Accuracy of MICROX-2 for PBR analysis using Monte Carlo technique

One of the important issues in a nodal diffusion analysis of a PER core is the generation of accurate nodal constants. As compared to a light water reactor (LWR) lattice, in which the variation of the core properties in the axial direction is relatively weak and therefore a 2-D modeling appropriate, the calculation at the core sub-region level for a PER needs to account for the third spatial dimension because of the complex geometry (double heterogeneity) of packed arrangements of spherical pebbles. The purpose of the present work was to assess the capability of the MICROX-2 code to generate accurate nodal cross sections for PER lattices. This preliminary evaluation was based on comparison to a continuous-energy, doubly heterogeneous MCNP reference model. The terms of the comparison were the infinite medium multiplication factor, the few-group cell-homogenized total, capture and fission cross-sections, and the spectral indices. The principal phenomena covered by these choices pertain to resonance treatment and double heterogeneity. The models developed here do not attempt to evaluate the effect of packing randomness at either heterogeneity level. Such a study will be discussed elsewhere. The double heterogeneity of the lattice cell was fully modeled in the MCNP reference model. The cell-homogenized few-group cross sections and spectral parameters were calculated in MCNP by using the reaction rates and flux tallies. A spherical geometry model was used with MICROX-2. The analysis was completed at cold, room temperature (296 K) and at hot operating conditions (1073 K). For consistency, the same cross section data files were used for generating both the pointwise cross sections for MCNP and the fine-group cross sections for MICROX-2. Results showed significant differences between the MCNP and the MICROX-2 results, especially in the thermal and resonance energy range.