A conservative approach to consider burnup credit in criticality studies

In order to avoid criticality risks, several facilities operating spent PWR fuels have been designed considering the fuel as fresh. This choice has obviously led to considerable safety margins. In the early 1980's, a method allowing to consider the changes in the fuel composition during the depletion with some very pessimistic hypothesis (only actinides were considered and the amount of burnup used in the studies was equal to the mean burnup in the 50-least-irradiated centimeters) was accepted by the French Safety Authorities. As many facilities still want to optimize their processes (e.g. transport, storage, fuel reprocessing), the main French nuclear companies, researchers and IRSN set up a working group in order to study the way to take into account Burnup Credit in the criticality calculations, considering some fission products and a more realistic axial profile of burnup. The article presents the different questions that have been raised while studying the possibility to set an axial shape of the burnup and the absorption of the neutrons by some of the fission products. However the questions would also apply to a method with actinides only. They are sorted in the following topics: (i) which axial shape of the burnup should be considered, (ii) which composition of the irradiated fuel should be considered for every axial sub-part of the assembly, and finally, (iii) which confidence can we have in the criticality codes used to determine the k-effective. The paper highlights the questions raised during the work of a French working group on Burnup Credit but doesn't present a recommended approach. This approach is to be issued by the end of the year. (author)