A New Approximation in the Calculation of Collision Probabilities in Cluster-Type Fuel Lattices

Abstract

A new approximation is proposed for calculating the first flight collision probabilities in cluster-type fuel lattices, based on the subcell model. In this method the unit cluster is divided into subregions of fuel pins (with or without cladding) arranged in rings, associated coolant, and homogeneous annular regions surrounding the fuel region. The fuel pins and associated coolant are treated separately, and each is distinguished ringwise as a different subregion. The collision probability is then calculated for a neutron proceeding from one subregion to another. The method treats rigorously the fuel annulus where a neutron starts with consideration given to the non-uniform structure of the annulus. The other fuel annuli, however, are treated approximately as if each annulus were homogeneous. The values of the homogenized cross sections of the annuli are determined in such manner that the number of outgoing neutrons from the homogenized annuli equals that from the actual non-uniform fuel annuli. The colli...