Adaptation of a High Frequency Ultrasonic Transducer to the Measurement of Water Temperature in a Nuclear Reactor

Most high flux reactors possess for research purposes fuel elements composed of plates. Their relative distance is a crucial parameter, particularly concerning the irradiation history. For the High Flux Reactor (RHF) of the Institute Laue-Langevin (ILL), the measurement of this distance with a microscopic resolution becomes extremely challenging. To address this issue, a specific ultrasonic transducer, presented in a first paper, has been designed and manufactured to be inserted into the 1.8 mm width channel existing between curved fuel plates. It was set on a blade yielding a total device thickness of 1 mm. To achieve the expected resolution, the system is excited with frequencies up to 70 MHz and integrated into a set of high frequency acquisition instruments. Thanks to a specific signal processing, this device allows the distance measurement through the evaluation of the ultrasonic wave time of fight. One of the crucial points is then the evaluation of the local water temperature inside the water channel. To obtain a precise estimation of this parameter, the ultrasonic sensor is used as a thermometer thanks to the analysis of the spectral components of the acoustic signal propagating inside the sensor multilayered structure. The feasibility of distance measurement was proved during the December 2013 experiment in the RHF fuel element of the ILL. Some of the results will be presented as well as some experimental constraints identified to improve the accuracy of the measurement in future works.