Viability of uranium nitride fueled high-conversion PWR

Abstract

Light water breeder reactors have been examined periodically for decades, and considerable research into the neutronics and thermal hydraulics of several designs has been performed. Currently, there is an interest in reduced moderation boiling water reactors in Japan and in the United States, where Department of Energy (DOE)-sponsored university research programs are examining proposed breeding BWRs.This study assesses the thermal hydraulic and neutronic feasibility of a nitride fueled pressurized water reactor (PWR) breeder design. Because of the higher fuel density, the use of nitride fuel would be preferable to the traditional oxide fuel for a high conversion PWR design. To illustrate the advantage, a design that uses large hexagonal assemblies was considered. The design has 14 inner seed pin rows and 4 outer blanket pin rows, leading to 505 inner seed pins and 366 outer blanket pins per assembly. Each assembly contains 6 thimble locations for control rods. The same assembly type can be used at all locations in the core. For this study a fuel pin diameter of 1.2 cm was used with a gap of 1 mm between pins. The seed regions had 12.75% percent of the heavy metal as fissile plutonium (Pu-239 and Pu-241), hosted in depleted uranium (0.25% U-235). The plutonium vector for the seed region was 2.7% Pu-238, 47.9% Pu-239, 30.3% Pu-240, 9.6% Pu-241, 8.5% Pu-242 and 1.0% Am-241. The Blanket regions contained only depleted uranium with 0.25% U-235.This assembly was modeled with the Monte Carlo depletion code Serpent to determine the initial to final fissile inventory ratio (FIR). The as-specified assembly model did not achieve an FIR value above 1.0, so the water in the thimble regions was changed to void, to lower the H/HM ratio. This led to FIR values above 1.0 for the oxide, the 85% theoretical density nitride (N85) and the 95% theoretical density nitride (N95). All designs had an FIR of 1.03 at 35 MWd/kgHM. However, the single batch discharge burnup of the assembly was much higher for the nitride cases. The burnups were 15.6, 24.5 and 32.2 MWd/kgHM for the oxide, N85 and N95 respectively.The KfK correlation for critical heat flux was applied to calculate the minimum departure from nucleate boiling ratio (MDNBR), whose value was set to be 1.45. With an acceptable core pressure drop of 400 kPa and acceptable vibration magnitude, the maximum power in an assembly and the maximum flow rate were determined. A nitride fuel had a larger MDNBR than the oxide fuel. This increase is likely a result of spectrum hardening, which was found to reduce the fractional amount of power generated in the seed region, where departure from nucleate boiling (DNB) occurs. Because of the longer neutron mean free path in the harder spectrum, more fission events occur in the blanket. The coolant density coefficient of the nitride fuel was found to have positive coefficient values in voided thimble tube cases. However, the power coefficient was negative and has the ability to limit power increases. The radially reflective assembly boundary may have made the analysis more conservative than full core analysis.