A Numerical Study of Heterogeneity Effects in Fast Reactor Critical Assemblies

An attempt is made of applying the shielding factor method with a coarse-group structure to cell calculations for plate-type fast reactor assemblies. The 24-group collision probability calculations based on the shielding factor method derived from the basic equation is shown to give results of practical validity for heterogeneity and bunching reactivities, as proved by comparisons with 1950-group collision probability calculations and experiments in the FCA assemblies V-2 and VI-2. The mechanism of the heterogeneity effect in the two assemblies is examined in detail. Most part of the heterogeneity reactivity is found to be due to the flux heterogeneity effect, whereas the reactivity associated with the change of plate pattern is mostly caused by the change of the flux heterogeneity effect in the case of the assembly V-2, and by the change of the heterogeneous shielding effect in the case of the assembly VI-2. In comparison with experiment the calculated results on the streaming of neutrons in the entire a...