Scientific Secretary of IWGFR
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XA0100185-ZOM International Atomic Energy Agency IWGFR/16 INTERNATIONAL WORKING GROUP ON FAST REACTORS Technical Committee Meeting on FUEL AND CLADDING INTERACTION Tokyo, Japan February 21-25, 1977 SUMMARY REPORT Scientific Secretary 32/07 of IWGFR Reproduced tiy the IAEA in Austria April 1977 77-2721 PLEASE BE AWARE THAT ALL OF THE MISSING PAGES IN THIS DOCUMENT WERE ORIGINALLY BLANK INTERNATIONAL WORKING GROUP ON FAST REACTORS Technical Committee Meeting on FUEL AND CLADDING INTERACTION Tokyo, Japan February 21-25, 1977 Chairman: Kunihiko Uematsu Fast Reactors Development Project PNC, Japan Scientific Secretary: E. A. Khodarev International Atomic Energy Agency Recording Officers: Hiromitsu Kaneko Fast Reactor Development Project PNC, Japan Saburo Kikuchi Fast Reactor Development Project PNC, Japan 1. SUMMARY AND RECOMMENDATIONS OP THE MEETING 1 2. APPENDIX I. AGENDA OP THE MEETING 6 3. APPENDIX II. SCHEDULE OP TECHNICAL VISIT 6 4. APPENDIX III. LIST OP PARTICIPANTS 7 5. APPENDIX IV. LIST OP PAPERS 8 6. PAPERS 9 Chemical and mechanical properties aspects of this interaction were SUMMARY AND RECOMMENDATIONS covered, as well as information on interactions involving candidate 1. INTRODUCTION. buffer/getter materials. The Technical Committee Meeting on Fuel and Cladding Interaction An introductory Japanese paper reviewed available information on was held at the headquarters of the Power Reactor and Nuclear Fuel the parameter-dependency of stainless steel cladding attack using avail- Development Corporation of Japan. The meeting, which was sponsored able world-wide data from irradiated oxide test pins. Important fuel by the International Atomic Energy Agency (IAEA) on the recommenda- pin parameters that were identified included cladding temperature, fuel tion of the International Working Group on Fast Reactors (IWGFR), O : M ratio, burn-up and cladding pin diameter (residence time); was attended by eighteen participants from Belgium, France, the Federal factors with a relatively small influence included fuel form, fuel density, Republic of Germany, Japan, the United Kingdom, the United States and linear heat rating and cladding composition and pre-treatment (degree two International Organizations: IAEA and OECD. The purpose of the of cold-working). Meeting participants expressed the view that linear meeting was to provide a forum for exchange of information on chemical heat rating should be recognised as an important parameter. interaction between fuel and cladding tubes and ways and means of handl- ing potential chemical interaction in designing LMFBR fuel pins. An FRG paper reviewed out-of-pile capsule results on chemical Although the main stress was concentrated on the chemical aspects, fuel- aspects of fuel cladding chemical interaction (FCCI). From this work cladding mechanical interaction was also touched upon at the meeting. Cs, Te and I were identified as the most important aggressive fission products, and oxygen availability was recognized as an essential require- The technical parts of the meeting were divided into four major ment for attack. An investigation of the mechanical properties of sessions, as follows: corroded cladding specimens showed more complex behaviour than ex- I. Out-of-Pile Experiments pected on the basis of simple wall thinning. A weakening coefficient II. Irradiation Results was proposed to account for these variations. III. Mechanisms of Interaction IV. Fuel Pin Design Capsule experiments performed in the British programme showed In each session, papers were presented by the specialists on behalf type-316 stainless steel cladding suffered intergranular penetration when of their countries. Each presentation was followed by an open discussion exposed to Cs, Te, I mixtures buffered by VO/V2O3, Fe/FeO, and Mo/ in the general area covered by the paper. MoO2 (678 C). Under these conditions tellurium was a necessary ingredient for deep intergranular penetration. Depending on the buffer The technical sessions were followed by a final summary session (oxygen activity), attack by the liquid phase produced deeper or shallower at which each of the four technical sessions was summarized, conclusions attack than the gaseous phase. Buffer concepts (Nb, Mn, VO, Ta, Mo, were drawn, and recommendations were made. The resulting summaries, W) were evaluated using thermodynamic and kinetic information. On the conclusions and recommendations, which were agreed upon by the dele- basis of low oxidation rates Mo and \V were eliminated as effective gates, are presented below. The meeting agenda, list of participants and inhibitors. technical papers are reproduced elsewhere in this document. A second Japanese paper presented results on mechanical proper- 2. SUMMARIES AND CONCLUSIONS ties and fracture behaviour of stainless steel tensile specimens corroded 2.1. Session I. Out-of-Pile Experiments. in liquid CsOII media. Corrosive reaction was severe. Pre-treatment conditions affected the character of corrosion zones and the stress/ Six papers were presented in this session which focussed on the strain behaviour of corroded specimens, and that the ultimate tensile results of out-of-pile experiments related to fuel cladding interaction. strength of corroded zones was 20% - 30% of that of uncorroded material. However, when the strain was less than 0.5% it was found that the strated that fission product Cs will associate strongly with Nb and Ti 2 stress/strain behaviours were very similar between corroded and un- oxidation products. Finally, compatibility tests between Nb, Ti, and corroded zones. Some peculiar features of the fracture behaviour, i.e., V, and type-316 stainless steel demonstrated that compatibility problems longitudinal cracks, are probably attributable to the use of CsOH(l) as between candidate buffer/getter materials and the cladding are not ex- corrodant. pected for temperatures up to 800 C. A variety of out-of-pile results directly and indirectly related to Only one paper dealt with carbide fuel. Both out-of-pile and in- fission product-induced cladding attack were described in an American pile experiments showed that carburisation of the stainless steel cladding paper. New oxygen potential data were presented for mixed oxide fuel should not cause greater problems in Me-bonded pins. The fuel, how- with and without dissolved fission products and for equilibria including ever, should not contain free metal.- The oxygen content should be possible fission product reaction phases. From these data conclusions lower than that normally encountered in fuels fabricated by carbothermic were drawn about the relative stabilities of different Cs compounds inside reduction processes (~35OO ppm) if high fuel centre temperatures are oxide fuel pins. Carefully controlled capsule experiments were used to to be allowed. At fuel-cladding interface temperatures higher than delineate the conditions for Cs, Te-induced attack of type-316 austenitic 950 C rapid liquid phase reaction has been observed both with hyper- stainless steel. An oxygen activity threshold was established for inter- stoichiometric and stoichiometric carbide. These results may have granular attack of the cladding, and also an oxygen activity dependence some relevance when safety aspects of fuel pin behaviour are considered. for the type and rate of attack. The fission product composition (Cs:Te ) and inventory were also shown to influence attack depth. Of the common 2.2. Session 11. Irradiation Results. fuel fabrication impurities carbon was found to have the greatest catalytic effect on Cs, Te-induced intergranular attack. C-14 labelling experi- Nine papers were presented in this session; seven concerning ments and SEM examinations of attacked stainless steel specimens showed in-pile fuel pin corrosion behaviour and two on fuel cladding mechanical that carbon impurity does not advance ahead of the visible attack front effects. and that, during intergranular attack, carbon is removed from the grain boundary carbides as they are attacked. Cold-work and carburisation In Belgium, France, Japan, the United Kingdom, and the U.S.A. of cladding surfaces leads to a diminution of attack depth, and evidence three general forms of clad attack were observed; grain boundary attack, was presented for a linear relation between attack depth and cladding general matrix dissolution or oxidation, and spalling in which the layer grain size. A variety of alternate cladding alloys were screened for of clad is detached from the cladding surface. susceptibility to attack by Cs, Te mixtures at "high" and "low" oxygen activities. At "low" oxygen activities the depth of intergranular attack The incidence of internal clad attack observed from the destructive decreases with increasing nickel content of the alloys. Results were examination of fuel pins showed a more consistent pattern from sub- repeated for Cs thermal migration from column of mixed oxide fuel into assembly irradiations than from trefoil or single pin irradiations. UO2+X blanket fuel. Cs thermal migration was rapid with low O : M fuel, and Cs+UO2 chemical interaction in the blanket was accompanied by +x The maximum depth of internal clad attack would appear to generally pellet swelling and cladding strain. Some evidence for cladding compo- be in the range 4-0 - 80 micron, although values as high as 210 micron nent (Fe, Cr, Ni) mass transport by liquid Cs-Te mixture was obtained have been reported. The depth of cladding attack increased in the tem- from annular temperature gradient capsule experiments. Temperature perature