The engineered barriers of a geologic repository for high-level radioactive waste are comprised of a number of components that may be identified in terms of their safety function. The main components are the waste form, the waste canister, and backfill material, as well as tunnel seals and plugs. The main safety function of the waste form is to provide structural stability and resistance to waste dissolution, slowing the release of radionuclides into the aqueous phase. Some fuel cycles produce liquid wastes that need to be stabilized in a solid form for disposal, often using materials such as borosilicate glass, ceramic, and glass ceramic. An understanding is needed of the long-term stability of such glass based waste forms.
A long-standing problem in the analysis of nuclear waste glass degradation (or corrosion) rates is the short-term time scales associated with laboratory studies. One approach to this problem has been to study archaeological glasses, particularly where the environment within which they existed can be well constrained over time. One such study has been presented by Verney-Carron et al. (2008), who focused on an 1800-year-old archaeological glass block (Figure 1) recovered from the seafloor of the Mediterranean in 2003.
In this study, concluded in 2014, we made use of a micro-continuum modeling approach to capture the spatial distribution and identity of reaction products developing over time as a result of the archaeological glass corrosion, while also matching the time scales of alteration where possible. Since the glass blocks sat on the Mediterranean seafloor for 1800 years, the physical and chemical boundary conditions are largely constant.
We focused on a single fracture within the glass block studied by Verney-Carron et al. (2008) and simulated it as a 1D system, with a fixed concentration (Dirichlet) boundary corresponding to the interior of the fracture (Figure 2). The assumption here was that the rate of replenishment of the fluid in the fracture is much more rapid than the rate of diffusion into and out of the glass block.
Preliminary results for the 1D diffusion-reaction simulations are shown in Figure 3. The pristine glass corrodes as a result of the diffusion of water into the glass matrix, forming hydrated glass as a reaction product. The hydrated glass then corrodes following an affinity or Transition State Theory (TST) rate law that is linked to the solubility of amorphous silica (Grambow, 2006). Calcite and smectite (Mg-saponite) form over time, in agreement with the observations reported in Verney-Carron et al. (2008). The formation of smectite (saponite) reduces the silica concentration in the pore space close to the corroding glass, thus accelerating the rate as a result of the TST rate law used. However, a larger effect potentially comes from the change in physical properties, especially diffusivity, when the silica gel converts to a crystalline phase. This may be relevant to the increase in corrosion rate observed in some experimental studies that is associated with the formation of secondary silicate phases (especially smectite).
The 1D micro-continuum reactive transport simulations suggest that the corrosion rate of ancient archaeological glasses like that described from the Mediterranean Sea by Verney-Carron et al. (2008) can be simulated. Most importantly, the model captures the approximate mineralogical zoning, as well as the identity of the newly formed secondary phase smectite. This may be important in future studies, since the formation of new secondary silicate phases appears to be associated with an increase in the glass corrosion rate relative to the “residual” rate (Verney-Carron et al., 2008; Verney-Carron et al., 2010; Gin et al., 2011).
Gin, S., C. Guittonneau, N. Godon, D. Neff, D. Rebiscoul, M. Cabie, S. Mostefaoui (2011), Nuclear glass durability: New insight in alteration layer properties. Journal of Physical Chemistry, C 115, 18696-19706.
Grambow, B. (2006), Nuclear waste glasses—How durable? Elements, 2, 357-364.
Verney-Carron, A., S. Gin, G. Libourel (2008), A fractured Roman glass altered for 1800 years in seawater: Analogy with nuclear waste glass in a deep geological repository. Geochimica et Cosmochimica Acta, 72, 5372-5385.
Verney-Carron, A., S. Gin, P. Frugier, G. Libourel (2010), Long-term modeling of alteration-transport coupling: Application to a fractured Roman glass. Geochimica et Cosmochimica Acta, 74, 2291-2315.