Reactivity Analysis Model Based on Finite Difference Method for Three-Dimensional Fast Breeder Reactor Core Deformation

Abstract

An analysis model has been proposed to evaluate reactivity due to horizontal fast breeder reactor (FBR) core deformation in seismic events by direct three-dimensional eigenvalue calculations, which are impossible for current neutronic analysis programs. The model is based on a current-centered finite difference neutron diffusion calculation method. Macroscopic neutron reaction cross sections are defined, which take into account changes in both mesh volume and material composition. Further, the expression of vertical neutron current is modified in such a way as to take into account changes in vertical mesh interface areas. By using these macroscopic neutron cross sections and the modified expression for vertical neutron current, it is possible to calculate the effective multiplication factor of a deformed FBR core within the bounds of a finite diffusion calculation method using the same mesh division used for the normal non-deformed core. Computation time and computer core memory required by the presented model are almost the same as in current finite difference methods. The calculated reactivities for simple one-dimensional slab, two-dimensional slab, and three-dimensional hexagonal systems agreed within 5% of those obtained by either a finite method or a finite difference method. The agreement was particularly good (within 2%) for cases in whichmore » fuel assembly pitches decrease around the horizontal core midplane; therefore, large reactivity is inserted.« less