Development of acoustic sensors for the extension of measurements to high temperature in the experimental reactors

This Ph.D. thesis is dedicated to the development of a new generation of ultrasonic sensors devoted to fission gas characterization. More generally, these studies concern the development of instrumentation for the Jules Horowitz material testing reactor (JHR) aiming to perform in-situ control of the released gas composition for optimization of burn-up rate and fuel rod lifetime. The operation temperature of this new reactor will be in the range of 200 °C - 400 °C. Hence, the main problem concerns the development of piezoelectric material, able to operate in the required temperature range, and its integration with a sensor device.To resolve this problem, we propose to use the sodium bismuth titanate (NBT) ceramic material developed by the screen-printing technique. Several versions of piezoelectric materials were produced during this research, with the purpose of optimizing material manufacturing conditions Each material was characterized (morphological, chemical, electric, dielectric, piezoelectric and electromechanical parameters) and “tests as a function of temperature” were carried out. After demonstrating repeatability in material fabrication, the protocol for NBT sensor production was determined and a prototype of the ultrasonic sensor was fabricated.The sensor was fabricated by deposition of an active material onto an alumina substrate. After characterization of sensor properties, acoustic tests showed a high sensitivity of measurements at ambient temperature (25 °C). Furthermore, by integration of sensors into a pressurized enclosure the possibility of gas detection in the range from 50 to 70 bars was demonstrated. To verify the sensor’s applicability to gas detection under harsh temperature environment, a theoretical model based on electromechanical properties and impedance matching equations was introduced. It was theoretically demonstrated that the sensor is able to perform gas measurements from ambient temperature up to 350 °C.