STRUCTURAL INVESTIGATIONS OF NUCLEAR WASTE  GLASS ALTERATION  LAYERS

Joelle Tammy Gail Reiser

Nuclear reactors provide reliable energy, but resultant nuclear waste requires safe disposal. Borosilicate glass is a current immobilization method, incorporating nuclear waste products into glass matrices. Understanding alteration mechanisms in aqueous media is essential to nuclear waste performance assessments to ensure radioisotopes are contained for extended periods of time. When exposed to aqueous solutions, borosilicate glass releases various ions into solution and alteration products (or alteration layers) are formed at the surface of glasses. Although the nuclear glass alteration community has agreed on the mechanisms during initial dissolution, the general mechanisms of the formation of alteration layers and their role in long-term glass alteration are still being debated. However, the community agrees that more information on physical properties of alteration layers is needed to further the understanding of their impact on overall glass alteration. Two main approaches were used in this work to understand the physical properties of alteration layers further. Firstly, positron annihilation spectroscopy (PAS) was used to evaluate pore volumes of various alteration layers as a function depth. Since this technique has not been previously used to on alteration layers, the potential of PAS analysis with various alteration layers is explored. Secondly, skeletal structures of alteration layers with skeletal structures of sol-gel synthesized gels were analyzed and compared using small angle X-ray scattering. A proposed alteration layer formation mechanism (the interfacial dissolution-precipitation) suggests that alteration layers are formed through mechanisms similar to those proposed sol-gel synthesized gels. Sol-gel derived silicate gels could be used as analogues glass alteration layers in future studies if alteration layers and synthetic gels demonstrate physical and chemical properties that are sufficiently similar. Skeletal structural similarities between alteration layers formed at very high pH (pH 11) and synthetic gels formed in neutral and high pH (pH 7 and 9) conditions indicate that they likely share similar formation mechanisms. However, alteration layers formed at pH 9 do not share these similarities. The results indicate that proposed formation mechanisms apply only to certain pH conditions, and that future proposed mechanisms should account for alteration layer formation behavior in various solution conditions.

STRUCTURAL INVESTIGATIONS OF NUCLEAR WASTE GLASS ALTERATION LAYERS

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