Résumé
Most of the fuel elements of Nuclear Research Reactors are made of plates undergoing during irradiation limited swelling. Measuring the fuel thickness or the inter-plate distance is then a challenging mean to obtain information on the element irradiation history. In order to respect the experimental constraints, a high resolution ultrasonic device with a 120 MHz central frequency was designed. It was thinned down to 1 mm in order to be inserted into a reactor water channel. Measurements were performed on a full size irradiated fuel element of the Institute Laue Langevin reactor and distances were identified through the ultrasonic waves' time of flight estimation.