Interplay of Variance Reduction and Population Control in Monte Carlo Neutron Transport

Monte Carlo methods are widely used for neutron transport simulations at least partly because of the accuracy they bring to the modeling of these problems. However, the computational burden associated with the slow convergence rate of Monte Carlo poses a significant challenge to running large-scale simulations. The continued improvement in high-performance computing capabilities has put exascale time-dependent Monte Carlo neutron transport simulations within reach. Variance reduction techniques (VRTs) have become an essential component to the efficiency of steady-state simulations, and population control techniques are an integral part of time-dependent simulations, but combining them can create algorithmic conflicts. This study investigates the performance of steady-state VTRs when extended to time-dependent problems and examines how variance reduction and PCTs combine to impact the effectiveness of time-dependent simulations. Simulations were conducted using various combinations of these techniques across multiple test problems to assess their performance. While this study does not examine all possible variance reduction and population control combinations, the findings emphasize the importance of carefully selecting algorithms to simulate large-scale time-dependent problems effectively. Notably, using weight windows with weight-based combing for population control can significantly hinder simulation performance, whereas pairing weight windows with uniform combing can provide the efficiencies necessary for successfully computing the results of massive problems. Further performance gains were observed when steady-state weight windows were replaced with time-dependent versions.