Improved design of LBE cooled solid reactor using 3D neutronics thermal-hydraulics coupling method

Abstract

The goal of this study is to improve a micro reactor core into uniform power distribution, a smaller and safer core. Neutronic thermal-hydraulics coupling is an effective means to study the characteristics of microreactors. This paper performs an improvement work of lead–bismuth eutectic alloy (LBE) cooled solid core using the three-dimensional neutronics/thermal-hydraulics coupling method. Based on the Monte Carlo code RMC and the three-dimensional thermal-hydraulics code STAR-CCM+, the coupling code processes the power distribution, fuel temperature, coolant temperature, and coolant density. The improvement of the solid core is carried out in these aspects: reflector material, reflector dimensions, fuel materials, coolant channel dimensions, the volume fraction of fuel phase in dispersion fuel, and critical enrichment. This paper studies the effect of reflector material and dimensions on the solid core and its mechanism. After improvement, the power peaking factor is effectively reduced, while the size and weight of the solid core are also reduced than before due to the excellent ability of beryllium oxide (BeO), making it compact and lightweight. In addition, the core eigenvalues keff and thermal characteristics of different ceramic fuels, uranium dioxide (UO2), uranium nitride (UN), and uranium carbide (UC), are compared. UO2 was replaced by UN, which improves the thermal conductivity and reduces the maximum temperature of the solid core. At the same pressure difference, the mass flow rate of the coolant and temperature distribution of the fuel and the variation trend of neutron characteristics are studied when the coolant channel diameter changes. The results indicate that the coolant channel diameter of 1.775 cm has the best safety effect on the core. The variation of critical enrichment and thermal conductivity of the fuel at different volume fractions is compared, and 60 % is chosen as the volume fraction of the fuel phase. The final design of the shutdown rod and control drums is performed to ensure that the core is subcritical under all conditions. The improved core can operate at full power for five years and is characterized by miniaturization, high thermal conductivity, low volumetric swelling rate, flattened power distribution, and abundant reactivity control measures.