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

Abstract

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.

Microbial carbon cycling in oligotrophic regional aquifers near the Tono Uranium Mine, Japan as inferred from δ13C and Δ14C values of in situ phospholipid fatty acids and carbon sources

“…Elevated concentrations of H 2 up to 7.4 mM have been attributed to water radiolysis in the Witwatersrand Basin, South Africa where U concentrations are <3 ppm . Although H 2 concentrations were not determined for KNA-6 waters, the high average U content of the rocks near the Tono Uranium Mine (2-12 ppm in the TG and up to 200 ppm in the ore deposits of the basal LTL) Sasao et al, 2006) are a potential source of radiolytic H 2 . It has been hypothesized that abiogenic H 2 supports primary production by a variety of chemolithoautotrophs which in turn may support methanotrophic and heterotrophic communities.…”

“…The geology, geochemistry, and hydrology of the Tono Uranium Deposit and surrounding area have been extensively described (Yoshida et al, 1994;Kanai et al, 1998;Iwatsuki and Yoshida, 1999;Iwatsuki et al, 2001Iwatsuki et al, , 2005Takahashi et al, 2002;Arthur et al, 2006;Sasao et al, 2006). The deposit is located in central Japan $40 km NE of Nagoya at the base of a 150 m deep sedimentary sequence.…”

“…The sedimentary sequence contains the Seto Group and the underlying Mizunami Group which is composed of the Oidawara, Akeyo, Hongo, and Upper and Lower Toki Lignite-Bearing Formations in descending order. The Toki Granite and ore deposits of the basal lignite-bearing Toki formation contain 2-12 ppm and up to 200 ppm U, respectively Sasao et al, 2006). Due to a lowpermeability disconformity between the Toki and Hongo Formations, groundwaters in the Toki Lignite Formation and upper Toki Granite are isolated from more shallow groundwaters.…”

Hydrochemical disturbances measured in groundwater during the construction and operation of a large-scale underground facility in deep crystalline rock in Japan

“…Precipitation of Fe-oxy-hydroxides and dissolution textures of carbonate minerals are observed up to 60 m below ground level, suggesting low pH and oxidizing conditions in these shallow sedimentary rocks. The diagenetic clay mineral assemblage in the Mizunami Group is dominated by the smectite group (Utada 2003;Sasao et al 2006).…”

Estimation of the paleostress field from the 3-D orientation distribution of microcracks and their geothermal conditions in the Toki Granite, central Japan

“…Lespinasse and Pecher, 1986;Kowallis et al, 1987;Jang et al, 1989;Ren et al, 1989;Jang and Wang, 1991;Vollbrecht et al, 1991;Takeshita andYagi, 2001 , Takagi et al, 2005;, 2006 HC OC SC Engineer. Geol., , 225-278. , 1969, .…”