Validation of simplified methods for fuel depletion calculations in gas-cooled fast reactors

In this article, an overview of recent results obtained in the study of core depletion calculations for gas-cooled fast reactors is presented. The objective is to validate simplified depletion methods which could be used to analyze a large variety of core designs with different geometry and fuel compositions in the full core simulations. The codes used to perform the fuel depletion were MCNPX 2.6.0 linked to CINDER90 and TRIPOLI-41 coupled to MENDEL depletion solver. In the case of TRIPOLI-4, three different numerical techniques for time integration of the fuel depletion calculation were applied; these are the standard Euler explicit method, the CSADA method, and the CELL-2 method. The results obtained with these three techniques were compared with CSADA method available in MCNPX-CINDER90. The standard Euler technique is a first order method, which assumes there is a constant neutron flux over the entire time step, while CELL-2 and CSADA methods are of second order. The depletion calculations were made for a simplified core configuration where each assembly was represented by a homogeneous volume. The effect of the number of fission nuclides that were tracked in the depletion calculation with MCNPX-CINDER90 was also studied and reported in this article. Results are presented for the effective multiplication factor as a function of irradiation time. Furthermore, the evolution of the atomic densities, for a selected group of isotopes, was also compared. It was found that there are no significant differences in the results obtained with the two codes, and that the Euler explicit method, which is an approximation of first order, resulted adequate to simulate the fuel depletion evolution obtaining a very good approximation in less time.