Neutronic performance of uranium nitride composite fuels in a PWR

Abstract

Uranium mononitride (UN) based composite nuclear fuels may have potential benefits in light water reactor applications, including enhanced thermal conductivity and increased fuel density. However, uranium nitride reacts chemically when in contact with water, especially at high temperatures. To overcome this challenge, several advanced composite fuels have been proposed with uranium nitride as a primary phase. The primary nitride phase is “shielded” from water by a secondary phase, which would allow the potential benefits of nitride fuels to be realized. This work is an operational assessment of four different candidate composite materials.We considered uranium dioxide (UO2) and UN base cases and compared them with the candidate composite UN-based fuels. The comparison was performed for nominal conditions in a reference PWR with Zr-based cladding. We assessed the impact of UN porosity on the operational performance, because this is a key sensitivity parameter. As composite fuels, we studied UN/U3Si5, UN/U3Si2, UN/UB4, and UN/ZrO2. In the case of UB4, the boron content is 100% enriched in 11B. The proposed zirconium dioxide (ZrO2) phase is cubic and yttria-stabilized. In all cases UN is the primary phase, with small fractions of U3Si5, U3Si5, UB4, or ZrO2 as a secondary phase. In this analysis we showed that two baseline nitride cases at different fractions of theoretical density (0.8 and 0.95) generally bound the neutronic performance of the candidate composite fuels. Performance was comparable with UO2. One notable difference observed was longer cycle lengths with the composite fuels (due to increased fuel loading). Another significant finding is that the nitride composites exhibited a harder neutron spectrum, which decreased the reactivity worth of burnable absorbers, soluble boron, and control rod materials relative to the UO2 case. In general, the full-core reactivity coefficients for the nitride and nitride composite fuels were within the design limits for the reference PWR and UO2-Zr fuel system. It is noted that the limits for the proposed advanced composites will likely be different than the reference UO2 fuel. The baseline UN and UN/ZrO2 cases, both with relatively high porosity in the nitride phase (20%), exhibited the strongest similarity to the reference UO2 case.