On the few-group cross-section generation methodology for PBR analysis

The generation of accurate few-group nodal parameters, which strongly influences the accuracy of a reactor core criticality analysis, constitutes an important aspect of the work in developing advanced neutronics methodologies for pebble bed reactors (PBRs). As compared to other methodologies currently used for light water reactors (LWRs), the calculation at the core sub-region level for PBRs needs to account for the third spatial dimension because of the double heterogeneity of the configuration. This paper considers two modeling approaches for few-group cross-section generation: Monte Carlo (the MCNP code) and deterministic transport (the MICROX-2 code). Results from these codes are used to discuss the effect of the modeling assumptions (parameters) on the accuracy of the solution. Few-group cross-sections and spectral indices are calculated and compared for different packing fractions of the pebbles in the pebble lattice cell and different values of the moderator-to-fuel pebble ratio, temperature, and fuel burnup.