Nitrogen Release from UO₂ Pellets at Elevated Temperatures

The mechanism and kinetics of nitrogen release from UO₂ pellets containing high nitrogen impurities have been investigated at elevated temperatures. A combination of metallographic and x-ray analyses showed that nitrogen present in the UO₂ exists as a second phase in the form of uranium nitrides. The kinetics of nitrogen release were measured at temperatures of 1000, 1200, 1400, and 1600°C and the mechanism of release was determined to be controlled by the diffusion of nitrogen through the UO₂ lattice. The variation in nitrogen pressure upon heating a UO₂ fuel element to elevated temperatures was studied. The data showed that a reversible equilibrium pressure-temperature relationship did not exist between the nitrides in the UO₂ and the nitrogen gas above it as occurs with pure uranium nitrides. The quantity of nitrogen which will be released in a UO₂ fuel element is determined by the kinetics of diffusion of nitrogen through the UO₂ lattice and may be computed for any given fuel element design utilizing a diffusion model similar to one used for computing release of fission gases.