The Molten Salt Reactor Experiment (MSRE) was a small circulating fuel reactor operated at Oak Ridge National Laboratory (ORNL) between 1965 and 1969. To do date it remains the only molten salt reactor (MSR) that has been operated for extended periods, on diverse nuclear fuels. Reactor physics in MSRs differs from conventional solid-fuelled reactors due to the circulation of hot fuel and delayed neutron precursors (DNPs) in the primary circuit. This alters the steady state and time-dependent behaviours of the system.
A coupled point kinetic-thermal hydraulic feedback model of an MSRE-like system was constructed in order to investigate the effect of uncertainties in the values of key physical parameters on the model’s response to step and ramp reactivity insertions. This information was used to determine the parameters that affected the steady state condition and transient behaviours.
The model was also used to investigate features identified in the frequency response, in particular a feature corresponding to fuel recirculation. Greater than expected mixing in the primary circuit has been previously proposed as an explanation for the lack of observation of this feature. A velocity-dependent turbulent dispersion term is proposed to increase dispersion of the fuel temperature field in order to suppress the recirculation feature in the frequency response. An additional semi-analytical model was constructed as a part verification of the mixing hypothesis - this model was also used to examine the stability of an MSRE-like design.
Finally the validated coupled system model was used to establish the just-safe combination of intrinsic source and ramp rate that does not exceed an estimated maximum permissible vessel temperature. The CALLISTO-SPK stochastic point kinetics code is used to demonstrate that the intrinsic source in an MSRE-like design is sufficient to reduce the probability of a rogue startup transient to an acceptably small value from the point of view of regulatory safety analysis. Such analyses may be used to support the case for extrinsic source deletion in future MSR designs.