Gamma radiolysis of aqueous media and its effects on the leaching processes of nuclear waste disposal materials

The effect of the presence of moderate gamma doses (6 X 104 Gy) on the leaching of glassy and ceramic waste forms has been investigated. It is found that the leach rates are enhanced by <50% in the case of alumina-free glasses, by a factor of 3 to 4 in the case of glasses that contain 5 to 8% Al2O3, and by a factor of 20 to 100 in the case of SYNROC-D (20% Al2O3). Buffer studies show the enhancement to be almost entirely due to a decrease in pH, and the composition dependence of the enhancement factor is interpreted in terms of the sharp rise in both alumina solubility and leach rates of alumina-containing materials with increasing acidity. The radiation-induced pH decrease is partially due to the formation of nitric acid but formic and oxalic acid are also observed to be produced. The concentration of carboxylic acids is as large as that of HNO3. A mechanism is proposed that assumes HNO3 is produced due to the oxidation of dissolved nitrogen, while formic and oxalic acids result from the reduction of CO2 by hydrated electrons. The mechanism is supported by scavenging studies with 2-propanol. The production of carboxylic acids increases the significance of radiation effects on waste form leaching because of the presence of CO2 in subsurface water and because of the tendency of these acids to form complexes, further enhancing the leach rates. Organic acid formation may be particularly important in the case of groundwater, which usually contains significant levels of dissolved CO2 and carbonates