Fig. 8.6. GMT: construction of the 1:10 scale concrete silo. The engineers in the hole are standing on the bentonite/sand buffer between the silo and the host rock (image courtesy of Nagra and RWMC).

seal weld, given that very high reliability is a necessity and, because of the high radiation field, the task will need to be done remotely. Only recently was friction stir welding finally chosen over electron beam welding as the preferred technique for further development (SKB, 2005).

8.3.2. Natural barriers

8.3.2.1. Geochemistry and groundwater flow

As noted in Chapter 3, the main safety functions of the geosphere are to:

isolate the wastes to avoid inadvertent human intrusion or malevolent use of the waste;

protect the engineered barriers i.e., provide an environment in which the engineered barriers can perform as required; and

contain the radionuclides in the waste by providing natural barriers such as low groundwater flow, sorption onto the surrounding rocks, etc.

The first of these functions is simply achieved by placing the wastes at such a depth that they are, and will remain, beyond the reach of most human actions.

The second function – protection of the engineered barriers – mostly relates to the compatibility of the engineered barriers with the geochemistry of potential sites. The proposed use of copper canisters for the containment of spent fuel in the Swedish and Finnish disposal concept provides a classical example. The containment function of the canisters will persist for at least one million years with respect to corrosion resistance provided that the copper is not exposed to high levels of dissolved oxygen or sulphide in the groundwater. These requirements, derived from the relevant R&D, are used to inform siting decisions. However, there is ongoing R&D to explore the situation whereby future

glaciations could force oxygenated water to the depth of a repository, where chemically reducing conditions exist today, or whereby other natural occurrences such as earthquakes could compromise the integrity of the containers emplaced in a repository. The output from such R&D is fed directly into the SA for such a concept.

Similarly, the required performance of the bentonite buffer in the most HLW disposal concepts in providing a low-permeability, diffusion-controlled, barrier can be compromised by degradation of the clay by high potassium-containing or highly saline groundwaters. Again the results of R&D are used to direct siting decisions and to provide the information necessary to model the deterioration of the buffer in SAs.

For cementitious repository concepts, another compatibility issue is the interaction of hyperalkaline leachates with the mildly alkaline to neutral host rock. There is now a significant body of data which indicates that that the secondary minerals that form due to the reaction of the plume with the host rocks have the general tendency to seal up the system (e.g., Linklater, 1998; Smellie et al., 2001; Ma¨der et al., 2004).

The general point to be made is that the geochemistry of the chosen site and the engineered barriers need to complement each other and this can generally only be decided following extensive R&D.

The third safety function of the geosphere, containment of radionuclides, is primarily achieved by low-groundwater flow through the repository and by retardation of radionuclides in the surrounding rocks which, again, is primarily a function of the local geochemistry.

Many radionuclides may be retarded during their migration through the EBS and the host rock by a range of chemical processes that are generically termed sorption (Fig. 3.6). To represent this, most safety assessment models use a factor, known as a Kd value, which represents the equilibrium distribution of radionuclide between solid and liquid phases. In the past, assignment of reliable Kd values or distribution coefficients has been problematic and based on observation from equilibrium sorption experiments and empirical corrections. This is discussed in reviews of the use of sorption data in safety assessments for crystalline rocks (Nagra, 2002a,b) and clays (Mazurek et al., 2006). In recent years, however, sufficient advances have been made in understanding chemical sorption mechanisms that Kd values used in assessments can now be supported by more mechanistic modelling specific to the materials and groundwater conditions that are being represented (NEA, 2001a, 2005a,b).

The results of the NEA Sorption Project Phase II show that the conceptual and methodological tools for characterising, interpreting and justifying Kd values provided for SA needs are largely available (NEA, 2005b). The final report recommends that future R&D efforts should be focussed on the development and demonstration of an optimised approach to experimental characterisation and interpretation of radioelement sorption on complex materials, guided by thermodynamic sorption modelling.

8.3.2.2. Gas transport and two-phase flow

As noted above gas will be produced in the EBS and it could lead indirectly to a radiological hazard if the gas flow were sufficient to disturb the natural groundwater flow pattern and cause contaminated groundwater to reach the surface more quickly than it would do otherwise. This has been the subject of a number of experimental and modelling studies (e.g., Nirex, 1996) and is still the subject of significant R&D effort. For example, in the (Gas Migration Test (GMT)) project at Grimsel, RWMC of Japan