Abstract
Metallographic and X-ray diffraction techniques were used to explore the extent of oxygen solubility in UO 2-ThO 2 solid solutions as a function of temperature and composition. The value of x in (Th, U)O 2+ x increases continuously from 0, at ThO 2, to a value of 0.25 for (U 0.9Th 0.1)O 2.25 at 1200° C. The entire portion of the UO 2-ThO 2-O ternary diagram bounded by this curve is the stability field of a single, continuous face-centered-cubic phase. This phase, for the most part, obeys Vegard's law; contours representing equal cell sizes are straight and uniformly spaced. High-urania solid solutions, between (U 0.9Th 0.1)O 2+ x and (U 0.5Th 0.5)O 2+ x , are in equilibrium with U 3O 8. There is a 3-phase field, in which U 3O 8, O 2 and U 0.5Th 0.5O 2+ x are in equilibrium. High-thoria solid solutions, from (U 0.5Th 0.5)O 2+ x , are in equilibrium with oxygen. The limits of solubility at 1200° C in air can be calculated and accurately predicted if it is assumed that it is controlled by a structural requirement that limits interstitial oxygens to 1 per unit cell and by the availability of uranium ions among the nearest neighbors to compensate for the excess charge. Subject to this structural requirement, interstitial oxygens can fill vacancies bounded by two or more uranium ions. It can also fill 18 % of the suitable vacancies bounded by only one uranium ion. In high-urania solid solutions, above U 0.9Th 0.1O 2+ x , anomalies appear. Vegard's law is no longer followed. The limits of the continuous cubic phase become dependent on temperature, reaching the U-O binary at the point shown by previous work on that binary. A three-phase field, in which U 4O 9, U 3O 8 and the cubic phase are in equilibrium, appears. These anomalies are probably an expression of the tendency to form an ordered phase, found in the vicinity of U 4O 9.
Published Version
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