Molten salt reactors (MSRs) feature high temperature, low pressure, high chemical stability, and nuclear non-proliferation, which make them promising for a wide range of applications. However, owing to the fluid nature of the fuel, changes in the core structure of MSRs are bound to affect the fuel distribution and loading, consequently affecting the physical parameters of the core. Currently, the research on the impact of structural changes of the MSR core, composed of dispersed graphite components, on reactivity is insufficient.

This study aims to explores the influence of core structure changes on reactivity.

The core design model of new solid hexagonal graphite module MSR was taken as a reference, the small geometric changes, such as deformation and displacement of the core components, caused by factors including thermal expansion, fluid erosion, core vibration, and graphite irradiation, were analyzed using MCNP code. Additionally, the impact of these structural changes in the core on reactivity was investigated in details.

The results indicate that the structural changes in the core caused by thermal expansion introduce negative reactivity. The reactivity introduced by fluid erosion and core vibration, causing graphite component displacement, fluctuates. However, the overall trend shows that a shift of the graphite components towards the center of the core introduces positive reactivity, whereas shift towards the outer periphery of the core introduces negative reactivity. Graphite irradiation-induced deformation initially decreases reactivity and then increases it. At the end of the core's lifespan, the reactivity is still lower than at the beginning of the lifespan. This reactivity change can be compensated for by online feed or control rod movement, which has a limited impact on the operation of the MSR. However, the critical issue of control should be considered when this batch of fuel is reinserted into a new core.

Results of this study suggest the necessity of constraining the graphite components within a certain range to ensure the safe operation of the reactor, providing an important reference for the design, operation, and maintenance of MSRs.