8.4 Natural analogues

Natural analogues can be used to develop an understanding of relevant phenomena and toprovide support for the assumptions made in the course of the safety assessment, and thusbuild confidence in its reliability8-3). They also provide more general support for the conceptof deep geological disposal, by indicating the ability of suitable geological environments toretain radionuclides (e.g. Miller et al., 1994).

Stability of the geological environment and its fundamental ability to retain radionuclides

Well known examples of relevant uranium ore-based natural analogues include the naturalreactors discovered in the Oklo ore deposit in Gabon in Africa (Kuroda, 1956; IAEA, 1975,1978; Isobe et al., 1995; Gauthier-Lafaye, 2000). The Oklo deposit is a high-grade uraniumdeposit formed approximately 2 billion years ago. It spontaneously underwent fission chainreactions, generating fission products and transuranic elements, many of which having beenretained in the geological environment up to the present, i.e. for 2,000 million years (see, forexample, Brookins, 1990; Hidaka et al., 1994; Curtis et al., 1989).

Other examples include the Koongarra ore deposit in Australia (e.g. Duerden et al., 1987;CEC, 1993; Ohnuki et al., 1995), the Cigar Lake ore deposit in Canada (e.g. Goodwin et al.,1989; Cramer and Smellie, 1994) and the Osamu Utsumi ore deposit in Brazil (e.g. Chapman,et al., 1990). All of these have shown that radionuclides such as uranium and thorium can beretained in suitable underground environments over geological time periods.

The Tono uranium ore deposit in Gifu Prefecture (e.g. Yamakawa, 1991; Yusa et al., 1993;Yoshida, 1994; Yoshida et al., 2000) is the largest known in Japan and is estimated to havebeen formed approximately 10 million years ago (Ochiai et al., 1989; Kobayashi, 1989). Theuranium deposits are distributed in the lowest part of the Mizunami Formation and areconsidered to have formed when uranium in the granite was leached by oxidizinggroundwaters, transported to the overlying sediments and then deposited and concentrated ina locally reducing environment (Katayama et al., 1974).

Between 5 and 15 million years ago, the area was uplifted and the upper part of the MizunamiFormation experienced denudation (Itoigawa, 1980). The repeated activity of the Tsukiyoshifault, formed after the uranium deposit, shifted the Mizunami Formation approximately 30 min a vertical direction. This fault also intersected and displaced the uranium ore body, but noevidence for uranium being transported significantly along the fault has been found (Shinjo etal., 1997). Following denudation up to approximately 5 million years ago, the area subsidedand rivers and lakes formed. The sand and clay deposited during this period (the SetoFormation) covered the Mizunami Formation (Itoigawa, 1974) and this sedimentationcontinued until approximately 100,000 to 700,000 years ago, when denudation recommenced.

The Tono uranium ore deposit has remained undisturbed since it was formed approximately10 million years ago, despite experiencing fault movement, uplift, subsidence, denudation,sedimentation and climatic and sea-level changes. This indicates that the conclusions of theperformance assessment analyses carried out for a Japanese HLW repository are at least

8-3) More general observations from nature are also used in H12. For example, in Japan, evidence of significant

erosion by glaciers can only be seen at high altitudes, which supports the assumption that erosion at potentialrepository sites is mainly caused by weathering.

credible.

Evolution of EBS components

Natural analogue research relating to individual EBS components, such as waste glass,overpack and buffer materials, has been carried out in Japan.

Studies have been conducted on the alteration rate of natural volcanic glass, which is similarin some respects to waste glass (Jercinovic and Ewing, 1988; Arai et al., 1989; Yusa et al.,1991). According to these studies, some natural glasses show dissolution rates that are lowerby several orders of magnitude than that assumed for vitrified waste in H12. In a case wherevolcanic glass had been buried for millions of years in marine sediments, similar tocompacted bentonite, only minor alteration of the glass matrix was observed (Kamei et al.,2000).

As an analogue for the overpack, cast iron pipes in freshwater clay have been studied toderive an empirical formula for corrosion (Sumiyama et al., 1997). Surveys have also beencarried out on the corrosion of archeological iron products (Johnson and Francis, 1980; Arakiet al., 1989). Research on copper has also been conducted both in Japan and abroad, forexample on the corrosion of archeological copper products in soil (e.g. Bresle et al., 1983;Mitsui, 1996a). The corrosion depths estimated from these studies are equal to or lower thanthose identified through the latest corrosion assessment for iron and copper, indicating that thecorrosions assessment in H12 is conservative (see Section 4.1.1.3 of Supporting Report 2).

There are also many examples of research on the natural thermal alteration of buffer materials(e.g. Pusch and Karnland, 1988; Kamei et al., 1992). Data obtained from this researchindicates that mineralogical alteration will not be a problem for the temperatures andgeochemical conditions expected in a repository (Mitsui et al., 1996b; Kamei et al., 1999).

Retardation of geosphere transport

Evidence for matrix diffusion, which is expected to play an important role in retarding nuclidemigration through fractured rocks, has been found in studies of natural systems, for exampleat the Kamaishi mine and in international research programs (see Sections 3.6 and 3.7 ofSupporting Report 1).