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measurement of in situ gas permeability and gas escape from a sealed tunnel via the EDZ;
determination of the sealing capability of different clay/sand mixtures at different compaction;
study of self-sealing of faults in argillaceous rocks and accompanying water-rock interaction and palaeo-fluid migration.
The last significant sediment type currently under study is salt. Here, large-scale experiments in the Asse Mine (see, e.g., Rothefuchs et al., 2004; CEC, 2004) have done much to confirm the feasibility of disposal of heat-generating waste in rock salt. The experiments were designed to simulate the direct disposal of SF in horizontal drifts and the disposal of vitrified HLW in 15 m deep boreholes; in total, six disposal cask mock-ups were used and these were heated for over 8 years. Both types of emplacement were at the Asse Mine’s 800 m level. Measurements of the coupled THM response of the crushed salt backfill and surrounding host rock were in reasonable agreement with model calculations based on laboratory tests. Three dimensional models were needed for the more complex geometry of the horizontal tests. The tests also allowed a demonstration of the engineering systems required to transport and handle waste canisters. Post-test examination of candidate material samples exhibited minimal corrosion.
Experience of disposal operations and the associated scientific investigations in salt URLs in the USA and the Czech Republic also contribute to knowledge of the disposal- in-salt option but the main push for new R&D is likely to come with the re-establishment of the German national programme, especially following the March, 2006, announcement to re-activate the Morsleben repository.
Finally, it is of note that new URLs are still being constructed for further R&D. In Korea, a generic URL is currently under construction (see http://www.kaeri.re.kr/ english/index.html for details) and, in Hungary, a site-specific URL is currently under construction. In Japan, two generic URLs are currently under construction by JAEA at Horonobe (sediments), on Hokkaido, and at Mizunami (crystalline), near Nagoya (see www.jaea.go.jp for details). In addition, JNFL constructed an experimental drift at their Rokkasho L/MLW site in north-eastern Japan in 2005. The site already hosts two near-surface LLW repositories and it is planned to conduct site-specific tests for the intermediate-depth L/MLW repository in the experimental drift, beginning in 2006.
8.5. Nature’s laboratories: studies of the natural environment
8.5.1. General
Radioactive waste disposal presents a considerable challenge to safety analysts because of the long timescales over which it is necessary to assure safety. These timescales – perhaps a million years in some cases – go well beyond those that might be considered in laboratory or field experiments. The challenge therefore is to provide evidence that, when models are used to extrapolate laboratory or field data into the far future, this is being done appropriately and credibly. One way of corroborating long-term estimates is through the use of natural and archaeological analogues (or NA). Apart from the scientific insights that they provide, NA studies also have considerable value in
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communicating with the public. Critical reviews of NA studies are provided in Miller et al. (1994, 2000) and NA literature lists are provided by Knight (1998) and Ruiz-Lopez et al. (2004).
8.5.2. Corrosion
Possibly two of the better known archaeological analogues for corrosion are the hoard of Roman iron nails (Fig. 8.8) found at Inchtuthil, Scotland (Angus et al., 1962) and the Kronan brass cannon (see Fig. 8.9) recovered from the Baltic Sea (Hallberg et al., 1988). There are, however, many other examples (e.g., Johnson and Francis, 1980; Yoshikawa et al., 2003). Comparison of laboratory, field, in-service and archaeological data for the corrosion of iron and brass objects provides evidence that the models developed from laboratory experiments are applicable to these longer timescales (Crossland, 2005).
Fig. 8.8. Roman blacksmiths disposing of over one million iron nails in a 5 m deep pit prior to abandoning, in AD 87, the most northerly legionary fortress of the Roman Empire at Inchtuthil, Scotland (image courtesy of Nagra).
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Fig. 8.9. The brass cannon from the Swedish man-of-war, Kronan, being lifted from the Baltic Sea, three
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centuries after the ship sank in the battle of Oland (Image courtesy of SKB).
Nevertheless, R&D remains to be done in this area as few of the artefacts studied are made of directly analogous materials (i.e., iron instead of steel and brass instead of copper3) and few, if any, report on the degree of pitting in the artefacts, evidence which is of much greater importance to SA than the average corrosion values normally reported (see comments in Alexander and McKinley, 1999).
8.5.3. Cement
Modern ordinary Portland cement (OPC) was invented in 1824 and is made by reacting limestone and clay at high temperature to form a clinker. Grinding this clinker to a powder produces Portland cement. OPC is widely used in radioactive waste management for waste immobilisation and as a repository buffer (see Chapter 5 for details). Issues relating to its long-term performance, i.e., issues where it is hoped that natural analogue information might be useful, include (i) its ability to provide long-term groundwater buffering, (ii) its long-term strength and resistance to cracking, and (iii) the evolution of a hyperalkaline disturbed zone around the repository.
Natural analogues for OPC occur in very few locations worldwide, but an extensive field of natural cement exists across Syria, Israel, Jordan and Saudi Arabia. Here, there are marls (clay-rich biomicritic limestones) that contain sufficient kerogen for it to be used as low-grade fuel (as was the case early last century when Turkish troops in what is today Syria and Jordan used the material to fuel their steam engines; Lawrence, 1935). Natural combustion of the kerogen can convert the marl to a cement clinker which, when re-hydrated, is mineralogically very similar to Portland cement. Despite the large extent
3 One study of a copper artefact has been reported recently (IAEA, 2005).
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of the cement clinker zone, only one area currently hosts an active groundwater system and this is at Maqarin, in northern Jordan. Rainwater flows through the cement clinker and emerges as hyperalkaline groundwater, directly analogous to cement leachates (maximum observed field pH of 12.9). The formation of secondary cement minerals in the cement clinker and the downstream hyperalkaline disturbed zone (or ‘‘plume’’) gradually blocks the flow paths but continual landform evolution – specifically, landslip caused by deepening of the adjacent river valley – causes the fractures to be reopened. Through this process, hyperalkaline groundwater is believed to have been flowing intermittently in the Maqarin area for over 100 thousand years and continues to flow to the present day. The natural concrete clinker at Maqarin and other sites in Jordan have been studied in some detail as analogues to a wide range of processes expected to occur in cementitious waste repositories (see Alexander, 1995 and Smellie et al., 2001 for details). Currently, R&D work continues at these sites (Pitty, 2007) on processes such as the retardation of 129I in the geosphere, matrix diffusion into the altered host rock and the potential degradation of the bentonite buffer by cementitious hyperalkaline leachates (as might happen if concrete and bentonite are used together as is foreseen in some designs – see Chapter 5).
While OPC is a modern material, similar ‘‘hydraulic’’ cements (so-called because they will set under water) were known and used in Roman times for harbours, baths and, most famously, in the dome of the Pantheon in Rome. This dome, completed in about AD 130, with a diameter of 43 m is said to be the largest dome ever covered by masonry. This example, supported by detailed examination of other concrete/cement structures, testifies to the durability of this material when it is employed carefully (Nirex, 2003). Although, as noted by McKinley and Alexander (1992) and Miller et al. (1994, 2000), great care must be taken not to extrapolate the information from such archaeological cement analogues too far as the mineralogy (and construction methods) are significantly different from modern cements.
More recently, so-called low pH cements have been investigated as alternatives to OPC as the cement leachates have a pH of 10.5 to 11 and are presumed to consequently produce much less interaction with bentonite or repository host rocks. These constitute a greater challenge to natural analogue-based R&D programmes as such cements are not known to occur naturally. Low pH leachates from ophiolite complexes have been studied in the past (e.g., Bath et al., 1988) and it seems likely that this is an area for future R&D efforts.
8.5.4. Clay materials
Reactions between bentonite and the surrounding groundwater (or cement porewater) at elevated temperatures can result in the process known as cementation, whereby the bentonite loses its plasticity and its ability to swell. As noted in Chapter 5, these are important properties that are needed to allow the saturated bentonite to achieve the required levels of permeability, strength and ability to filter colloids produced by the waste form. A NA study at Kinnekulle, Sweden has been used to help understand this long-term process (Pusch et al., 1998). About 300 million years ago, a nearhorizontal intrusion of magma flowed into rocks lying 90 m above a 2 m thick layer of bentonite. The heat from this intrusion, together with the hydrochemical conditions produced by the adjacent rocks, caused cementation reactions to occur in the bentonite.