Probing Corium in severe nuclear accident experiments: Development of acoustic techniques resisting very high temperatures

Abstract

Since the Fukushima-Daiichi accident, severe accident research has been reconsidered from another perspective. In the case of a severe accident in a nuclear reactor, the core can melt to form corium, a complex mixture of molten fuel and structural materials. In order to improve reactor safety and mitigate the accident on site, it is important to understand the behaviour of corium in the configurations. In the context of in-vessel retention (IVR) studies, the scientific community has limited knowledge of immiscible phase stratification phenomenon. This knowledge could be further developed thanks to experiments with dedicated online instrumentation to follow the metal relocation transient occurring below the oxide phase and the stratification inversion kinetics. This instrumentation has to resist an aggressive liquid and high temperatures (1700–3000°C). The study of an ultrasonic technique, based on time-of-flight measurements, using a refractory material waveguide is in progress. This paper describes the feasibility approach used and the resulting choice of waveguide (able to withstand high temperatures, no interaction with steel, and transmission of the acoustic wave in the waveguide and at the waveguide/ steel interface), the choice of ultrasonic transducer, the numerical simulations, and the results of preliminary experiments performed at high temperature (>1500°C).