Abstract
In order to develop the dissolution process of irradiated fuels under mild conditions, the dissolution behavior of UO2 pellet with clad in HNO3 solutions has been examined under various conditions. The dissolution rate at the boiling point (b.p.) was confirmed to be almost the same as that at 90°C and such a phenomenon was found to be due to the effect of HNO2, because the concentrations of HNO2 in the dissolvent solution at 90°C is maximally 50 times that at b.p. The UO2 pellets containing 13 elements as simulants of fission products(FP) were dissolved in HNO3 solutions at various temperatures to examine the behavior of undissolved residues under the mild dissolution conditions. It was found that a total amount of undissolved residues at 90°C is less than that at b.p. and that main components of undissolved residues in the dissolution of simulated fuels are almost the same as those of irradiated fuels. Furthermore, corrosion rates of materials such as SUS304ULC and SUS310Nb were examined in the HNO3 solutions containing simulated FP ions at various temperatures. It was clarified that the corrosion rates of these materials at 90°C are almost 1/10 those at b.p. and that the retardation of corrosion rate is attributed to the effect of lowering temperature and HNO2. From these results, it is expected that the dissolution of irradiated fuels becomes possible under milder conditions than those in conventional reprocessing processes and this results in improvements of confidence and safety in the dissolution process.
