Analysis of Control Rod Ejection Accident in MOX and MOX/UOX Cores with Time-Dependent Multigroup Pin-by-Pin SP₃ Methods

Multigroup, pin-by-pin transport methods based on the SP₃ approximation have been implemented in the U.S. NRC neutronics code PARCS and applied to analysis of a control rod ejection transient for MOX/UOX and UOX fueled cores. The purpose of this paper is twofold. First, comparisons are made of rod ejection transient results for the MOX/UOX and UOX cores in order to assess the impact of MOX on core transient performance. Results here indicate that the MOX/UOX core shows an earlier and smaller power peak compared to the UOX core. Point kinetics parameters were generated from the spatial kinetics results in order to help explain the observed behavior. The second purpose of this paper is to assess the importance of multigroup transient transport calculations for highly heterogeneous problems. A comparison of the 8-group pin-by-pin SP₃ with the conventional two group nodal diffusion methods indicates that the difference of the predicted control rod worth is a large source of the discrepancy between the higher and lower order results. (author)