Irradiation impact on the leaching behavior of HLW glasses

Fission products and minor actinides arising from spent fuel reprocessing are immobilized in a borosilicate matrix known as high level nuclear waste glass (i.e. HLW glass). Glass packages are intended eventually for disposal in a geological repository. The long-term behavior of HLW glass subjected to radiation by long-life radionuclides must thus be investigated with respect to geological disposal. The present article focuses on HLW glass alteration under irradiation conditions by providing a general description of the methodology and illustrating it through examples. The scientific approach adopted to study irradiation potential effects combines experiments performed on radionuclide-doped borosilicate glasses and non radioactive glasses externally irradiated, focusing on all the different leaching stages (initial, drop and residual steps). Different parameters, such as the cumulative dose, the dose rate, the electronic and the ballistic impacts, are taken into account. As under disposal conditions the glass radioactivity will be quickly governed by alpha decay (after 300 years), the results presented here to illustrate the methodology concern the behavior of alpha-doped glasses. Concerning the initial alteration rate, both the alpha activity and the alpha cumulative dose have been studied and no significant effects have been observed. The structural variations observed on the irradiated solid are thus not important enough to induce any changes in the initial chemical reactivity between glass and water. Concerning the drop and residual alteration steps, the behavior of a ²³⁹Pu-doped glass having an alpha dose rate corresponding to a HLW glass after around 1500 years has shown no effect on the kinetics, nor on the altered layer morphology. Moreover, the plutonium appears to be highly retained in the alteration layer (∼ 95%). (authors)