Transport through deep aquifers of transuranic nuclides leached from vitrified high-level wastes

The results presented in this paper are the first output of a new program carried out at the Joint Research Centre of Ispra on underground migration of actinides related to risk evaluation of the geological waste disposal option. In order to simulate the expected conditions of glass leaching and underground transport in the laboratory, a water pathway is established which flows over the actinide-bearing borosilicate glass, through a filter and then up through columns containing soil samples. Water fractions are continuously collected and analyzed at the column outlet; the water velocity through the columns corresponds to a groundwater movement of about 25 m/y. The experiments are carried out over periods of 1 to 6 months, and at the end the columns are cut into thin sections and the distribution profile of transuranic isotopes is measured. These initial results already demonstrate a certain number of phenomena; firstly, that the soil matrix studied effectively retains the transuranic nuclides. However the more effective processes seem not to be related to the high effective filtering ability of sandy clays. Further the data appears to show that both for plutonium and americium cationic species are only present at very low levels and negatively charged species appear, for plutonium at least, to be the main mobile form. For both plutonium and americium a soluble anionic form having a concentration of 10-13M was detected which was not retained by the columns. Under the present experimental conditions, this concentration is about four orders of magnitude lower than the Maximum Permissible Concentration for drinking water