Physical properties and leaching behaviour of spent fuel BISO coated particles

The safe disposal in a geological repository is proposed for the spent fuel elements obtained from operation of High Temperature Reactors. The behavior of the fuel particles under disposal conditions is a key question for the long-term nuclear waste disposal. In the present work, the spent fuel BISO coated particles, which have been irradiated to a burn-up of 10% FIMA, were studied. The size and morphological characteristics of the coated particles were investigated by the using of optical and SEM microscopy. The distribution of the ¹³⁷Cs amount in the coated particle was studied in detail. It was shown the activity was concentrated mainly inside the kernels and in the carbon buffer layer, while the outside carbon layer contained 0.1% of the total ¹³⁷Cs only. Further, the thoria-based (Th_0.834U_0.166)O₂ kernels were mechanically isolated from the coated particles, and their solution behavior was studied using the flow through experiments. In all experiments the average flow rate was ∼7–8 ml/day. Dissolution of irradiated and unirradiated kernels in HCl solution with the different value of pH (from 0 to 5) was investigated at the temperatures 90, 55 and 20 °C. The amounts of the radionuclide leached in solutions were determined by ACP-MS, γ- und α-spectrometry. On the basis of the obtained results the important leaching characteristics such as the normalized leaching rate, the activation energy value for the release of the different radionuclides were calculated.