Surface layers on a borosilicate nuclear waste glass corroded in MgCl2 solution

Surface layers on the French borosilicate nuclear waste glass, R7T7, corroded in MgCl2 solution were studied to determine the composition, structure and stability of crystalline phases. The characteristics of the phases constituting the surface layer varied with the parameter S/V X t, the glass surface area (S) to solution volume (V) ratio, times time (t). At low S/V X t values (< 360 days/m; less than or equal to 36 d) the surface layer was thin and contained mainly iron hydroxide particles and hydrotalcite crystals. At an intermediate S/V X t value (2800 d/m; 5.5 y) the surface layer contained hydrotalcite- chlorite- and saponite-type phases. At the highest S/V X t value (10(7) d/m; 463 d) the major phases were saponite, powellite, barite and cerianite solid solutions. About 95% of the uranium and > 98% of the neodymium released from the glass were precipitated in the surface layer. In the 463 day experiment, 86% of the neodymium in the surface layer was in solid solution with powellite, the rest with saponite. Uranium was contained exclusively in saponite. High S/V ratios, typical of disposal conditions for vitrified high-level radioactive waste, favor retention of actinides in fairly insoluble corrosion products. Observation of similar corrosion products on natural glasses as on nuclear waste glasses lend support to the hypothesis that the host phases for actinides observed in the laboratory are stable over geological periods of time.