Nitrogen under cladding of fuel pins with mixed uranium-plutonium nitride

In gas-bonded fuel pins the interior space is filled with helium in order to provide heat removal from the fuel. Under irradiation, as a result of gaseous fission products release, the initial gas composition under the fuel pin cladding changes, which leads to a change in its thermophysical characteristics. Post-irradiation examinations have shown that the partial pressure of nitrogen under the fuel pin cladding increases with fuel burn-up increase, since nitrogen atoms also release from uranium-plutonium nitride fuel under the cladding, in addition to the krypton, xenon and helium inert gases. The mechanisms of release of inert gases and nitrogen from nitride are different: the diffusion mechanism for inert gases and the knocking out of fission fragments for nitrogen. The difference in the gas release mechanisms leads to a significant quantitative difference in the gases release under the cladding. The specific nitrogen yield under the fuel pin cladding is significantly lower than the specific yield of other gases, but its presence is an important factor, since the dissociation temperature of the mixed nitride, as well as the nitriding of the inner surface of the cladding, depends on the nitrogen pressure under the fuel pin cladding. A generalization and analysis of the results of post-irradiation examinations of the nitrogen content in the gas mixture under the claddings of 87 investigated fuel pins after irradiation in the BN-600 reactor as part of 17 experimental fuel assemblies with mixed uranium-plutonium nitride fuel, irradiated to maximum fuel burn-up from 3 at. % to 9 at. % is carried out in the paper.