Advanced Nodal Methods of the Few-Group BWR Core Simulator NEREUS

A few-group nodal BWR core simulator NEREUS, which is based on the analytic polynomial nodal method, has been developed. The analytic polynomial nodal method is applicable to multigroups and has good accuracy for analysis of cores having large spectral mismatch between fuel assemblies, since the intranodal thermal flux can be correctly represented. In NEREUS, the following advanced methods are adopted to overcome the shortcomings of the conventional analytic polynomial nodal method. The flux solution iteration matrix is cast into the form of finite difference, by using the flux discontinuity factors which correct the finite difference error as well as the homogenization error. The intranodal burnup and spectral history distributions are considered in the method, and source moments are obtained by orthogonal expansions. Burnup calculations are made by looking-up exposure dependent tables for macroscopic cross sections, which are prepared by single assembly spectrum and depletion calculations. A unified mod...