Solubility Phenomena in The Context of Nuclear Waste Disposal

Safety concepts regarding nuclear waste disposal in underground repositories generally rely on a combination of engineered and geological barriers that minimize potential radionuclide release out of the containment-providing rock zone and transport through the biosphere. The presence of water (e.g., groundwater and pore water of repository rocks), however, may alter the engineered barrier system, dissolve radionuclides, and facilitate radionuclide transport that over millennia may allow small fractions of water-soluble radionuclides to permeate to the biosphere. So while barrier systems aim to prevent or hinder water from contacting the waste, the possible intrusion of aqueous solutions must be considered for several safety case scenarios impacted by the long-term evolution of a repository. Dissolution, precipitation and solubility phenomena thus arise as important processes controlling the chemical behaviour of radionuclides and other key materials of relevance in the context of such repositories and their safety assessments. The solubility and aqueous speciation of radionuclides is of particular interest as they provide upper limits for water-transportable concentrations of radionuclides.For many radionuclides, solubility limits are only reached after release from the disposed waste products close to the disposal locations, where expected maximal concentrations are highest. It is then often the solubility of a secondary phase, precipitated after dissolution of a primary phase in the waste matrix, that controls the maximum transportable radionuclide concentration close to the disposal location.Solubility phenomena of radioactive and other gases in water provide important constraints on gas transport and pressure build-up. Moreover, due to its thermodynamic foundation, solubility assessments offer insight into time-independent constraints (e.g., maximum concentrations of radionuclides or gases in a fluid phase) on the evolution of the disposal system. Particularly important in this regard is the geochemical modelling of radionuclide behaviour in the engineered barrier system as well as in natural aquatic systems along the transport path of radionuclides to the biosphere. Despite its simplicity, the concept of solubility is difficult to apply in natural water systems, as various water constituents will influence solubility which may vary over the very long times considered in safety assessments of disposal. For instance, the interaction of natural waters with the components of the engineered barrier system surrounding the disposed waste products will influence solution pH, redox potentials or ligand concentrations. Information obtained from studies evaluating radionuclide behaviour provide valuable inputs that appropriately constrain safety analyses of nuclear waste disposal.In this series of articles, solubility phenomena related to different domains and key components in the context of nuclear waste disposal are presented. Several of these contributions provide insight on the chemical, thermodynamic and (SIT, Pitzer) activity models describing the investigated systems. This emphasizes the usefulness of solubility studies for the determination of thermodynamic properties, which can be implemented in thermodynamic databases (e.g., NEA-TDB, THEREDA, ThermoChimie, JAEA-TDB or PSI-Nagra, among others) and further in geochemical calculations of relevance for nuclear waste disposal.