In the frame of the fourth generation forum, France decided to develop sodium fast nuclear reactors. French Safety Authority requests the associated monitoring of argon gas into sodium. This implies to estimate the void fraction, and a histogram indicating the bubble population. In this context, the present letter studies the possibility of achieving an accurate determination of the histogram with acoustic methods. A nonlinear, two-frequency mixing technique has been implemented, and a specific optical device has been developed in order to validate the experimental results. The acoustically reconstructed histograms are in excellent agreement with those obtained using optical methods.

Leaky Lamb waves are proven effective to carry out nondestructive testing especially on parallel and immersed plates. To detect and localize defects in such a set, this work associates for the first time the topological energy method and leaky Lamb waves. This methodology is applied in a single immersed plate to validate its application. Firstly, Lamb mode A1 is generated in the plate, and the reflected waves on the defect are measured. A first case is examined where the edge is considered as a defect to be localized. Then, measurements are taken on a plate where a notch is machined. The measurements are time reversed and reinjected in a finite-element simulation. The results are then correlated with the direct problem of the topological energy method that is also simulated. In both cases, the defects are precisely localized on the energy images. This work is the preliminary step to an application of the topological energy method to a set of two parallel and immersed plates where the research defect is located in the second plate.

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).

In 4th generation nuclear reactors cooled with liquid sodium, argon microbubbles are present in the primary sodium. Due to the opacity of liquid sodium, acoustic control methods are chosen for operating inspections but this bubble presence greatly affects the acoustical properties of the medium. It is therefore required to characterize the microbubble cloud, i.e., to provide the bubble’s volume fraction and the size distribution. Safety demands the proposed method to be robust and applicable with few assumptions (about the bubble populations) as possible. The objective of this study is to evaluate the performance of spectroscopic methods (based on celerity and attenuation) in the presence of bubbles whose sizes and surface (or volume) contributions are very different. Two methods of evaluating the histogram and the void fraction are compared. The first is based on the inversion of the integral equation of the complex wave number derived by Commander and Prosperetti. The second, which assumes the populations to have log-normal or sums of Gaussians distributions, performs an adjustment of the distribution’s parameters to fit attenuation and celerity curves measurements. These methods are compared with experimental data obtained using ACWABUL facilities at CEA Cadarache.