Overview of the ultrasonic instrumentation research in the MYRRHA project

The Belgian Nuclear Research Center SCK·CEN is in the process of developing MYRRHA, a generation IV fast flux research reactor to replace the aging BR2. MYRRHA is conceptualized as an accelerator driven system cooled with lead bismuth eutectic mixture (LBE). As LBE is opaque to light, visual feedback during fuel manipulations is not available and must therefore be replaced by a system that is not hindered by the opacity of the coolant. In this respect, ultrasonic measurement techniques have been proposed and even developed in the past for operation in sodium cooled reactors. No such systems have ever been deployed in lead based reactors and SCK·CEN is the first to have a research program in this direction as is detailed in this paper. An extensive literature study shows that the acoustic properties of LBE are in many ways superior to the properties of liquid sodium and water. Hence, an ultrasonic system for operation in liquid sodium has the potential to work even better in LBE, which allows us to leverage on the results of half a century of ultrasonic research for sodium cooled reactors. Moreover, as the speed of sound in water and LBE are comparable, new and innovative ultrasonic techniques can first be tested in water before their final validation in hot LBE. In this paper, we will discuss an experiment where a single ultrasonic transducer is scanned in a pool of LBE to render ultrasonic images of submerged stainless steel objects. The quality of the ultrasonically generated images directly proves the feasibility of ultrasonic techniques in LBE systems. This experiment has also shown that the acoustic behavior at the surface of submerged stainless steel objects undergoes constant change. Hence, the acoustic transmission at the surface of different metals submerged in LBE is further investigated in a new LBE experiment, where the acoustic behavior of aluminum, titanium and two types of stainless steels is studied in more detail. We will present the measurement methodology and show the results after one month of submersion of the samples. The second part of the paper focuses on the engineering issues related to deployment of ultrasonic techniques in the MYRRHA reactor. We describe the two main ultrasonic systems that will be used in MYRRHA: the ultrasonic fuel identification system and the ultrasonic lost fuel localization system. Finally we briefly touch upon the design of LBE compatible transducers and the use acoustic retro-reflectors.