$In~Situ$ High-Resolution Measurement of HFR Nuclear Fuel Plates’ Spacing

Most of the high-performance research reactors are made of plates undergoing limited structural modifications during irradiation. Measuring the fuel thickness or the interplate distance is then a promising 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 a 1 mm thickness in order to be inserted into the reactor water channel. The system was then excited with a 120-MHz central frequency burst and distance measurements were carried out through ultrasonic waves' time-of-flight estimations. 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.