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MAINTENANCE DEVELOPMENTFOR MOLTEN-SALT BREEDER REACTORS ~mmRobert Blumberg

.ABSTRACT

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The maintenance.system of the proposed molten-salt breeder reactors will be based upon the technology in.use and experience gained from the Molten-Salt ReactorExperiment.- The unit replacement scheme, long-handled-tools, movable maintenance shields,and the means for handling contaminated equipment will..be-similar..for manyoperations. The techniques must be improved and extended.and-new techniques must be developed for maintaining--some of-the-larger more radioactive components.of the breeder reactors.. Remote welding-is needed for major component replacement,- Methods-must be:.available for replacing the core and for the repair.of heat exchangers. ,Finally, a,general development.and design surveillance program-:.wiU..be,required.~ These Dromams are described and their..cost is estimated.

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of a preliminary nature and was prepared Ridge National Laboratory.’ It is sub eh’,’ does not represent a final report. f reprinted or otherwise given public disthe ORNL potent branch, Legal and infor-

L E G A L NOTICE T h i s report was prepared as an account of Government sponsored work.

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any information, opporatus, method. or process disclosed in this report. A s used in the above, "person acting o n behalf of the Commission"

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TABLE OF CONTENTS

Page INTRODUCTION

PHILOSOPHY OF MAINTENANCE

DESCRIPTION OF PRESENT TECHNOLOGY

MSRE MAINTENANCE EXPERIENCE AS R E W E D TO MOLTEN-SALT BREEDER REACTORS

DISCUSSION OF ANTICIPATED PROBLEMS

Piping Connections and Vessel Closures

Replacement and Repair of Components

Large Component Replacement

Core Replacement

Heat Exchanger Replacement

Repair of Components

Radioactive Component Examination

Improved Performance

INCREASED RADIATION LEVEL

GENERAL MAINTENANCE DEVELOPMENT AND DESIGN SURVEILLANCE

PROGRAM COSTS AND SCHEDULE

INTRODUCTION

One of t h e basic differences between a molten-salt reactor o r any circulating-fuel reactor and t h e more widely u t i l i z e d , s o l i d o r stationary f u e l reactor, i s i n how each contains f i s s i o n products. One of t h e important ramifications o f t h i s difference i s i n t h e area of maintenance. The c i r c u l a t i n g f'uel deposits some f i s s i o n products i n t h e reactor system, drain tanks, offgas system, and t h e f u e l processing system. Questions n a t u r a l l y a r i s e . Is it feasible t o maintain such radioactive systems? From our experience with Homogeneous Reactor Experiment-2 (HIIE-2 or HRT) and t h e Molten-Salt Reactor Experiment (MSIIE), t h e answer i s an unqualif i e d yes. Another question i s , "Are such systems much more expensive t o maintain than s o l i d m e 1 systems?" This question cannot be adequately assessed because of t h e complexity of t h e economics, but when one accounts f o r t h e c o s t s involved with replacing spent f u e l elements, t h e answer i s no longer c l e a r l y i n favor of the s o l i d f'uel system. It may well be cheaper t o maintain a c i r c u l a t i n g f u e l reactor. A reference conceptual design f o r a 1000-Mw(e) molten-salt thermal breeder reactor (MSBR) i s described i n 0 ~ ~ ~ 3 9 9The 6 .program ~ for developing t h e breeder through a molten-salt breeder experiment ( MSBE) i s summarized i n ORNL-TM- 18%. The following report describes a program of development of methods and equipment f o r maintaining t h e radioactive equipment of t h e MSBE. A major c r i t e r i o n f o r t h e program i s t h a t only scaleup o f t h e equipment should be necessary t o s a t i s f a c t o r i l y maintain t h e l a r g e r MSBR.

PHILOSOPHY OF MAINTENANCE

The prime philosophy of maintenance of f a i l e d radioactive components i n molten-salt breeder reactors i s simply stated as remove, replace and r e p a i r o r discard. We do not plan t o r e p a i r those components i n place. They w i l l be removed and replaced by a new o r repaired unit and then w i l l be repaired i n specially equipped f a c i l i t i e s o r discarded depending on t h e s i z e and cost of t h e uni i f f i c u l t y of making t h e r e p a i r , and the value of t h e repaired un h i s philosophy was adopted because it ng r e p a i r s quickly t o get t h e plant appears t o be t h e best way o back i n t o operation.

One of t h e mgst import s which a f f e c t s t h i s maintenance components reliable so repairs philosophy i s t h e ' e f f o r t t o are infrequent -and discard not prohibitively expensive. The program f o r engineering development w i l l place emph'asis on establishing a predictable and p r a c t i c a l l i f e f o r t h e components permitting us t o e s t a b l i s h a philosophy which includes discard of t h e f a i l e d component i f it i s t o o radioactive f o r d i r e c t o r semi-direct maintenance. Emphasis a l s o w i l l be on rapid replacement t o educe t h e down time f o r t h e system. The reactor vessel, t h e heat exchangers f o r t h e f'uel and blanket systems, the drain tanks, and t h e pump rotary elements w i l l become so radioactive t h a t they w i l l normally be discarded. It may be necessary

t o disassemble and examine a f a i l e d component t o determine t h e exact cause of f a i l u r e but t h i s would not require t h a t t h e component be reassembled. Possibly t h e f a i l e d component w i l l be stored i n t h e same c e l l with t h e reactor t o a w a i t some decay before examination. The offgas system and t h e chemical process plant w i l l contain many Where p r a c t i c a l , components of these systems w i l l be decontaminated and repaired w i t h a minimum of shielding, otherwise they w i l l be discarded and a new component w i l l be installed.

of t h e f i s s i o n products and w i l l be very radioactive.

The coolant system ordinarily should not be very radioactive. There i s some activation of t h e sodium but with t h e system drained t h e residual a c t i v i t y should be low enough t o permit d i r e c t maintenance. The pressures i n t h i s system are maintained above t h e opposing pressures across t h e tube w a l l s i n the f u e l and blanket heat exchangers so t h a t any leakage would be out of t h e coolant system. Thi.s arrangement should prevent contamination of the coolant s a l t with f i s s i o n products. I f perchance f i s s i o n products should get i n t o t h e coolant system, then some decontaminat i o n would be necessary before maintenance would be possible. The steam and turbine generator system should never become radioactive since it i s separated f r o m t h e prime sources of a c t i v i t y by t h e coolant system and from d i r e c t neutron activation by t h e shielding o f t h e c e l l w a l l s . Conventional methods of d i r e c t maintenance w i l l be used here.

I n t h i s program w e s h a l l concentrate on developing t h e techniques f o r maintainingthe highly radioactive components such as t h e core, pump, drain tanks, and heat exchangers. DESCRIPTION OF PRESENT TECHNOLOGY The present s t a t u s o f t h e technology of maintenance of molten-salt reactors i s l a r g e l y embodied i n t h e maintenance scheme f o r t h e MSRE. Methods and equipment i n use at t h e MSRF: are based on extensive experience gained on t h e aqueous Homogeneous Reactor Experiment No. 2. Experience w i t h remote maintenance of t h e radiochemical plants at Hanford and Savannah River, r e p a i r of t h e Sodium Reactor Experiment, and remote dismantling of various reactor assemblies contributes i n a general, and often important way, t o t h e development. $0 achieve a p r a c t i c a l level of maintainability, uniform methods are provided far gaining access t o , removing, and replacing a31 of t h e equipment i n the radioactive areas of the reactor. A t t h e MSRF,, t h i s includes t h e reactor c e l l , t h e drain tank c e l l , t h e offgas system and t h e chemical process system. The general philosophy i s t o remove a failed component and replace it with an interchangeable spare. A considerable emphasis w a s placed i n design and construction phases on making components r e l i a b l e so t h a t t h e need for replacement i s infrequent-and discarding f a i l e d components i s not prohibitively expensive. However, f a c i l i t i e s have been provided f o r some some decontamination and r e p a i r of equipment.

3 Reducedto fundamentals, t h e MSRE i s a collection of component p a r t s which a r e capable of being disconnected and reconnected remotely. Access t o these u n i t s i s provided through removable shielding sections t h a t make up t h e roofs of t h e various c e l l s . A portable maintenance s h i e l d i s ins t a l l e d over t h e component, the roof section i s removed, and long-handled t o o l s are used t o do t h e manipulations t h a t are required. T h i s port b l e s h i e l d provides 1 2 i n . of steel f o r shielding (attenuation f a c t o r 1 0 -lO5), t o o l access holes, l i g h t i n g , and maneuverability. The long-handled t o o l s a r e , f o r the,most p a r t , simple, strong, and single purpose. Periscopes and lead glass windows i n t h e shield provide viewing i n the work area. A l l preparations f o r removal a r e donehcompletely with t h e portable shield. After l a r g e components a r e prepared f o r removal w i t h the same technique, they are removed from t h e i n s t a l l e d position by means of a crane operated by personnel inside a shielded control room with closed-circuit t e l e v i s i o n and l i q u i d - f i l l e d windows f o r viewing. Small components are removed by use of s u i t a b l e transport shields. A hot-equipment storage c e l l and a decontamination c e l l can be reached by t h e crane so t h a t contaminated equipment can be disposed of conveniently.

The a b i l i t y t o completely disconnect a p a r t i c u l a r component i s basic t o t h i s system. The disconnects must be remotely operable by t h e longhandled t o o l s . They must be r e l i a b l e both f o r t h e service conditions and f o r t h e high radiation and i n some cases must satisfy nuclear s a f e t y considerations of containment leak tightness and leak d e t e c t a b i l i t y . A number of d i f f e r e n t disconnects a r e used at t h e NSRE f o r t h e various applications. Almost a l l t h e piping i n such auxiliary systems as t h e offgas, l u b r i c a t i n g o i l , a i r , and cooling water systems have standard r i n g j o i n t flanges, with minor modifications. Special designs were used f o r leak detector tubing, thermocouple, e l e c t r i c a l and instrument leads. The disconnects f o r t h e 5-in. sched-40 piping a r e c a l l e d freeze flanges. They are l a r g e diameter, unheated and uninsulated flanges. The clamping device, a U-shaped spring clamp, and the r i n g gasket s e a l a r e near t h e perimeter and operate at a much lmer temperature than t h e bore of t h e pipe. The oversize flanges take up much space, have l a r g e temperature gradients, and require l a r g e clamping forces. Much deyelopment was required t o obtain t h e desired s e r v i c e a b i l i t y and maintainability, but f i v e p a i r s of flanges a r e now i n service at t h e MSRE and they work w e l l . While they have never been broken and remade remotely i n a radiat i o n f i e l d , t h e long-handled t o o l s which were developed f o r t h i s purpose were used f o r the assembly of t h e reactor and t h e i r operability was established t o t h a t extent. I

The drain and s t t h e reactor i s connec d with 1-1/2-in. sched-40 piping. It is p l i n t a i n t h i s system by remotely c u t t i n g and brazing these l i n e s . t o accomplish t h i s i s on hand3 and has been extensively t e s t e d i n mockups, but not y e t i n a radioactive situation.

The maintenance philosophy.in use f o r most p a r t s of the WRE is t o replace a f a i l e d , contaminated u n i t with a spare component. Spares are b u i l t i n j i g s t o assure interchangeability. Pieces t h a t a r e small and not t o o radioactive a r e p a r t l y decontaminated and repaired by d i r e c t contact with t h e help of l o c a l shielding t o reduce t h e radiation level. To satisfy t h e requirements of t h e MSRE, a constant. review was made o f t h e component and i n s t a l l a t i o n design t o insure t h a t i t was maintainable and, where.necessary, mockups were constructed t o a s s i s t ' i n guiding t h e designers. -

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MSRE MAINTEEJAMCE EXPERIENCE AS RELATED TO MOLTEN-SALT BREEDER REACTORS

The MSRE has been successfully operated and maintained during t h e past year and a half. Several different items of equipment have been replaced o r repaired p d several d i f f i c u l t operations were completed which were unanticipated i n our planning f o r maintenance. To evaluate our experience i n the l i g h t of t h e needs of t h e breeder, one may divide all of R E i n t o two classes. The first class includes t h e equipment of t h e E all t h e large salt-containing vessel and piping complexes. These are t h e three major components in t h e reactor c e l l , t h e drain tanks and t h e interconnecting piping. While these complexes have t h e most d i f f i c u l t maintenance jobs, they are a l s o low frequency jobs. We have not yet maintained these large components. The second c l a s s includes all of the r e s t of t h e removable equipment; items such as control rods, heaters, valves, auxiliary l i n e s , offgas component, e t c . This i s where most of t h e maintenance work w i l l be dane because of t h e higher failure r a t e . The maintenance capability has been c l e a r l y demonstrated f o r the second c l a s s of equipment. Based upon many hours of actual work experience, a detailed knowledge of t h e magnitude of t h e radiation and contamination l e v e l s , and a first hand knowledge of t h e a b i l i t y of t h e system t o handle unanticipated problems, we make t h e following statements regarding WRE maintenance.

1. We believe t h a t t h e demonstration(of maintenance of t h e major f u e l components (i.e. , t h a t which we have not yet done) i s merely a matter of doing it when t h e occasion arises. It presumably w i l l be more d i f f i c u l t and w i l l require more t i m e but nevertheless i s w e l l within our capability. J

2. The E R E maintenance system possesses several a t t r a c t i v e q u a l i t i e s , including r e l i a b i l i t y , simplicity, ruggedness and f l e x i b i l i t y .

3. There a r e two weaknesses of t h e These are t h e l e v e l s of radiation around of disposing of contaminated equipment. be improved quite readily through design

system which have been recognized, t o o l penetrations and t h e method These are weaknesses t h a t can and procedural changes.

4. The MSRE can continue t o supply information of value t o t h e MSBR program. It i s planned t o conduct experiments, perform maintenance tasks and gather data, during t h e remainder of t h e operating l i f e of t h e MSRE. Projects of t h i s nature include demonstrating t h e replacement of a major component, mapping t h e gamma radiation l e v e l s i n a portion of t h e reactor c e l l and the offgas system, and continuing t h e p l o t t i n g and analysis of i n - c e l l radiation l e v e l s . The p o s s i b i l i t i e s of decontaminating components of t h e f u e l and offgas systems t o a l e v e l which would permit d i r e c t maintenance w i l l be investigated.

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5 . We believe t h a t t h e requirements of t h e MSBR can best be fulf i l l e d with a system based g e n e r d l y on t h e one i n use at t h e MSRE. The equipment must be modified, a f course, t o meet increased requirements i n performance and i n s i z e , weight and radiation c a p a b i l i t i e s . Finally, it must be modified t o r e f l e c t t h e specific design problems of t h e breeder. DISCUSSION OF ANTICIPATED PROBLEMS

We propose t h a t t h e system for maintaining t h e radioactive components o f t h e MSBR be based on t h e technology and experience of t h e EasRE. The overhead access, movable maintenance s h i e l d , separable components and long handled t o o l s t o accomplish i n c e l l manipulations w i l l be retained. We know t h a t some new techniques must be developed and e x i s t i n g techniques must be improved. However, the d e t a i l s of the maintenance system must be based upon a more detailed design of t h e reactor than now e x i s t s . The MSBR w i l l be l a r g e r . The pumps, heat exchangers, and reactor vessel w i l l be l a r g e r and heavier, so t h e maintenance equipment must have increased c a p a b i l i t i e s . For example, t h e reactor vessel f o r a 250 Mw(e) IBBR module weighs 71 tons compared t o 9 tons f o r t h e MSRE. Radiation l e v e l s w i l l be higher, so t h e shielding must be increased. The power l e v e l i n an MSBR module is higher by a f a c t o r of about 80 and t h e residual a c t i v i t y a f t e r t h e f u e l s a l t i s drained would be correspondingly higher. While t h i s would require some additional thickness in t h e portable maintenance s h i e l d , t h e important e f f e c t w i l l be t h e a t t e n t i o n which m u s t be given t o t h e cracks around t h e t o o l s a t penetrations. Economic factors and some nuclear requirements d i c t a t e a compact design f o r t h e f u e l , blanket, and some auxiliary equipment and systems. This tends t o make maintenance more d i f f i c u l t . Finally, economic considerations and program objectives place more emphasis on e f f i c i e n t maintenance. The following is a discussion of t h e places where problems are anticipated, proposed sblutions t o t h e problems, and the development required. This discussion i s concerned p r i marily w i t h t h e l a r g e breeder reactors. The MSBE w i l l have t h e same problems but on a smaller scale and the research and development w i l l i n most instances be done on MSBE scale.

Piping Connections and Vessel Closures

The unit replacement sch equires piping connections and vessel closures t h a t a r e highly r e l i i n service and are capable of being maintained remotely. I n t h e MSBR these connections w i l l be needed i n t h e main f u e l and blanket r e c i r c u l systems, t h e drain and storage systems, t h e offgas system, .the f u e l an e t processing systems, and i n t h e p a r t s of t h e coolant and other ary systems t h a t must be located i n radioactive areas. Vessel closures w i l l be needed on t h e reactor and on t h e f u e l and t h e blanket heat exchangers. For l i n e s no l a r g e r than those i n t h e MSRE and i n s t a l l e d i n areas e r e t h e ambient temperature i s below about 400째F, use can be made of e n t and techniques t h a t w i l l have been proven at t h e ERE. However, t h e design of t h e MSBR imposes t h r e e new d i f f i c u l t i e s : (1) The 2b-in.-diam piping i s considerably l a r g e r than has been used with remotely disconnectable j o i n t s . ( 2 ) The 1150'F ambient

6 temperature proposed f o r t h e reactor c e l l i s considerably higher than has been used i n t h e past. ( 3 ) No vessel closures approaching t h e size needed f o r t h e MSBR have been developed f o r remote operation and elevated temperatures. I n t h e reference design of t h e MSBR, Six connections are required i n t h e reactor c e l l i n t h e large l i n e s t h a t j o i n t h e reactor vessel t o t h e heat exchangers and t h e heat exchangers t o t h e coolant system. Remote welding, we believe, i s t h e best way t o make s a t i s f a c t o r y j o i n t s i n those l i n e s . Welding a l s o appears t o be t h e best way of making r e l i a b l e vessel closures and it i s possible t h a t t h e design of a vessel closure s e a l can be made fundamentally t h e same as t h e piping connection. While considerable development w i l l be required, t h e program seems t o be straightforward and t h e goal reasonably attainable. Development of s a t i s f a c t o r y flanges f o r those l i n e s would a l s o be d i f f i c u l t and probably would require considerably more long-term t e s t i n g . Once developed f o r t h e l a r g e r closures, remote welding can be used on t h e smaller l i n e s i n all t h e radioactive systems. The development w i l l be of considerable value t o t h e e n t i r e nuclear industry. Some development has already been done on remote welding. Atomics International Division of North American Aviation, Inc. , h e equipment f o r remote welding of small tubing f o r repairing heat exchangers. They are deeply involved i n automatic welding development including t h e joining of 4O-ft-long pieces of k-in.-diam pipe f o r deep-well casings by welding from t h e i n ~ i d e . The ~ PAR Project advanced t h e technology t o t h e point of completing many seal w e l d s and t e s t welds on l a r g e and small The pipeline industry has pipes with remotely operated equipment. automatic equipment t h a t w i l l make high quality welds on 30-in.-diam pipe. North American Aviation, Inc., has used t h e "skate welding'' method f o r fabricating missiles where t h e welding i s controlled from a remote location. 6

The welding development w i l l be a j o i n t e f f o r t of t h e Materials Development Program and t h e Maintenance Development Program. It w i l l consist primarily of: 1. designing and qualifying t h e weld j o i n t s ,

supporting t h e improvement o r modification of e x i s t i n g automatic welding apparatus,

adding the j i g s and fixtures required t o align and hold t h e pipe o r vessel and t h e manipulative devices t o operate t h e torch, and

making- t e s t welds t o improve t h e techniques u n t i l good welds can be made consistently.

The maintenance development w i l l a l s o include t h e devices f o r c u t t i n g t h e seals and machining t h e ends t o t h e specified configuration. A j o i n t design using a s e a l weld with a mechanical clamping device t o provide t h e strength i s a possible alternative t o t h e multi-pass welding of thick w a l l members.

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Development of techniques f o r inspecting and repairing welds i n radioactive areas must accompany t h e e f f o r t on remote welding. Visual inspection via closed c i r c u i t t e l e v i s i o n and dye penetrant and ultrasonic t e s t i n g techniques appear t o be applicable, whereas radiography does not appear feasible. An intensive study of t h e j o i n t configuration may reveal a design t h a t w i l l allow complete confidence i n t h e j o i n t without t h e detailed evidence of a t o t a l l y inspected weld. A leak detectable buffered j o i n t i s one example of such a design. One arrangement f o r making welded connections would involve i n s t a l l i n g at each j o i n t a built-in t r a c k o r guide, upon which a wheeled o r geared carriage containing welding, cutting, and inspection heads would r i d e . The t r a c k would provide accurate positioning i n t h e r a d i a l and circumf e r e n t i a l directions. Long-handled t o o l s would lower and i n s t a l l t h e various heads upon t h e built-in t r a c k s and would provide means f o r routing purge, power, coolant, and instrument leads. Remote t e l e v i s i o n o r o p t i c a l equipment could be used t o monitor t h e automatic control of t h e process. The development e f f o r t w i l l consist of at least t h r e e stages of t e s t i n g : (1)bench t e s t s of automatic welding equipment t o e s t a b l i s h t h e basic parameters of control of t h e welding process sucb as voltage, current, purge and coolant r a t e s ; (2) t e s t s of welding, cutting, inspection, and r e p a i r on f u l l - s i z e pipes and vessels using t h e p r e i n s t a l l e d guides and remote controls; ( 3 ) f u l l y remote shakedown of reactor grade equipment and procedures. The magnitude of t h e supporting design e f f o r t would depend upon t h e success of t h e tests i n t h e two early stages. The development work w i l l be done on j o i n t s of t h e s i z e s required f o r t h e NSBE,making certain t h a t t h e r e s u l t s can be applied t o t h e l a r g e j o i n t s of an MSBR. Service t e s t s must be made on a l l j o i n t s i n t h e various systems. Equivalent l i f e cycles of these j o i n t s w i l l be run t o establish compatibility of t h e j o i n t , i t s method of operation, and i t s service requirements. While t h e remote welding i s t h e f i r s t - l i n e approach, some study Will be made of two additional approaches. The f e a s i b i l i t y of remotely disconnectable mechanical j o i n t s for t h e intended service w i l l be investigated. Also'a braze s e a l with mechanical support w i l l be considered f o r use i n t h e auxiliary systems i n t h e reactor (both salt and non-salt carrying) and as a backup t o remote welding. It i s well t o note t h a t remote welding and remote brazing'are techniques r a t h e r than designs f o r specific applications. As such, they have a wide v a r i e t y of p o t e n t i a l uses i n radioactive environments for incorporation i n t h e o r i g i n a l designs and f o r modifying o r repairing e x i s t i n g equipment. Replacement and Repair of Components

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In keeping with t h e long-range goals of t h e program, we m u s t develop t h e a b i l i t y t o m a i n t a i n t h e reactor quickly t o avoid down time penalties and e f f i c i e n t l y t o lower t h e overall maintenance costs. The present plan of maintenance of radioactive systems c a l l s f o r replacement of a f a i l e d component with another l i k e unit and then e i t h e r repairing or discarding the f a i l e d component. The following i s a discussion of the probl e m s of t h i s plan.

.. i,

LJ.

Large Component Replacement To replace any large u n i t we do t h e following basic operations. Separate t h e u n i t from i t s connecting l i n e s by remote cutting. Using long-handled t o o l s , detach a l l minor connections and prepare for l i f t i n g . Remove t h e u n i t t o a previously prepared area i n t h e c e l l o r take it out of the c e l l t o some other storage area. The l a t t e r choice involves t h e transport of a very large, very radioactive component, shielding of maintenance and non-maintenance personnel, and control of contamination i n areas which are used daily. A new u n i t must then be i n s t a l l e d and reconnected t o t h e piping by remote welding. Development of t h e means f o r t h i s capability w i l l begin a s a design study. The sizes, weights, and expected radiation l e v e l s of t h e components w i l l be studied along with t h e various handling methods t h a t are available. A t t h e MSFiE, measurements w i l l be made of t h e effectiveness of flush salt operations, radiation l e v e l s w i l l be measured and experience w i l l be gained i n handling radioactive components. From t h i s experience, t o o l designs, shielding requirements, procedures, and requirements f o r equipment such as cranes, supports, in-cell jacks, and alignment devices w i l l be specified. Questionable areas w i l l be mocked up and tested. For instance, It is e x p e c t e d t h a t t e s t s m u s t be run on equipment t o a l i g n large vessels and equipment t o e f f e c t t h e necessary displacements. Tools and techniques must ultimately be t r i e d and demonstrated i n MSBE s i z e equipment and f i n a l l y on t h e components o f t h e Engineering Test Unit. Core Replacement In t h e MSBR of reference design t h e core i s an assembly of graphite f u e l tubes or c e l l s t h a t are joined t o Hastelloy-N plenums. F i r s t , each graphite tube i s joined by a threaded and brazed j o i n t t o a Hastelloy-N tube. The r e s u l t i n g elements are assembled i n t o a reactor core by screwing, welding o r brazing t h e Hastelloy N tubes t o t h e plenum header. The core assembly i s then i n s t a l l e d i n t h e reactor vessel and connected t o t h e f u e l entrance plenum by a gasketed o r seal welded j o i n t . Finally, t h e t o p head i s i n s t a l l e d t o close t h e reactor vessel. Means m u s t be provided f o r replacing t h e core i f one o r more of t h e graphite elements breaks o r develops l a r g e leaks. Problems of containment, shielding, removal of f i s s i o n product decay heat, e t c . influence t h e choice of a method f o r safely removing, transporting and disposing of t h e core. One method c a l l s f o r replacing t h e e n t i r e core and reactor vessel assembly and f o r storing t h e used u n i t i n a morgue within t h e reactor building. This scheme would use t h e vessel f o r containment of t h e f i s s i o n products and would ease some of t h e problems of removing decay heat, transporting and s t o r i n g t h e core. With t h i s concept t h e reactor vessel could be of all welded construction, thereby eliminating t h e need for l a r g e , remotely assembled vessel and plenum closures.

9 b

A second proposal c a l l s f o r removing t h e core assembly from t h e reactor vessel, i n s t a l l i n g a new one and discarding t h e old. This method requires t h e l a r g e closures and may a l s o make solution of t h e other problems more d i f f i c u l t . An early study w i l l be made of t h e problems and t h e economics of t h e two methods, a choice w i l l be made f o r use i n t h e reactor design, and equipment w i l l be developed f o r accomplishing t h e maintenance. Heat Exchanger Replacement

To r e p a i r a leak i n one tube of a heat exchanger one must do t h e following: \ 1. open t h e vessel t o gain access t o t h e tube sheets,

f i n d t h e tube which i s leaking,

seal t h e ends of t h e tube,

r e s e a l t h e vessel.

The reference design heat exchanger i s not w e l l s u i t e d t o r e p a i r because of very poor a c c e s s i b i l i t y t o t h e tube ends. Many compromises would have t o be made i n t h e design of t h e heat exchanger t o make it more e a s i l y repairable. The first choice f o r a maintenance method f o r t h e radioactive heat exchanger i s t h e replacement of t h e e n t i r e heat exchanger bundle. T h i s requires t h e removal of t h e pump rotary element from t h e pump bowl, opening t h e j o i n t i n t h e piping t o t h e core, opening t h e vessel closure i n t h e exchanger s h e l l and disconnecting several s m a l l service l i n e s . Then t h e heat exchanger would be removed t o an examination f a c i l i t y o r t o a storage area. The capability of replacing t h e e n t i r e heat exchanger m u s t be available and t h e necessary steps t o do so w i l l be developed.

Components t h a t can be e a s i l y decontaminated w i l l be repaired and sed as spare p a r t s . Components t h a t can be repaired by use of simple t o o l s behind a small amount of shielding or a r e small enough t o be handled i n a small hot c e l l may a l s o repaired f o r reuse.

tu4 D

Whether t o r e p a i r or disc t h e radioactive components from a l a r g e breeder plant has not been firmly established but d i s c a r d , i s t h e first choice at t h i s time. Studies are required of t h e f a c i l i t i e s f o r making repairs and of t h e costs i n a r r i v i n g at a firm decision. Measurements w i l l be made of t h e effectiveness of flush s a l t operations and decontaminat i o n procedures i n reducing t h e a c t i v i t y of contaminated p a r t s from t h e MSRE. The l e v e l s of neutron-induced radioactivity w i l l be calculated. Making use of these data, some designs w i l l be made of hot c e l l s and t h e equipment f o r making t h e repairs. This involves t h e application of hot

I I

-i

c e l l techniques t o tasks that are ordinarily done i n a heavy equipment shop. Total costs of making the r e p s i r s w i l l be estimated and compared with the value of equipment t h a t would be salvaged. Results of these studies w i l l be used i n specifying and developing equipment and f a c i l i t i e s f o r t h e MSBE. Experience with that reactor w i l l strongly influence what i s done f o r large breeder reactors.

Although our maintenance proposals are based on.removal and replacement of major equipment i n the plant, some a t t e n t i o n w i l l be given t o in-place repair. Studies w i l l be made of core designs and heat exchanger designs t o b e t t e r determine whether in-place r e p a i r of the graphite f u e l c e l l s and the heat exchanger tubes can be made practicable. Radioactive Component E x a m b a t ion The experimental nature of t h e MSBE requires t h a t c a r e f u l examination be made of any failed component t o determine t h e cause of failure so the cause can be corrected i n flrture components. An examination c e l l w i l l be required a t t h e reactor s i t e and it must at least be equipped t o dismantle equipment so t h a t parts can be sent t o other hot c e l l f a c i l i t i e s , f o r d e t a i l e d examination. Depending on t h e types of f a i l u r e s , repair of some radioactive components could a l s o be demonstrated i n t h i s c e l l .

Specifications w i l l be prepared f o r the f a c i l i t y and t h e equipment required. Some development of very s p e c i a l equipment i s anticipated and procedures w i l l be prepared f o r operating t h e equipment. Improved Performance

I n a power reactor the importance of making repairs quickly must be taken i n t o account. I n t h i s respect the record of t h e radioactive maintenance of t h e I S R E has been encouraging i n spite of several negative elements. Because it was an experimental r e a c t o r , ' t h e r e was l i t t l e e f f o r t t o provide anything above the minimum l e v e l of maintainability. The tight t i m e schedule and l o w budget did not allow much t e s t i n g and practice a t the reactor, and the very crowded condition i n t h e reactor c e l l is not conducive t o e f f i c i e n t maintenance. The handling of components where maintenance w a s anticipated such as control rods, space coolers, valves, heaters, and piping spool pieces has a l l gone smoothly. The a b i l i t y t o u t i l i z e e x i s t i n g craft forces with- modest t r a i n i n g was encouraging. Thejre i s no doubt, however, t h a t the performance can be improved. Among the items that w i l l be studied are t h e increased use of shielding t o cut down radiation l e v e l s , better mobility of the roof shield and the maintenance shield, and perhaps more than one maintenance shield. O f course, in t h e design of a l l the t o o l s , components and equipment, t h e speed o f t h e completion of the operation w i l l be considered.

I I

11 INCREASED RADIATION LEVEL

A general a r e a for study arises fâ&#x20AC;&#x2122;romthe increase i n t h e radiation level which is expected i n the WBR. The geometry f o r shielding maintenance personnel i s shown i n Figure 1. The radiation l e v e l where personnel w i l l operate t h e long-handled t o o l s a r i s e s from sources i n t h e reactor c e l l and varies inversely with t h e distance and t h e attenuation factor of t h e shielding. Gamma l e v e l s at the MSRE a s measured by in-cell ion chaaibers ,indicate t h a t t h e shielding provided there i s adequate. When t h e reactor i s operating a t 7 Mw, the l e v e l i s 60,000 r/hr. This drops t o 4000 r/hr immediately after draining the fuel. During a recent shutdown t h e radiation level i n the c e l l was 2000 r / h r seven days a f t e r shutdown while maintenance operations were i n progress and t h e work was accomplished without undue exposure of personnel.

For the MSBR, t h e radiation l e v e l s w i l l be considerably higher. This w i l l require additional shielding. A study w i l l be made t o evaluate t h e source strength during shutdown and methods for reducing t h e radiat i o n l e v e l s , such as using a flushing material, decontamination systems, and f l u i d shielding (perhaps a molten salt w i t h a low melting point). Also involved with an increldse i n radiation a r e d e t a i l s of t h e design of t h e long-handled t o o l s and t h e penetrations through t h e maintenance shield. I n t e r n a l voids i n t h e t o o l s and cracks i n t h e penetration represent radiatfon leakage paths, and e f f o r t m u s t be taken t o avoid them. otection of t h e maintenance crew w i l l begin Development of bet t h e radiation levels a t the MSFE with an analysis of-dat and the experience w i t h enance there. This information w i l l then radiation l e v e l s i n t h e MSBR and the MSBE, be applied t o analysis hielding and t o o l s w i l l be studied and Then the designs of sp s a r y shielding. Some new devices o r new improved t o provide t h approaches t o special shielding problems can be e x p e c t e d t o evolve, and mockups w i l l be b u i l t t o t e s t them. GENERAL MAINTENANCE DEVELOPMENT. AND DESIGN SURVEILLANCE

maintenance development program involves important part o t o r t o make c e r t a i n that t h e maintenance e design of t h are satisfied hen designing and tes ng the special This a c t i v i t y is ntenance operat ions 001s t o do a wide v a r i e t y e n t i r e l y concerned w i t h t h e breeder experiment; however, the experience loped are expected t o be useful f o r gained and t h e gener t h e -1-scale reactors.

u *

Fig. 1. Shielding Configuration f o r Maintenance Operations.

13 m

L J *

This a c t i v i t y w i l l be carried out i n about t h e following sequence.

1. Review preliminary flowsheets and equipment and plant designs t o make c e r t a i n that t h e maintenance requirements a r e integrated i n t o t h e design of t h e plant. Items of special importance include t h e shielding arrangement with emphasis on special shielding f o r maintenance, t h e shielding penetrations, t h e component support s t r u c t u r e s , t h e cranes and other equipment f o r handling radioactive components, and t h e viewing devices. 2.

Prepare proposals f o r a l l maintenance operations. and special disconnects.

Compile l i s t s

of problems, t o o l s

3. Review f i n a l designs as they a r e being made. requirements as necessary.

Revise maintenance

4. Build required special t o o l s , j i g s and fixtures. full-scale mockups and on the Engineering T e s t Unit.

T e s t them i n

5. Follow t h e construction of the MSBE and t h e i n s t a l l a t i o n of t h e equipment t o be c e r t a i n t h a t maintenance i s properly considered i f changes are made. t

6, Prepare procedures f o r maintaining the equipment i n

t h e MSBE.

PROGRAM COSTS AND SCHEDULE A preliminary estimate of t h e cost of t h e maintenance devebopment program f o r t h e MSBE i s shown i n Table 1. The a c t i v i t i e s a r e f i t t e d i n t o t h e schedule proposed i n ORNL-TM-1851 (Ref 2) f o r designing and building t h e plant. Although t h e maintenance considerations influence t h e design, t h e r e i s only one c r u c i a l development. The f e a s i b i l i t y of remotely welding the j o i n t s i n t h e main f u e l , blanket, and coolant l i n e s and vess e l s o r some s u i t a b l e a l t e r n a t i v e must be established before t h e design can be completed. The program i s designed t o demonstrate a s a t i s f a c t o r y j o i n t by t h e end of FY 1970, although considerable t e s t i n g and improvee expected-'to follow t h e demonstration. ment of equipment and t e c

I n t h e other areas w e and provide convincing evide schemes by t h e end of FY t o o l s and procedures w i l

e s t a b l i s h maintenance requirements the p r a c t i c a l i t y of t h e maintenance ever, t h e development o ished while t h e plant

Table 1. Estimate of Costs for Maintenance Development Program ( $ Thousands)

Remote Welding

30,

400

370

270

190

160

120

230

250

180

150

100

Core Replacement and Repair

160

100

Maintenance of the Heat Exchangers and Other Components

100

120

300

610

630

General Maintenance Development and Design Surveillance

15 I

8 $'.

REFERENCES

IP. R. Kasten, E. S. B e t t i s and R. C. Robertson, Design Studies of lOOO-Mw( e ) Molten-Salt Breeder Reactors , 0RN~-3996 (August 1966).

*R. B. Briggs, Summary of Objectives and a Program of Development of Molt en-Salt Breeder Reactors , ORNL-TM-1851 (June 1967) . 3E. C. R i s e , F. W. Cooke and R. G. Donnelly, "Remote Fabrication of Brazed S t r u c t u r a l J o i n t s i n Radioactive Piping , I 1 ASME 63-WA-53 (September 1964)

4E. H Seidler , "Welding ,I1 Advanced Fabrication Technolorn , Atomics International (Brochure). 5E. H. S e i d l e r , Layout and Maintenance, WCAP-1104, Vol. I V , Part 1 (March 1959) 6R. R. Irving , "Welding Reacts t o New Demands ,I1 Iron Age 191(14) ,

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1nt ernal Distribution 1.-50.

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76. 77. 78. 79.

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R. C. C. C. R. H. H. C.'

18 r â&#x20AC;&#x2122;

145. 146 147. 148. 149.

J. McHargue E. McNeese S. Meyer L. Moore P. Nichols 150. E. L. Nicholson 151. L. C. Oakes 152. P. Patriarca 153- A. M. Perry 154. H. B. Piper 155 B. E. Prince 156. J. L. Redford 157 M. Richardson 158. R. C. Robertson 159 H. C. Roller 160. H. C. Savage 161. C. E. Schilling 162. Dunlap Scott 163. H. E. Seagren 164. W. F. Schaffer 165. J. H.* Shaffer 166. M. J. Skinner 167. G. M. Slaughter 168. A. 8 . Smith 169. F. J. Smith 170. G. P. Smith

C. L. A. R. J.

171. 0. L. Smith 172. 173

P. W. I. R. H. J. E.

174. 175 176. 177 178. 179 R. 9

180.

181. C. 182.

183. 184. 185. 186. 187. 188. 189. 190.

191

B. A. J. W.

K. M. J. L. G. H.

G. Smith

F. Spencer Spiewak C. Steffy H. Stone R. Tallackson H. Taylor E. Thoma S. Watson F. Weaver H. Webster M. Weinberg R. Weir J. Werner W. West E. Whatley C. White V. Wilson Young C. Young

192. -193.

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E x t e r n a l Distribution 207.-208. 209. 210. 2ll.-225. 226. 227.-228. 229. 230. 231.-245. 246. 247.-248.

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