Microreactor Benchmark Problems and Effects of Core Environment on Multigroup Cross Sections

A set of single-assembly and full-core benchmark problems that are based on a heat pipe–cooled microreactor design are developed and presented. Monte Carlo solutions are given that were obtained using the continuous-energy Serpent 2 code. The compiled results consist of eigenvalues, flux spectra averaged over fuel and moderator volumes within fuel assemblies, and fuel pin fission density distributions for each problem. In addition, corresponding multigroup macroscopic and microscopic cross sections in fuel pins, moderator rods, and heat pipes located in selected positions within a single fuel assembly and those corresponding positions in multiple such fuel assemblies at selected locations within a whole-core configuration are provided. Using these results, the effects of the core environment on multigroup cross sections are determined. It was found that multigroup cross sections obtained with the B-1 leakage correction for a given material varied as a function of axial and radial location in the whole-core problems. Core environment effects on microscopic and macroscopic cross sections were manifested as significantly strong variations with energy group and location. For example, the effects on fuel macroscopic fission cross sections ranged from zero to 59.7% across all groups and locations. The effects with large magnitude occurred in regions close to the core axial and radial peripheries, where neutron leakage is expected to be strong and thus spectral shifts to be important. The Monte Carlo–derived cross sections provided in this paper were generated via whole-core continuous-energy Monte Carlo calculations, and therefore avoid the fidelity and accuracy shortcomings encountered when calculating cross-section libraries using multigroup deterministic lattice physics transport codes. The Monte Carlo benchmark data, including cross sections and solutions, should be suitable to serve as a resource for benchmarking microreactor physics methods that are based on the two-step approach. In microreactors, small size and heterogeneity at the assembly and core levels imply significant core environmental effects (e.g. large neutron leakage and spectral effects), which amplify the importance of on-the-fly corrections to data libraries, a need that is substantially more severe than in the case of currently operating light water reactors.