Phase Relations in the System UO ₂ ‐UO ₃ – Y ₂ O ₃

Abstract

The phase relations in the system U02‐U03‐Yz03, particularly in the Y203‐rich region, were examined by X‐ray and chemical analyses of reacted powders heated at temperatures up to 1700°C in H 2 , CO 2 ‐CO 2 and air. Four phases were identified in the system at temperatures between 1000° and 1700°C: U308, face‐centered cubic solid solution, body‐centered cubic solid solution, and a rhombohedral phase of composition (U,Y) 7 O 2 ranging from 52.5 to 75 mole % Y 2 O 3 . The rhombohedral phase oxidized to a second rhombohedral phase with a nominal composition (U,Y), at temperatures below 1000°C. This phase transformed to a face‐centered cubic phase after heating in air above 1000° C. The solubility of UO, in the body‐centered cubic phase is about 14 mole % between 1000° and 1700°C but decreases to zero as the uranium approaches the hexavalent oxidation state. The solubility of Yz03 in the face‐centered cubic solid solution ranges from 0 to 50 mole % Y2O3 under reducing conditions and from 33 to 60 mole % Y2O3 under oxidizing conditions at 1000°C. At temperatures above 1000° C, the face‐centered cubic solid solution is limited by a filled fluorite lattice of composition (U,Y)O 2 . For low‐yttria content, oxidation at low temperatures (<300°C) permits additional oxygen to be retained in the structure to a composition approaching (U,Y)O 2.25 A tentative ternary phase diagram for the system UO 2 ‐UO 3 ‐Y 2 O 3 is presented and the change in lattice parameter and in cell volume for the solid‐solution phases is correlated with the composition.