onceptual Design and Preliminary Feasibility Study of Fluid-Driven Suspended Control Rods for Molten Salt Reactors

Molten salt reactors, being the only reactor type among Generation IV advanced nuclear reactors to utilize liquid fuel, offer inherent safety, high-temperature and low-pressure operation, as well as the capability for online fuel reprocessing. However, fuel salt flow results in the decay of delayed neutron precursors (DNPs) outside the core, leading to fluctuations in the effective delayed neutron fraction and consequently impacting reactor reactivity. Particularly under accident scenariossuch as combined pump shutdown and inability to rapidly scram the reactorthe reliance solely on negative temperature feedback may cause a substantial increase in core temperature, posing a threat to reactor safety. To address these issues, this paper introduces an innovative design for a passive fluid-driven Suspended Control Rod (SCR) aimed at dynamically compensating for reactivity fluctuations caused by DNPs flowing with fuel flow. The control rod operates passively by leveraging the combined effects of gravity, buoyancy, and fluid dynamic forces, thereby eliminating the need for any external drive mechanism and allowing direct integration within the cores active region. Using a 150 MWth thorium-based molten salt reactor as the reference design, a mathematical model was developed to systematically analyze the effects of key parametersincluding the SCR's geometric dimensions and densityon its performance, examine its motion characteristics under different core flow conditions, and assess its feasibility for dynamic compensation of reactivity changes caused by fuel flow. The studys results demonstrate that the SCR can effectively counteract the reactivity fluctuations induced by fuel flow within molten salt reactors. Sensitivity analysis revealed that the SCRs average density exerts a profound impact on its start-up flow threshold, channel flow rate, resistance to fuel density fluctuations, and response characteristics, underscoring the critical need to optimize this parameter. Moreover, by judiciously selecting the SCRs length, number of deployed units, and placement, one can achieve the necessary reactivity control while also maintaining a favorable balance between neutron economy and heat transfer performance. Ultimately, this study provides an innovative solution for passive reactivity control in molten salt reactors, offering substantial potential for practical engineering applications.