Group. It has been suggested that the best approach in biosphere assessment is to create a range of credible illustrations for the biosphere, thereby exploring the uncertainty inherent in its future evolution, e.g., within the Swiss programme (Sumerling et al., 2001). These stylised approaches are generally accepted internationally by national implementers (e.g., NEA, 2000) and regulators (e.g., HSK and KSA, 1993) alike as an appropriate way in which to consider the biosphere in repository SAs. Thus the major requirement for any further R&D is to test the credibility of such illustrations and to show any significant sensitivity to assumptions rather than to attempt detailed predictive modelling.

A methodology for developing such ‘‘Reference Biospheres’’ has been developed internationally, and worked examples provided, within the IAEA BIOMASS Project (IAEA, 2003). The methodology has been applied to a range of different biosphere conditions based on actual site characteristics at five sites across Europe within the EUfunded BioMoSA (Biosphere Models for Safety Assessment) Project (Pro¨hl et al., 2005; Olyslaegers et al., 2005).

In some national programmes, there is a move towards considering potential doses to other species and also actual site characteristics in more detail, (e.g., see SKB, 2004). The first of these represents an area where further R&D is still required.

8.4. Underground rock laboratories

Over the past 30 years, Underground rock laboratories (URLs) have been created to provide access to an underground environment that is similar to that of a repository. This makes it possible to do the work necessary to build an adequate understanding of features and processes that could influence repository safety. These features include geochemical and physical characteristics such as mineralogy, fractures and faults; the processes include groundwater and gas flow, response to excavation disturbance and heat conduction. URLs also allow construction and operation activities such as excavation and waste emplacement to be tested and demonstrated in a realistic setting (see, e.g., Boxes 8.1 and 8.2). Table 8.1 lists some of the types of information that may be obtained from URLs with examples of each. International collaboration is a feature of URL research and countries that do not themselves operate a URL will often be found as partners in specific URL experiments.

In NEA (2001b), 18 generic URLs worldwide are listed of which, at that time, 12 were still in operation. Twelve of the 18 had been developed from existing structures like mines and road tunnels and 6 (all still in operation) were specially built. A further 8 URLs were associated with actual or potential disposal sites. The geological environments that they cover correspond closely to the main anticipated repository environments, namely crystalline rocks, bedded evaporites, salt domes, plastic clay and indurated clay. The USA is unique in its focus on an unsaturated host rock at Yucca Mountain and this is covered in Box 8.2.

The distinction between generic and site-specific URLs is an important one that very much influences the approach to the work carried out there. Generic URLs are primarily research facilities; they are not intended for waste disposal but, rather, are designed to gain experience of underground construction, develop measurement techniques and test models and methodologies. They may also be used for developing waste handling

that it was possible to construct suitable openings at repository depths (NEA, 2001c) – there being some doubt about this at the time. Consequently, much of its early work was concerned with investigating geo-mechanical models.

PRACLAY (PReliminAry demonstration test for CLAY disposal of HLW) is a good example of key R&D support of the stepwise design development, focusing as it did on experimental and modelling studies that help to confirm the general feasibility of HLW disposal in plastic clay (see Bastiaens and Demarche´, 2003 for details). The experiment consisted of a 30 m long, 2 m diameter dummy disposal gallery constructed at Mol. The experiment aimed to demonstrate, first, that the construction, operation and sealing of a disposal gallery are technically and economically feasible using existing industrial techniques. Second, the experiment contributes to the evaluation of long-term safety and performance of the disposal system through a better understanding of the processes involved in the disposal system and an attempt to validate mathematical models. The low-mechanical strength and high-water content of plastic clays means that coupled processes tend to be more important than in hard rocks (either crystalline or indurated clays). Consequently, the thermo-hydromechanical (THM) behaviour of the clay and the EDZ around the gallery is of particular interest and a focus for further R&D.

The RESEAL project (Volckaert et al., 1998) aimed to demonstrate the feasibility of making effective, large-scale, in situ bentonite seals under semi-industrial conditions. The project included the sealing of a 1.4 m diameter shaft with a combination of clay powder and high density pellets. As the last stages of hydration of the shaft seal are slow – the last 5–10 per cent takes as long as the first 90 per cent – the project has been extended to 2007, so new R&D on this theme is unlikely before then.

Other clay URLs are constructed in indurated clays whose plasticity is much more limited and, while there may also be mineralogical differences, mechanical properties are the key difference between, say, Mol and Mont Terri (Volckaert et al., 2004). Mont Terri (see www.mont-terri.ch) and Tournemire have been used to improve basic knowledge of low-permeability indurated clays that, in the past 10 years, have come to be increasingly favoured as potential repository host rocks. In situ examinations were considered necessary for progress to be made on (i) understanding the flow and radionuclide transport properties, (ii) characterising the rock response to the excavations and operations, and (iii) assessing potential construction techniques for this type of rock. More recently, construction of the Meuse/Haute-Marne (Bure) URL (MoR, 2003) has extended the range of clay rocks investigated. The following list of activities, based on the current Mont Terri work programme, is typical of ongoing R&D in sediments:

detailed examination of porewater chemistry;

diffusion experiments in undisturbed rock;

measurement of in situ stress;

mine-by tests to examine the formation and evolution of the EDZ;

investigation of methods of diminishing the EDZ;

heater experiments to measure the thermo-hydro-mechanical (THM) response in buffer and host rock;

investigations of clay interactions with cement leachates;

ventilation experiments designed to follow the response of the host rock to desaturation and resaturation;

demonstrations of installation methods for engineered barriers;