Comparative Analysis of Experimental Data on the Sublimation of Uranium Carbonitrides and Uranium–Zirconium Carbonitrides at High Temperatures

The review is devoted to a comparison of new experimental data on the sublimation of uranium–zirconium carbonitrides with different contents of carbon, nitrogen, and oxygen impurities at high temperatures (1700–2300 K), we obtained in the past 2 years, with data of previous reported works on the sublimation of uranium carbonitrides, we and other authors prepared using mass spectrometry and some other methods of thermodynamic analysis. The main attention is paid to the consideration of the composition of the gas phase and the analytical dependences of the partial pressures of its components on temperature, as well as the chemical mechanism and heats of sublimation. The essential feature of the sublimation process of all materials based on uranium carbonitride (both pure and doped with zirconium) is its incongruent nature, due to the loss of nitrogen, which leads to a shift in their compositions towards the phase with higher carbon content. The chemical mechanisms of sublimation of carbonitrides of both types are considered, according to which oxygen impurities in these materials bring about the appearance of oxide components UO, UO₂, and CO in the gas phase and additional release of nitrogen. The introduction of zirconium into uranium carbonitride and an increase in the carbon content in it lead to a decrease in the partial pressures of uranium monoxide and nitrogen, which increases the thermal stability of this innovative fuel material.