[en] The reactivity of the uranium monocarbide and mono-nitride has been studied under air and argon or oxygenated nitrogen in order to determine their inflammability temperature and to search safe experimental conditions for the manufacture and reprocessing of these compounds. These compounds will certainly be used as fuels in the future nuclear reactors. During their treatment, they are pyrophoric and can become strongly overheated when they are in the state of finely separated powders. Experimental techniques as the TGA, the DTA and the DSC have revealed a strong reactivity under air of the UC powder at 200 C. For the UN powder, the inflammability occurs above 300 C. The influence of the oxygen amount of the scanning gas and the velocity of the temperature increase have been studied. A study of the ternary diagram U-C-O has been carried out and a mechanism based on the progressive or sudden rupture of the UO₂ formed layer is proposed. (O.M.)

[en] Mixed plutonium and uranium carbide (UPuC) is considered as a possible fuel material for future nuclear reactors. However, UPuC is pyrophoric and fine powders of UPuC are subject to temperature increase due to oxidation with air and possible ignition during conditioning and handling. In a first approach and to allow easier experimental conditions, this study was undertaken on uranium monocarbide (UC) with the aim to determine safe handling conditions for the production and reprocessing of uranium carbide fuels. The reactivity of uranium monocarbide in oxidizing atmosphere was studied in order to analyze the ignition process. Experimental thermogravimetric analysis (TGA) and differential thermal analysis (DTA) revealed that UC powder obtained by arc melting and milling is highly reactive in air at about 200 deg. C. The phases formed at the various observed stages of the oxidation process were analyzed by X-ray diffraction. At the same time, ignition was analyzed thermodynamically along isothermal sections of the U-C-O ternary diagram and the pressure of the gas produced by the UC + O₂ reaction was calculated. Two possible oxidation schemes were identified on the U-C-O phase diagram and assumptions are proposed concerning the overall oxidation and ignition paths. It is particularly important to understand the mechanisms involved since temperatures as high as 2500 deg. C could be reached, leading to CO(g) production and possibly to a blast effect.