Advancement in molten salt reactor physics: new coupled MSRE model incorporating xenon and void transport

Researchers from the University of Shanghai for Science and Technology and the University of Illinois Urbana-Champaign have developed a couple system model tailored to the unique behavior of liquid-fueled molten salt reactors (MSRs). This coupled model—incorporating neutron kinetics, thermal hydraulics, xenon behavior, and void transport—has been validated using experimental data from the Molten Salt Reactor Experiment (MSRE), a landmark demonstration conducted at Oak Ridge National Laboratory.

Bringing New Clarity to Reactor Behavior
Traditional system analysis tools are not well-suited to model the dynamic behavior of MSRs, which differ significantly from solid-fueled reactors. The new model developed in Simulink explicitly simulates the transport of xenon and delayed neutron precursors (DNPs), improving the prediction of reactivity feedback during transient events such as pump start-up, coast-down, and control rod operations.

“This work helps us better understand how circulating fuel, xenon removal, and void reactivity affect reactor stability and control,” said Dr. Jia-Qi Chen, the study’s lead author. “It provides a validated model and open-sourced model for analyzing MSR dynamics and supporting future reactor designs.”
The validated model was used to simulate various operating scenarios, revealing how initiating events like off-gas system blockages or loss of gas voids can affect the safety of molten salt reactors. These insights are critical for the safe and flexible operation of advanced reactors, especially those intended for load-following in modern electricity grids.

Unique Behaviors of Molten Salt Reactors

The newly developed dynamic model significantly advances the understanding of molten salt reactor (MSR) behavior by accurately capturing the interplay between xenon transport, delayed neutron precursor (DNP) circulation, and thermal-hydraulic feedback. Validated against historical data from the Molten Salt Reactor Experiment (MSRE), the model reproduces power-to-reactivity frequency responses at both zero power and operating conditions (up to 8 MW) for reactors fueled with both ²³³U and ²³⁵U. Notably, it correctly predicts the system’s resonant frequency near 0.23 rad/s, which is tied to the out-of-core DNP circulation time, and matches experimental trends in reactor gain and phase lag across multiple dynamic scenarios.

The study reveals that higher operational power increases reactor stability due to stronger thermal feedback, whereas lower-power conditions are more sensitive to void and xenon transients. For example, the model predicts that a loss of gas voids can produce an immediate power spike, particularly at low operating power, followed by a gradual decrease in power due to increased xenon poisoning over several hours. Under an off-gas blockage scenario, xenon accumulation led to a power reduction of over 20% after 30 hours of operation in the absence of control rod movement.

Overall, this work demonstrates that a lumped-parameter Simulink-based MSR model, when properly calibrated and validated, can accurately simulate the complex multi-physics interactions in liquid-fueled reactors. The model offers a robust and extensible platform for investigating control strategies, system stability, and safety margins in advanced reactor designs. As global demand for flexible, low-carbon nuclear energy rises, this model provides a valuable foundation for the analysis, design, and licensing of next-generation molten salt reactors.   The complete study is accessible via DOI: 10.1007/s41365-025-01680-w.