Thorough analyses and resolution of various errors in pin-homogenized multigroup core calculation

Abstract

The errors in pin-homogenized multigroup calculation (PHMGC) from various sources are thoroughly analyzed to find the optimal neutronics solution method. The dependence of the errors due to the use of pin-homogenized multigroup cross sections on the energy groups are investigated first to clarify the need for proper equivalence factor and more groups in pinwise calculation. Various transport approximations including the simplified PN (SPN) and the discrete ordinate method (SN) are then examined with different degrees of spatial discretization. It is demonstrated that the diffusion calculations involve significant error cancellation which would result in seemingly good results. It turns out that SP5 is not any better than SP3 and SN yields intolerable errors due to the inconsistency of single assembly based SPH factors especially noted in the fast energy range. Through the analyses, the source expansion nodal method solver of the SP3 equations is determined as the optimal solver for PHMGCs.