Overview of actinides (Np, Pu, Am) and Tc release from waste glasses: influence of solution composition

Abstract

Studies on the release of actinides from nuclear waste glasses are numerous and essentially focus on the retention of actinides in the surface layer, their mobility, solubility and speciation in the medium of interest. Even if the chemical properties of the actinides, such as solubility and speciation in groundwaters, have been studied by many researchers, everything is not yet understood. Indeed, the radionuclide behaviour during glass alteration depends on many parameters like the nature of the solution in contact with the glass (pH, E h, complexing agents, colloids). Several radionuclides are redox sensitive and their mobility is linked to the local redox conditions, expected to be reducing in deep repository environments. Their chemical species and solubilities will be greatly influenced by the redox conditions. Laboratory tests on glass corrosion in pure water under oxic or anoxic conditions show Tc and Np to be released to a greater extent than Pu or Am. Most of the Np and Tc are found in a dissolved form whereas Pu and Am are present as colloidal material with a mobility varying according to the nature of the solutions. The presence of carbonates increases more significantly the mobility of the actinides compared to phosphate and sulfate ions. Most of the released radionuclides will also adsorb on environmental materials or form colloids like Pu and Am, often associated with colloids attributed to the spallation from the alteration layer of the active glass. Parameters affecting the actinide speciation and release (redox conditions, complexation effects, colloid formation) are overviewed through a survey of literature data on active leaching tests.