A trigonal nodal SP₃ method with mesh refinement capabilities - Development and verification

The neutronics model of the nodal reactor dynamics code DYN3D developed for 3D analyses of steady states and transients in Light-Water Reactors has been extended by a simplified P₃ (SP₃) neutron transport option - to overcome the limitations of the diffusion approach at regions with significant anisotropy effects. To provide a method being applicable to reactors with hexagonal fuel assemblies and to furthermore allow flexible mesh refinement, the nodal SP₃ method has been developed on the basis of a flux expansion in triangular-z geometry. In this paper, the derivation of the trigonal SP₃ method is presented in a condensed form and a verification of the methodology on quasi-pin level is performed by means of two single-assembly test examples. The corresponding pin-wise few-group cross sections were obtained by the deterministic lattice code HELIOS. The power distributions were calculated using both the trigonal DYN3D diffusion and SP₃ solver and compared to the HELIOS reference solutions. Close to regions with non-negligible flux gradients, e.g., caused by the presence of a strong absorbing material, the power distribution calculated by DYN3D-SP ₃ shows a significant improvement in comparison to the diffusion method.