It was possible to show, however, that the reactions produced no significant changes in the physical properties of the bentonite. Unfortunately, this study, and most other NA studies like it, are of only limited value as the analogy with a repository is not very close. For example, the temperature in the molten magma is about an order of magnitude greater than that expected in the EBS of a HLW repository and the bentonite mineralogy and physical state, while similar, is not too close to the industrial bentonite used in a repository. This is even more of a problem when recent repository designs which incorporate bentonite-sand mixes, rather than pure bentonite buffers, are considered (see Chapter 5 for details). Clearly then, further R&D work in this area seems appropriate.

8.5.5. Degradation of organic materials

The hyperalkaline chemical and microbial degradation of organic wastes, especially cellulose, produces degradation products that could increase the solubility of actinide elements. For the purposes of repository SAs, it is usually assumed that the end-product of cellulose degradation is iso-saccharinic acid (ISA), which is probably the strongest of several complexants. ISA may be removed from the repository by groundwater flow or it may itself be degraded – ultimately to carbon dioxide and water – by microbial action. In determining the concentration of ISA present, the key parameters are the long-term degradation rate of cellulose and the degradation rate of ISA.

Existing NAs include spills of ‘‘black liquor’’, a waste product of the Kraft paper pulping process. Soil and lake sediments polluted by black liquor have been found to contain several microbial strains capable of metabolising ISA under near-neutral chemical conditions (Bailey, 1986, 1987). Microbes capable of degrading ISA under alkaline conditions have been extracted from alkaline lakes in Africa (Greenfield et al., 1995). However, while these studies indicate that such metabolising bacteria exist, no information is available for repository-relevant hyperalkaline environments. Work has been carried out on microbes at the Maqarin site in Jordan (e.g., West et al., 1995; Pedersen, 2000) and no ISA metabolisers were identified but, as the researchers noted that the methods utilised were not yet optimised for use in hyperalkaline environments, this should not be taken as proof as yet. Additional, focused R&D work on this question at this site would be of use in answering this open question.

8.5.6. Glass corrosion

Borosilicate glass, used to form vitrified HLW, is chosen because of its ability to meet processing requirements for working temperature and viscosity, but also because of its resistance to radiation, its low solubility in water and its ability to take high loadings of fission products.

For any glass, composition is the most important determinant of its durability and, therefore, on the basis of its silica content, which (at around 50 per cent) is broadly similar to that of the borosilicate glasses used for HLW, numerous natural basaltic and archaeological glasses have been studied. The properties of particular interest are its rate of dissolution in groundwater and the nature of the secondary alteration products. Comparisons of the corrosion layers on natural basaltic glass with those found on borosilicate glass that has been reacted with water in the laboratory show them to be

Fig. 8.10. A goblet and decanter made from uranium-rich (up to 5 per cent) glass, potential archaeological analogues of HLW glass (Images courtesy of Ken Tomabechi).

very similar (Lutze, 1988), thus increasing confidence in the laboratory data (although some significant differences in the glass chemistry mean that the analogy should not be pushed too far).

To date, little has been done on trying to assess radionuclide uptake in the secondary clays and, consequently, this mechanism is generally ignored in SA. Further R&D in this area, perhaps along the lines of the work of Crovisier et al. (2003), might be worthwhile, as would be studying glasses which have high radionuclide contents (e.g., Fig. 8.10) and are thus more directly analogous to HLW glasses.

8.5.7. Radionuclide migration

Uranium deposits are an obvious source of long-term information on radionuclide migration. They may be used to examine a wide range of processes invoked in SAs, such as containment by the buffer and the geosphere, redox effects, colloids, microbiology etc. (see, for example, Alexander et al., 2006, for further details). The oldest analogues (described next) offer convincing evidence for the long-term stability of uranium dioxide, an analogue of SF, under repository-relevant conditions.

The Oklo (Gabon) analogue is unique in that it contains natural fission reactors in which nuclear chain reactions took place 2000 million years ago – at that time the fissionable (235U) content of uranium ores was still high enough to induce criticality. The reactors occur in reducing conditions at a sandstone/shale interface; locally they are surrounded by clay that has allowed most of the uranium and some fission products to remain in place over this very long timescale. The Cigar Lake (Canada) analogue is