Study of the sintering of hyper-stoichiometric mixed oxides of uranium and plutonium

The aim of this work is to study the reactive sintering, in oxidizing conditions, of compounds (U1-yPuy)O2+x, more or less homogeneous, where y is about 0.06 and x, positive, and is inferior to 0.215. It has been shown, at least for the low values of y considered, that it is possible to anticipate the properties influencing the sintering of the oxides, with as support those relative to the sur-stoichiometric uranium oxides. The use of gaseous mixtures presenting an adjustable oxygen potential, is an interesting solution to adjust x to the stoichiometry of the oxides. Either the thermodynamic equilibrium is not reached in the used experimental conditions, the samples, sensitive to the characteristics of the scanning gas, can then evolved by loss or gain of oxygen. The sintering of the considered oxides is carried out at a temperature all the more lower, and with a kinetics all the more high, that the effective ratio of composition O/(U+Pu) is high. Nevertheless, when the ratio to stoichiometry is important (2.215), the presence of the oxide M4O9-z in high proportion compared with the oxide MO2+x during all the thermal treatment cycle slows it considerably. These results are in a direct correlation with the activation energy values of sintering, measured by the temperature methods and constant heating velocities. Indeed, it decreases of 560 kJ.mol-1, for O/M ratios between 1.98 and 2+ε (ε∼0.001), the most probable value for the stoichiometric oxide being of 465 kJ.mol-1; it is constant and equal to 430 kJ.mol-1 when x passes from 0.001 to 0.16; then it increases a lot and in continuous for compositions such as the oxide M4O9-z is majority compared with MO2+ and approaches 640 kJ.mol-1 for a O/M ratio of 2.217. For some samples in the usual oxidizing conditions, whose sintering is ended at about 1100 - 1200 C, a secondary de-densification phenomenon ('solarization') appears if the temperature rising is followed. It is an irreversible process bound to the appearing of a particular microstructure, formed of thin grains at the exterior of the sample and of big grains in the center. At the triple joints of these grains, pores appear and growth progressively under a high carbon monoxide pressure. This is closely bound to the presence inside the sample of trace amounts, in relatively important quantity, of carbon on the form of impurity, brought particularly by the used plutonium oxide. It is slightly amplified by the decomposition in UO2+x, due to a thermal effect, of the oxide U3O8 introduced voluntary in samples to adjust the initial x value to the stoichiometry. (O.M.)