Development and applications of retroreflective surfaces for ultrasound in LBE

The Belgian Nuclear Research Centre SCK-CEN is in the process of developing MYRRHA a fast flux research reactor, cooled by lead bismuth eutectic mixture (LBE). As LBE is opaque to visual light, ultrasonic techniques will be employed to provide feedback to submerged operations. One of the tasks tackled using ultrasound is locating a potentially lost fuel assembly. Unfortunately, standard machined stainless steel objects, such as a fuel assembly, reflect an ultrasonic pulse in a specular way which induces stringent requirements on the alignment of the ultrasonic sensor with respect to the fuel assembly. An alternative approach consists in relying on reflections from the known surroundings: a lost fuel assembly will block the line-of-sight to the diaphragm resulting in an anomaly in the reflection. In that way, it suffices to align the sensor with the surroundings. The alignment requirement in the latter approach can be further relaxed by using a tiling of retroreflectors on the lower surface of the diaphragm. In this paper, we present the first results in developing a retroreflecting surface for ultrasound in LBE. We also demonstrate how retroreflective surfaces can be used for localizing lost objects.