Amélioration de la transmission des ultrasons dans les métaux liquides dans un environnement à haute température

Abstract

To ensure the integrity of fast neutron reactor, the immersion of ultrasonic probes in liquid metals stands as a major challenge in this field of energy. Molten sodium is the liquid metal envisaged for cooling new 4th generation nuclear reactors. Numerous experiments have been performed with this liquid metal, demonstrating the difficulty of obtaining good ultrasonic transmissions due to unstable acoustic coupling. Moreover, the propagation of acoustic waves in liquid metals is still a poorly covered topic in the literature, and the results obtained are still too few. Furthermore, conducting experiments with liquid sodium is very restrictive, as it is highly reactive in air and therefore requires a neutral environment to be used. The discovery of new, non-toxic liquid metals with interesting properties is relevant as they can replace most common human-dangerous liquid metals (sodium, mercury). It is in this perspective that this study using galinstan has been carried out. This gallium-based alloy has excellent characteristics, including good chemical stability and the ability to be easily handled in ambient air. Being liquid at room temperature, it was possible to conduct experiments over a wide temperature range. However, galinstan has the drawback of oxidizing in the presence of oxygen, forming a thin passivation layer on its surface to isolate it from ambient air. The challenge of this project is to improve ultrasound transmission in liquid metals in a high-temperature environment, while handling the effects of oxidation. To achieve this, an experimental protocol was used to identify and optimize parameters that improve the acoustic coupling between the probe and the galinstan. This was followed by experiments immersing ultrasonic probes in galinstan at temperatures ranging from 22°C to 200°C. The results show that it is the viscoelastic properties of the galinstan's oxide layer that limit ultrasound propagation in the liquid metal by promoting the formation of air bubbles at the interface. Depositing soldering flux on the immersed surface of the probe drastically improved results by 72% at room temperature and 50% at 200°C. The use of flux proved to be a robust method during successive immersions. Finally, this study showed that optimizing key parameters for acoustic coupling significantly improved ultrasound transmission in galinstan at both room temperature and high temperature (200°C).

Date	8 Oct 2024
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Pierre Bélanger (Supervisor)