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

Highlights: • The pinwise diffusion solver involves significant error cancellation. • The SP₅ is not better than SP₃ in the two-step calculation with the SPH factors. • The use of S_N for the two-step calculation with the SPH factors is not practical. • The SP₃ source expansion nodal method is determined as the optimal solver. • The spectral leakage correction technique is required for further improvement. 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 P_N (SP_N) and the discrete ordinate method (S_N) 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 S_N 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.