Global variance reduction for Monte Carlo reactor physics calculations

The development of hybrid Monte-Carlo-Deterministic (MC-DT) approaches, taking place over the past few decades, have primarily focused on shielding and detection applications where the analysis requires a small number of responses, i.e. at the detector location(s). This work further develops a recently introduced global variance reduction approach, denoted by the SUBSPACE approach, and extends its application to reactor analysis problems, where responses are required everywhere in the phase space. In this proof-of-principle study, the SUBSPACE approach is shown to reduce the excessively long execution time of Monte-Carlo reactor physics calculations for simplified reactor geometries significantly. By way of demonstration, the SUBSPACE approach is applied to assembly level calculations used to generate the few-group homogenized cross sections. These models are typically expensive and need to be executed in the order of 10³–10⁵ times to properly characterize the few-group cross sections for downstream core-wide calculations. Applicability to k-eigenvalue core-wide models is also demonstrated in this work. Given the favorable results obtained in this work, we believe that the SUBSPACE method significantly enhances the state of the art of Monte-Carlo reactor physics analysis with particular focus on reducing the necessary runtime for achieving accurate results