Kinetic Studies of the Nitriding–Denitriding of Uranium Mononitride

The nitriding–denitriding kinetics of uranium mononitride (UN), UN + N₂ → U₂N₃ + UN₂ → UN + N₂↑, has been studied. Despite a high thermodynamic probability, UN weakly interacts with nitrogen at 500°C, and the reaction is slow (~10 h) at 600°C. The optimum nitriding temperature is 800–850°C. At this temperature, the nitriding time is ~1 h. Denitriding in an argon flow (U₂N₃ + UN₂ → UN) proceeds very slowly, in 8–10 h, even at 1200°C. The nitriding–denitriding of UN pellets in EP823 steel tubes does not lead to their spontaneous falling from the tubes. The actual increase in the UN volume during nitriding is significantly greater than the theoretical increase (30%) due to the formation of a loose structure. If the pellets are in a steel clad, this leads to self-compression of the material and difficulty in removing it. Despite the use of high-purity nitrogen and argon (O₂ \(\leqslant \) 0.0001%), a UO₂ impurity is always found in the reaction products. Uranium effectively absorbs oxygen traces even from high-purity gases.