An improved core design of a 50 kWth heat pipe cooled micro Molten Salt Reactor (micro-MSR)

The micro-MSR is a small (50 kWth) fast molten salt reactor cooled by heat pipes, and has certain attractive features, such as liquid fuel without need of fuel assembly fabrication, inherent safety, strong environmental robustness and high reliability, which make it potentially be implemented in the applications in space, ocean and other off-grid remote areas. Several design aspects of the micro-MSR, including the heat pipe arrangement, fuel salt and reflector candidates, methods of reducing core mass and heat pipe dimension, are optimized in this work in order to achieve a compact and light-weighted core with a uniform power and heat distribution. Firstly, a new configuration of concentric annular heat pipes is employed to replace the regular hexagonal prism layout in the original design, and the power distribution is evaluated with regard to effectively reduce localized power hot spots. Under such an improved core configuration, some important physical parameters, including criticality and uranium loading, are compared for both fluoride-based and chloride-based fuel salts. The results indicate that the LiF-UF4 salt with the composition of 72-28 mol% is more favorable for the micro-MSR application owing to its relatively high density and low melting point. Afterwards, several reflector materials are analyzed and optimized to mitigate the power peaking caused by BeO reflector in the original design. It can be found that the MgO reflector can substantially reduce the power peaking near the active core edge and meanwhile provide an acceptable increment of core mass from 1330 kg to 1813 kg. Subsequently, three parameters concerning the optimization of the design of MgO-reflected micro-MSR, reflector thickness, active core height to diameter (H/D) ratio and thickness of alloy around the active core, are studied to lower the core mass. The results show that the core mass can diminish to an equivalent and even less level of about 1330 kg by simultaneously or solely enlarging the active core and H/D ratio, and decreasing the alloy thickness. Moreover, based on the above improved core configuration with MgO reflector, an evaluation of a larger number of thin heat pipes with a smaller inner diameter is carried out for enhancing the efficiency of heat transfer from fuel salt to heat pipes, and the result demonstrates that the new design of heat pipe is favorable for enhancing the thermal performance without deteriorating the neutronics characteristics. Eventually, the principal data for the improved micro-MSR core are summarized.