Study of the low temperature oxidation of uranium powders and its application to the sintering of uranium oxide powders

The uranium oxygen reaction has been studied with a view to obtaining U-UO2 samples containing about 20 per cent by weight of UO2 starting from spherical grain uranium powder (36 μ < Φ < 50 μ). The techniques used are micrography, thermogravimetry, sintering under pressure, radio-crystallography. At 170 deg. C in air or argon + oxygen mixtures, the uranium oxide formed is always UO2 and it is uniformly distributed around the initial uranium spheres. These mixed powders can easily be sintered under pressure in the γ-phase. The density of the samples obtained is 85 to 90 per cent of the theoretical density. The influence of UO2 on the properties of uranium has been shown by the use of dilatometry and thermal cycling in the α phase. The temperatures at which the phase changes α ↔ β and β ↔ γ occur are lowered, the remnant expansion is decreased. High density samples resist well to thermal cycling; the characteristic defects of uranium: high distortion, wrinkled surface, have almost disappeared. Heat treatments in a secondary vacuum at 1050 deg. C cause crystallization of UO2 in a geometrical form and the appearance of a phase of the F.C.C. crystalline type having the composition UWCXOYNZ. This phase causes a new decrease in the α ↔ β, β ↔ γ transformation temperatures for the uranium. After ten dilatometric cycles the remanent expansion of the sample is about 0.5 per cent. The resistance to thermal cycling of a low density sample which has been heat-treated is similar to that of a high density sample which has not undergone a heat treatment. (author)