Natural analogue / Uranium deposit / Long-term geochemical condition / Uranium solubility / Safety assessment Summary.A key concept underpinning most safety assessments of geological disposal systems for high-level nuclear wastes is that the maximum aqueous concentrations of actinide elements released from the waste will be limited by the solubility of the corresponding tetravalent oxide or hydrous oxide. The validity of this concept for U is evaluated in the present study using hydrochemical and mineralogical data from the Tono uranium (U) deposit in Gifu Prefecture, Japan. In addition, the long-term stability of U mineralization in the Tono deposit is assessed in terms of plausible range of U solubilities in paleogroundwaters migrating through the deposit.A thermodynamic evaluation of the aqueous-speciation and solubility behaviour of U indicates that U concentrations in the coexisting groundwaters greatly exceed the solubility of both uraninite [UO 2 (c)] and coffinite [USiO 4 (c)]. The dissolved U concentrations appear instead to be controlled by the solubility of amorphous hydrous U(IV) oxide [UO 2 (am)], which is metastable with respect to uraninite. Mineralogical and microbiological investigations suggest that pH-E h conditions similar to those observed presently in groundwaters of the lower Toki Lignite-bearing Formation (pH ≈ 8-10; E h ≈ −250-−410 mV) have been buffered, possibly since the present geological structure formed about 5-0.7 Ma, by microbial sulphate reduction and precipitation/dissolution of calcite. If so, corresponding U concentrations controlled by UO 2 (am) solubility are calculated to have been in the range of 10 −6.1 and 10 −8.7 mol kg −1 depending on the possible environmental conditions, i.e., E h , pH and carbonate concentration.

No abstract

Equilibrium geochemical speciation and mass transfer calculations show that porewaters analogous to those in the candidate high-level nuclear waste repository at Yucca Mountain, Nevada, can be strongly affected by open-system boiling. A modified Rayleigh distillation equation appropriate to reactive volatile species is derived and applied to calculate pH ranges during C02-H20 volatilization in a system of fixed alkalinity. Variations in pH by as much as + 2,3 units are caused by volatile weak electrolyte interactions as C02(g) exsolves from the aqueous phase. Protolysis reactions and precipitation of secondary minerals moderate the calculated increases in pH by less than 1 unit. The host rock mineralogy can be altered by boiling because differences in the chemical potentials of components in the aqueous phase and in minerals are created by pH changes. Experiments using pressurized carbonated solutions in closed systems may be inappropriate for simulating water-rock interactions in the proposed repository.

The Tono uranium deposit (Gifu Prefecture, central Japan) was formed about 10 million years ago and available evidence suggests that this deposit has remained stable, i.e. uranium has not migrated significantly since then. Substantial field and modeling work is being carried out to better understand the hydrogeological and geochemical processes affecting or governing uranium transport and immobilization in and around the Tono uranium deposit. A systems analysis approach has been applied to the Tono deposit natural analogue project. Application of a systems analysis approach to a generic research project is novel and is different from prevailing applications. Previous applications of a systems analysis approach have been principally concerned with performance and safety assessments for radioactive waste disposal. The systems analysis approach provides a convenient framework for this project, having been used in the Tono Natural Analogue Project (TAP) to: • define the system to be studied; • identify features, events and processes (FEPs) associated with this system; • identify external factors (EFEPs) which are likely to have influenced the Tono area or study site or uranium deposit; • define interactions between pairs of FEPs; and • develop credible scenarios for evolution of the area or uranium deposit.The results from the systems analysis approach allow us to: • develop possible scenarios for how the Tono area or study site evolved, • evaluate uncertainty and how it propagates backwards in time,• link field characterization data with predictive modeling, and • identify gaps in data and, hence, identify additional characterization needs.