The Density and Compressibility of Liquid (U,Pu)-Mixed Oxide

A sample of (U,Pu)-mixed oxide was fission heated along the saturation line and into the compressed liquid regime. The density of the sample at temperatures around 7000 K was determined from the onset of the liquid-phase pressure signal. Values for the compressibility of the (U,Pu)-mixed oxide were determined from the slope of the measured pressure/time signal. Based on these results and earlier measurements, new relations are proposed for the following properties: the density/enthalpy, density/temperature, thermal expansion/temperature, and isothermal compressibility/temperature relations. The ranges of validity are 1400 to 3600 kJ/kg and 3120 to 7600 K, respectively. The property relations are recommended for both liquid UO₂ and (U,Pu)O₂ with y_Pu ≤ 0.25. The measured data strongly suggest that the critical temperature of (U,Pu)O₂ is well above 8000 K.