Russia’s Techsnabexport (Tenex) in a consortium with specialists from key institutes have completed a two-year project to study the effect of storage conditions on the characteristics of modelled fuel-containing materials (corium). The institutes include the VG Khlopin Radium Institute (St Petersburg), the Research Institute of Atomic Reactors in Dimitrovgrad (NIIAR - part of Rosatom’s scientific division), the AP Aleksandrov Scientific Research Technological Institute (NITI in Sosnovy Bor) and the St Petersburg branch of the Institute of Geoecology of the Russian Academy of Sciences. The project administrator was Japan’s Mitsubishi Research Institute.
Scientists made samples of silicate-containing fuel debris (based on the characteristics of fuel debris formed after the accident at the Fukushima-Daiichi NPP) and studied their properties during ageing in three environments. The resulting mathematical model makes it possible to predict the behaviour of materials during extraction, transportation and storage with discrete steps of 10, 20, 30 and 50 years.
“The programme of experimental handling of fuel-containing materials, significant in volume and duration, has been completed. The results of the project are important both from the point of view of research value and from the point of view of practical applicability,” noted Elena Artemova, Deputy General Director of Tenex for back-end development.
Experiments have shown that long-term storage of corium under water leads to its partial leaching, as well as to secondary phase formation in the surface layer. Later, when removed and dried, the surface of such material can turn into dust. The conclusions obtained as a result of the project will make it possible to make experimentally substantiated decisions in the subsequent choice of technology for the safe extraction of corium from the NPP units.
“The results of our research will help our Japanese colleagues to take into account the possible risks associated with the formation of radioactive dust and the occurrence of any uncontrolled situations leading to secondary pollution of the territory during the decommissioning of the station,” said Albert Aloy, chief researcher in the Department of Applied Radiochemistry at the Khlopin Radium Institute. “The results obtained will also make it possible to minimise personnel exposure and reduce the financial costs of eliminating the consequences of the accident. The presence of a silicate component in the solidified corium expands the applicability of the results and makes it possible to predict the stability of vitrified high-level wastes during their long-term storage before final disposal.”
Corium (fuel-containing material) is a lava-like material formed in the core of a nuclear reactor during severe accidents with core melt. In 2011, during the accident at the Fukushima-1 nuclear power plant, the temperature inside the reactor rose above 2000 °C, causing the uranium fuel to melt and further react with the vapour-oxidised zirconium cladding, which in turn led to the formation of corium.
The study was carried out at with a subsidy from the Mitsubishi Research Institute (Japan) in 2019 for implementation of the project “Decommissioning and handling contaminated water (development of technologies for analysing and assessing the properties of fuel fragments (development of technology for assessing ageing properties of fuel fragments) ".