SUMMARY

This thesis focuses on the development of hybrid depletion-based methodologies for Monte Carlo (MC) reactor physics simulations. More specifically, a hybrid depletion MC that leverages both the high-fidelity continuous energy (CE) and quick multigroup (MG) is developed here. The novel efficient time integration scheme removes the need to use the expensive CE simulations and replaces them with more frequent and relatively cheap MG simulations. The time integration scheme incorporates a sub-stepping technique, in which both spectrum and spatial updates to the reaction rates are combined using these low-cost MC solutions. This is accomplished by generating on-the-fly homogenized groupwise macro- and micro-cross sections for various regions using the high-fidelity CE MC approach. Groupwise microscopic cross sections are then used to perturb the material composition after solving the Bateman equations, and thus also allowing for the update of the groupwise macroscopic cross sections. The updated macroscopic cross sections are then used in a reduced order multigroup Monte Carlo simulation for the substeps. Getting the spectrum from the substeps then allows for the one group microscopic cross sections to be updated continuing to the next substep or step. The treatment of how to homogenize a region and perturb the macroscopic cross sections is important and rigorously tested in this thesis.