Characterization of Aerosols Emitted during Core Boring of Prototypic Fukushima Daiichi Fuel Debris Simulants
Résumé
One of the important challenges for the decommissioning of the damaged reactors of the Fukushima Daiichi Nuclear Power Station (1F) is the fuel debris retrieval. In this context, the URASOL project (acronym coined from URAnium and aeroSOL) has been undertaken. It aims at acquiring basic data on the generation and characteristics of radioactive aerosols from the thermal or mechanical processing of fuel debris simulant blocks. Prototypic fuel debris blocks were fabricated at VULCANO facility. Here, we focus on blocks based on the average value computed for the lower head compositions in the OECD/BSAF benchmark for Fukushima Daiichi unit 2. Core boring of these blocks were carried out in a dedicated test section (FUJISAN for Fukushima Daiichi Uranium-containing sample Joint Investigations of the Source term of Aerosols and their Nature) at CEA Cadarache. Aerosols were sampled, collected and characterized. Particle size distribution determined from impactors show a bimodal distribution with a major mode around 4 µm and a minor around 0.4 µm. STEM-EDS and ICP-MS/OCP-AES have been carried out on the collected aerosols. Zirconium, uranium and iron are the major elements in the aerosols as well as in the fuel debris prototypes but in significantly different proportions. Boron concentration in the aerosols is larger than that in the blocks, suggesting a preferred aerosolization of borides. Fission product prototypes such as tellurium and yttrium have also a higher concentration in the collected aerosols than that in the block. Chemical composition is also significantly varying with the size of aerosols. From these tests, it is assumed that some of the phases of the fuel debris blocks are more likely to become aerosols than others. Knowing the estimated isotopic composition of each element in the Fukushima Daiichi unit 2 fuel 10 years after shutdown, it is possible to estimate the aerosol activity from its composition. More than 85% of the radioactivity is carried by Am, Cm and U which are represented by depleted uranium in these tests. Tin-121m is the most active non-actinide isotope in these aerosols.