Multi-physics coupled analyzes of research nuclear reactors based on steady-state and kinetics models

Nuclear research reactors are the workhorses of nuclear material irradiation and isotope production, but the complex assembly geometries and physical coupling effect adds difficulties to their accurate safety analysis. In this study, a multi-physics coupled method for assessing the steady-state and kinetic behaviors of the LVR-15 and MPRR research reactors is proposed. Two-dimensional whole-core Monte Carlo models are utilized to produce homogenized assembly cross-sections in order to describe complex core geometries. Utilizing a three-dimensional variational nodal transport method with unstructured triangular-z meshes, the neutronic field is calculated. Besides, a multi-channel model and a finite difference heat conduction model are employed to determine the temperature distributions of coolant and fuel. Based on these models, the MORPHY (Multi-purpose Reactor multi-PHYsics analysis) code platform is created and verified using the NEACRP neutronics/thermal-hydraulics benchmark. The steady-state and transient behaviors of LVR-15 and MPRR are analyzed using MORPHY. The steady-state results are found to agree well with Monte Carlo reference solutions. For LVR-15 and MPRR, the keff errors are 377 pcm/383 pcm, the maximum assembly power errors are 6.49%/10%, and the maximum control rod worth errors are 31 pcm/136 pcm, respectively. In the meantime, transient processes including unprotected loss-of-flow accidents and reactivity-initiated accidents are studied. Simulation results for both reactors exhibit reasonable negative feedback phenomena. Moreover, it is discovered that despite the fact that the two reactors have distinct core designs, their accident-related behaviors are similar due to the use of the same types of fuel material and water coolant.