Secondary Phase Formation During Nuclear Waste-Glass Dissolution

Abstract

Abstract--Secondary minerals formed during simulated weathering of nuclear waste #asses have been identified by analytical electron microscopy. A complete description of the reacted #ass, from the outermost surface in direct contact with the leachant solution to the reacting front that migrates into the bulk #ass, was obtained. Manganese and iron oxyhydroxide phases and saponite were found to have precipitated onto the residual #ass surface from the leachant solution. Iron-bearing smectite, serpentine, and manganese and uranium-titanium oxyhydroxides formed in situ in the #ass 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 #ass into crystalline phases did not occur at the interface with fresh #ass. 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.

The effects of the glass surface area/solution volume ratio on glass corrosion: A critical review

“…However, the thinness of the layers formed at 50 and 100 n-I may have prevented resolution of similar sublayers. The alteration phases observed on the sample reacted for 100 days at 10 rn-1 are consistent with those formed on SRL 131 glass under the same test conditions after longer reaction times [62]. Analysis of samples reacted for shorter time periods showed the layer to evolve from a single clay phase to complex layer containing several distinct phases.…”

Formation of Allophane and Beidellite during Hydrothermal Alteration of Volcanic Glass Below 200°C

“…Recently, Murakami et al (1989) and Copyright 9 1992, The Clay Minerals Society Banba et al (1990) examined the experimental alteration of synthetic borosilicate glass and observed both that an amorphous fibrous phase was initially produced within a "mottled phase" and that crystalline materials such as nontronite, chlorite, septechlorite, and/or stilpnomelane appeared to precipitate. Abrajano et al (1990) also reported secondary phyllosilicates precipitated from the leachant solution during simulated weathering of nuclear waste glasses. Tazaki et al (1989) observed the domain structure of crystalline regions within the noncrystalline volcanic glass matrix and the formation of clays with 14 and 16.7 ]k spacings and suggested that the crystalline regions represented noncrystalline matrix directly transformed into clay minerals.…”

Critical review of glass performance modeling

“…In some open-system glass dissolution tests, the gel layer appears to serve as a transport barrier that limits the overall dissolution rate [13]. However, in most closed-system experiments, elemental release data and electron microscopic examination of the surface layers show that the overall reaction rate is not controlled by diffusion of elements through the alteration layers [14][15][16][17]. Note, however, that some recent Product Consistency Test (PCT) data [17] show a significant effect of surface layer formation and possible diffusional rate control for some simple borosilicate glass compositions.…”