Thermal stability investigation technique for uranium nitride

•Thermochemical stability is needed for the prediction of nitride fuel behavior.•A reliable technique that determines the temperature and parameters of dissociation is developed.•Dissociation of UN is found to start even prior to reaching saturation at 1500°С.•This process comprises two stages: solid nitrogen and liquid uranium evaporation.•Uranium release from UN depends on exposure temperature and initial N/U ratio. Thermochemical stability of nitride nuclear fuel is an important factor that ought to be considered in fabrication processes, operation and more importantly under accidental conditions. Development of a reliable technique which determines dissociation kinetic parameters and initial temperature of UN disassociation using synchronous thermal analysis apparatus with quadruple mass-spectrometer and SEM investigations was performed. The dissociation process is found to start at temperatures higher than 1500°С. The process comprises two stages, with a significant increase in mass loss rate at the second stage, which is due to the additional contribution of liquid uranium evaporation. The duration of isothermal exposure before uranium release depends on temperature and the initial N/U ratio and varies from several hours to a few minutes. Intense interaction of the uranium nitride with tungsten crucible was observed after 30min exposure at 2300°С.