment. This third salt is a eutectic mix-, ture of nitrite and nitrate salts (KNOa-NaNOa-NaN03). "he primary purpose of this third salt loop is to capture tritium which is generated in the primary salt and diffuses through the heat exchange surfaces of the primary and secondary systems and into the third salt system. It would migrate into the steam system if this third salt, having oxygen available to tie up the tritium, were not present. The nitrite-nitrate salt cannot be used as a secondary coolant for reasons that will be discussed later. The steam system is conventional. It has high-, intermediate-, and low-pressure turbines, coupled to a generator, which take steam from the steam generator-superheaters, and reheaters at a temperature of 900째F. The high-pressure turbine throttle pressure is 2400 psia. The boilers and superheaters are of the once-through type with recirculation of water through the boiler for flexibility in control. The condenser, deaerator, water treatment, and feedwater heater chains are conventional and will not be described. The steam conditions were chosen somewhat arbitrar as being suitable for a first-of-a-kind plant. One of the essential auxiliary systems for a molten-salt reactor is the cover-gas system for the primary salt circuit. Since salt must be kept free from owgen, an inert atmosphere (helium) must be maintained -in the gas space associated with the f'uel-salt system. Many of the fission products are bolatile and these highly radioactive gases must be cooled, stored, contained, and safely disposed of by either radioactive

L d

8 decay o r p e m n e n t storage.

This off-gas system, which w i l l be described

i n detail l a t e r , must have a guaranteed heat removal system, f i s s i o n product absorber system, pressure regulating system, and means of separ a t i n g l i q u i d ( s a l t ) entrainment. The off-gas system i s quite involved and must have an extremely high degree of dependability that makes a c e r t a i n mount of redundancy necessary.

. c

A second auxiliary system which i s extremely important i s the a f t e r -

heat removal system. Because t h e f u e l can be drained fr salt reactor and because of the low power densLty,there i s no need f o r an emergency core cooling system, but the drain tank f o r t h e primary salt must have a cooling system t h a t i s positive and independent of t h e

power supply or operating machinery, i f possible.

The afterheat,removal system i s thus one of the e s s e n t i a l auxiliary systems.

The fact that there a r e no s o l i d f u e l elements t o be fabricated, loaded, reprocessed, and refabricated makes the molten-salt reactor unique. Many advantages accrue from this f a c t , but it a l s o sary an on-site chemical processing plant f o r maintaining t a clean and operating condition. This chemical processing plant i s another auxiliary system which i s e s s e n t i a l t o t h e p l a n t .

When a moltens a l t reactor i s t o be used as a breeder, the c h e d c a l processing system

becomes r e l a t i v e l y involved.

I n addition t o keeping the salt clean and

low i n oxygen, and adjusting the uranium inventory, the f i s s i o n product poisons must be removed from t h e primary system on a f a i r l y short time cycle. If, on t h e other hand, the plant i s designed only as a converter: having a breeding r a t i o of l e s s than unity, t h e f i s s i o n products can be removed on a long time cycle and the chemical plant becomes much more simple. Even i n a converter reactor it i s uneconomicalto allow t h e f i s s i o n product poisons t o remain i n t h e primary salt f o r t h e l i f e of the plant. The f u e l c a r r i e r s a l t w i t h the f i s s i o n product poisons i s therefore d i s carded a f t e r about 8 years of operation. Provision must be made f o r recovering t h e f i s s i l e material and f o r disposing of t h e radioactive c a r r i e r s a l t . Such a chemical processing plant i s an i n t e g r a l p a r t of t h i s reactor power plant design. The processes involved are those which were used successfully t o process t h e s a l t i n t h e MSRE.

--I

9 i

f 5

: r

A nuclear power plant requires other auxiliary systems such as control, instrumentation, and safety systems. Basic and inherent safety features of. the molten-salt reactor permit the safety system to be considerably less complicated than it is for somenuclear reactors. Because of the necessity to bring the reactor to a temperature above the melting point of the molten salt before the system can be loaded, a startup heating system is required, The MSDRplant is housed in two buildings, the reactor building and the turbine building. Since the latter-is a conventional mill-type structure,.little effort was put into designing a building for this part of the plant. Considerable attention was given to design of the reactor building. It 'consists pf a cylindrical shell with a hemispherical top. Contain. ment integrity is provided by a l/2-in.-thick steel liner, completely surrounded by concrete. The .concrete is for biological shielding as well,as providing strength for resisting tornadic winds and missiles, in accordance with the accepted standards for design of reactor buildings. One unique feature of the reactor building shown in this report is that it is supported by a large circumferential concrete ring, with about one-third of the structure hanging from this ring below grade and two-thirds of the build$ng,extending above/the support ring. It was felt that this building configuration would have better seismic resistance than one which totally extended above the support. Obviously this feature is not mandatory and the design of .,a particular reactor building would depend on the topography, geology, and seismology of theâ&#x20AC;&#x2DC;site. The cylindrical part of the,building contains all the radioactive . systems and components, such as the reactor; primary and secondary heat exchangers, off-gas system, chemical processing plant, and/ primary drain tank. '- The cells for this equipment have a sealed steel containment in , 1 addition to-the building containment. On'the thick con&e ring, but : * ~' outside the building containment, are located the steam generator cells; E the control room, and water-cooled heat sinks that provide for afterheat . ; .removal. '

10 â&#x20AC;&#x2DC;\

.,PRll$RY SYgTEM.

.- _

The reactori. is one of the &.&lest c.&onents in the entire plant due to the basic simplicity-of the circulating fuel concept. In'previous _ molten-salt concepts the reactor has not been fullydescribed. For this reason the mechanical design of the reactor- is given a more thorough Other parts of the plant have received a'much treatment ,in this report. '. ,. more cursory treatment since have been more fully discussed . these '_ items .~ . transfer that takes in previous design studies. There is little~heat the fissioh heat being transported out-' place within the reactor itself, side the The only heat' '. reactor vessel by the circulating fuel salt. transferred within the reactor vessel is that produced by g& and , By making 'the neutron :. _.heating of the gr'aphite: and the 'vessel-walls. .. I power density of the reactor low, the-graphite will not undergo exces.: sive radiation damageduring the life of the plant. Hence, the reactor tank can be an all-welded vessel. Also, since the vapor pressure of " the fuel salt is low even at high operating temperatures, the reactor need not operate at a high pressure and the walls can be relatively thin. The temperature coefficient of expansion of .graphite is only about one-third that of the Hastellgr of which the vessel is made, however, and this requires somedesign ingenuity to.prevent the differential expansion from causing problems when the reactor is brought to temperature. The reactor consists of a cylindrical vessel, with di'shed heads, which is filled with a matrix of graphite slabs that form flow passages for'the fuel salt. The inner surface of the vessel is lined with'an average thickness of 2-l/2 ft of graphite as a reflector. This_1reflector ,: conservesr neutrons by reducing leakage to a very low value. and thereby._ also re-duces the amount of radiation reaching the vessel wall. The.._.. reflector . , as:is attached to the vessel so that it moves with the vessel they expand during heatup. The internal structure of the reactor is madeup of the core matrix, the axial and radial graphite reflectors, and two internal metal dished heads which locate and support the graphite of the axial reflectors.

Figure 2

an elevation of the reactor vessel showing the i n t e r n a l

graphite structure.

Figure 3 i s a plan view of t h e reactor vessel.

It i s important t o maiptain controlled flow passages i n the reactor c

vessel and t o regulate t h e flow so as t o get a uniform temperature r i s e i n a l l flow passages a s t h e s a l t flows through the reactor. Since t h e power distribution i s non-uniform, t h i s requires d i f f e r e n t v e l o c i t i e s of s a l t i n the various flow passages. It i s a l s o desirable t o minimize cross flow and t o have a x i a l flow from bottom t o top of t h e reactor core. The flow i n the r e f l e c t o r and along the vessel wall should be straight through from bottom t o top, The graphite should be mounted within the core i n a manner t h a t w i l l preserve t h e geometry of the flow passages, y e t , a t the same time,

accommodate the dimensional changes due t o temperature differences and radiation damage i n t h e graphite. The f a c t o r of about 3 x 10" in./in./"F difference i n thermal expansion between graphite and Hastelloy must a l s o be taken care of i n the design. A further limitation i s t h a t graphite, of the required grade, cannot be made i n large monolithic blocks. The r e f l e c t o r should have a minimum salt volume i n it i n order t o keep t h e f i s s i o n r a t e low.- Ideally, only such salt a s i s necessary t o cool the r e f l e c t o r should be present, This makes it desirable t o have a few large pieces of graphite i n the r e f l e c t o r r a t h e r than many small

pieces,

For t h i s reason t h e graphite of t h e r e f l e c t o r i s laminated of

smaller pieces cemented together, baked, and machined t o shape a f t e r baking. The low flux encountered i n the r e f l e c t o r permits t h i s fabrication technique t o be used. By t h i s lamination procedure, blocks of the correct s i z e and shape for the axial and r a d i a l r e f l e c t o r sections a r e made up from graphite slabs approximately 2 i n . thick by 12 i n . wide and from 4 f t t o 21 ft i n length. I n order t o control t h e unavoidable gaps t h a t r e s u l t from the expansion of t h e vessel r e l a t i v e t o t h e graphite, both radial and a x i a l r e f l e c t o r blocks a r e attached t o the vessel and move with it while t h e 5

core matrix remains stationary. The method of accomplishing t h i s w i l l become evident i n the description of t h e r e f l e c t o r s that follows. Modified dished heads having a f l a t center area and an overall s i z e s l i g h t l y smaller than the vessel heads are used t o hold t h e graphite

12 _-

L, ORNL DWG 72-2829

-I?=.Fig. 2.

Reactor Vessel Elevation.

I f

14 of the axial r e f l e c t o r s . Tap and bottom heads a r e exactly a l i k e and have r i b s welded on t h e i r convex surfaces t o form bearing surfaces t h a t f i t against the reactor vessel heads. The concave surfaces of these

b .

heads a r e machined t o form concentric rings having f l a t "lands" o r surfaces about I 2 i n . wide t o which graphite sectors a r e attached. The

graphite sectors, having radii corresponding t o t h e radius of t h e ring t o which they fasten, a r e mounted on these heads w i t h gaps of varying widths between t h e concentric rings formed by t h e graphite. Spaces equal t o the gap width between rings a r e a l s o l e f t between t h e ends of adjacent graphite sectors i n each ring as they a r e assembled on the head. The method of mounting t h e ' sectore on the heads i s sh.&n i n Fig. 4. Three holes a r e d r i l l e d i n each graphite sector from t h e center of the surface that i s t o be i n contact w i t h the head.

One i s a t the center

of t h e sector and one i s near t h e end, Into t,hese holes a r e placed H a s t e l l o y plugs 2 i n . i n diameter by 4 i n l long. These plugs a r e a r e d r i l l e d and tapped a x i a l l y f o r 3/4-in. cap screws They a l s o have

3/4-in,-diam transverse holes a t a distance of 3 i n . f r o m t h e tapped end. The graphite sectors have horizontal ho1e.s so that 3/4-in,-diam pins inserted through those holes w i l l pass through t h e transverse holes i n the metal plugs and f i x them i n t h e graphite. Holes i n the heads permit the graphite sectors t o be fixed t o t h e heads by cap screws that engage t h e metal plugs i n t h e graphite.

The holes f o r the screws t h a t

engage the plugs i n the ends o f t h e graphite sectors a r e s l i g h t l y overs i z e t o permit d i f f e r e n t i a l expansion between t h e graphite and t h e metal head. The widths of the s l o t s between t h e concentric rings of r e f l e c t o r were computed t o provide salt flow t o the core i n accordance w i t h t h e radial power distribution i n the core. I n making these computations,

5 p s i . The AP across t h e holes i n t h e head under the s l o t s was also assumed t o be 5 p s i and t h e E across t h e r e f l e c t o r was assumed t o be

the number of holes required t o Arrnish t h e required flow was calculated with t h e r e s u l t s shown i n Table 1.

After the graphite has been i n s t a l l e d i n t h e r e f l e c t o r head, t h e complete bottom assembly i s lowered i n t o t h e reactor vessel.

Working

through the bottom hole i n the vessel, the center gussets of the

- *

16 Table 1. Axial Reflector Hydraulic Data. Rings from Center

Slot Width (in.)

4 5 6

(fP4

0.14 0.22

5 *1 6.6

0.25

7.1 6 *9

0.24 0.24 0.22

Velocity

7 6 *9 6.8 0

Number of 1-In. Holes per Ring

18 36 51 63 71 76

0.20

0.16

6.2 6.2

11 12

0 .ll 0.07

5.4 4.6

13 14

0.03

1.2

0.02

0 07

8 9

0.16

63 43 28

Substitute many s m a l l holes f o r equivalent area.

r e f l e c t o r head a r e welded t o matching overlapping gussets i n t h e vessel head, r i g i d l y attaching the r e f l e c t o r head t o t h e vessel a t t h e center. The periphery of t h e r e f l e c t o r head has 8 s l o t s which permit it t o be lowered i n t o t h e vessel, the s l o t s sliding over the 8 v e r t i c a l r i b s on t h e cylindrical w a l l .

After having been lowered i n t o position the

r e f l e c t o r head i s pinned a t these 8 points, tying the head t o the vessel but permitting r a d i a l expansion as the pins move i n r a d i a l s l o t s i n the ribs.

Figure 5 shows t h i s method of attachment. The radial r e f l e c t o r i s made up of laminated blocks of two d i f f e r e n t

shapes, plus some odd-shaped f i l l e r blocks used t o wedge t h e r e f l e c t o r t o the vessel w a l l .

The outer r e f l e c t o r i s made up of laminated wedges

about 10 f t long and about 3 f t high. .These wedges a r e machined t o f i t t h e vessel w a l l a t the 8 places where they a r e i n s t a l l e d around the inside vessel perimeter. By f i t t i n g each wedge approximately 1/16 i n .

ORNL DWG 72-3580

18 from the wall, any out-of-roundness

of the vessel i s accommodated.

When

a l l pieces have been put i n t o place, a retaining "T" i r o n i s fixed t o

t h e v e r t i c a l metal r i b s and wedging pieces of graphite are inserted behind t h e "T" iron. Thus t h e outer r e f l e c t o r i s made t o move out with t h e vessel w a l l a s it expands. Figure 6 i s a plan view of the' core

w h i c h shows the r e f l e c t o r graphite as well as the reactor core matrix.

The inner layer of r e f l e c t o r i s made up of columns of graphite approximately 1 ft wide by 2 f t t h i c k by 21 ft long. These blocks a r e a l s o laminated from slabs similar t o those used elsewhere i n t h e core. These v e r t i c a l blocks a r e doweled i n t o t h e bottom axial r e f l e c t o r pieces, but t h e dowels a r e loose enough t o permit the columns t o f l o a t up from the axial r e f l e c t o r while s t i l l being t i e d t o it radially. The columns a r e i n s t a l l e d w i t h 1/8-in. gaps between columns and t h e back wedges and between adjacent columns. Also the bottom edge of each block has a 2i n . by 1/4-in. slot machined on it to provide flow access to the Sertical s l o t should any block f a i l t o f l o a t when t h e reactor i s full of s a t . A t t h e top of these columns there a r e horizontal slabs of graphite approximately 1 in. thick by 3 in. wide by about 2-1/2 f t long. These slabs f i t i n milled s l o t s and plug t h e gaps between columns a t t h e top. They a r e doweled t o one of the columns and t o the wedge of graphite adjacent t o t h e tank w a l l i n order t o provide o r i f i c i n g and t i e the columns t o the wedges a t t h e top t o maintain the desired location of t h e columns. The slabs r e s t against t h e top r e f l e c t o r when t h e reactor i s f i l l e d w i t h salt, and they provide a 1/4-in.-thick

salt plenum over

t h e radial r e f l e c t o r graphite. The core matrix i s made up of a square array of slabs approximately l-3/4.dn. by 9-3/8 i n . i n cross section. These slabs a r e stacked i n square c e l l s approximately 11-1/2 i n . by 11-1/2 i n .

The slabs a r e sepa-

rated by dowels which provide v e r t i c a l flow passages approximately 0.142 i n . wide running from bottom t o top of the core. The plan arrangement of t h e core i s shown i n Fig. 7 and Fig. 8.

I n addition t o t h e slabs,

there a r e posts approximately 2-1/2 i n . square that define t h e corners of the c e l l s . These posts have 6-in. dowels 1-1/4 i n . i n diameter on the bottom end which f i t i n t o holes i n t h e corners of a graphite "egg crate" g r i d t h a t r e s t s on t h e bottom axial r e f l e c t o r . This g r i d defines

ORNL DWG 72-2826

Fig.

Reactor Core and Reflector Plan.

ORNL DWG 72-2830

Fig.

7. Core Cell Plan.

ttIn

i rl

PI rl

rl ai a,

PI k a, .rl

k"0 u

a3 M

;r;l

22 t h e core and prevents accumulation of radial expansion tolerances i n any region of t h e core. Figure 9 i s a d e t a i l of the g r i d . Around the top of t h e core i s a graphite "collar" which prevents spreading of t h e v e r t i c a l core pieces i n t o the annulus between the core and r e f l e c t o r . This annulus, approximately 0.75 i n . Wide, r e s u l t s from the d i f f e r e n t i a l expansion between the metal vessel and t h e graphite.

The c o l l a r a l s o blocks t h i s annulus t o prevent excessive flow i n this path of low pressure drop. Flow f r o m t h e annulus has t o cross through the radial s l o t s i n the r e f l e c t o r and out the e x i t s l o t s above t h e wedge r e f l e c t o r pieces. Figure 10 i s a d e t a i l of t h i s c o l l a r structure. The graphite core i s assembled i n t h e vessel and then o r i f i c e p l a t e s a r e put on top of t h e core t o make t h e core flow match t h e radial power distribution. These f l a t p l a t e s a r e approximately 11-1/2 i n . by 1 1 4 2 i n . by 1-1/2 in., with undercut edges t o f i t down i n t o t h e c e l l s formed by t h e v e r t i c a l posts. These plates have no intentional gaps and are f i t t e d as closely as tolerances permit. Resting on top of the c e l l side p l a t e s , they leave a 1/8-in. gap between t h e i r underside and t h e top of t h e f i v e slabs making up t h e c e l l structure. Short dowels on the underside of the p l a t e s prevent the slabs from f l o a t i n g up and closing t h i s gap. I n the center of each p l a t e i s an o r i f i c e . The diameter of this hole i s 2.05 i n . f o r t h e center c e l l s and 0.74 i n . for the peripheral c e l l s , w i t h intermediate c e l l s having holes of diameter proportional t o the radial power distribution. These cover p l a t e s , i n addition t o providing o r i f i c i n g f o r t h e d i f f e r e n t c e l l s , perform an additional function of tying t h e core together a t the top and preventing accumulation of expansion opening i n one location. On one side of each p l a t e i s a 3/4-in. dowel extending 1-1/2 i n . above t h e top of t h e p l a t e . After these o r i f i c e p l a t e s have been placed on top of the core, rectangular t i e blocks 1-1/2 i n . t h i c k ,

3 i n . Wide, and 6 i n . long, with two holes i n them, a r e placed over t h e dowels of adjacent p l a t e s t o t i e the c e l l s together.

A t t h e same time,

t h e t i e s a r e not r i g i d and w i l l allow some movement of t h e core i f needed.

These l i n k s cannot float free of t h e dowels even i f some c e l l

or c e l l s s t i c k and do not f l o a t t o t h e underside of t h e top axial r e - ' flector.

Figure 11 shows the o r i f i c i n g and tying of the graphite.

23 OWL DWG 72-3579

I -

'+\

-4 I

-7

. -

Fig. 9.

Bottom Graphite Grid.

._.--

ORNL DWG 72-2828

Fig. 10.

Graphite Collar.

26 Six c e l l s i n t h e core a r e d i f f e r e n t from t h e other 375 c e l l s .

These

contain the sockets i n t o which the s i x control rods are inserted. They are made up of s o l i d square prisms, laminated out of t h e core slabs, and bored and machined according t o t h e design shown i n Fig. 12. These blocks

a r e made i n sections a s long a s p r a c t i c a l t o machine, and t h e sections are then joined t o make a continuous cylindrical hole i n t o which each control rod i s inserted. The holes a r e 7 i n . i n diameter and have a 1/2 i n . o r i f i c e i n the bottom t o permit proper flow of salt t o the channel. The control rods a r e cruciform i n shape and 6-1/2 i n . wide. This provides a @-in.

r a d i a l clearance between t h e rod and the hole i n t o

which each i s inserted.

The rods are made of 1/8-in. Hastelloy N en-

closing a 1/2-in. f i l l e r of boron carbide,

When f'ully inserted, t h e rods extend t o within 3 f t of t h e bottom of t h e core. When f'ully withdrawn, t h e bottoms of t h e rods are j u s t within t h e t o p r e f l e c t o r . Pipe extensions around the control rod extensions penetrate t h e top of the reactor c e l l shielding and the control rod drive mechanisms a r e located outside the reactor c e l l on top of t h e shielding. The rods w i l l have t o be replaced periodically as t h e high f l u x w i l l destroy

t h e d u c t i l i t y of t h e Hastelloy. One rod i s s u f f i c i e n t t o shut down t h e reactor, and only one rod w i l l be used a t any one time f o r control of t h e reactor. Primary Heat Exchanger .

Primary salt i s pumped through the three primary loops by three

salt pumps.

Each pump discharges s a l t i n t o two primary heat exchangers i n p a r a l l e l . There a r e two heat exchangers i n each l e g i n order t o make Each exchanger i s about 26 i n . i n diameter and i s i n the form of a "U" with a tube length of about 30 f t . Each has 1368 tubes of 0.375 i n . OD with a wall thickness

the fabrication of these u n i t s more p r a c t i c a l .

of 0.035 i n .

Table 2 gives t h e data on the primary heat exchangers.

Table 3 gives the physical properties of the s a l t on which t h e design of the heat exchangers i s based. Figure 13 i s a view of t h e heat exchaker and Fig, 14 shows t h e d e t a i l of the head closure.

.qcs

28 Table 2. MSDR Primary Heat Exchanger Design Data

U-tube, U-shell, countercurrent, one-pass shell-andtubes with disk-and-doughnut baffles Rate of heat transfer per unit Mw

‘

Btu/hr Tube-side conditions Hot fluid Entrance temperature, OF Exit temperature, OF Pressure drop across exchanger, psi Mass flow rate, lb/hr Shell-side conditions Cold fluid Entrance temperature, OF Exit temperature, OF Pressure drop across exchanger, psi Mass flow rate, lb/hr Tube material Tube OD, in. Tube thickness, in. Tubesheet-tb-tabesheetdistance, ft Shell material Shell thickness, in. Shell ID, in. Tubesheet ‘material Number of tubes Pitch of tubes, in. Total heat transfer area, fta Basis for area calculation

Type of baffle Number of baffles Baffle spacing, in. Disk OD, in. Doughnut ID, in. Overall heat transfer coefficient, U, Btu hr” ft’a Volume of fuel salt in tubes, fta

125

4.3 x lo8 Fuel salt 1250 1050 127 6.6 x lo6 Coolant salt (2 LiF-BeFa) 900 1100 115 3.7 x lo= Hastelloy N

375 0 0035 30 0

Hastelloy N 0 05

26.32 Hastelloy N

1368 o .672 (triangular) 4024 Outside of tubes Disk and doughnut

47 7.7 19.0 18.6 700 20.8

Table 3.

Physical Properties of t h e Fuel and Coolant S a l t s Used i n t h e MSDR

Fuel Salt

Composition

LiF-BeFa -ThF4 -UF4 (71.5-16.0-12 .O-0.5 mole $)

Density, l b / f t 3

236.3

Viscosity, lb hr-'

ft-'

Specific heat, B t u lb-' I-),'( Thermal conductivity, Btu hr-'

ft-'

(OF>-'

- 2.33

0*2637 0.32 0 -75

x 10'aT (OF) 7362 459.7 + T (OF)

LiF-BeFa Coolant Salt

Density, l b / f t 3 Viscosity, l b hr-'

(66-34 mole 5 ) (99.99 + % 'Li) 138.68 - 1.456 X 10-aT 6759 0.2806 exp 459.7 +T 0.57 0.58 'LiF-BeFz

Composition

( O F )

ft-'

( O F )

Specific heat, Btu lb-'

(OF)-' Thermal conductivity, B t u hr-' ft-' (OF)-'

Nitrate-Nitrate Coolant Salt

Composition (eutectic) Density , l b / f t 3 Viscosity, l b hr-'

KNG-NaNOa-

Specific heat, Btu lb-'

(OF)-'

Thermal conductivity, Btu hr-'

ft-' (OF)-'

(44-49-7 mole %)

x 10-'T ( O F ) 0.1942 e q 4 3821.6 59.7 + T (OF) 0.37 0 -33

130.6

f't-'

- 2.54

NaNOa

ORNL DWG 71-5032

Fig. 13.

Primary Heat Exchanger.

Fig, 14.

Primary Heat Exchanger Head Closure Detail.

The heat exchangers a r e mounted horizontally, one l e g of t h e "U" being under t h e other.

The tubesheet ends of the exchanger a r e access-

i b l e through a hole i n t h e c e l l w a l l by removing a shield plug from the hole. The head i s flanged and bolted with metal "0"-ring s e a l s backed up by a t h i n s e a l weld. This permits r e l a t i v e l y easy removal of t h e heads for plugging a tube i n the event o f a leak. Primary PumpS Sump-type centrifugal pumps f o r c i r c u l a t i n g molten salts have been employed f o r many years i n experimental r i g s and have been used successfully i n two molten-salt reactors, t h e ARE6 and the MSRE. These moltensalt pumps a r e described i n the l i t e r a t u r e , '

and t h a t material w i l l not

be repeated here.

I n the MSDR the pumps a r e mounted symmetrically on t h e top of the reactor vessel. Each pump i s submerged i n a tank that has excess gas volume t o accommodate expansion of t h e s a l t .

This volume communicates

with the primary system by a 6-,in. l i n e from each pump bowl t o the center dome'in the top of the reactor vessel. During normal operation, flow down these l i n e s returns the fountain purge flow from each pump t o the top head of t h e reactor. The pump suction l i n e communicates with the pump sump through a small leakage path i n order t o drain t h e sump when t h e primary system i s drained. Each pump has a capacity of 8100 gpm and develops, a. 150-fi head. order t o purge xenon from t h e salt, about

lo$

of the flow from each pump i s bypassed d i r e c t l y from pump o u t l e t t o i n l e t . The bypass l i n e contains a bubble generator f o r admitting gas t o t h e salt and a bubble stripper f o r removing gas. The gas s t r i p p e r removed about two volumes of salt' with each volume of gas removed, and t h i s mixed stream i s directed i n t o the drain tank.

A venturi i n t h i s l i n e i s provided with

'James A . Lane, H. G MacPherson, and Frank Maslan, eds , -.Fluid Fuel Reactors, Chapter 16, "Aircraft Reactor Experiment, I' AddisonWesley Publishing Company, Inc. (Sept 1958).

6Molten-Salt Reactor Program Staff, Roy C . Robertson, ed., Conce t u a l Design Study of a Single-Fluid Molten-Salt Breeder Reactor, ORNL- 5 1, p . 58 f f (June 1971).

. L

a 1-in. connection t o t h e gas space i n the pump bowl. This arrangement makes it possible t o mix the pump s e a l purge gas w i t h the stripped gas and t o send it t o t h e drain tank and thence i n t o the off-gas system. T h i s l i n e a l s o serves as a syphon break t o prevent draining the system through $he gas separator discharge l i n e i n t h e case of a stopped pump i n any of t h e primary c$rcuits. I n addition t o providing the primary salt flow and t h e bypass flow, a l i n e f r o m each pump discharge i s manifolded i n t o a l i n e which drives two j e t pumps i n the drain tank. Each of these l i n e s has a b a l l check valve inserted i n it upstream of the manifold. The j e t pumps i n t h e drain tank return t h e entrained l i q u i d i n the'gas separator discharge t o the primary c i r c u i t , as w i l l be described i n the discussion of the drain tank. SECONDARY CIRCUITS

Heat from each primary c i r c u i t i s transferred t o a secondary s a l t system. The salt i n each secondary loop i s circulated through the s h e l l s of the primary heat exchangers and t h e tubes of the secondary heat exchangers by a pump similar t o t h e primary pump. The secondary c i r c u i t i s designed t o contain a r e l a t i v e l y small volume of salt. The salt used i n the secondary loop i s 7LiF-BeFa (66-34 mole $). Although it would be possible t o use a d i f f e r e n t s a l t i n t h e secondary

c i r c u i t , it was believed t h a t the advantages accuring from using the 7LiF-BeFs salt j u s t i f i e s i t s use i n t h e f i r s t demonstration p l a n t . By using t h i s salt, any primary heat exchanger leak w i l l be much l e s s troublesome than $f a d i f f e r e n t secondary salt were used. Also, t h e LAP-BeFz salt w i l l remain nonradioactive, except f o r very short-lived a c t i v i t i e s , and thus maintenanae of t h e secondary heat exchangers w i l l be l e s s complicated. There are two secondary heat exchangers per seccondaryloop. These a r e "U"-tube exchangers of the same design as the primary exchangers. *

Because of t h e differences i n t h e salt heat transfer properties, they a r e s l i g h t l y l a r g e r than t h e primary exchangers,

Table 4 gives the

design data f o r these exchangers.

It may be noted that t h e secondary c i r c u i t i s o l a t e s t h e primary c i r -

c u i t from t h e r e s t of t h e system.

Because of the s e n s i t i v i t y of the

34 Table

MSDR Secondary Heat Exchanger Design Data

. -

U-tube, U-shell countercurrent, one-pass shell and tubes with disk-anddoughnut baffles ,

m e I

Rate of heat transfer per unit Mw

4.3 x lo8

Btu/hr Tube-side conditions Hot fluid Entrance temperature, OF Exit temperature, OF Pressure drop across exchanger, psi Mass flow rate, lb/hr Shelloside conditions Cold fluid Entrance temperature, OF Exit temperature, 'F Pressure drop across exchanger, psi Mass flow rate, lb/hr Tube material Tube OD, in. Tube thickness, in. Tubesheet-to-tubesheet distance, ft Shell material

900

80 3.7 x lo6

Hitec

700 1000

3.8 x iog .

Hastelluy N 0 0375 0 *035

37.5 Hastelluy N

0- 5

Shell thickness, in. Shell ID, in. Tubesheet material Number of tubes Pitch of tubes, in.

30.5 Hastelloy N

Total heat transfer.area, ft2 Basis for area calculation

m e of baffle Number of baffles Baffle spacing. in. Disk.OD, in. Doughnut ID, in.

2LiF-BeF2 Sa 1100

Overall heat transfer coefficient, U, Btu hr" f't-2 Volume of 2LiF-BeF2 salt in tubes, ft3

1604 0.7188 (triangular) 5904 Outside of tubes Disk and doughnut 52

8.6 22.o 21.6 500

30.5

LJ :

primary salt to oxygen, this is a desirable, if not necessary, feature. Also, the secondary and tertiary salt circuits provide:a double barrier between the low-pressure fuel-salt system and the relatively high-pressure steam system. 'TERTIARYSALTCIRCUIT

The primary purpose of the tertiary salt circuit is to provide a tritium trap to prevent diffusion of tritium from the primary circuit into the steam system via the circulating salt systems. The tertiary circuit uses a eutectic mixture of KNOscNaNOz-NaNOs (44.2-48.9-6.9 mole &), which has the commercial name of "Hitec." The oxygen in this salt combines with the tritium to form tritiated water, which can be recovered from the system. Although the primary function of the tertiary salt loop is to trap tritium, it also has certain advantages which offset, at least to a degree, the complication of the additional pumps and heat exchangers required for the third loop. The advantages derive chiefly from the fact that the liquidus temperature of the nitrite-nitrate eutectic is 288'~, which relieves considerably the possible problems of salt freezeup in the steam generators. An additional advantage concerns the fact that water will simply vaporize from the nitrite-nitrate salt and the consequencesof a steam leak are thus much less severe. In fact, this salt could possibly be used with steam as a cover gas. : The nitrbte-nitrate salt cannot be used as a secondary coolant because it would react with the primary ,' ..' salt to precipitate thorium and uranium oxides in the event of a primary heat . exchanger leak. : 'There is the additional danger of a reaction with the graphite of the core;sh&ld nitrite-nitrate eutectic get into the primary system, The nitrite-nitrate saltwill.remain nonradioactive, except for ,. _ : ',.I sometritium, and thus the tertiary loop can penetrate the building containment~ This permits the location of all the steam system equipment outside the containment and makes it accessible for .,.direct maintenance. It also removes the possibility of a pressure rise within the containment due to a leak in the steam system. The corrosion resistance to Hitec is a consequenceof the establishment of a passivating film on 'the containment metal. Cheaper

materialscan'be used for the piping and steam-generating equipment-of this loop than is required in'the primary and secondary systems. -,It. is intended to make the secondary exchangers out of 316. stainless steel _I because of the secondary,salt in the tubes. The shells and piping, .: however, could be made of Croloy. The tertiarypumps could also be fabricated of Croloy. ._ ,As previously mentioned, the low melting point (228°F) of the Hitec -' .' :' permits the feedwater to enter the boiler at temperatures that are in .._ Since the wall At must be held. line with current steam plant practice'. to a reasonable value (2 2OOO'F),the salt temDerature at the exit - of'the boiler must be of the order of 700°F ^. if the'feedwater enters at about heat exchanger 500'F. This 7OO'F salt cannot'be returned to the secondary .: : To avoid because the freezing point of the secondary salt ii'856°F.e : this low temperature and still maintain an 'acceptable At across the tubes : in the boiler, a bypass stream of hot tertiary sait ' is '. mixed with'the cool (700'F) salt to raise the temperature of the mixture‘to an acceptable level before returning it to the secondary heat exchanger. The total flow-in the tertiary system is about 40,500 gpm., Of this,, 27,qOO gpm flows through the steam generating equipment and is cooled to 700'F. Ihe bypass loop mixes 13,500 ~pm of 1OOO"F‘salt with'the 27,000-gpm 700'F stream and raises its temperature to 8009 before it enters the secondary heat exchanger. A throttle valve in this bypass line , permits temperature control for partial loading and transient conditions. Data on the decomposition and corrosion of Hitec are insufficient to be absolutely certain of the highest temperature that is all&able .,,. and the type of cover gasrequired over the salt. It is intended to : limit the temperature to 1000°F and to provide an Ns overpressure until more experience is gained. Indications are that a temperature *.of llOddF, is tolerable, and, if this is verified,'stesm conditions for a -'moltensalt plant could be significantly &proved'. A report, OF&L-TM-3777, ' which is in preparation, will present the knowri information on.Hitec.. .' temperature limitations. *, : L

u c T

37 STEAM SYSTEM system conditions at the turbine throttle were chosen to be 90O0F/24O0 psi with WOOF reheat, but the steam system was not specially designed or optimized for the demonstration plant. Steam flows and temperatures are shown on the simplified flowsheet of fig. 1. The steam generating equipment is, of course, quite different from that in a fossil-fired plant in that the heat is extracted from a circulating molten salt. On a ompletely arbitrary basis the steam load f o r the turbine-ge vided between 6 boiler units, 6 superheater units, and 6 r its. All these units are heated by the three salt circuits comprising the tertiary salt system of the plant. The boilers are divided into two units each simply because is results in units of a size more easily handled. The entire steam stem where both the water and salt sides of the preheater, boiler, and superheater are in series. Feedwater from the conventional feedwater chain enters the tube side of a U-tube preheater at a temperature of 480째F and a pressure of 2600 tube boiler and sses from this unit first into a s from that into the U-tube superheater. These thre its each have 100 tubes in shells about 12 in. ID. Additional data are given in Table 5 . Table 5 .

Steam Generating Data

23.8

x .IO6

At the side-stream exit of the boiler section there is a bypass through a recirculator pump t o the inlet of the preheater. This bypass is for the purpose of control for partial load operation and for such

contingencies as a breaker trip.from full load. The shell sides of all these units (preheater, boiler, superheater) are also connected in series so that the 1000°F tertiary salt flows countercurrent to the water and exits from the preheater at 700°F at design load., _ : .,. The steam generating cells are outside the reactor,containment building but are closely adjacent to,provide close coupling. The interiors ._ of the steam cells are not heated ,'like ..'the reactor cell, but. . heaters and insulation areinstalled on the piping and components. All steam and water lines are runfrom from. ,. cells : . . each piece of equipment in the ste.am The ,. manifolds, stop valves, etc., located : outside , :._ . the.steam cells. rupture discs on the tertiary salt system for relievingt steam pressure :,_ , ,'-in the event of a leak are -installed in the steam cells.in~order to contain the salt within the cell. -.~ , < ._ : The turbine-generator isshown as a tandem unit withLone high-pressure, I : .. one intermediate-pressure, and two low-pressure casings ,on ,one shaft . The specific turbine-generator has not driving a single gene,rator unit. .. and been designated. As has been stated, the usual extractions points ., I' conventional feedwater handling are employed. No effort-has been made to detail the regenerative feedwater heating system ‘.or ,other.[aspects of _,.~ the steam system. ., : REACTOR BUILDING The reactor building follows what has come to be a rather traditional Basically it is a,cylindrical reinforced concrete cqntainment structure. structure with a hemispherical dome. 'A sealed steel membrane,4/2~in. . thick; lines the entire building and provides the containment. The con' crete.provides.protection'against missiles resulting~fromtornadic winds. The building isabout 150ft tall and about 112 ft in diameter. The building is suspended'from a large concrete ring about 176 ft This arrangement in outside .^ _ gives _~1. _,a_.low ." --. diameter and about 8 ft .~thick. Although seismic analyses have not center of gravity to the building. been made,,it is believed that this method-of construction may-be more stable to a seismic disturbance. The samebasic design"of~the:building I~ could be used if subsequent analyses or considerations related to a.par-‘ titular site indicate that the building should be set on a foundation

39 a t the bottom of t h e structure.

Figures 15 and 16 show the elevation and

plan of the reactor building.

The steam c e l l s a r e appended symmetrically outside the containment and r i s e from the concrete ring. Also mounted

on the ring and external t o the containment a r e the three water tanks which provide the heat sink f o r t h e drain tank cooling system. The control room i s a l s o b u i l t on t h i s ring outside the containment building. Inside t h e reactor building a r e the c e l l s containing t h e radioactive There are 5 sealed c e l l s which have a common the reactor c e l l , the 3 heat exchanger c e l l s , and the drain

portions of the plant. atmosphere: tank c e l l .

These c e l l s have water-cooling c o i l s embedded i n the concrete w a l l s with water plena a t the ends o f the c e l l s f o r removing the heat that leaks through the thermal insulation.

l i n e d with a 1/2-in.-thick

Each c e l l i s completely

304 s t a i n l e s s s t e e l membrane which seals t h e

c e l l s from the remainder of the building. Penetrations i n t o the c e l l s have been kept t o a minimum and t h e interconnecting sleeves between c e l l s have bellows which permit d i f f e r e n t i a l movement between t h e membranes of t h e various c e l l s . These sleeves provide t h e necessary passages f o r t h e s a l t piping.

Figure 17 i s an elevation of t h e reactor c e l l and one of

the 3 heat exchanger c e l l s . drain tank c e l l .

Figure 18 i s an elevation and plan of the

The reactor i s hung from a ledge by means of a large flange near the top of t h e vessel. This i s seen i n the elevation section of the building, Fig. 15. I n the heat exchanger c e l l s a l l u n i t s a r e mounted from a superstructure rising f r o m the floor of t h e c e l l . This mounting method i s shown i n Fig. 19. The drain tank s i t s on t h e floor of t h e drain tank c e l l , t h e load being carried through a s k i r t and 8 lugs which transmit t h e load through the seal membrane and insulation t o the conc r e t e bottom of t h e c e l l .

CELL HEATING AND COOLING All

5 c e l l s a r e heated by circulating the c e l l atmosphere (mostly

nitrogen) over e l e c t r i c a l heaters

The circulation i s accomplished by

3 large blowers, each of which discharges 32,500 cfln of gas a t a temperature of 1050째F i n t o t h e reactor c e l l . The reactor c e l l has 4 gas outlets, one t o each heat exchanger c e l l and one i n t o t h e drain tank

T I ’

Fig. 15.

Reactor Building Elevation.

1’1

I ,

PHOTO 79090

Fig. 18. i

Drain Tank Cell Elevation.

m I cu

P-

---

-r .i

'0; 'io;

t. B

c e l l , Return ducts from each of these c e l l s t o t h e blower suction make a closed circulation system which i s p a r t of t h e c e l l containment. Pertinent data f o r t h i s c e l l heating system are shown i n Table 6. Heating the c e l l s by forced circulation greatly reduces t h e number of c e l l penetrations necessary when compared t o heating by space heaters i n the c e l l s . I n addition t o heating, it i s desirable t o cool the c e l l s a t times.

The data for cooling, using the same circulation system, a r e shown i n Table 7 . The circulation duct i s divided i n t o two sections, and a simple b u t t e r f l y type damper d i r e c t s the a i r through e i t h e r t h e heater section or t h e cooling section. The e l e c t r i c heaters and the reentrant tube cooling water u n i t s each f i t i n t o thimbles which a r e welded i n t o t h e top of the duct. I n each case (heating or cooling), heat t r a n s f e r i s by radiation. Neither t h e heaters nor the water piping penetrate the ducting so t h a t repairs can be made without breaking t h e containment. DRAIN TANK SYSTEM

One of t h e most important systems i n the molten-salt reactor plant i s the f u e l - s a l t drain tank with i t s cooling system and provisions for gas holdup.

(It i s somewhat analogous t o the emergency core cooling

system of a s o l i d f u e l reactor.) It provides automatic removal of the afterheat i n t h e event of a f a i l u r e of t h e main circulation system. Also during normal operation the v o l a t i l e f i s s i o n products and t h e stripping gas a r e held up for about 6 hours i n the drain tank. The decay heat from t h i s operation i s removed by t h e natural convection cooling system. The drain tank cons s i t s i n s i d e , a secondarjt t

separate tank The primary tank cible " The r a i n tank has a f l a t

top with thimbles located i n a tubes each cover t h e top

e t r i c a l pattern.

Sixty u n i t s of 6

f t h e drain tank.

Figure 20 i s a plan view showing the arrangement of these thimbles with the cooling headers i n

place. Figure 21 i s an elevation of the drain tank. In order t o s t a b i l i z e these thimbles against seismic shockc, they a r e interlocked a t t h e bottom i n such a w a y as t o allow d i f f e r e n t i a l axial movement

but no individual vibration.

The 360 thimbles, therefore, a c t together

Table 6.

Heater Design Data f o r MSDR Containment Cells

Estimated normal containment heat l o s s ( t o t a l ) , kW Design heat loss, kW Number of heater c e l l s

1200

Capacity of each heater c e l l , k W

400

Circulating gas Gas i n l e t - o u t l e t temperatures a t heaters, OF

Gas flow r a t e through each heater c e l l , cf’m

Heater element

Heater length, f t Number of heater elements per heater c e l l Heater element arrangement p e r heater c e l l Pressure drop i n c i r c u l a t i n g gas, in. &O Heater c e l l width and depth, f t

100~1100 32,500 (at 1050OF) 1-in.-OD cartridge with Incoloy 800 cladding

3 150 12 rows of 12 or 13 elements on 3-in. A p i t c h

9 06 3.25 x 2.6

Thermal conductivities f o r c e l l wall (k) , Btu hr-’ ft-’ ( OF)” 1/2-in. s t a i n l e s s s t e e l c e l l l i n e r 5-in.-thick f i b e r g l a s s c e l l insulation Prestressed concrete with 2-in. sched 40 carbon-steel water cooling pipes on 6 i n . centers located 4 i n . fronl inside concrete face

12.4 0.034 1.12 (concrete) 25.9 ( s t e e l )

Assumed heat t r a n s f e r coefficient i n water pipes, Btu hr-’ ft‘a

7 -9

Maximum concrete temperature, OF

150

Table 7. Cell Cooling System

Initial heat rate, Btu/hr

3.5 x loe

Number of cooler cells

Number of thimbles per cell

225

Length of thimbles, ft

Diameter of thimbles, in.

Pitch of tubes in square array, in.

Temperature of cooling water, "F

225

Gas flow rate per cell, cfm

32,500

AI? across thimbles, in. HzO

Emissivity of thimble

0 07

Volume of gas to be cooled, f t 3

150,000

Temperature Na at start, "F

1000

Temperature I% after 2.75 hr,

740

O F

Temperature reactor at start, "F Temperature reactor after 2.75 hr,

1000 O F

998

ORNL DWG 72-3582

Fig. 20.

Drain Tank Plan.

Fig. 21.

Drain Tank Elevation.

as a u A i t as far as vibration i s concerned, and t h e n a t u r a l frequency of t h i s structure i s high enough t o provide protection against earthquake

Lad L

damage A cooling module consists of a manifold and

6 reentrant tubes f o r

each of t h e 60 groups of thimbles which penetrate t h e drain tank.

Each

- o f these cooling modules i s connected by two b i n . pipes t o a section of pipe jrn t h e water heat dump outside the,building containment. These cool&g modules contain NaK as the c i r c u l a t i n g heat transport f l u i d . Each module has an expansion tank and shuV-off valve so that each a c t independently of the others. Three water tanks 25 ft x 25 f t t h e reactor building.

x &2 f t deep are located oulxitle

Water flows through these tanks i n an open c i r c u i t .

The water is taken from the same scxlrce a s the turbine condenser cooling e r going through t h e tanks, â&#x20AC;&#x2DC;it i s returned t o the stream. a t e r tanks are given i n Table 8, and the tank arrangement Table 8. Dump Tank Heat Sink Data Size of tank, f t Depth of water over tubes, f t Number of tubes Arrangement of tubes

25 X 25 X 12

5 576 for each of 2 tanks; 672 f o r t h i r d 48 tubes per row, 12 rows f o r 2 tanks, and 48 tubes per row with 14 rows f o r t h i r d tank.

Total volume of water f o r a l l tanks, fts

12,920

Thimbles i n row Row spacing, i n . NaK pipes i n thimbles

3-in. sched-10

NaK entering temperature,

12 2-in4 sched-40 pipe O F

NaK exiting temperature, OF

Water entering temperature, Water exiting temperature, NaK flow r a t e , gpm

Water

flw

r a t e , gpm

pipe on 6-in. centers

OF OF

532 450 80 100 2800

-Lid

i s shown i n Fig. 22.

Two of t h e 3 tanks a r e s u f f i c i e n t t o provide cool-

Heat i s transferred from the drain tank t o the NaK, and from the NaK t o t h e water by radiation. Thus ing under maximum heat load conditions.

the NaK has a double b a r r i e r between it and t h e contents of the drain tank and a l s o between it and the water i n the cooling water tanks. The crucible i n which the drain tank i s suspended i s a double-walled tank. It i s a l s o cooled by NaK, having i t s own independent l i n e s t o t h e water tanks. transfer

The crucible cools t h e drain tank walls by radiative

The drain tank connects w i t h the primary system i n three completely d i f f e r e n t ways i n order t o perform the three d i s t i n c t l y d i f f e r e n t functions for which it i s used. The f i r s t of these f'unctions i s t o provide a receptacle i n t o which the primary s a l t may be drained whenever it i s necessary f o r any reason t o remove t h e salt f r o m t h e primary system. A 6-in. l i n e runs from the bottom of the reactor i n a generally horizontal direction i n t o t h e drain tank p i t . A t t h i s point a drain valve i s i n s t a l l e d and a v e r t i c a l l i n e connects t h e drain valve t o t h e drain tank. The drain valve i s a c r i t i c a l p a r t of t h e system since it operates but infrequently and y e t must operate when required. To provide some redundancy, the drain valve i s bypassed by a rupture disc valve which can be used i n an epiergency i f t h e drain valve f a i l s t o open. Both the drain valve and the rupture disc valve a r e replaceable by remote maintenance methods a f t e r the primary system 5 s drained. The drain valve i s a combination mechanical valve freeze valve, As shown i n

Fig. 23, t h e poppet does not s e a l mechanically but r e l i e s on a frozen salt film t o s e a l . This s e a l i s frozen by circulation of a coolant It i s thawed by circulation of hot f l u i d . i n the body of the p The poppet i s actuated by a positive e l e c t r i c drive and the movement i s sealed by a bellows which cannot be contacted by salt but only by

the cover gas. The second use f o r %he drain tank i s one which i s extremely unlikely but nevertheless must be provided f o r safety considerations. I f there i s any leak of primary s a l t f r o m t h e primary system, the s a l t

must be put i n t o t h e drain tank.

Therefore, t h e reactor c e l l , heat

.. .

...

'-0-,E2

__ .

0 n4

54 exchanger c e l l s , and the s a l 3 piping above the drain tank c e l l are equipped with catch pans which communicate with the drain tank through a rupture disc i n t h e top of the tank. The t h i r d use of t h e drain tank Concerns the holdup of t h e f i s s i o n gases and t h e separation of entrained l i q u i d from these gases,

About 8 MW

of heat from these f i s s i o n products must be disposed of and t h e drain,tank cooling system i s used t o dissipate t h i s heat. This i s a continuous flmct i o n of the drain tank while t h e plant i s i n operation. As has been mentioned, the primary salt i s sparged w i t h helium t o remove xenon from the s a l t . This sparging i s accomplished by introducing gas bubbles i n t o a salt bypass loop around each pump of t h e primary c i r c u i t . I n t h i s bypass a centrifugal separator extracts t h e gas from the s a l t . The separator constantly extracts about 10 gpm of salt along with t h e gas. This two-phase stream from each pump i s sent t o the drain tank where, because of t h e s i z e of t h e drain tank, the s a l t separates from the gas by gravity, and, after an average residence time of about 6 hours, t h e gas passes thruugh a p a r t i c l e t r a p . About half t h e gas i s returned d i r e c t l y t o t h e bubble generator and t h e other half passes through a cleanup system and i s returned t o t h e pump purge system by means of a gas compressor. The flow of l i q u i d t o the drain tank may vary over a range from 0 t o about 32 gpm. A means must be provided f o r returning t h i s s a l t t o the primary system without returning any of t h e radioactive gas. A system of j e t pumps with automatic flow control i s used t o accomplish t h i s . This j e t pump system i s i n s t a l l e d as a u n i t i n the center of t h e drain tank and can be removed f o r replacement o r r e p a i r should any of the u n i t become defective. "his u n i t i s shown i n plan and elevation i n f i g . 24. A 19-ft-long by 6-in.-dim pipe, closed a t t h e bottom end and open a t t h e top, a c t s as a sump i n the center of the drain tank. On t h e outside of t h i s sump there a r e two j e t pumps which are driven by a common s a l t source taken from the output of t h e three primary pumps.

A line

f r o m t h e discharge of each primary pump, af'ter passing through a b a l l check valve, i s manifolded i n t o one common l i n e which feeds these two

u i

56 One of the j e t pumps has i t s suction i n a small sump i n t h e bottom of t h e drain tank and discharges tangentially i n t o t h e 6-in.-diam sump

u i

near t h e top.

This pump has a capacity greater than t h e flow of l i q u i d

i n t o t h e tank so it keeps the drain tank sump-empty. Because the'capacity

of t h e pump exceeds t h e l i q u i d flow, t h i s pump a t times pumps gas, but the gas simply discharges back i n t o the drain tank atmosphere. The second j e t pump i s located about midway up t h e 6-in.-diam sump and sucks out of t h e bottom of this pipe. The discharge of t h i s pump goes i n t o one of the pump bowls of the primary system. The j e t pump suction can change from about plus 14 ft t o a minus 5 ft. Therefore, t h e pump discharges s a l t a t a r a t e proportional t o t h e suction head and t h i s r a t e can be anything between 0 and 32 gpm. Thus only l i q u i d and never hot gas i s returned t o the primary system. This automatic regul a t i o n takes care of t h e l i q u i d pumpback f o r a l l conditions-of operation. A t h i r d j e t pump i s mounted w i t h t h i s assembly, but it i s us&dt only f o r f i l l i n g the primary system or f o r transferring t h e contents of the drain tank t o t h e chemical processing c e l l . This operation i s described below, alang with t h e chemical processing system. Because t h i s plant i s operated as a converter having a breeding r a t i o l e s s than unity, the chemical processing required i s very much l e s s involved than t h a t required for a breeder. Three things a r e required of t h e chemical treatment: F i r s t , means must be provided f o r removing oxygen from t h e primary o r secondary s a l t i n case e i t h e r becomes contaminated. Second, when the f u e l s a l t has accumulated enough f i s s i o n products t o cut significantly i n t o the neutron econow, this salt must be replaced with uncontaminated c a r r i e r s a l t .

The uranium must be

extracted f o r return t o the reactor before the old salt i s discarded. Third, there must be provision f o r storing the discarded c a r r i e r s a l t ,

a t l e a s t as long a s cooling of t h i s salt i s required. I n addition t o these three primary functions there i s a further requirement f o r a storage f a c i l i t y where primary salt may be placed i f it i s necessary t o do maintenance of any kind on the drain tank. Since t h i s storage tank i s required i n any event, it was decided t o use it as t h e chemical processing tank f o r fluorination of the primary s a l t t o remove uranium.

The cooling system for the storage tank can be quite simple since the salt can be held in the drain tank until the heat rate is of the order of 1 MW. This situation exists after approximately 30 days of cooling. The 1 MWcan be successfully transferred from the surface of the storage tank by radiation to the NaK-cooled jacket surrounding the storage tank. After removal of the uranium, the contaminated carrier salt must be put into disposable cans. These cans, about 2 f-t in diameter by 10 ft long, are placed in tanks located in an annulus around the top part of the storage tank. Figures.25 and 26 are plan and elevation of the storage tank and the disposable cans. These cans,have interconnecting l-in. lines so that salt can be transferred to them from the storage tank by gas pressure. .After cooling, the lines can be cut and welded, sealing the spent salt permanently in the disposable cans, After the cans have been sealed they can be stored under the reactor In the storage vault or transported to salt mines or other permanent storage facility. New cans can be installed by direct maintenance once the full cans have been removed from the location. OFF-GASSYSti Helium is used as a cover gas over the primary and secondary salt. It is also used as a purge; flowing inward around the pump seals, and as a sparge for'removing xenon from the primary salt. This gas system is a closed system, and storage and cleanup equipment must be provided. The bubble injection and separation and the holdup of the gas in the drain tank have already been described. Whenthe gas leaves the drain tank it goes to one of two particle traps...These traps can be isolated by valves for removal'from theâ&#x20AC;&#x2DC;system for repair. The purpose . of these particle traps is to catch the solid daughters and granddaughters of the noble gases and also such noble.metal particles as are carried by the.gas as it leaves the drain tank. 'Theseparticle traps have adequate size to accumulate solid particles for about three months before ,excessive pressure drop would require replacement of the trap. To remove the heat generated by.these particles, the traps have NaK cooling. The heat load is of the order of 400 kW. These particle traps have not been

ORNL DWG 70-12199

. -.-

-..

.. . . . ..

1 2016"

. 1

Fig. 26.

Processing and Storage Tank E l e v a t i o n .

C"H

designed since t h e exact calculations on which a design i s based have not been made. Past experience with MSRE shows t h a t t h e design i s not complex and the configuration does not present a problem. The gas l i n e downstream of t h e p a r t i c l e t r a p divides i n t o two equal branches. About 1 cfm of t h e gas goes d i r e c t l y t o the bubble generators i n each of t h e three pump bypass l i n e s . When t h i s gas i s removed by t h e bubble separators, it recirculates t o the drain tank, through t h e p a r t i c l e trap, and t o t h e bubble generators again. T h i s gas i s very radioactive, t h e only l o s s i n radioactivity being that which has decayed i n the approximately 6-1/2 hr t r a n s i t time f o r t h e loop. The other gas stream fromthe p a r t i c l e t r a p enters one of two charThe two beds a r e valved so that e i t h e r can be used. It i s imperative t h a t the krypton and xenon i n t h i s p a r t of t h e gas be coal absorber beds.

held up f o r 90 days because it must be e s s e n t i a l l y clean before it goes t o the gas compressor and storage tank f o r recirculation t o t h e pump seals. The head end section of the holdup bed i s cooled by NaK while t h e downstream sections of t h e bed can be cooled by natural convection of the gas atmosphere i n t h e off-gas c e l l . These beds a r e i n t h e form of calandria with the charcoal surrounding t h e tubes.

The a c t u a l design of t h e charcoal beds, as i n t h e case of t h e p a r t i c l e t r a p , has not been done. Again, there has been enough experience with such absorber beds

a t ORNL t o show t h a t they a r e feasible and a r e not excessively large. For d e t a i l s concerning gas cleanup systems, reference i s made t o ORNL4541 ( r e f . 6). CONTROL RODS

There a r e 6 control rod penetrations i n the reactor core.

These

a r e 7-in.-diam holes i n graphite blocks i n which cruciform control rods are suspended. There i s a 1/4-in. radial clearance around t h e rod, and

it i s f r e e t o r o t a t e i n the hole. S a l t flows up aromd t h e control rod a t a velocity of about 7.5 f'ps and keeps it cool. A control rod i s made of up t o 1/2 in. of BqC clad w i t h a 1/8-in. thickness of Hastelloy N. Each control rod i s worth about 3% AK/K and any one rod w i l l shut t h e reactor down.

W a

Each rod i s suspended on a Hastellay cable which i s taken up on a

12-in.-diam drum driven through a worm gear drive. The motor i s outside the reactor c e l l shielding, but the winch and cable a r e located within the containment. The motor and drive i s a l s o enclosed and a positive gss pressure i s maintained t o produce inleakage around t h e drive seal. A spring loaded monitor cable attached t o a synchro provides position

indication f o r the rod. One end of the cable i s attached t o the top of t h e rod and by actuation of t h e synchro gives remote indication of t h e rod position a t a l l times. This cable i s not strong enough t o supp o r t t h e rod; therefore, it cannot prevent the rod from f a l l i n g i n t o place. The control rod has a f i n i t e l i f e and so must be replaced a f t e r it has received a fluence of about loB1 neutrons/cm2. The redundancy furnished by the 6 rods makes it possible t o hang the rods on cable without providing a positive drive f o r rod insertion. A magnetic clutch can be provided between the winch and the gear drive i f it I s desirable t o drop the rod instead of relying on t h e slower i n s e r t i o n provided by the drive mechanism. INSTRUMEPJTATION

Instrumentation f o r the plant has received l i t t l e a t t e n t i o n since

it probably poses few problems t h a t are d i f f e r e n t from other nuclear plants. One exception i s that the reactor i s so heavily reflected that f o r i n i t i a l startup a f i s s i o n chamber w i l l probably have t o be inserted i n one of the control rod sockets i n order t o get a good enough signal i n t h e i n i t i a l c r i t i c a l i t y run. The nuclear instrumentation should not be more complicated than f o r other nuclear plants f o r the reactor has a prompt negative temperature coefficient, and +he very l a r g e heat capacity of t h e primary system prevents rapid temperature excursions. A thorough study of t h e control and safety instrumentation must be done. It appears t h a t t h e instrumentation required w i l l not exceed the existing a r t .

62 MAINTENANCE

A circulating-fuel nuclear reactor i s inherently a highly radio-

active system, and t h e radioactivity i s more dispersed than it i s under normal conditions i n a solid-fuel reactor. For this reason, a l l maintenance on the primary c i r c u i t must be done by remote means. By designing t h e MSDR t o operate a t a low enough power density so t h a t graphite replacement i s unnecessary, t h e maintenance i s greatly simplified. Also by designing t h e heat exchangers so that tubes can be plugged, t h e maintenance i s made more p r a c t i c a l . Experience w i t h the MSRE showed that remote maintenance of t h e radioactive equipment and systems of a small molten-salt reactor i s feasible and p r a c t i c a l . The techniques and types of t o o l s successfully employed there 'should be satisfactory f o r many of t h e remote maintenance operations on t h e MSDR, but they w i l l have t o be adapted f o r handling the l a r g e r and heavier equipment and piping. New t o o l s w i l l have to be developed f o r such operations as remote cutting and welding of piping and plugging of heat exchanger tubes. Experience with t h e development of automatic cutting and welding equipment a t ORNL indicates t h a t these more d i f f i c u l t operations can be accomplished. PERFORMANCE

A summary of t h e nuclear data with f u e l cycle costs i s shown i n

Table 9.

No c r e d i t was taken f o r r e c l a W n g t h e discarded c a r r i e r salt

a f t e r it i s removed from the reactor.

This s a l t has some very valuable

constituents i n the 7 L i and beryllium and when molten-salt reactors come i n t o general use it i s p r o b a b l e t h a t t h i s salt could be reclaimed a t

an economic benefit. It i s a l s o almost certain that future reactor plants could u t i l i z e s u p e r c r i t i c a l steam systems and r e a l i z e t h e improved e f f i c i e n c i e s of these systems. As has been stated, t h i s "first-of-a-kind" demonstration plant was designed on a conservative basis. The question of plant safety has not been t r e a t e d here. Certainly safety considerations were of prime concern i n t h e design study described i n t h i s report, but t h e subject i s so important as t o merit separate

Table 9. Lifetime Averaged Performance of a 750-MW(t.) Molten-Salt Demonstration Reactor Identification ~38 Description Dimensions, cm Core, radius 340 2.54 Plenum-annulus, thickness Reflector, thickness 7599 Salt volume fractions 0.10 Core 1.o Plenum-annulus 0.01 Reflector Composition of carrier salt, mole $ LiF 67 BeFa 23 10 Tu4 Core moderator ratio, atoms C/Th 277 0.8 Plant factor 36.6 Thermal efficiency, $ 24 Core life, ef'p years 6 Processing, batch cycle time, ef'p years Fuel enrichment, * 3 6 ~ ,$ 93 volume of fuel salt, ft' Reactor (spherical model) 760 Piping and heat exchangers 320 m Total Performance a 0.88 Conversion ratio, lifetime averaged Fuel cycle costs,b mills/kWhr Inventory Fissile 0.62 Salt 0.071 Replacement Fissile 0.18 0.13 Salt \ 1.o Total fuel cost 0.1 Processing cost, estimated 1.1 Total fuel cycle cost Lifetime fissile material balance, kg Initial inventory 710 Further purchases 940 Recovery at end of life 700 Net fuel requirement 950 aAssuming a processing loss of fissile uranium of 0.01% per cycle.

bFrom a present-worth calculation of fissile, fertile, and carrier salt purchases and/or sales over the reactor lifetime, compounded quarterly at a discount rate of 0.07 per annum. Inventory charge rate 0.132. and 13.8 for a33U. Fissile nuclide value in $/kg, 11.9 for '"U,

64 study. An in-depth safety study of the molten-salt concept is in progress and will be the subject of an extensive report.

, t

DISTRIBUTION

ORNL-TM-3832

MSRP Distribution D W. B. Cottrell H. C. McCurdy A . J. Miller 213. M. Sheldon 214. D. A. Sundberg 215. G. D. Whitman

1-209. 210. 211. 212.

us-AEC

216. D, F. Cope, ORO-SSR 217. D. Elias, RDT, Washington, D. C, N. Haberman, RDT, Washington, D. C, W. H. Hannum, RDT, Washington, D. C. J. Neff, RDT, Washington, D. C. M. J. Whitman, RDT, Washington, D. C. Director, Division of Reactor Licensing, Washington, D. C. Director, Division of Reactor Standards, Washington, D. C. Executive Secretary, Advisory Committee on Reactor Safeguards, Washington, D. C. Research and Technical Support Division, OR0 228. 229-230. Technical Information Center

218 219 220 221. 222-223 224-225 226-227

Industry a

231. J, A. Acciarri, Continental Oil Co., Ponca City, OK 232 R. M. Bushong, Carbon Products Division, UCC, P. 0. Box 6116, Cleveland, OH 44101 233 G, C. Clasby, Byron Jackson, P, 0. Box 2017, Terminal Annex, Los Angeles, CA 90054 234-236 D. R. deBoisblanc,Ebasco Services Inc,, Two Rector Street, New York, NY 10006 237 B. Deering, Black & Veatch, P. 0. Box 8405, Kansas City, MO

64114

238. Bernard Mong, Babcock and Wilcox, P. 0. Box 1260,Lynchburg, VA 24505

T. K. Roche, Cabot Corp. , Kokamo, IN 4691 240. A . E. Swanson, Black 80 Veatch, P. 0. Box 8405, Kansas City,

239

64114