Accelerated and heterogeneous corrosion of Cr-doped uranium nitride fuel pellets

The steam oxidation of Cr-doped UN fuel pellets is analysed during sequential isothermal holds up to 720 °C. In situ neutron diffraction results show how Cr is accommodated in a secondary U₂CrN₃ phase, leading to the formation of a duplex UN/U₂CrN₃ microstructure. Under corrosion, the oxidation of the two phases begins at 400 °C for UN and 430 °C for U₂CrN₃, respectively. Because the UN phase is preferentially oxidised in the presence of U₂CrN₃, addition of Cr in UN based nuclear fuel is found to accelerate the corrosion rate. At 430 °C the oxidation of UN in the UN/U₂CrN₃ microstructure is ∼5 times faster than pure UN, increasing to ∼19 times faster at 460 °C. The oxidation of U₂CrN₃ produces UO₂ via the formation of two transient intermediate phases. In situ neutron diffraction enables oxidation processes of UN and U₂CrN₃ components to be followed separately within the two-phase system.