Alexei M. Potapov

High-temperature processing (HTP) of nitride spent nuclear fuel (SNF) is a series of head-end operations, as a result of which the fuel cladding is removed and the main components of the fuel (uranium and plutonium) are converted into pressable UO2 and PuO2 oxides. Based on the properties of uranium nitrides and oxides, a high-temperature processing (HTP) scheme for nitride spent nuclear fuel is proposed. This scheme consists of nitriding, denitriding, cladding separation, uranium oxidation to U3O8, and subsequent reduction to UO2. Theoretical and thermodynamic justification for each HTP stage is provided.

The efficiency of the high-temperature treatment (HTT) of mixed uranium–plutonium nitride spent nuclear fuel (MUPN SNF) to separate the fuel compound from the cladding and remove volatile fission products from the fuel was evaluated. The high-temperature treatment of SNF allows separation of the fuel compound from the cladding. The degree of removal of cesium nuclides from the fuel compound does not exceed 7%, whereas tritium, radiocarbon, and ruthenium are removed from the fuel compound to 94.2, 89.9, and 95.5%, respectively, relative to their initial content in SNF.

The thermodynamic modeling of uranium mononitride (UN) oxidation by gas mixtures (Ar + O₂) at different temperatures and oxygen contents in the mixture is performed. The oxidation is found to proceed via several consecutive stages, and each stage includes a number of parallel reactions. At the majority of stages, the equilibrium composition of oxidation products is complicated and includes nonstoichiometric compounds. The exception is the composition with the ratio O/U = 2 at which stoichiometric UO₂ is formed. The kinetics of UN oxidation by the (Ar + 20% O₂) gas mixture is studied. According to differential thermal analysis results, uranium mononitride is slowly oxidized in a temperature range of 300–400°C. As a temperature of 420°C is reached, the sample weight increases sharply accompanied by a significant heat evolution. The maximum reaction rate is achieved at 432°C. The maximum increase in the sample weight is 12.4%, which exceeds the theoretical value in the reaction UN → U₃O₈ (+11.26%) but is lower than that in the reaction UN → UO₃ (+13.49%). The sample weight begins to decrease with further heating above 500°C and decreases to the end of experiment by 0.5% of maximum values. Flue gases leaving a simultaneous thermal analyzer are examined using a quadrupole mass spectrometer. These gases, except for argon, oxygen, and nitrogen, contain impurities of nitrides (NO, NO₂, and, possibly, N₂O). The oxidation of UN by the (Ar + 20% O₂) gas mixture at 450°C is studied by thermogravimetry. The stages of the process are revealed. The maximum oxidation rate is 0.4%/min.

—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.

The electrical conductivity of LiCl–ZrCl₄, NaCl–KCl (1:1)–ZrCl₄ and KCl–HfCl₄ molten mixtures has been studied for the first time as a function of temperature (770–1100 K) and the concentrations of ZrCl₄ (0–25 mol%) or HfCl₄ (0–30 mol%). A conductometric cell of an original construction has been used. The electrical conductivity of these mixtures ranges from 0.9 to 3.1 S/cm. The increase in the temperature results in the increase in the electrical conductivity, whereas the increase in the ZrCl₄ or HfCl₄ concentration results in its decrease. The obtained results on the electrical conductivity of MCl–ZrCl₄ (M – alkali metals) melts are summarized, taking into account our previous measurements, and discussed on the basis of the available data on the structure of such melts.