Surface Layer Effects on Waste Glass Corrosion

Abstract

Water contact subjects waste glass to chemical attack that results in the formation of surface alteration layers. Two principal hypotheses have been advanced concerning the effect of surface alteration layers on continued glass corrosion: (1) they act as a mass transport barrier and (2) they influence the chemical affinity of the glass reaction. In general, transport barrier effects have been found to be less important than affinity effects in the corrosion of most high-level nuclear waste glasses. However, they can be important under some circumstances, for example, in a very alkaline solution, in leachants containing Mg ions, or under conditions where the matrix dissolution rate is very low. The latter suggests that physical barrier effect may affect the long-term glass dissolution rate. Surface layers influence glass reaction affinity through the effects of the altered glass and secondary phases on the solution chemistry. The reaction affinity may be controlled by various precipitates and crystalline phases, amorphous silica phases, gel layer, or all the components of the glass. The surface alteration layers influence radionuclide release mainly through colloid formation, crystalline phase incorporation, and gel layer retention. This paper reviews current understanding and uncertainties.

Proposed research and development plan for mixed low-level waste forms

“…The controlled crystallization also produces a glassy matrix that is high in silica and alumina and is extremely durable (Feng 1994b). Vitreous ceramics have been tested with extended long-term PCT and vapor hydration tests, and all the results suggest greater durability than HLW glasses, with uranium releases as low as 0.000029 g/m2/day (Feng 1994~). The volatility of Cs in the process is high, but a special formulation of nepheline-vitreous ceramics for treating Cs wastes showed less than 1% Cs,O loss after melting the vitreous ceramic, with 35 wt?!…”

“…The devitrification is controlled in such a way that most radionuclides and hazardous elements are incorporated in the crystalline phases, and the network formers such as silica and alumina and any other components that do not dissolve into the crystalline solid solutions remain in the glassy matrix (Feng 1994b). Glass ceramics are, therefore, thermodynamically more stable and usually more durable than the parent glass (Feng 1994~). …”