Development and application of a coupled point-reactor and multi-channel delayed neutron precursor transport model in molten salt reactors

The flowability of molten salt reactor fuel (MSR) leads to distinct transport and distribution characteristics for delayed neutron precursors (DNP) compared to solid-fuel reactors, which has important implications for reactor safety analysis. Traditional point-reactor model, when applied to MSR, fails to account for the effects of neutron importance at different core locations and has limitations in describing the spatial redistribution of delayed neutron precursors caused by fuel circulation. To investigate the reactivity loss and redistribution characteristics of DNP under various conditions in MSR, this study develops a coupled point-reactor and multi-channel DNP transport model. By incorporating shape functions and adjoint fluxes, the model accounts for the spatial variation in neutron importance within the core and the effects of flow distribution among different system components. Steady and transient validation was performed using Molten Salt Reactor Experiment (MSRE) data. The results indicate that the difference between the simulated and measured changes in flow reactivity, after accounting for multi-channel adjoint flux, is below 5 pcm. Additionally, the distribution of precursor residence time outside the core has a certain impact on the transient reactivity changes during the pump start-up process. Both of these reflect the necessity of multi-channel delayed neutron precursor transport.