Abstract
Pellets of uranium-plutonium mixed oxides with Pu/( U + Pu) ratios from 0.05 to 0.97 have been reduced in a hydrogen atmosphere at temperatures between 1400° C and 2000 °C. The results of ceramographic and X-ray diffraction analysis were used to construct the partial U-Pu-O phase diagram at room temperature for O/ U + Pu) ⩽ 2.00. Mixed oxides containing less than 20% Pu remain single phase fcc until all Pu has been reduced to the trivalent state. A large region containing two cubic phases extends from Pu/( U + Pu) − 0.20 up to the binary Pu-O system. These phases are a fcc phase with O/( U + Pu) = 1.985, in equilibrium with another fcc phase in the range 0.2 ⩽ Pu/( U + Pu) ⩽ 0.5 and with a bcc phase at higher plutonium contents. For Pu/( U + Pu) ⩾ 0.5 and low O/M values, a bcc single phase region extends up to Pu/( U + Pu) = 0.95. At Pu/( U + Pu) = 0.97 this phase is represented by a line with O/( U + Pu) − 1.51. A hexagonal phase of the β-Pu 2O 3 type was found for Pu/( U + Pu) ⩾ 0.97. The samples reduced below the line UO 2.0− PuO 1.5 contain a metallic phase. DTA measurements performed at temperatures up to 1500 °C and at different O/(U + Pu) ratios show that the region of two cubic phases has the shape of a tunnel whose top is at 620–640 °C in the binary Pu-O system and decreases progressively in the direction of increasing uranium content.
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