Phase Relations in the System UO₂‐UO₃– Y₂O₃

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 H2, C02-CO, 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)70,z ranging from 52.5 to 75 mole yo Yz03. The rhombohedral phase oxidized to a second rhombohedral phase with a nominal composition (U,Y),015 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 yo between 1000" and 1700°C but decreases to zero as the uranium approaches the hexavalent oxidation state. The solubility of Y z 0 3 in the face-centered cubic solid solution ranges from 0 to 50 mole yo Y z O~ under reducing conditions and from 33 to 60 mole yo Y203 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 ( <3OO0C) permits additional oxygen to be retained in the structure to a composition approaching (U,Y) 02.z~. A tentative ternary phase diagram for the system U02-U03-Y203 is presented and the change in lattice parameter and in cell volume for the solid-solution phases is correlated with the composition.

Thermodynamic assessment of the U–Y–O system

“…The phase diagrams of Bartram et al [6] and Aitken and Joseph [7] however, partially disagree, with the fluorite phase in the latter containing significantly more yttrium than that of the former. Since there is a significant difference between the proposed phase regions, and Aitken and Joseph [7] only determined the presence of the fluorite and rhombohedral phases, the phase equilibria of Bartram et al [6] were used in the current assessment. A limitation in the work of Bartram et al [6] however, is that no three-phase regions appear in their proposed diagram, whereas that would be expected based on observations in similar systems containing rare earths (RE) [8].…”

“…The models of the phases and their thermodynamic functions are optimized with respect to observations [6,7,9] in the system to obtain a self-consistent description and are compared to the results of the KEMS of Nakajima et al [10].…”

“…For consideration of the Y-O system the assessment of Djurovic et al [5] was utilized. The high-temperature phase relations in the U-Y-O ternary have been characterized in two separate studies [6,7]. The phase diagrams of Bartram et al [6] and Aitken and Joseph [7] however, partially disagree, with the fluorite phase in the latter containing significantly more yttrium than that of the former.…”

“…The high-temperature phase relations in the U-Y-O ternary have been characterized in two separate studies [6,7]. The phase diagrams of Bartram et al [6] and Aitken and Joseph [7] however, partially disagree, with the fluorite phase in the latter containing significantly more yttrium than that of the former. Since there is a significant difference between the proposed phase regions, and Aitken and Joseph [7] only determined the presence of the fluorite and rhombohedral phases, the phase equilibria of Bartram et al [6] were used in the current assessment.…”

“…Since there is a significant difference between the proposed phase regions, and Aitken and Joseph [7] only determined the presence of the fluorite and rhombohedral phases, the phase equilibria of Bartram et al [6] were used in the current assessment. A limitation in the work of Bartram et al [6] however, is that no three-phase regions appear in their proposed diagram, whereas that would be expected based on observations in similar systems containing rare earths (RE) [8]. The only other thermodynamic data for this system are from thermogravimetric determinations of oxygen potential vs O/M of Hagemark and Broli [9] and partial pressure determinations of U, UO, and UO 2 species from the Knudsen effusion mass spectrometry (KEMS) of Nakajima et al [10].…”