Comparisons of Reduced Moderation Small Modular Reactors With Heavy Water Coolant

This paper presents the comparison of two reduced moderation small modular reactor concepts with heavy water coolant. Two reduced moderation small modular reactors, RMSMR-Th and RMSMR-MOX, are proposed for the sustainable utilization of nuclear resources. The design concepts are established on modifications of the well-experienced pressurized water reactor technology. Tightly-packed lattice and heavy water coolant are employed to yield a hard neutron spectrum, which proved advantageous for increasing the conversion ratio as well as lowering the burnup reactivity swing between beginning of cycle and end of cycle. Thorium-uranium dioxide fuel and MOX fuel are compared using the same core arrangement, and the small modular reactor concept is adopted to reduce void coefficients. Radial blanket region and axial blanket region are adopted to enhance the fissile breeding and a three-zone fuel configuration is adopted to flatten the power distribution. Core burnup calculations were carried out to investigate the available cycle length, the conversion ratio, the power peaking factors, reactivity coefficients, etc. Light-water based thermal-hydraulic models were employed to examine the safety features of the concepts. Numerical simulations indicate that both RMSMR-Th and RMSMR-MOX can sustain the power generation of 100MWe by 7 years without refueling. Compared with thorium-uranium dioxide fuel, MOX fuel is helpful in reducing the burnup reactivity swing, increasing the conversion ratio and increasing the mDNBR value. However, the positive void coefficient becomes a serious problem making RMSMR-MOX less attractive than the RMSMR-Th concept.