Fig. 3.7. Emplacement of bentonite pellets around the waste package. The canister and bentonite/sand support (middle) is surrounded by pellets of bentonite/sand (right) blown from a hopper by the bentonite emplacement wagon (left) (Image courtesy of Nagra).

by the repository tunnels and providing required structural elements (floors, liners, etc.). Previously, all use of cementitious material was excluded from the waste deposition zone – due to concerns about bentonite alteration – but now this is considered impractical and efforts are focused on providing a suitable compromise between operational and post-closure safety requirements (Walker and Metcalfe, 2004).

These two examples illustrate the process by which repository designs, particularly of the engineered barrier system (EBS), developed in the past. A more comprehensive overview of EBS roles and modern design approaches is provided in Chapter 5.

3.3.3. Roles of the geosphere in disposal options

A geological repository consists of a series of engineered barriers, of the sort described above, which are set within a further ‘‘natural’’ geological barrier. The attraction of such a natural barrier is that geological formations have shown stability for periods well beyond the long timescales needed for decay in toxicity of the longest-lived waste – something that cannot be claimed for any man-made structure. Such stability arguments can be supported by natural analogues, which indicate that the geochemical anomaly represented by a repository is, in many ways, quite similar to that represented by ore bodies which can persist over hundreds of millions of years (see Miller et al., 2000, for details).

Other than in the ‘‘absolute containment’’ options of a salt host rock (section 3.3.3.2), it has been argued (Nagra, 1994) that the three most important safety features provided by the geosphere are ‘‘. . . mechanical protection (of the EBS), adequate geochemical conditions and sufficiently low groundwater flow rates’’. Simple physical isolation is increasingly seen as important – particularly minimising the risks of human intrusion and associated security concerns. Additionally, the geosphere adds another barrier to any radionuclides released by the EBS, providing a zone of retardation due to uptake on minerals (Fig. 3.5) and diffusion into the rock matrix (for non-interacting radionuclides such as 129I), along with dilution/dispersion along the groundwater flow path to the accessible environment.

favourable attributes, causing self-sealing of fractures and healing of mechanical damage caused by the construction of the repository.

3.3.3.2. Hydrogeology

Three key parameters influencing the performance of the geosphere are the flux, flow rate and flow path of groundwater. Rocks can be classified as having effectively no groundwater, having negligible flux of groundwater or having a significant groundwater flux.

Rocks with effectively no groundwater include some evaporites (e.g., salt, anhydrite) and permafrost. Under undisturbed conditions, such properties would give extremely high barrier performance (effectively complete containment of all radionuclides). All these options are, however, vulnerable to perturbations which could, potentially, cause massive barrier disruption (e.g., permafrost melting) or even complete failure (groundwater intrusion into salt). Such failure mechanisms could, in some cases, result in large advective flows of water to the EBS although, in others, such fluxes might be inherently limited by the geological setting (see below).

The groundwater flux is defined by macroscopic characteristics – the hydraulic conductivity and the hydraulic gradient. Very low fluxes can result from either – or both – of these parameters having very low values. Extremely low conductivities are exhibited by many clay-rich sediments and a number of ‘‘unfractured’’ crystalline rock bodies. Effectively ‘‘zero’’ gradients can be found under the sea bed and in the middle of large, flat continental basins (e.g., in central Australia). In both cases, advective flow can be so small that solute transport is dominated by the process of molecular diffusion. Such a diffusion-dominated system inherently leads to high performance of both the EBS and the geological barrier.

Many of the low-conductivity formations studied to date would be classified hydrogeologically as completely impermeable. Two features are responsible for this: high clay content and the plastic nature of the rocks. The Boom Clay in Belgium is an example of plastic clay (i.e., readily flows to seal any openings/fractures) whereas others, although not classified as plastic clays, exhibit sufficient creep (i.e., can flow over geological timescales) that fractures are rapidly re-sealed, re-establishing a diffusion-dominated system. Studies in the Opalinus Clay formation in Switzerland, strictly classified as a siltstone, indicate that all fractures at depth are sealed and, indeed, often have a lower permeability than the rock matrix due to the concentration of clay minerals at the fracture face (e.g., Blu¨mling et al., 2002).

Impermeable rocks are generally favourable for construction, but care has to be taken to ensure that the presence of the repository does not disrupt this favourable property (e.g., EDZ formation, fracturing by gas, etc.). More permeable rocks, which have low fluxes due to their setting, are less favourable for construction, as the head of water above open tunnels will drive flow even in the absence of a gradient in the undisturbed system. On the other hand, post-closure performance will be relatively unaffected by mechanical disturbances to the rock, although such performance will be vulnerable to repository-derived pressure gradients (e.g., due to temperature, gas).

A special case of very low flux occurs for disposal above the water table in a desert environment (e.g., YM; Box 3.1). The average flux may be extremely low but, in this case, diffusion does not dominate and solute transport occurs predominantly in short, episodic advective flow events. Although geological barrier performance may be quite

good, it depends on prevalence of the present desert climate and could, potentially, degrade very rapidly if rainfall increased – particularly associated with a rise in the water table (a clear concern in the light of the unpredictable consequences of anthropogenic global warming).

If the flux is significant, the overall performance of the geological barrier depends on the properties of the flow paths in which advection occurs (see detailed discussion in Chapter 6 – Box 6.1). Particularly important are the extent of flow localisation, the microscopic characteristics of flow paths and their length until the accessible environment (i.e., the biosphere) is reached. Construction can be problematic in such rocks, but they may be less vulnerable to perturbation by the repository. Although high barrier performance is still possible, this may require a robust EBS and/or extensive characterisation of the retardation properties of the host rock. Due to their inherent heterogeneity, explorability of such rock types can be a problem (especially if they do not outcrop at the surface) and the detection of water-conducting features can be difficult. Consequently, definition of representative transport properties may be uncertain until an underground characterisation facility has been constructed at a chosen site.

3.3.3.3. Geochemistry

Although the geochemistry of a rock and its associated groundwater is a highly complex field, as far as barrier performance is concerned the key characteristics are redox, pH and total salinity.

The performance of both the engineered barriers and the geosphere as a retardation barrier for radionuclides is improved considerably when conditions are chemically reducing. This is because corrosion processes are generally slower under such conditions and many key radionuclides are both less soluble and more highly sorbed. Repositories, particularly for more toxic radwaste, are sited to take advantage of this – the one exception being YM (Box 3.1). Redox can be characterised by an ‘‘Eh value’’, but this is notoriously difficult to define in natural groundwater. In general, however, the more evidence for reducing conditions and redox-buffering minerals in the host rock, the better the geological barrier performance is likely to be. The aspect of redox buffering is particularly important as oxidants can be introduced to the system during construction and operation, from repository processes (e.g., radiolysis) and as a result of geological processes (e.g., groundwater flushing below the nose of a retreating glacier). Evidence of the capacity to buffer such perturbations will help to strengthen a safety case for any type of repository.

The pH of groundwater is a less sensitive parameter and is unlikely to have much direct impact on performance of the total system unless it lies at a very high ( >10, say) or very low (< 4) end of the range. Such cases are rare but, if encountered, might make development of a safety case much more difficult (apart from anything else, due to the lack of international precedents). The vulnerability of the host rock to ‘‘pH perturbations’’ is, however, a more significant factor (see, e.g., Guimera` et al., 1999). The positive process of redox buffering mentioned above can, for example, lead to production of highly acidic waters if it involves oxidation of pyrite (a very common process in underground constructions, leading to ‘‘acid mine drainage’’). If pyrite oxidation is extensive, resultant sulphuric acid can cause both disruption of engineered barriers and the formation of short-circuits through the geosphere (and a direct environmental pollution risk). It is therefore advantageous if the rock has the capacity to buffer any pH excursions caused by either perturbations to the rock itself (probably acidic) or directly