Eiji Sasao

Geochemical constraints on the origin and stability of the Tono Uranium Deposit, Japan

Data characterizing the mineralogy, hydrochemistry and geomicrobiology of the Tono region of central Japan were used to interpret geochemical constraints on the origin and stability of the Tono Uranium Deposit. The derived constraints are compatible with models of deposit formation, which call for leaching of uranium from the upper weathered zone of the Toki Granite by relatively oxidizing groundwaters that are near-neutral to moderately alkaline and carboniferous. The oxidizing groundwaters then migrate into mudstones and sandstones of the overlying Toki Formation, where the uranyl species is reduced by water–rock–microbe interactions to uranous species, sorbed by various detrital and authigenic phases and eventually precipitated as uraninite, coffinite and the metastable, amorphous hydrous oxide, UO2(am). Formation of the Tono deposit may have been more or less continuous up to the present time. The modern hydrochemical system, upon which the genetic model is based, began to evolve about 15 Ma when seawater was flushed out of the sedimentary cover and basement granite by fresh, meteoric waters during a period of uplift and erosion preceding Pliocene to Pleistocene sedimentation. Recharge with meteoric water continued to the present, which suggests that palaeohydrochemical conditions were probably similar to those observed in the region today when the Tono deposit began to form about 10 Ma. Redox environments in the Tono region inferred from in-situ Eh measurements in deep boreholes and calculated potentials for the SO42−/HS− redox couple appear to be controlled by heterogeneous reactions involving Fe(III)-oxyhydroxides. Metastable equilibria and particle-size effects associated with these reactions produce a range of possible redox environments that are equally compatible with both the relatively oxidizing and reducing groundwaters of the Toki Granite. This compatibility extends to sedimentary porewaters, where the redox environment is also controlled by microbially mediated sulphate reduction, oxidation of organic matter and precipitation of sulphide minerals. Redox conditions have been stable during at least the past several tens of thousands of years based on palaeoredox indicators interpreted from the trace element contents of fracture calcites. The pH and carbonate contents of palaeogroundwaters and modern groundwaters of the Tono region were, and are, controlled mainly by calcite equilibrium.

An overview of a natural analogue study of the Tono Uranium Deposit, central Japan

The basic concept of deep geological disposal of high-level radioactive waste is to isolate the waste from the human environment for the long term. Because the Japanese islands are located in a geologically active area, geological phenomena such as exhumation and fault activity must be considered by any safety assessment connected with deep geological disposal. The Tono Uranium Deposit, central Japan, has been affected by such geological phenomena during the interval since its formation, and so it is a suitable analogue for evaluating how this might be done. The present natural analogue study of the Tono Uranium Deposit (Tono Natural Analogue Project) was started in 2001 with the main aim of studying a so-called ‘worst-case scenario’ for performance assessment (PA). The project involved characterizing the geology, hydrogeology, geochemistry and microbiology of the deposit and obtaining quantitative information about specific times in the past, as a means for developing, and building confidence in, conceptual and numerical models. This project applied systems analysis, which has been widely undertaken in PAs of deep geological isolation. Systems analysis involves a systematic identification, classification and screening of features, events and process (FEPs) that occur or have occurred in and around the deposit. Based on the site data, important FEPs were identified.

A system model for the origin and evolution of the Tono Uranium Deposit, Japan

Data from the Tono Uranium Deposit of central Japan were used to develop an improved approach for simulating uranium migration and retardation, while taking into account both long-term environmental changes and uncertainties in data. Based upon extensive field and laboratory investigations, conceptual and numerical models for environmental perturbations, including uplift, subsidence and faulting, were constructed. Model development was based on a novel adaptation of a safety assessment methodology that previously has been applied to radioactive waste repositories. A ‘reference scenario’ was developed using a systems analysis approach. This scenario is a best estimate of how the geological system and the uranium deposit reached their present states and includes descriptions of all major environmental perturbations. Uranium is mobilized from the uppermost Toki granite under relatively oxidizing conditions, and is then transported by groundwater into overlying sedimentary rocks. There, reducing conditions promote uranium deposition. A specially designed numerical model simulated the main features of this scenario. Many simulations were performed to identify key uncertainties to which the timing of ore deposition and uranium distribution are sensitive. A key finding is that retardation of U by processes other than precipitation of discrete U minerals, most probably sorption on solid phases, contributes significantly to the stability of the ore deposit. Sorption could potentially be important for confining the U within the sedimentary rocks in spite of environmental changes such as exhumation and seismic pumping. The approach could be used elsewhere, to assess the safety of deep geological high-level radioactive waste (HLW) disposal. A related application would be at potential future waste disposal sites, to prioritize site characterization so that the most safety-relevant uncertainties are reduced. There are also possible applications in other fields, most notably to assess the implications of alternative ore genetic models.

Hydrothermal chloritization processes from biotite in the Toki granite, Central Japan: Temporal variations of of the compositions of hydrothermal fluids associated with chloritization

Abstract This paper describes the biotite chloritization process with a focus on mass transfer in the Toki granitic pluton, Central Japan, and also depicts the temporal variations in chemical characteristics of hydrothermal fluid associated with chloritization during the sub-solidus cooling of the pluton. Singular value decomposition (SVD) analysis results in chloritization reaction equations for eight mineral assemblages, leading to the quantitative assessment of mass transfer between the reactant and product minerals, and inflow and outflow of components through the hydrothermal fluid. The matrices for SVD analysis consist of arbitrary combinations of molar volume and closure component in the reactant and product minerals. The eight reactions represent the temporal variations of chemical characteristics of the hydrothermal fluid associated with chloritization: the progress of chloritization results in gradual increase of silicon, potassium, and chlorine and gradual decrease of calcium and sodium in the hydrothermal fluid with temperature decrease. The biotite chloritization involves two essential formation mechanisms: chlorite formation (CF) mechanism 1, small volume decrease from biotite to chlorite and large inflow of metallic ions such as Al3+, Fe2+, Mn2+, and Mg2+ from the hydrothermal fluid, and CF mechanism 2, large volume decrease and large outflow of the metallic ions into hydrothermal fluid. Chlorite produced with CF mechanism 1 dominates over that of CF mechanism 2, resulting in the gradual decrease of the metallic components in the hydrothermal fluid with chloritization progress. The chloritization reactions also give the temporal variations in physicochemical parameter of the hydrothermal fluid: a gradual decrease of pH and a gradual increase of redox potential in the hydrothermal fluid as chloritization proceeds. The combination of continuous reactions based on compositional variations in chlorite together with corresponding continuous AlIV variations gives an indication of the temporal variations in rates of decreasing and increasing concentration of chemical components in the hydrothermal fluid associated with chloritization. The biotite chloritization and resultant temporal variations of chemical and physicochemical characteristics in hydrothermal fluid act as a trigger for the successive dissolution-precipitation process of a granitic rock.

Redox Buffer Capacity in Water-Rock-Microbe Interaction Systems in Subsurface Environments

An incubation experiment was conducted to estimate redox buffer capacity of “water-rock-microbe” interaction systems in sedimentary rocks. The water chemistry, microbial growth and community structure were analyzed during the incubations. The dissolved oxygen (DO) concentrations and oxidation-reduction potential (ORP) values decreased notably in the presence of active microorganisms, whereas abiotic reactions did not lead to reducing conditions during incubation. The change in microbial community structure suggests that nitrate-reducing and sulfate-reducing bacteria played an important role in reduction of water by using lignite-derived organic matter. These results show that the microbial role is extremely important for the redox buffering capacity in sedimentary rock environments.

The Long-Term Stability of Geological Environments in the Various Rock Types in Japan From the Perspective of Uranium Mineralization

Long-term stability of the geological environment is one of the important keys for deep geological disposal of high-level radioactive waste in the Japanese Islands due to their location in a tectonically active island-arc. Uranium occurrences in Japan have been subjected to many geological processes inherent to the island-arc setting. Geological environments associated with uranium mineralization are considered favorable for HLW disposal, because uranium mineralization is considered a natural analogue of the radionuclides in HLW. Studies on the long-term stability of the uranium mineralization in Japan can be instructive as these could provide useful information on the long-term stability of the geological environment. Information on host rock and mode of occurrence of uranium mineralization was compiled from published data. The mineralization occurs in these types of deposits, i.e., sedimentary formations, association with metallic ore mineralization of magmatic origin and stratiform manganese mineralization, pegmatite, and alluvial placer deposit. The mineralization occurs in various geological settings in Japan. This fact suggests that geological environments suitable for geological isolation are widely distributed in the Japanese Islands, despite their location in a geologically active area. This study will support building confidence in HLW disposal in the Japanese Islands.Copyright