Release of Cs, Sr and Rb from two high–burnup spent nuclear fuels: Effect of the dishing and the internal grain boundaries

This work presents the results of the release of Cs, Sr and Rb from two different spent nuclear fuels (BWR fuel with a burnup of 58 GWd(tU)−1 and a PWR fuel with a burnup of 63 GWd(tU)-1). Dishing effect was observable in the experiments with a 58 GWd·(tU)−1 spent fuel but not with the 63 GWd·(tU)−1 spent fuel, perhaps due to the decrease of the gap with burnup which could block the release of the radionuclides from the void between two consecutive pellets. The fitting of the SERNI model [12] to the experimental data was found to be a useful tool in order to separate the contribution of the gap and of the internal grain boundaries. Actually, it was possible to establish the fraction of the radionuclides in the internal grain boundaries: 0.8-2.0% for Cs, 0.22-0.89% for strontium, and 0.36-0.90% for rubidium. The fraction of such radionuclides in the gap was calculated to be 0.39-2.58% for Cs, 0.04-0.20% for strontium, and 0.05-0.27% for rubidium. In addition, the release from internal grain boundaries was found to be relatively slow (e.g. only 18% of the caesium was found to be already released to the solution from internal grain boundaries after 200 days).