The thermal diffusivity and heat capacity of uranium dioxide have been measured from 500 to 2900 K with an advanced laser-flash technique. These two quantities were determined simultaneously by means of an accurate numerical fitting of the experimental thermograms. At high temperatures the precision of the method used is much better than that associated with conventional laser-flash measurements. It was found that the heat capacity continues to increase even at temperatures above the expected lambda transition (2670 K). The inverse of the thermal diffusivity increases linearly with temperature up to 2600 K, whilst at higher temperatures the slope markedly decreases. A new expression for the thermal conductivity as a function of temperature is proposed, which is corroborated by some theoretical considerations on the underlying heat transport mechanisms.

A laser-flash apparatus has been constructed for the measurement of thermal diffusivity. The apparatus is specially designed to operate under conditions imposed by the requirement to measure the thermal diffusivity of highly radioactive reactor-irradiated nuclear fuels. Among the various requirements, relating to the measurement of irradiated samples, were the ability to characterize sample platelets of irregular contours and different sizes, to make these measurements in a sufficiently short experimental time frame, and to maintain good experimental accuracy while keeping pulse laser energies at low levels. This article shows that improvement of key components above the current standards—in particular of the laser-beam homogeneity and of the transient temperature detector—makes it possible to create more flexible and controllable experimental conditions, enabling reliable measurements to be carried out in a broad range of modes. The method used to analyze the collected temperature pulse data is based on a least-squares fitting procedure of integrals of the temperature diffusion equation with realistic boundary conditions. Numerical analysis techniques have been employed to interpret the experimental data, measured in different modes, as imposed by the above-mentioned conditions. The testing and characterization of the machine using POCO graphite and UO2 are presented.

A subsecond laser heating technique has been successfully applied for graphite melting under controlled isobaric conditions. During the applied pulses, relatively large amounts of graphite were melted and subsequently solidified under good stability conditions of the liquid mass. The solid–liquid-vapor triple point was determined. From metallographic analysis of the quenched liquid, the expansion upon melting could be estimated. A mathematical model was then applied to analyze the measured thermograms and the thermal conductivity of liquid carbon was deduced. Both experimental observations and calculation results indicate a nonmetallic nature of liquid carbon in the pressure range of 110–2500 bar. Finally, an analysis of the melting line Tm(p) based on Simon’s empirical equation of state confirms the self-consistency of all results obtained.

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