In situ neutron powder diffraction experiments are applied to physical, kinetic, and microstructural characterization of uranium mononitride as a promising light water reactor fuel material. The temperaturevariable coefficient of thermal expansion and isotropic Debye Waller factors are obtained by sequential Rietveld refinement over 499-1873 K. Oxidation of a UN pellet (95.2% density) under flow of 11 mg/min D 2 O is observed to initiate above 623 K and the rate increases by a factor of approximately 10 from 673 to 773 K, with activation energy 50.6 +/- 1.3 kJ/mol; uranium oxide is the only solid corrosion product.Crown Copyright