Enhanced fidelity of Monte Carlo coupled multi-physics simulations in the MCS code

This study integrates previously developed methods to enhance the fidelity of direct whole-core Monte Carlo coupled multi-physics simulations in the MCS code. First, it introduces multi-physics simulations with spatially continuous material properties by using the Functional Expansion Tally combined with delta-tracking. Second, it incorporates on-the-fly thermal expansion of reactor core components during Monte Carlo particle tracking. To evaluate the accuracy and overall performance improvement of the framework, several numerical experiments were conducted at both the assembly and whole-core levels. The incorporation of spatially continuous material properties produces eigenvalue solutions that asymptotically converge to those from conventional cell-based discretized simulations with infinitesimally small cells as demonstrated in the assembly and whole-core problems. In the whole-core problem, the framework reduces simulation times by around threefold and requires 80 % less memory than the traditional cell-based discretization using very small cells, while maintaining the high-fidelity solutions. Whereas the numerical results for on-the-fly thermal expansion demonstrate that the observed trends in reactor reactivity due to thermal expansion align with previous studies. These findings suggest that integrating the multi-physics framework into reactor modeling can enhance simulation fidelity while reducing simulation time.