A review of the oxidation of uranium dioxide at temperatures below 400°C

On the relation between lattice parameter and O/M ratio for uranium dioxide-trivalent rare earth oxide solid solution

The effect of gadolinium content on the thermal conductivity of near-stoichiometric (U,Gd)O2 solid solutions

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 H2, CO2-CO2 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)7O2 ranging from 52.5 to 75 mole % Y2O3. 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)O2. 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)O2.25 A tentative ternary phase diagram for the system UO2-UO3-Y2O3 is presented and the change in lattice parameter and in cell volume for the solid-solution phases is correlated with the composition.

Phase Relations in the System UO2‐UO3– Y2O3

Uranium oxide-lanthanum oxide compositions were investigated to determine the phase relations and crystal chemistry in the system under a variety of experimental conditions. X-ray diffraction, polished-section, and chemical analysis techniques were used as tools of investigation. Compositions of known metal atom concentration were subjected to oxygen, air, hydrogen, and vacuum environments at elevated temperatures. The dependence of lattice parameter on a stoichiometry for a U/sub 0.5/La/sub 0.5/ O/sub 1.75 +x composition was determined. Cubic fluorite solubility was found over a considerable portion of the system uranium oxide lanthanum oxide. Results of lattice parameter measurements, stoichiometry determinations, and microstructure examinations are also presented. (auth)

I.F. Ferguson

Papers

Letter to the editorAnionic vacancies in fluorite-type oxides