INFORMATION NEXT

PROBLEMS

OXIDE

FUEL I.

S.

IN THE

ELEMENTS

DEVELOPMENT FOR

FAST

POWER

OF REACTORS

Golovnin

One of the chief f e a t u r e s in the c u r r e n t development of n u c l e a r power is the c r e a t i o n of nuclear p o w e r stations with fast r e a c t o r s . C o n s i d e r a b l e advances have b e e n made in a n u m b e r of countries as to the technical design of such r e a c t o r s . E x p e r i m e n t a l industrial stations with powers of the o r d e r of 300 million (electrical) kW each a r e a l r e a d y in the stage of adoption in the USSR, Britain, and F r a n c e . E x p e r i e n c e in the u s e of t h e s e installations and the m o r e powerful v e r s i o n s now being built in the USSR and USA (BN-600 and F F T F ) should o v e r the next decade provide all the data r e q u i r e d to c r e a t e and int r o d u c e l a r g e e l e c t r i c a l p o w e r stations of optimum p a r a m e t e r s with fast sodium b r e e d e r r e a c t o r s using mixed oxide fuel. At the p r e s e n t t i m e s c i e n t i s t s a r e a s s i d u o u s l y exchanging the r e s u l t s of r e s e a r c h and technical exp e r i e n c e . Several r e c e n t l a r g e international c o n f e r e n c e s have, in effect, s u m m a r i z e d p r o g r e s s up to 1975 and pointed the way to future d e v e l o p m e n t s . This s p e c i a l l y applies to the winter s e s s i o n of the A m e r i c a n Nuclear Society (Washington, October 27-31, 1974),* the F i r s t E u r o p e a n Nuclear Conference ( P a r i s , April 21-25, 1975), ~ and the C o n g r e s s of Specialists f r o m the International Agency on Atomic Energy Regarding P o s s i b l e D a m a g e to F a s t - R e a c t o r Fuel (Seattle, USA, May 12-16, 1975). E a r l i e r inv e s t i g a t i o n s w e r e mainly a i m e d at d e m o n s t r a t i n g the potentialities of fast r e a c t o r s and elucidating the b a s i c p r o b l e m s , e s p e c i a l l y those concerning the rue1 e l e m e n t s . The limited amount of i n f o r m a t i o n a v a i l a b l e as to the influence of r e a c t o r conditions on the p r o p e r t i e s of m a t e r i a l s p r e v e n t s all the effects which b e g a n to a p p e a r on i n c r e a s i n g the power, the neutron flux density, and the b u r n - u p of the fuel in the e x p e r i m e n t a l installations f r o m being t a k e n into account at the s a m e t i m e . One of the m o s t i m p o r t a n t effects, which has not as yet b e e n studied o v e r a sufficiently wide r a n g e of i r r a d i a t i o n (up to 4.1023 n e u t r o n s / c m 2 at E > 0.1 MeV), is the softening of the construction m a t e r i a l s , i . e . , the fall in l o n g - t e r m s t r e n g t h and ductility, the a c c e l e r a t i o n of h i g h - t e m p e r a t u r e e m b r i t t l e m e n t (austenitic s t a i n l e s s steels), and c r e e p . Effects which have b e e n d i s c o v e r e d include the neutron-induced swelling of c o n s t r u c t i o n m a t e r i a l s and the effect of the initial purity of the fuel and the a c c u m u l a t i o n of f i s s i o n f r a g m e n t s on the compatibility of the fuel with the m a t e r i a l of the f u e l - e l e m e n t can. On the whole t h e s e effects lead to a c e r t a i n (though not critical) reduction in the efficiency of fast r e a c t o r s as c o m p a r e d with the original o p t i m i s t i c e s t i m a t e s , although the exact extent of the effect cannot yet be a s s e s s e d . A p a r t f r o m design and technological d e v e l o p m e n t s , i m p o r t a n t f e a t u r e s in the c r e a t i o n of efficient fuel e l e m e n t s include the study of m a t e r i a l s and s t r u c t u r e s (both inside the r e a c t o r and a f t e r r e m o v a l ) , analytical investigations into efficiency b a s e d on e x p e r i m e n t a l data r e g a r d i n g the p r o p e r t i e s of m a t e r i a l s , and an a n a l y s i s of e m e r g e n c y situations f r o m the point of view of t h e i r influence on the efficiency of the installation as a whole and its safety under s e r v i c e conditions. E x p e r i m e n t a l investigations b a s e d on both c o o p e r a t i v e and independent p r o g r a m s a r e being c a r r i e d out by s c i e n t i s t s in the United States, Britain, F r a n c e , West G e r m a n y , Japan, Italy, and the USSR. Many y e a r s ' e x p e r i e n c e in the u s e of the f i r s t fast sodium r e a c t o r s has shown that the n u m b e r of f u e l - e l e m e n t f a i l u r e s involving oxide and mixed oxide fuel is l e s s than 1% for b u r n - u p s of m o r e than 10% of the heavy a t o m s . In individual c a s e s fuel e l e m e n t s r e m a i n efficient up to a b u r n - u p of 15-18% of the heavy a t o m s . D a m a g e only a p p e a r s visually in 0.1% of the total n u m b e r of fuel e l e m e n t s studied. *See At. ]~nerg., 38, No. 4, 268 (1975). ~See At. ]~nerg., 3_99, No. 3, 230 (1975). T r a n s l a t e d f r o m Atomnaya ]~nergiya, Vol. 40, No. 1, pp. 78-80, January, 1976.

9 Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. No part of this publication may be reprodueed, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $15.00.

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T h e r e a r e two m a i n causes of damage: produetion defects and the exhaustion of efficiency. E x p e r i ence in the m a n u f a c t u r e of 25,000 fuel e l e m e n t s for the Phenix r e a c t o r (France) r e v e a l e d that, subject to qualified technological monitoring, production defects could be completly e l i m i n a t e d . As yet it has not b e e n found p o s s i b l e to p r e d i c t the limit of efficiency exhaustion with a sufficient a c c u r a c y , since d a m a g e c r i t e r i a have not b e e n finally established, nor have the d a m a g e m e c h a n i s m s b e e n c l e a r l y f o r m u l a t e d . F u r t h e r m o r e , e x p e r i m e n t a l data r e g a r d i n g the influence of i r r a d i a t i o n on the p r o p e r t i e s Of m a t e r i a l s a r e as yet insufficiently r e l i a b l e . Hence the limiting d e g r e e of b u r n - u p of the fuel in the fuel e l e m e n t s is tO a l a r g e extent chosen on an a p p r o x i m a t e o r e m p i r i c a l b a s i s .~ F o r example, in the French Rhapsodie, Phenix, and Superphenix r e a c t o r s the b u r n - u p i s s p e c i f i e d as 8, 6.5, and 9% of the heavy a t o m s r e s p e c tively. C r i t e r i a r e l a t i n g to can d a m a g e depend on the m e a n s of loading, and m a y be d e t e r m i n e d , f i r s t l y , by r e f e r e n c e to the d e g r e e of nonuniformity of the p l a s t i c d e f o r m a t i o n of the m a t e r i a l arid the ductility limit, secondly f r o m the d e f o r m a t i o n r a t e , thirdly f r o m the steady c r e e p velocity, and fourthly f r o m t h e yield s t r e s s of the m a t e r i a l . Detailed study has b e e n devoted to the i n t e r a c t i o n between the mixed oxide fuel c o r e s and the a u s t e n itic s t a i n l e s s s t e e l cans which a r i s e s at t e m p e r a t u r e s exceeding 500~ mainly as a r e s u l t of the action of volatile cesium, iodine, and t e l l u r i u m f r a g m e n t s accumulating in the r e a c t i o n zone at the high oxidizing potential of the m e d i u m . The r a t e of i n t e r c r y s t a l l i t e p e n e t r a t i o n is high until roughly 3% of the heaxT a t o m s have b e e n consumed, a f t e r which it rapidly d i m i n i s h e s . F o r a l m o s t s t o i c h i o m e t r i c fuel c o m p o s i tions the depth of i n t e r c r y s t a l l i t e p e n e t r a t i o n m a y amount to 70-120 ~ for deep burnup, but it is much l e s s for s u b s t o i e h i o m e t r i c fuel. For s u p e r s t o i c h i o m e t r i c fuel, however, a u n i f o r m frontal oxidation of the inner s u r f a c e of the can o c c u r s to a depth of 10 ~, without i n t e r c r y s t a l l i t e r u p t u r e . A reduction in the density of the fuel, a r i s e in the linear power of the fuel element, and an i n c r e a s e in the gap between the. can and the c o r e , e s p e c i a l l y in the p r e s e n c e of e c c e n t r i c i t y , intensify the interaction. West G e r m a n r e s e a r c h showed that the i n t e r e r y s t a l l i t e p e n e t r a t i o n depended on the composition of the steel. G e r m a n t y p e s of steel m a y be placed in the following s e r i e s of i n c r e a s i n g i n t e r a c t i o n (the b r a c k e t s indicate foreign analogs to the West G e r m a n b r a n d s of steel): 1.4988 (AISI-31~, FV-548); 1.4919 (AISI-3Ig); 1.4981 (OKh16N15M3B); 1.4970 (121R72HV). The difference in the depth of i n t e r c r y s t a l l i t e p e n e t r a t i o n amounted to as much as 20% for different types of steel. The i n t e r a c t i o n due to the p r e s e n c e of f i s s i o n f r a g m e n t s does not lead to any direct d a m a g e to the oxide-fuel fuel e l e m e n t s , but it weakens the can. A change:to the use of ear'bide fuel fails to eliminate interaction: In this c a s e c a r b u r i z a t i o n of the inner s u r f a c e of the can m a y occur to a depth approaching 40 #. Visible d a m a g e to the fuel e l e m e n t s consists of longitudinal c r a c k s in the u p p e r and central p a r t s of the cans a r i s i n g as a r e s u l t of the exhaustion of efficiency in the m a t e r i a l . The central c r a c k s c o r r e s p o n d to the r e g i o n of m a x i m u m d i a m e t r a l d e f o r m a t i o n of the fuel e l e m e n t s and a r e m o s t probably a s s o c i a t e d ~tth a gradual i n c r e a s e in the m e c h a n i c a l action of the swelling c o r e and the gas p r e s s u r e inside the fuel e l e ment. The c r a c k s in the upper, hot p a r t , in which the oxide fuel is a l m o s t completely softened, a r e a s sociated with the t r a n s i e n t operating conditions of the fuel e l e m e n t , involving a considerable loss of ductility by the can (thermal r a t c h e t effect). T h e s e m e c h a n i s m s go hand in hand, and it is as yet hard to say which is the m o s t s e r i o u s . F o r boosted i r r a d i a t i o n conditions, c r a c k s in the hot p a r t of the fuel e l e m e n t p r e d o m i n a t e . C r a c k s in the middle a r e usually f o r m e d a f t e r d i a m e t r a l distortions of over 1%. The p e r m i s s i b l e d e f o r m a t i o n of the m a t e r i a l in the u p p e r p a r t of the element can hardly be t a k e n as g r e a t e r t h a n 0.1%. Under n o r m a l s e r v i c e conditions the initiation and development of d a m a g e in one fuel element does not have any m a j o r effect on the whole a s s e m b l y . The t i m e between the loss of h e r m e t i c p r o p e r t i e s a n d the opening of a c r a c k m a y extend to s e v e r a l months. Investigations into the b e h a v i o r of fuel e l e m e n t s with a r t i f i c i a l defects have r e v e a l e d a r e l a t i o n s h i p between the d e v e l o p m e n t of d a m a g e and the f o r m a t i o n of sodium uranoplutonate under the can. Only if the coolant flow is reduced or stops altogether, or if individual open c r o s s sections of the a s s e m b l y b e c o m e clogged, is it possible for d a m a g e to p r o p a g a t e to the neighboring fuel e l e m e n t s ; in the case of oxide fuel the e m e r g e n c y is localized within a single a s sembly. The individual m e c h a n i s m s leading to the exhaustion of the efficiency r e s e r v e of the fuel e l e m e n t s a r e quite closely a s s o c i a t e d , for example, with the r e d i s t r i b u t i o n of the fuel within the i n t e r i o r of the fuel element, the a c c u m u l a t i o n of fission f r a g m e n t s , gas evolution under the can, and so on. At the s a m e t i m e the influence of the neutron-induced swelling of the cans, which p r e d o m i n a t e s over mechanical d e f o r m a tion at t e m p e r a t u r e s below 525~ and a l s o the r o l e Of radiation-induced c r e e p , have not yet been studied anything like sufficiently. One has the i m p r e s s i o n that the m a i n effort should be directed at creating v e r y strong and ductile c o n s t r u c t i o n m a t e r i a l s , although combating the swelling effect is still a m a t t e r of no

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m e a n i m p o r t a n c e . It is quite p o s s i b l e that it will p r o v e d e s i r a b l e in the future only t o use low-swelling m a t e r i a l s for the housings of the whole f u e l - e l e m e n t a s s e m b l i e s . T h e o r e t i c a l investigations play a m a j o r p a r t in e s t a b l i s h i n g the m e c h a n i s m s of f u e l - e l e m e n t d a m a g e . M a t h e m a t i c a l m o d e l s and computing p r o g r a m s developed by s c i e n t i s t s in v a r i o u s countries have in g e n e r a l p r o v e d p e r f e c t l y adequate. D i f f e r e n c e s between v a r i o u s t h e o r i e s have in the m a i n been a s s o c i a t e d with the i n t e r p r e t a t i o n and a s s e s s m e n t of individual p r o p e r t i e s , as well as the computing speed and a c c u r a c y . Some p r o g r a m s have b e e n developed in g r e a t detail, and account for quite refined phenomena (the effect of changes in the isotopic c o m p o s i t i o n of the fuel, the continuous change in contact t h e r m a l conductivity, r a d i a l nonuniformity of the neutron-induced swelling, etc.). T h e s e useful computing p r o g r a m s cannot yet be fully exploited in view of the a b s e n c e of adequate e x p e r i m e n t a l data r e l a t i n g to the p r o p e r t i e s of the m a t e r i a l s . This f a c t o r a l s o i m p e d e s the e s t a b l i s h m e n t of the limiting and s t a t i s t i c a l l y a v e r a g e d values of the f u e l - e l e m e n t efficiency c r i t e r i a . Special attention should be given to the b e h a v i o r of fuel e l e m e n t s under t r a n s i e n t conditions of o p e r a tion. T h e s e include the rapid t r a n s i e n t p r o c e s s e s a s s o c i a t e d with the ~riggering of the e m e r g e n c y p r o t e c tion s y s t e m , failure in the p u m p s of the f i r s t circuit, a s u r g e in the power of the r e a c t o r , p a r t i a l or c o m plete blocking of the coolant flow through the f u e l - e l e m e n t a s s e m b l y , and finally r e p e a t e d changes in power level. The a i m of t h e s e investigations (the i m p o r t a n c e of which it is h a r d to o v e r e s t i m a t e ) is to e s t a b l i s h the limitations which will enable f u e l - e l e m e n t efficiency to be maintained right up to the onset of an e m e r gency situation. Such c r i t e r i a include, for e x a m p l e , the limiting p l a s t i c d e f o r m a t i o n of the c o n s t r u c t i o n m a t e r i a l s in v a r i o u s t e m p e r a t u r e r a n g e s , for v a r i o u s loading r a t e s , and for v a r i o u s doses of neutron i r r a d i a t i o n and d e g r e e s of c h e m i c a l i n t e r a c t i o n with the ambient; the p e r m i s s i b l e p o w e r s u r g e s ; the p e r m i s s i b l e s c a l e s of fuel melting; the r u p t u r e of the can by gas p r e s s u r e on overheating, and so on. The extension of s t e a d y - s t a t e and q u a s i - s t e a d y - s t a t e t h e o r e t i c a l models to the c a s e of rapidly changing p a r a m e t e r s r e q u i r e s c o n s i d e r a b l e d e v e l o p m e n t and c a r e f u l c o m p a r i s o n between the analytical r e s u l t s and e x p e r i m e n t . It is i m p o r t a n t , for e x a m p l e , t o allow for the t r a n s i e n t nature of gas evolution, the r e d i s t r i b u t i o n of p o r o s i t y in the fuel, m a s s t r a n s f e r on melting, the t r a n s i e n c e of the m e c h a n i c a l i n t e r a c t i o n between the c o r e and the can, the growth r a t e of the gas p r e s s u r e (which m a y amount to hundreds of a t m o s p h e r e ) , and SO

on,

A n u m b e r of w e s t e r n c o u n t r i e s and a i s o J a p a n have u n d e r t a k e n combined and independent p r o g r a m s for studying the efficiency of fuel e l e m e n t s under t r a n s i e n t conditions, including e x p e r i m e n t s inside the r e a c t o r involving p o w e r s u r g e s and coolant l o s s e s , and investigations a i m e d at establishing the influence of rapidly changing p r o c e s s e s on the hydraulic and t h e r m o p h y s i c a l c h a r a c t e r i s t i c s of the f u e l - e l e m e n t a s s e m b l i e s . It is a l s o intended to c a r r y out e x p e r i m e n t s on the heating of fuel e l e m e n t s i r r a d i a t e d to v a r i o u s b u r n - u p s outside the r e a c t o r . The p r o g r a m s envisage the use of the TREAT pulse r e a c t o r (USA), which enables p o w e r s u r g e s to be introduced in a controlled way, with a visual a s s e s s m e n t of the r u p t u r e p r o c e s s , and a l s o loops of the e x p e r i m e n t a l F r e n c h t h e r m a l r e a c t o r s , in which it is intended to c a r r y out s i m i l a r e x p e r i m e n t s with fuel e l e m e n t s p r e v i o u s l y i r r a d i a t e d in the fast Rhapsodic r e a c t o r . Individual e x p e r i m e n t s have a l r e a d y b e e n c a r r i e d out. The f o r m i n g of the i r r a d i a t e d fuel on melting and the r u p t u r e of the can by gas p r e s s u r e at points of overheating have b e e n studied, and the r i s e in gas p r e s s u r e in the fuel e l e m e n t following a p o w e r s u r g e in the T R E A T r e a c t o r has b e e n e s t i m a t e d . P r e l i m i n a r y e x p e r i m e n t s have shown that the r u p t u r e of the fuel e l e m e n t s under t r a n s i e n t conditions m a y be a s s o c i a t e d with the rapid evolution of gas f r o m the m o l t e n fuel. Gas evolution f r o m the solid fuel begins playing a m a j o r p~rt in the t r a n s i e n t p r o c e s s b e f o r e the onset of melting, leading to additional m a s s t r a n s f e r ; the onset of can r u p t u r e depends on the original s t r u c t u r e of the c o r e . A b r i e f power s u r g e does not c a u s e any s e r i o u s d a m a g e to the f u e l - e l e m e n t cans. The effect of the efficiency of the fuel e l e m e n t s on the question of safety in the running of fast r e a c t o r s is of i m p o r t a n c e in connection with the p o s s i b l e extension of any e m e r g e n c y situation (including that a s s o c i a t e d with a n o m a l o u s working conditions) beyond the confines of the f u e l - e l e m e n t a s s e m b l i e s , to e m b r a c e p a r t or the whole of the a c t i v e zone itself. Models a r e being developed to r e p r e s e n t the m e c h a n i s m s underlying the development of such hypothetical e m e r g e n c i e s in o r d e r to e s t a b l i s h the conditions under which the i r r e g u l a r i t y m a y be e n t i r e l y contained within the f u e l - e l e m e n t a s s e m b l i e s , so that a c o m p l e t e d a m a g e d a s s e m b l y m a y be r e p l a c e d without disturbing the running c h a r a c t e r i s t i c s of the r e a c t o r as a whole. It is c o n s i d e r e d that when using oxide fuel it should be p e r f e c t l y possible to m a k e a r e l i a b l e d e t e r m i n a t i o n of the location and extent of the d a m a g e and to r e p l a c e the f u e l - e l e m e n t a s s e m b l y quite safely, subject to the development of the n e c e s s a r y m e a s u r i n g devices r e c o r d i n g the p r e s e n c e and type of activity of the f i s sion f r a g m e n t s in the r e a c t o r .

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