Abstract
Most of the High Performance Research Reactors (HPRR) are made of plates undergoing a limited swelling during irradiation. Measuring the fuel thickness or the inter-plate distance is then a promissing way to obtain information on the fuel element irradiation history. The experimental constraints are however quite heavy due to the aimed resolution and the element geometry. In order to perform such measurements, a high resolution ultrasonic device was designed. It was thinned down to 1 mm in order to be inserted into the reactor water-channel. The system is then excited with a 120 MHz central frequency burst and the distance measurement is carried out through the ultrasonic waves' time of flight estimation. A series of experiments was performed on a full size irradiated fuel element of the "Institut Laue-Langevin" reactor proving the feasibility of real-time in-situ measurements.