Research progress and development trends in the anti-oxidation mechanism and performance enhancement of uranium nitride as an accident-resistant nuclear fuel

The Fukushima accident in 2011 prompted significant efforts to develop accident-tolerant fuel (ATF). The U.S. Department of Energy defines ATF as a fuel that can withstand a longer reactor core cooling loss time than the standard fuel system (UO2–Zr) in the event of an accident. These advanced fuels must maintain their thermodynamic and mechanical stability under extreme conditions to prevent or mitigate accidents. Various new nuclear fuels and claddings have been proposed and developed. Among the many ATF candidates to replace traditional UO2 nuclear fuel, uranium nitride (UN) stands out owing to its high uranium density, thermal conductivity, and melting point, making it one of the most promising accident-resistant nuclear fuels. Despite its significant potential in enhancing nuclear safety and its applicability in various other fields, poor high-temperature oxidation and corrosion resistance remain urgent challenges for the UN as a candidate accident-resistant nuclear fuel. Given the low oxidation resistance of uranium nitride-based nuclear fuel under in-pile service conditions, this paper first discusses the oxidation process and mechanism of UN. The oxidation products of UN may vary depending on the experimental conditions, but they typically include UO2, U2N3, U3O8, and UO3. It was observed that UN is highly prone to oxidation, with an initial oxidation temperature of 200 ℃ leading to the formation of oxynitride and U2N3. At 250 ℃, the oxidation rate accelerates, nitrogen is released, and U3O8 is eventually formed at 400 ℃. The calculation of the oxidation reaction of UN indicates that its oxidation is thermodynamically favorable and can occur even at low oxygen partial pressures, further demonstrating that UN readily reacts with oxygen in air. Oxidized UN typically exhibits a “sandwich” microstructure with an outer UO2 layer, an intermediate U2N3 layer, and an inner UN layer. The results of oxidation experiments on UN single crystals suggest that the oxidation process involves the inward diffusion of oxygen atoms, whereas nitrogen atoms are released either as gas or partially dissolved in the lattice. This study summarizes three main methods for improving the oxidation resistance of UN: compounding UN with ceramic compounds, doping UN with metal elements, and applying surface coating technology. We focus on the oxidation resistance of composite fuels formed by the sintering of uranium nitride with various ceramic antioxidant phases, including uranium dioxide, triuranium disilicide, and other ceramic antioxidant materials. Among these, the uranium nitride-uranium dioxide composite fuel demonstrates a higher oxidation initiation temperature than pure uranium nitride in high-temperature, high-pressure steam environments, indicating superior oxidation resistance. Therefore, we focused on exploring the microscopic mechanisms and improvement strategies related to the influence of the composition and microstructure of uranium nitride–uranium dioxide composite fuel on oxidation resistance. Although the formation of the uranium trinitride phase plays a key role in enhancing the oxidation resistance of the uranium nitride–uranium dioxide composite, its high-temperature stability remains a subject of controversy. Finally, this study summarizes the scientific challenges that need to be addressed in the current research on improving the oxidation resistance of uranium nitride-based advanced fuels.