Prefabricated EBS module development was driven, in many cases, by the desire to improve the practicality of remote emplacement of engineered barriers in a safer and quality-assured manner. This is particularly critical in wetter host rocks, where handling of compacted bentonite becomes problematic. Such modules have also been designed for much larger spent fuel overpacks. A clear variant would thus be modules containing several HLW packages in a single overpack – which could lead to significant savings of cost and handling time.

The lack of precedent for HLW disposal has, however, led to a call for slow, staged implementation which is monitored and easily reversible for a long institutional control period. To provide a disposal system which meets this requirement needs alternative repository design options, which could have additional attractions from the viewpoint of long-term safety. This long-term institutional control certainly raises issues associated with operational safety during this period (especially considering perturbations such as earthquakes, civil disturbances, etc.), but also has significant advantages due to its small footprint and, possibly, the long-term performance of the sealed system (Masuda et al., 2004).

Taking the catalogue of the design options, NUMO, e.g., have formulated a logical system for guiding the concept development process (Figure 5.10). As indicated, there are pre-defined exclusion criteria which would result in locations which are unsuitable in terms of geological stability being excluded from further consideration. Sites which are sufficiently stable may, nevertheless, have clearly better or poorer geological/hydrogeological conditions. Sites which are less favourable may be placed in a reserve class. Conceptual studies have indicated that various options exist to compensate for poorer site characteristics by enhancing engineered barriers (e.g., hydraulic cage or barrier concepts). Extensive R&D would be required to fully assess the feasibility of such designs.

In response to the challenge to derive safe and acceptable repository concepts for any potentially suitable location, advantage can be taken of international knowledge databases to develop alternative repository design options. Preliminary work has confirmed that many options are available to tailor designs to both the geological environment available and the particular socio-political constraints set, e.g., by the host community. The extra workload on the scientists and technologies involved is significant but will be completely justified if the resulting repository project can be implemented with the full support of the local community.

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