Global Core Environment Effects on Multigroup Cross Sections for Finer Energy Structures within Microreactors

This paper uses a combined single-assembly and full-core benchmark problem based on a heat-pipe-cooled microreactor design to study the effect of core environment on homogenized multigroup macroscopic cross sections generated in a fifteen-group energy structure by the continuous-energy Monte Carlo code Serpent 2. These cross sections are evaluated in fuel pins, moderator rods, and heat pipes in selected locations within the single assembly and those corresponding locations in multiple such fuel assemblies at a set of locations within the full core. Effects of core environment on multigroup cross sections are determined using the obtained results. As expected, it is found that the effects of core environment on fifteen-group cross section data are much less marked than the same effects observed in published three-group cross sections. This finding applies across several reaction types in all selected materials. The maximum effect due to core environment found is 7.5% using fifteen-group cross section data versus 26.2% using three-group data. However, the effects remain significant even in the fifteen-group structure, which unequivocally demonstrates the imperative need for applying correction methods. Typical energy bins and reactions where this prominently occurs are intermediate and fast capture and fission in fuel, and intermediate scattering in heat pipes. This study confirms that many-group cross section libraries obtained from single assembly transport calculations produce full-core modeling results that are less sensitive to spectral effects arising from core environment than those generated using fewer group libraries. This study also demonstrates that on-the-fly correction of these libraries is still required for accurate modeling of an entire microreactor core.