Comment on the Long-Term Chemical and Mineralogical Stability of the Buffer

This report examines concepts and data that SKB may use to assess the long-term chemical and mineralogical evolution of bentonite barriers in a KBS-3 repository for spent nuclear fuel. Three interrelated topics are considered: mineral chemistry of the smectite clays; thermodynamic stability of the smectite clays; and bentonite-water interactions during the early thermal period of repository evolution. Smectites are complex solid solutions having variable compositions resulting from ionic substitutions on exchange, octahedral and tetrahedral sites in the crystalline lattice. Although little is known about the mechanisms and rates of reactions involving the latter two sites, abundant observational evidence from natural systems suggests that such reactions could occur to an appreciable extent in the buffer over the million year time frame being considered for an intact canister. We are not aware of any efforts in SKB's current modeling strategy to account for such reactions, and therefore question whether the strategy is appropriate for modeling the long-term chemical evolution of the buffer and associated potential effects on the desirable physical and rheological properties of this barrier material. The variable chemistry of smectites affects their thermodynamic stability. Models of smectite-water equilibria use either a fixed stoichiometric composition to approximate representative smectite varieties, or account for compositional variations using solid solution models and ideal mixing relations among thermodynamic components. In either case the thermodynamic properties of a specific smectite composition or of individual solid-solution components must usually be estimated. Recent reports suggest that SKB will not account explicitly for the thermodynamic properties of smectite in its models of bentonite-water interactions. Rather, the models will assume that this clay mineral has a fixed, though unspecified, composition representing an ion-exchanger phase. This phase is assumed either to be stable or to alter to a less expandable mineral at an extremely slow, but predictable, rate. Unfortunately, this approach appears to have two inherent limitations: 1) it cannot be used to relate specific conditions of smectite stability/instability to evolving geochemical conditions in the near field, and 2) it may overemphasize the importance of accessory minerals and surface reactions in controlling the long-term chemical and mineralogical evolution of bentonite barriers. Dissolution, transport and precipitation processes during the resaturation phase and transient thermal period of repository evolution could cause individual clay particles in the buffer to become cemented together. Cementation could adversely affect the physical and microstructural properties of this barrier. SKB's current approach for modeling bentonite-water interactions may not be an appropriate basis for evaluating the potential long-term effects of cementation on buffer performance, due to the basic limitations in this approach noted above.