Determination of solidus-liquidus temperatures in the uo2 + x system (−0.50<x<0.20)

Abstract Solidus and liquidus temperatures for urania have been determined to ±15 °C over the composition range UO 1.50 to UO 2.23. The congruent composition is UO 2.00 (2865 °C) and the monotectic composition is UO 1.46 (2425 °C). Data were obtained by a thermal arrest technique, using samples sealed in tungsten or rhenium. The absolute accuracy of the temperature measurements was verified by melting Ta, Mo and Al 2 O 3 (sapphire) as standards. For oxygen-excess compositions, data for Wcontained and Re-contained samples agree well in spite of the extremely different types of interaction in the two systems. Rhenium dissolves in molten UO 2 + x and separates out as the element on cooling with no net change in the urania. Tungsten dissolves in the melt and separates out during cooling as a mixture of W and U x WO 3 . The remaining urania is stoichlometric.