Development and Application of Reduced-Order Models for Thermal-Fluid Dynamics in Molten Salt Reactors

Molten salt reactors (MSRs) present significant modeling challenges because of the inherent coupling between neutron transport and thermal hydraulics, stemming from the use of a circulating liquid fuel salt. The large-scale multiphysics simulations required demand extensive computational resources, making routine analysis of these systems impractical. This research explores the development of reduced-order model(ing) (ROM) techniques for the thermal-fluid analysis of MSRs, specifically focusing on the Molten Salt Fast Reactor (MSFR). The study aims to balance computational efficiency with model accuracy through several key tasks. To begin, a Reynolds-averaged Navier-Stokes (RANS) model of the MSFR is created using the spectral element computational fluid dynamics code nekRS. This model is then verified against large eddy simulation results from nekRS and RANS results from OpenFOAM. The MSFR model is further enhanced by incorporating delayed neutron precursor and decay heat precursor transport, enabling future multiphysics coupling. A ROM technique for thermal-hydraulic behavior is then introduced using a proper orthogonal decomposition and Galerkin projection procedure. This technique is tested on both two-dimensional and three-dimensional models of the MSFR. ROM stabilization methods are shown to be needed to generate sufficient small-scale dissipation, with Constrained Optimization ROM (C-ROM) emerging as the preferred method because of its efficiency and accuracy. The results demonstrate that C-ROM achieves high accuracy in predicting reactor behavior, comparable to the full-order model, while significantly reducing computational resources. This approach lays the groundwork for future multiphysics simulations in MSR analysis, enabling more efficient and accurate assessments of reactor performance and safety.