Speaker
Description
Low and medium activity nuclear sludges generated during the reprocessing of nuclear spent fuel have been stored since the 1960’s in bitumen. Indeed, bitumen presents many advantages regarding criteria for a long-term storage: high agglomerating power, high chemical inertness, non-permeability, low solubility in water, high confining power… In practice, coprecipitation salts are added to nuclear sludges (BaSO4, NaNO3…) to avoid any solubilization of liquid wastes in bitumen matrix. Even if it seems that on a chemical point of view, the bitumen packages are stable, radiolysis due to self-irradiation occurs and leads to the formation of gas bubbles (essentially hydrogen) which can significantly elevate the fire and explosion risk when released. Consequently, the role of these bubbles has to be precisely determined. More precisely, the aim of this work is to better understand the mechanisms of formation and evacuation of radiolysis gases when the temperature increases. Obviously, direct optical observation of bubbles is not possible because of bitumen opacity.
Some bitumen samples had been made in order to study these bubbles. Only X-ray tomography can be used on these small samples, previously irradiated to form H2 bubbles with various radii and volume fractions. This type of studies has been widely investigated in previous studies. Even if X-Ray tomography is the most actual accurate tool, it is not possible to perform measurements online during irradiation. Furthermore, no study at high temperature (namely 100-150°C) to evaluate bubble rising in the event of fire is available. To address this issue and refine existing results in term of bubbles properties versus irradiation dose or temperature, and to improve understanding of bitumen package thermal behaviour, we have proposed to use ultrasonic waves.
In this context, we will investigate the influence of bubbles on ultrasonic wave velocities in a first step, in order to detect bubbles in an elementary representative volume. In this framework, the ARISE project, supported by the National French Research Center (CNRS) and the Institute of Radioprotection and Nuclear Safety (IRSN), began in 2023 to explore the feasibility of ultrasonic methods to detect the appearance of radiolysis bubbles under gamma irradiation and track them in real-time during temperature increases.
In the present communication we present very first results concerning bitumen irradiated in IRMA French irradiator, from IRSN BOREE experimental platform, with total integrated doses of 6, 50 and
100 kGy. Regarding our previous work on non-irradiated bitumen, investigations were conducted with 500 kHz transducers in transmission mode. Ultrasonic velocity measured is very sensitive to bubbles (their presence and morphology was proved with X-Ray comparative tests) and a decrease of
around 350 m.s-1 was observed between non-irradiated bitumen and irradiated bitumen at 100 kGy.
These first results, which clearly demonstrate the potentialities of ultrasound for radiolysis bubble cloud evaluation, will be presented and discussed. First interesting elements concerning the minimum dose leading to H2 bubbles appearance and nucleation will be given.
