Theoretical Foundations of the Nitriding–Denitriding of Nitride Spent Nuclear Fuel

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Abstract
—Nitriding is the oxidation of UN-based nitride fuel by nitrogen at elevated temperatures. In this case, uranium mononitride UN is oxidized to sesquinitride U2N3, and the fcc structure of UN changes into a less dense bcc structure of U2N3, which is accompanied by an increase in mass by about 4% and an increase in fuel volume by more than 30%. The replacing one crystal structure with another and increasing the fuel volume lead to fuel fragmentation and the expansion of fuel-element cladding. The next operation is to remove part of nitrogen and to perform the reverse U2N3 → UN transformation (denitriding). Higher uranium nitrides are known to be unstable upon heating in an inert medium. The transformation from U2N3 to UN, which is again accompanied by a change in the crystal structure and a decrease in the fuel volume, occurs at elevated temperatures in an argon atmosphere. This process further contributes to the dispersion of fuel and its separation from fuel-element cladding fragments, the diameter of which was increased at the nitriding stage. Crushing of the fuel and a twofold change in its structure promotes the release of volatile fission products, such as inert gases, Cs, CsI, and Cd. Another advantage of this method is that nitrogen does not interact with the cladding and, hence, the cladding components do not pollute the fuel. In this work, the theoretical aspects of nitriding and denitriding are considered and thermodynamic modeling is performed. The next work in this series will present the results of an experimental test of the proposed method.

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