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Leachability of Actinides and Fission Products from Spent HTR Fuel

The aspect of radionuclide mobilization in spent HTR fuel in case of the unlikely event of water ingress into a salt mine was investigated. A salt brine thus formed could corrode the waste package and reach the HTR fuel particles which would, however, be still protected by their particle coating layers. Only in the case of breaking these coatings the fuel as such would be exposed to the brine. For the present study single bare fuel kernels were isolated from irradiated fuel elements and subsequently leached with a saturated salt brine at different pressures and temperature. This procedure corresponds to the most conservative scenario for the definition of the source term for radionuclide migration from a drowned repository with spent HTR fuel elements. Leachability was found nuclide-specific and increases with temperature and pressure. Cesium, antimony and europium are relatively well mobilized, whereas ruthenium and americium remain better fixed in the solid matrix. Cerium, neptunium, plutonium and curium were mostly found at intermediate levels of solubility. After 100 days at 90°C/13 MPa some kernels had disintegrated and released a large fraction of their radionuclide inventory.