Benchmarking a point reactor kinetics method with delayed neutron precursors transport using data from molten salt reactor experiment

Abstract

A consistent point reactor kinetics (PRK) model-based method was developed for analyzing the transient behavior of circulating fuel systems such as molten salt reactors (MSRs). The consistent PRK model was derived based on the factorization approach and was exclusively used for calculating neutron flux amplitude, while the delayed neutron precursors were treated as part of the thermal-hydraulics model to better count for the spatial effects of species transport phenomena occurring in the circulating fuel systems. The mathematical models developed in the method was implemented using COMSOL Multiphysics platform featuring neutronics, thermal-fluidics, and species transport. An efficient reactivity feedback model is established within the platform to provide thermal feedback from the system-level thermal-hydraulics model to the PRK based neutronics model. The results produced by the computational models were validated against Molten Salt Reactor Experiment (MSRE) experimental data.

The developed models were first employed to examine MSRE responses at steady-state conditions, and then benchmarked with a couple of transient tests that took place during the MSRE ²³³U phase of operations: the reactivity insertion tests and the low power natural circulation test. In steady-state conditions, a parametric study examined the effects of flow rates on kinetics parameters and showed that delayed neutron fractions strongly correlated with flow rate, while prompt neutron generation time exhibited minor sensitivity. In reactivity insertion tests, different parameter sets were analyzed to assess the impact on reactor response, showing graphite temperature feedback has limited influence whereas fuel temperature feedback provides more impact. Additionally, the reactivity insertion rate has effect on power responses, especially at higher power levels. Also, the effect of circulating void fraction was addressed as a random process. In natural circulation test, the MSRE response to a stepwise increase of the heat rejection rate was tested. The flow in the primary loop was maintained by fuel density variations and the reactor power was controlled by the heat load through thermal feedback mechanisms. This transient is used to validate the heat transfer correlation and natural flow capabilities. All the results highlight the efficiency and accuracy of the developed consistent PRK method, validating its use in MSR-type advanced reactor safety analysis and design optimization.