SECONDARY PHASE FORMATION DURING NUCLEAR WASTE-GLASS DISSOLUTION

Secondary minerals formed during simulated weathering of nuclear waste glasses have been identified by analytical electron microscopy. Manganese and iron oxyhydroxide phases and saponite were found to have precipitated onto the residual glass surface from the leachant solution. Iron-bearing smectite, serpentine, and manganese and uranium-titanium oxyhydroxides formed in situ in the glass in several distinct bands at different depths beneath the original surface. This sequential development of secondary phases displays a clear trend toward more order and crystallinity in the phases farthest from the reaction front and indicates that complete restructuring of the glass into crystalline phases did not occur at the interface with fresh glass. Additionally, the formation of a discrete uranium-bearing phase, as opposed to uranium uptake by precipitated phases, suggests that stable actinide phase formation rather than ion exchange may be a possible mechanism for retarding radionuclide release to the environment. -from Authors