Kunio Ota

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.

Development of Comprehensive Techniques for Coastal Site Characterisation: Integrated Palaeohydrogeological Approach for Development of Site Evolution Models

Radioactive waste repository designs consist of multiple safety barriers which include the waste form, the canister, the engineered barriers and the geosphere. It is widely considered that the three most important safety features provided by the geosphere are mechanical stability, favourable geochemical conditions and low groundwater flux. To guarantee that a repository site will provide such conditions for timescales of relevance to the safety assessment, any repository site characterisation has to not only define whether these features will function appropriately today, but also to assess if they will remain adequate up to several thousand to hundreds of thousand years into the future, depending on the repository type. The case study described here is focussed on the palaeohydrogeology of the coastal area around Horonobe in northern Hokkaido, Japan. Data from JAEA’s ongoing underground research laboratory project is being synthesised in a Site Descriptive Model (SDM) with new information from the collaborating research institutes to develop a Site Evolution Model (SEM), with the focus very much on changes in the Sea of Japan seaboard over the last few million years. This new conceptual model will then be used to assess the palaeohydrological evolution of the deep geosphere of coastal sites of Japan.Copyright

Development of the Quality Management System for Borehole Investigations: Part 1—Quality Assurance and Quality Control Methodology for Hydraulic Packer Testing

A quality assurance and quality control (QA/QC) system for the hydraulic packer tests has been established based on the surface-based investigations at JAEA’s underground research laboratories in Mizunami and Horonobe. The established QA/QC system covers field investigations (data acquisition) and data analysis. For the field investigations, the adopted procedure is selection of a test section based on a detail fluid logging and checking with tally list, followed by inspection of test tools such as pressure transducers and shut-in valves, etc., test method selection using a “sequential hydraulic test” for deciding appropriate method, and finally data quality confirmation by pressure changes and derivatives on a log-log plots during testing. Test event logs should also be described during testing for traceability. For the test data analysis, a quick analysis for rough estimation of hydraulic parameters, and a detailed analysis using type curve and/or numerical analyses are conducted stepwise. The established QA/QC system has been applied to the recent borehole investigations and its efficiency has been confirmed.© 2010 ASME

Development of a Quality Management System (QMS) for Borehole Investigations: Part 2—Evaluation of Applicability of QMS Methodology for the Hydrochemical Dataset

An appropriate QMS, which is among the first tools required for repository site characterisation, will save on effort by reducing errors and the requirement to resample and reanalyse–but this can only be guaranteed by continuously assessing if the system is truly fit-for-purpose and amending it as necessary based on the practical experience of the end-users on-site. A QA audit of hydrochemical datasets for boreholes HDB-1–11 from Horonobe URL project by JAEA has been carried out by the application of a formal QA analysis which is based on the methodology previously employed for groundwaters during the recent site characterisation programme in Sweden. This methodology has been successfully applied to the groundwaters of the fractured crystalline rocks of the Fennoscandian Shield and has now been adapted and applied to some of the ground- and porewaters of the Horonobe URL area. This paper will present this system in the context of the Japanese national programme and elucidate improvements made during hands-on application of the borehole investigation QMS. Further improvements foreseen for the future will also be discussed with a view of removing inter-operator variability as much as is possible. Only then can confidence be placed in URL project or repository site hydrochemical datasets.Copyright

The Study of Radionuclide Retardation in Fractured Rock by Means of In Situ Resin Impregnation

Traditional in situ tracer tests estimate contaminant retardation by analysis of the degree and form of tracer breakthrough after transport through the rock. Unfortunately, this approach does not allow direct examination of in situ retardation mechanisms and, in the case of strongly retarded radionuclides, is highly impractical as tracer breakthrough may take months to decades. An alternative method to study retardation is therefore required in such a case and Nagra and PNC have recently employed one such variant to study radionuclide transport in fractured crystalline rock. Here, direct, detailed, examination of in situ radionuclide retardation following tracer injection is carried out by immobilising and recovering the intact fracture and associated rock matrix [1,2].The material can then be studied in the laboratory by standard surface analytical and radiochemical methods and the degree and form of radionuclide uptake can be readily assessed. As part of this work, Nagra and PNC have invested significant effort over the last four years in developing appropriate means of immobilising water-conducting fractures and undisturbed low porosity crystalline rock matrix in a manner which minimises physico-chemical disturbance[3]. After examining a range of options, it was decided to employ in situ resin impregnation as the immobilisation medium as this produced the best results with respect to minimising physico-chemical disturbance of the system while at the same time ensuring impregnation of very fine water saturated pore space. In addition, the polymerised resins improve the rigidity and strength of the rock such that the water saturated structures (pores, fractures or fault gouges) survive the subsequent overcoring and sub-sampling. Two experiments will be discussed: the first has been recently completed in Nagras underground laboratory in the central Swiss Alps (the Grimsel Test Site, or GTS) and the second is currently ongoing at PNCs Kamaishi In Situ Test Site (KTS) in north-east Japan. In the GTS, retardation of radionuclides is being studied in the Radionuclide Retardation Project (RRP) and two resins have been formulated for different aspects of the study. An epoxy resin has been injected into a complex water-conducting shear zone in a granodiorite following the injection of a cocktail of strongly retarding radionuclides (including 60 Co, 237 Np, 234. 235 U, 99 Tc, 152 Eu, 113 Sn and 75 Se [1,2]). To negate the hydrophobic nature of the epoxy resin, a trick has been imported from soil science where isopropanol is first injected to replace the water and only then is the epoxy resin injected. Laboratory tests showed that neither the isopropanol nor the resin should disturb the in situ radionuclide distribution, a result which has since been verified in the field. In parallel with this work, the low porosity ( in situ injection of an acrylic resin. The very low viscosity of the specially developed acrylic resin allows impregnation (and subsequent visualisation) of the connected microporosity of the matrix, so allowing detailed in situ examination of the depth of available matrix behind the shear zone. These methods have been further refined in the KTS and are currently being applied to several different types of water conducting features. The form and type of connected porosity in the associated granodioritic rock matrix is also being examined in detail [4]. As with the GTS work, the results of the in situ experiments will be compared with laboratory data on retardation and matrix diffusion to assess the transferability of the large volume of laboratory data to the field. The development of the various resins will be discussed along with the applicability of these specially developed resins to other rock types. Finally, the results of the recently concluded GTS tests and the ongoing KTS tests will be presented.

Study on the Estimation Error Caused by Using One-Dimensional Model for the Evaluation of Dipole Tracer Test

In-situ tracer tests are a valuable approach to obtain parameters for a performance assessment of nuclear waste repository. A one-dimensional model is simple and is commonly used to identify radionuclide transport parameters by fitting breakthrough curves simulated using the model to those obtained from tracer tests. However, this method can increase uncertainty and introduce errors in the estimated parameters. In particular, such uncertainties and errors will be significant when evaluating parameters for tests conducted in a dipole (two-dimensional) flow field between injection and withdrawal wells. This paper describes a numerical analysis investigation into the effects of various experimental conditions on parameters estimated using a one-dimensional model for cases involving tracer tests in a two-dimensional fracture plane. Results show that longitudinal dispersivity tends to be overestimated by the one-dimensional model analysis. This overestimation is the result of several factors: smaller pumping rate, larger dipole ratio, stronger heterogeneity of the fracture hydraulic conductivity, and greater orthogonally-oriented background groundwater flow. Such information will help us to better plan and design tracer tests at underground research laboratories located in both Mizunami in central Japan and Horonobe in northern Japan. Understanding appropriate experimental conditions will help decrease the uncertainty in the results of tracer tests.Copyright

Development of Comprehensive Techniques for Coastal Site Characterisation: Part 1—Strategic Overview

Any assessment of long-term repository safety will require development of a set of analyses and arguments to demonstrate the persistence of the key safety functions of the geological environment up to several hundred thousand years into the future. However, likely future global climatic and sea-level fluctuations and uplift/subsidence would result in a dramatic change in the location of the current coastline with a subsequent significant change to hydraulic and hydrochemical conditions at coastal sites. It is thus of great importance in the Japanese disposal programme to establish comprehensive techniques for coastal site characterisation. To this end, a systematic framework, which is known as a ‘Geosynthesis Data Flow Diagram’, has been formulated, which outlines a basic roadmap of the geosynthesis methodology for characterising temporal and spatial changes of various properties and processes at coastal sites, with particular focus on the palaeohydrogeology. A basic strategy for stepwise surface-based investigations has also been proposed, which incorporates the geosynthesis methodology in an effective manner. This technique has been introduced in an ongoing collaborative programme for characterising the coastal geological environment around Horonobe in northern Hokkaido, Japan, and now tested and optimised based on accumulated technical knowledge and experience during the progress of the investigations.Copyright

Development of Comprehensive Techniques for Coastal Site Characterisation: Part 3—Conceptualisation of Long-Term Geosphere Evolution

A critical issue for building confidence in the long-term safety of geological disposal is to demonstrate the stability of the geosphere, taking into account its likely future evolution. An ongoing collaborative programme aims to establish comprehensive techniques for characterising the overall evolution of coastal sites through studying the palaeohydrogeological evolution in the coastal system around the Horonobe area, Hokkaido, northern Japan. Information on natural events and processes related to the palaeohydrogelogical evolution of the area have been integrated into the chronological tables and conceptual models that indicates the temporal and spatial sequences of the events and processes, such as climatic and sea-level changes, palaeogeography, and geomorphological and geological evolution in the area. The methodology for conceptualisation of the geosphere evolution will be applied to other analogous coastal areas on Japan’s western seaboard to produce comprehensive techniques to support understanding the geosphere evolution of potential coastal sites for deep geological repositories.Copyright