Advances in the use of laser-flash techniques for thermal diffusivity measurement

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.