Taking burnup into account in criticality studies using an axial profile of burnup and some fission products.
C. Lavarenne, D. Biron and D. Janvier, R. Cousin, M. Doucet, J.P Grouiller, A. Lebrun, N. Thiollay, E. Guillou, G. Leka, H. Toubon, ANS Nuclear Crticality Safety, 11/15/11 2001, Reno, Nevada, USA.
In order to avoid criticality risks, a large number of facilities using spent fuels have been designed considering the fuel as fresh. This choice has obviously led to considerable safety margins.
In the early 80's, a method was accepted by the French Safety Authorities allowing operators to consider the changes in the fuel composition during depletion with some very pessimistic hypotheses: only actinides were considered and the amount of burn-up used in the studies was equal to the mean burn-up in the 50-least-irradiated centimeters.
As many firms still want to optimize their processes (e.g. transportation, storage, fuel reprocessing), the main companies involved in the French nuclear industry and irsn set up a Working Group in order to define the way burn-up could be taken into account in the criticality calculations, considering some fission products plus a more realistic axial profile of burn-up.
The first part of this article introduces the current French method used to take burn-up into account in the criticality studies. The second part is devoted to the studies achieved by the Working Group to improve this method, especially concerning the consideration of the neutron absorption of some fission products and of an axial profile of burn-up: for that purpose, some results are presented related to the depletion calculations, the definition of an axial profile for the calculations, the knowledge of the fission products absorption and the calculation tools. In the third part, the results (keff) obtained with the new method are compared to those obtained with the current one.
The conclusions presented are related to the present state of knowledge and may differ from the final conclusions.