A Reactor Transient Benchmark for Molten Salt Reactor Experiment Pump Transient Tests

The molten salt reactor is one candidate among the Generation IV nuclear reactor designs, with its deployment relying on advanced computational tools to capture the unique behavior of the circulating fuel system. The Molten Salt Reactor Experiment (MSRE) provides valuable experimental data for validating these computational tools. This work develops a reactor transient benchmark based on the MSRE pump transient tests. Two computational models are evaluated in the benchmark: a simplified one-dimensional (1D) system-level model and a more detailed R-Z axisymmetric model using the porous medium approximation. The models are used to evaluate the impact of spatial resolution on predicted reactivity responses during the transient. Several impactful factors are examined during the benchmark evaluation, including the neutron diffusion multigroup energy structure, delayed neutron precursor (DNP) diffusion, DNP group structure, bypass flow, and transient flow rates. The reactivity predictions using the computational models are compared to the experimental data. The mean errors in the predicted reactivity responses ranged from 11 to 21 pcm (1 pcm = 10⁻⁵) for the pump startup transient and 5 to 13 pcm for the pump coastdown transient. These results indicate that the 1D model can provide adequate accuracy on MSRE pump transients with limitations in predicting the rate of reactivity at the early stage of the transient, while the higher-order model improves this capability by incorporating the influence of radial salt flow distribution and bypass flow on transient reactivity.