High temperature chromium volatilization from Cr _2O _3 powder and Cr _2O _3-doped UO _2 pellets in reducing atmospheres

摘要

Chromium volatilization from Cr _2O _3 powder and Cr _2O _3-doped UO _2 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 Cr _2O _3-doped UO _2 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 UO _2. For all the oxygen potentials, the mass loss of Cr _2O _3 powder was found to start at temperatures as high as 1470 °C due to Cr _2O _3 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 Cr _2O _3-doped UO _2 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 UO _2 pellets, the liquid phase CrO _((l)) seemed to appear at a lower oxygen potential than in Cr _2O _3 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 Cr _2O _3 powder, due to the part of chromia not solubilized in the UO _2 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 Cr _2O _3 powder. Thus the volatilization of chromium from doped UO _2 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 Cr _2O _3 particles, and Cr volatilization from doped UO _2 particles; then chromium volatilization from the doped UO _2 ceramic during further densification process.