Effect of testing conditions on the diffusivity measurements of nuclear fuels via the flash method

The nuclear fuel is a ceramic produced by powder metallurgy route. The knowledge of its thermal conductivity is a fundamental data for a better prediction of the fuel performance in reactor. The thermal conductivity is derived from a thermal diffusivity measurement, whose value may be affected by parameters such as the origin of the powders, the fabrication process used, the fraction of porosity of the sintered fuel....and must be measured accurately.Within the Fuel Studies Department, the Uranium Fuels Laboratory has acquired a thermal diffusivity Light Flash Apparatus (LFA HyperFlash 467, manufactured by the Netzsch company) to complete its range of characterization capabilities devoted to nuclear fuels.The device implements the flash method, which is a standard method for measuring the thermal diffusivity. The test consists of subjecting the lower surface of a small and thin sample, with plane and parallel faces, to an energy pulse of very short duration (provided here by a xenon flash lamp). The resulting temperature rise (thermogram) is recorded on the opposite upper face of the sample (using here an InSb infrared detector cooled with liquid nitrogen).The thermal diffusivity value is inferred from the interpretation of the thermal curve. The analysis software provides different theoretical models (such as those of Cowan and Cape-Lehman) for the interpretation of the experimental thermograms.Measurements can be performed up to 500DC under controlled atmosphere. Several samples (up to 16, depending on their geometry) can be characterized in a single test.In the present work, aiming at a better understanding of the factors that affect thermal diffusivity measurements using the flash method, the effect of parameters related either to the specimen (coating, thickness, porosity,) or to the operational conditions were investigated on uranium-based ceramics.