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UK Nirex Limited (Nirex) has applied for permission to construct a Rock Characterisation Facility (RCF) at Sellafield in Cumbria (UK). The RCF is part of the extensive science programme that will enable Nirex to decide whether or not to make a planning application to develop a deep repository for disposal of radioactive waste at Sellafield. The RCF will provide data on the characteristics of the potential host rocks. Three phases are planned. Phase 1 comprises construction of two shafts, and connection of the shafts at depth. Phases 2 and 3 involves construction of underground galleries and an intensive programme of scientific investigation. All three phases integrate a range of scientific activities within construction.
This paper describes the geotechnical data acquisition to be undertaken within Phase 1 of the proposed RCF. Geotechnical data are required to increase our knowledge, and validate models, of the development of the Excavation Disturbed Zone.
United Kingdom Nirex Limited (Nirex) is responsible for developing and managing a national disposal facility for solid intermediate-level (ILW) and low-level (LLW) radioactive waste (Nirex, 1992). Investigations began in 1989, initially at two sites (Sellafield in West Cumbria and Dounreay in Northern Scotland), and in 1991 Nirex announced that it was to concentrate its investigations at Sellafield (Figure 1). A range of surface-based investigations are proceeding in and around the Potential Repository Zone (PRZ - the block of rock within which the repository could be located) at Sellafield to provide data that will enable a Post-Closure Performance Assessment of the site to be undertaken to establish whether a repository at the site could meet regulatory requirements (Hickford, 1994). These investigations have defined the geological structure and succession of the site. The potential repository host rock is the Borrowdale Volcanic Group (BVG) of rocks which are Ordovician rocks around 440 million years old and comprise mainly andesitic and dacitic, pyroclastic and volcaniclastic rocks. These are unconformably overlain by the Permian Brockram (a basal breccia of BVG clasts set in a muddy, silty matrix) and St Bees Shales which are themselves overlain by the Triassic St Bees Sandstones (red, fine to medium grained sandstones). This is shown schematically in Figure 2.
The surface based investigation programme has enabled Nirex to gain a very good appreciation and understanding of the geological and hydrogeological conditions that exist at the site within the following limitations: the boreholes are only able to sample a limited volume of rock, and borehole testing is unable to provide direct evidence concerning the spatial variability and length of fractures and of the ways they form the networks of connected fractures that control groundwater flow. Investigations to date have been very successful, however there remain 3 key areas of uncertainty that cannot be resolved adequately soley on the basis of surface investigations: (a) groundwater flow and radionuclide transport, (b) natural and induced changes to the geological barrier, and (c) design and construction of the repository.
