High temperature chromium volatilization from Cr2O3 powder and Cr2O3-doped UO2 pellets in reducing atmospheres

Highlights: ► Chromium volatilization from Cr2O3 powder and chromia-doped UO2 pellets. ► Cr2O3 decomposition in metallic gaseous chromium and oxygen. ► Appearance of a CrO(l) liquid phase enhancing the doped-UO2 sintering process. ► Competition between the volatilization of Cr and sintering kinetics of UO2. ► Distinct volatilization phenomena before and after the porosity closure in the UO2. - Abstract: Chromium volatilization from Cr2O3 powder and Cr2O3-doped UO2 pellets during sintering in reducing atmospheres has been studied by thermogravimetry (TG) coupled with differential thermal analysis (DTA) up to 1700 °C. The sintering of Cr2O3-doped UO2 pellets was also followed by dilatometry. Oxygen partial pressures in the range 10−20–10−11 atm (10−15–10−6 Pa) have been fixed in all the experiments thanks to mixtures of hydrogen and carbon dioxide. A linear heating rate of 20 °C min−1 was applied to all the experiments. The dopant amount was in the range 0.18–0.9 mol% Cr in UO2. For all the oxygen potentials, the mass loss of Cr2O3 powder was found to start at temperatures as high as 1470 °C due to Cr2O3 dissociation, the lower the oxygen potential, the lower the starting temperature and the higher the volatilized amount. For intermediate oxygen potentials, an exothermic DTA peak observed during cooling, from 1700 °C to room temperature, attested for the crystallization of a liquid phase which was attributed to CrO(l) according to thermodynamic predictions. Then, the dilatometry and TG studies allowed following the behavior of Cr2O3-doped UO2 pellets. The mass loss at temperatures higher than 1470 °C was attributed to chromium volatilization for all the doped samples. During the sintering of doped UO2 pellets, the liquid phase CrO(l) seemed to appear at a lower oxygen potential than in Cr2O3 powder, which probably contributed to enhance the densification rate. For the highest dopant amount, 0.9 mol% Cr, the volatilization process was found to be rather similar to that of Cr2O3 powder, due to the part of chromia not solubilized in the UO2 crystal. Moreover, as the initial pellets were not dense, as long as the pellet porosity remained open, the chromia particles were able to dissociate as in the Cr2O3 powder. Thus the volatilization of chromium from doped UO2 pellets under sintering in reducing atmospheres must be understood as the result of several phenomena whose contribution depends on temperature, oxygen potential and heating rate: before the porosity closure, both dissociation of chromia into Cr(g) and oxygen from excess Cr2O3 particles, and Cr volatilization from doped UO2 particles; then chromium volatilization from the doped UO2 ceramic during further densification process.