Evaluation of clays and clay minerals for application to repository sealing

Clays are promising candidate materials for use in sealing nuclear waste repositories. Clay seals can be constructed with internal hydraulic conductivities lower than 10-9 cm/sec, approaching conductivities of candidate repository host rocks. Other favorable properties of clays include high sorptivity, high compressibility, and in some cases, high swelling capacity. Although not all clay stability issues are resolved, the potential of clays and clay minerals for long-term sealing is inferred from their geologic persistence, their low solubility in repository-like environments and their slow reaction kinetics. Moreover, expectations of long-term seal performance can be enhanced by prudent selection of clay mineralogy, clay content, moisture content, and emplacement technique. Techniques for emplacing clay-rich barriers include in-place compaction, emplacement in precompacted form, and slurry injection. Technologies for most of these techniques already exist. The sorptivity of clays with respect to radionuclides may be closely related to the cation exchange capacity of the constituent clay minerals. The sorptive capacity of a material may be gauged also by reference to a sorption coefficient. Laboratory studies have shown clays to be efficient sorbers of selected radionuclides although performance depends on radionuclide concentration, temperature, Eh, pH, fluid composition, and clay mineralogy. High priority research items include: (1) thermodynamic properties of clays, and their thermal stabilities and phase transformations, bearing on longevity; (2) seal-rock interface permeability, and permeability at extended times and elevated temperatures, bearing on performance of clay as a barrier to ground water movement; and (3) sorptivity under realistic repository conditions, bearing on performance as a chemical barrier. 139 references