B N Theory: Advances and New Models for Neutron Leakage Calculation

Abstract

Nuclear reactor physics design and analysis requires broad knowledge of parameters affecting reactor operation. Power distributions, control rod worth, shut down margins, isotopic depletion rates, etc., have to be determined throughout the reactor cycle. The efficiency of predicting these quantities depends strongly on models used to determine neutron density in space, direction and energy. When thermal-hydraulic characteristics of the reactor and nuclear data are available it seems as if one should be able to solve the three-dimensional transport equation. Unfortunately this is not true. The use of direct methods for solving the three-dimensional transport equation is limited because of the complexity in explicit modeling of every fuel pin, control rod, burnable poison rod and coolant channel. Although considerable progress has been made in recent years to increase the capabilities of digital computers, the magnitude of the computational problem created by explicit modeling is such that even most sophisticated computers are still not capable of evaluating reactor parameters to a satisfactory degree of accuracy. In order to overcome this computational problem of explicit geometrical modeling, most reactor analysis methods are based on coupling geometrically simple and energy dense calculations with geometrically complex and energy scarce calculations by way of spatial homogenization more » and energy group collapsing. In other words the neutronic calculations for a nuclear reactor are separated into two global steps. First, the fine multigroup transport calculation of assemblies or cells is carried out, with 60 to 100 energy groups, or more, where each cell consists of fuel pin, clad, coolant and/or moderator. This calculation is followed by a space homogenization and an energy group condensation of cross sections using the neutron spectrum obtained for these assemblies or cells. « less