Chemical interaction between the oxide and the clad in PHENIX fuel at burnup up to 60,000 MWd/t

In every fuel element there is a potential problem of chemical interaction between the fissile portion and the clad. As a matter of fact, even if the choice of materials is made after having established a satisfactory chemical compatibility between the fuel- (UO2 (U,Pu)O2, (U,Pu) C, . . .) and the clad (stainless steel, zircaloy, . . . ) out of pile, it is difficult to guarantee this compatibility after operation in the reactor due, on one hand, to the presence of fission products and, on the other hand, to impurities which are always present in the fuel to a greater or lesser degree. The fuel element currently chosen for the sodium-cooled fast reactors ((U,Pu)O2 in stainless steel clad) does not avoid this problem, in particular because of the relatively high temperatures envisioned for this type of reactor - the clad temperature is about 650 deg. C. Since it is considered as a demonstration reactor, Phenix should be able to provide additional information on this phenomenon, and one will see that we have been able to shed light on some points which the experiments or irradiations made to date have been unable to explain. However, before presenting the experimental results obtained with Phenix fuel end drawing conclusions, we shall give a brief resume of the expected behavior of this fuel with respect to the phenomenon of interest. (author)