Dissolution of high burn-up spent nuclear fuel at high-pH

The release of radionuclides from high-burnup spent nuclear fuel (SNF) segments was studied at pH = 13.2 as well as the effect of the presence of calcium and silicon. The aim was to ascertain the dissolution of SNF in solutions corresponding to a high-level nuclear waste repository including concrete in different structural parts. The release of uranium at pH = 13.2 was higher than at pH = 8.4 in bicarbonate medium, while the presence of calcium resulted in a decrease of the uranium concentrations in solutions, assumed to be the consequence of the formation of a secondary solid phase such as Ca2U2O7. Caesium release was found higher at pH = 13.2 as well, but it was not influenced by the presence of Ca and Si at long term. On the other hand, actinide elements (plutonium, neptunium and americium) dissolution decreased at pH = 13.2, probably because of the formation of secondary solid phases. On the contrary, ruthenium and technetium release at pH = 13.2 was found to be much higher than the measured at lower pH, perhaps due to the higher dissolution kinetics of the metallic inclusions at such pH.