An Analysis of Reflector Homogenization Techniques for Full Core Diffusion Calculations

Today's reactor core calculations are done in diffusion with a few coarse groups and require the homogenization of the core assemblies as well as a correct representation of the reflector. In industrial applications a homogeneous reflector is often used with cross sections obtained from transport calculations and adjusted to fit in-core measurements. However, the need for better precision in the core diffusion calculations and the emergence of new reflector concepts, such as for the European Pressurized Reactor (EPR), require an increase in the number of coarse groups for novel loading patterns and a rethinking of how to define the equivalent reflector. In this work we analyze and extend current techniques for the reflector homogenization for core calculations. Following the adopted industrial methodology, we have perfected a technique for the determination of an equivalent homogenous reflector by implementing a Particle Swarm Optimization Algorithm and showed its limitations through the analysis of an academic slab reactor model and of a realistic two-dimensional representation of the EPR. We have compared the precision of the resulting core calculations to transport reference calculations as well as to diffusion calculations using a multigroup albedo boundary condition. We have also explored the use of current-preserving flux discontinuity coefficients at the core-reflector interface in conjunction with an equivalent reflector.