Rutherford backscattering investigation of the corrosion of borosilicate nuclear waste glass

The corrosion properties of several borosilicate nuclear waste glasses exposed to aqueous environments have been investigated using a combination of Rutherford backscattering (RBS) depth profiling and solution analysis. Using the technique of RBS, the concentration of individual elements in the glass can be quantitatively profiled within a 1–2 μm surface layer. All of the glass surfaces were corroded in distilled water at 90°C for times ranging from 0–28 days. The evolution and composition of the altered layer that formed on the glass surface was of particular interest, since this layer can be crucial in determining whether or not the remainder of the glass undergoes further rapid corrosion. Within the altered surface layer produced by the corrosion process, there was an enhancement in the concentrations of U, Ti, Sr, Nd, Gd, and Ca and a depletion of Na, Si, Cs, (and probably B) relative to the concentrations present in the bulk glass. The U concentration, for example, was often 3–4 times greater in the altered surface layer than in the bulk. The layer apparently forms due to a local reaction that results in the formation of insoluble complexes on the glass surface. A simple model of the corrosion process has been used to identify the most likely insoluble metal complex for each metal present in excess in the alteration layer. The mechanical and chemical stability of this layer are also discussed.