Drift and diffusion coefficients for asymmetric cells of fast critical assembly.

Abstract

A calculational method of diffusion and drift coefficients for asymmetric slab cells has been derived on the basis of the first-flight collision probability method. The calculated values of the diffusion and drift coefficients for the asymmetric cells used in the fast critical assembly ZPPR-9 are shown. Drift coefficients calculated based on the original Duracz method and Duracz' cell averaging method are numerically compared. Flux distributions for one-dimensional slab models composed of the asymmetric cells such that U-238 plates are bunched at the core center are calculated with and without drift coefficients. It is seen that the Duracz original drift coefficient has well predicted the flux dip at the core center caused by neutron capture in the U-238 plates in lower energy, but the Duracz cell averaged drift coefficient is not able to predict the flux dip.