Ultrasonic method for nuclear fuel rods pressure and gas composition released measurement

The objective of this work was to develop a non-destructive acoustic method giving an easy access to two important pieces of information in the irradiated fuel rods: the pressure and the composition of the internal gas mixture in the upper plenum of a standard LWR fuel rod. A first attempt started in 1993 made possible the development of a focused sensor, able to inject acoustic power from a piezoelectric transducer, through the fuel rod cladding and excite the internal gas mixture. This step was achieved in 1999 and covered by a first patent. However, the initial problem was not totally solved because of the presence of the upper spring in the LWR fuel rod. This spring mainly induced a large decrease in the acoustic amplitude response and this solution was not applicable. The method has been revised recently for several aspects: the design of the sensor to optimize the acousmtic power injection through the cladding rod, and the entire experimental protocol, including the signal processing in the time and frequency spaces. A single λ/2 layer (instead of the standard solution in λ/4) of low impedance material (compared to the transducer and tube wall impedance) as water, was found to be a better matching layer.