Density functional theory study of uranium mono-nitride

Uranium nitride is next generation accident tolerant fuel (AFT) candidate designed to prevent loss of radioactive material by maintaining integrity during loss-of-coolant accidents (LOCAs). This is due to its high thermal conductivity, high specific heat capacity, low thermal expansion and low overheating. UN’s low oxidation resistance poses however a threat to its use in light water reactors (LWRs), since corrosion by high temperature pressurised water or steam following a breach in the cladding releases fission products to the coolant. It is therefore necessary to reduce UN oxidation rate to acceptable levels prior to its large-scale deployment in LWRs by developing an understanding of corrosion under reactor conditions. We perform DFT simulations to understand the complex processes that underpin the oxidation process in UN. Careful consideration is given to the DFT model employed to ensure the avoidance of meta-stable states. We investigate defect formation energies for the intrinsic defect species and show how these defects accommodate non-stoichiometry under different conditions. We rely on the UN bulk and defective model to investigate the interactions between H2O and UN surfaces and the oxidation mechanisms at the core of those oxidation processes leading to UN corrosion in working conditions.