Performance Evaluation of a High-Temperature Ultrasonic Transducer for In-Situ Defect Detection in Lead–Bismuth Eutectic alloy

Non-destructive monitoring of reactor vessel walls in heavy-liquid-metal-cooled systems requires ultrasonic probes capable of continuous operation at elevated temperatures under challenging acoustic coupling conditions. In lead–bismuth eutectic (LBE)-cooled reactors, defect detectability is influenced by temperature gradients, oxide-layer formation, wetting behavior, and coupling-interface changes. This study evaluates a custom-developed high-temperature ultrasonic transducer for in-situ detection of notch-type defects in a stainless-steel reactor-vessel mock-up under laboratory and LBE conditions. The methodology combines cyclic laboratory testing in air, thermomechanical modeling of the mock-up during immersion in the Helios 3 LBE facility, in-situ ultrasonic measurements, and analytical/experimental assessment of coupling-layer thickness effects. Laboratory tests confirmed repeatable defect detection over multiple heating–cooling cycles up to 200°C, with amplitude reduction during heating and near-complete recovery during cooling, indicating no irreversible transducer degradation. In the Helios 3 LBE facility, the defect remained detectable during two heating–cooling cycles, although the reflected amplitude recovered only partially after cooling. Probe inspection revealed no permanent degradation, suggesting that incomplete recovery was primarily caused by acoustic coupling changes rather than transducer failure. Analytical modeling and validation experiments demonstrated high sensitivity of reflected amplitude to coupling-layer thickness, with an 18 µm increase producing approximately −6 dB signal loss. These micrometer-scale coupling gap changes are sufficient to explain the observed amplitude variations. The results demonstrate that the developed transducer is suitable for high-temperature defect detection in representative LBE environments and that coupling-layer stability is critical for reliable ultrasonic monitoring of LBE-cooled reactor components.