The geology and hydrogeology of Sellafield: An overview

Nirex is responsible for providing and managing a national facility for solid intermediate-level and low-level radioactive waste. Geological and hydrogeological investigations have been in progress at Sellafield in west Cumbria since 1989 aimed at determining whether or not the site is suitable for such a deep repository. Geological investigations have included the drilling of 20 deep boreholes with over 20000 metres of drilling, together with almost 2000 line kilometres of seismic surveys and over 8000 line kilometres of airborne geophysical surveys. Hydrogeological testing and groundwater sampling and testing have provided additional information on the ground conditions at the site. The results from the investigations show that the Borrowdale Volcanic Group of rocks, the top surface of which is 400-600 m beneath the site at Sellafield, holds good promise as an eventual location for the repository. Further investigations are required before the suitability of the site can be confirmed. These additional investigations will include the construction of an underground rock-characterization facility for which Nirex applied for planning permission to construct in 1994. The investigations carried out to date have defined the subsurface structure of the area and have defined a stratigraphic subdivision of the Borrowdale Volcanic Group. Groundwater flow within the volcanic rocks has been shown to be occurring through a limited number of fractures and preliminary work has been undertaken to characterize these hydrogeologically significant fractures. Hydrogeochemical studies have begun to characterize the groundwater types and preliminary results have indicated that the groundwaters at depth are saline with residence times in excess of tens of thousands of years.