Accident scenario analysis and control scheme design for a micro Molten Salt Reactor

Combining with the appealing technologies of the liquid-fuel molten salt reactor and the passive heat transfer of heat pipes, the micro Molten Salt Reactor (micro-MSR) has prospective applications in space, the ocean and other off-grid remote areas. Due to the distinctive features of these applications and liquid fuel in a micro-MSR, special attention should be paid to the safety issues of the micro-MSR. The study aims to evaluate the safety features of the micro-MSR core under various accident scenarios and to further develop effective control schemes. The core reactivities under cool-down conditions are initially assessed based on the micro-MSR core optimized in the early work. The results indicate that the micro-MSR is slightly subcritical or supercritical due mainly to a shrunken salt volume. Then, an analysis of the core reactivities under various submersion and flooding accidents is performed. It is found that the micro-MSR would be highly supercritical under the accident scenarios where the core is submerged in wet sand, seawater or fresh water and meanwhile, the heat pipes are saturated with water. The core reactivity would grow by up to 28563 pcm in the worst-case scenario where the core is surrounded by wet sand and the heat pipes are saturated with seawater. Two control schemes, including the safety-rod and the fuel-out schemes, are proposed to redesign the core and prevent inadvertent criticality accidents before startup. Both control schemes can ensure an adequate shutdown margin under all accident scenarios by introducing more neutron poisons within the core or lowering the initial fuel loading. The two control schemes are also compared in terms of core size, initial fuel distribution, battery’s capacity and startup sequence. The fuel-out scheme is considered superior to the safety-rod scheme owing to its smaller size, lower weighted mass and safer startup sequence.