Implementation of second-order discontinuity factor for simplified P3 theory in Nem

The simplified P3 (SP3) neutronics solver has recently been developed in the Nodal Expansion Method (Nem) for providing the pin-cell level solutions in reactor core calculations. In this study, effort was made to improve the SP3 solution accuracy by taking into account the second-order Discontinuity Factor (Df) in addition to the zeroth-order Dfs, aiming to preserve the first-order and third-order angular moments of SP3 at a surface of the homogenized pin. More specifically, side-dependent Dfs are calculated for each homogenized cell based on the surface fluxes obtained in the heterogeneous lattice calculation using the neutron transport code Dragon [1] and the homogenized pin-cell calculation using Nem. Detailed calculation procedures are developed with appropriate approximations for the practical application of this method. The verification results for the C5G7 benchmark problem show that the second-order Df can improve the solution accuracy of the Nem SP3 solver significantly, make the solver a promising and affordable option for the direct pin-by-pin full core calculation. (Author)