Multigroup Diffusion and SP3 Solutions for a PWR MOX/UO2 Benchmark with the Code DYN3D

The core neutronic and thermal-hydraulic simulator code DYN3D, which has been developed for three-dimensional analyses of steady states and transients in thermal reactors with quadratic or hexagonal fuel assemblies, is based on nodal methods for the solution of the two group neutron diffusion equation. Loading cores with higher content of MOX fuel, the increase of the fuel cycle length, and the consideration of new reactor types require improvements of these standard methods. Therefore, DYN3D has been extended to use more than two energy groups. Furthermore, a nodal expansion method for solving the equations of the simplified P3 approximation (SP3) of the multigroup transport equation was developed for quadratic fuel assembly geometry. The new extensions of DYN3D are verified with calculations of the OECD/NEA and U.S. NRC PWR MOX/UO2 Core Transient Benchmark. The nodal SP3 method in DYN3D is outlined and results for a steady state are shown, which are compared with the solutions of the diffusion and transport codes used by the participants of the benchmark. Improved DYN3D-results are obtained with the new extensions.