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Investigations to date
Consideration of land contamination at Sellafield began in 1976 as a result of activity detected during the construction of an extension to waste storage silos in the Separation Area. Further investigation proved a silo leak, which is believed to have started in 1972. A working group was set up to determine the origin of contamination including the location and extent of any leaks and the possibility of remedial action. The drilling of site wide hydrogeological boreholes was started in 1976 by the Institute of Geological Services and a total of 60 groundwater monitoring wells were installed between 1977 and 1979. These were intended to allow environmental monitoring and determine whether any radioactivity attributable to the silo leak was escaping from the site. In addition a number of steel tubes were inserted into the ground adjacent to the leak site to allow monitoring by radiological (gamma) detection instruments.
In 1978 the feasibility of installing a barrier in the ground around the Silo was considered to seal off escaped activity. Options included excavating a trench and backfilling it with impermeable clay (“bentonite wall”) or injecting cement grout into closely spaced boreholes (“grout curtain”). The idea was abandoned because of the risks of adversely affecting the silo structure and the lack of guarantee that all leaks would be sealed. Excavation of soil was also ruled out as the dose rates were deemed to be too high and there was no authorised place to dispose of the soil.
In 1979 the hydrogeological investigations of the silo leak revealed that radioactive liquid had also leaked into the ground from another radioactive waste facility in the Separation Area. The volume that had escaped was estimated to be 12 to 24 m3, but it was suggested that an intermittent leakage from this facility may have been occurring since 1971. Consideration of land management now included this leak plus more minor leaks that were also known to have occurred.
Concreting of the two main leak areas was undertaken in 1980 to reduce rainwater infiltration and to provide additional shielding from radiation. In 1981 grout injection trials were undertaken but the technique proposed was not deemed to be feasible for the silo leak and an alternative technique was suggested for this using a coarse grout curtain placed by injection around the highly contaminated zone. This would have been placed in such a way as to enhance the chemically retentive (sorptive) properties of the soil without significantly reducing its permeability, thereby reducing the rate at which radioactivity could pass into the groundwater.
Initially these contingency schemes were addressed in isolation. However, as the understanding of the site geology and hydrogeology developed it became clear that the application of the proposed contingencies could be detrimental to the other areas of contamination. For example, the use of barriers around the silo leak was discarded as computer models of groundwater flow indicated that this could result in the build up of water in the Separation Area.
In the mid-1990s the Sellafield Site Management Project (SSMP) was set up to understand the total liabilities associated with the land and groundwater contamination in order to clarify the corporate risk. The work for the SSMP was split into two main tasks:
- Consolidate the available information from the individual site projects
- Develop a site wide strategy for all contaminated land at Sellafield
Following consolidation of the available information, a comprehensive range of management options were identified and evaluated. By the middle of 2002 the process had highlighted three preferred management options:
- In-situ encapsulation - contaminated land immobilised in-situ, for example by the addition of cement
- In-situ disposal - contaminated land contained in-situ, for example by underground barriers
- Near surface disposal - remediation of ground contamination with residual contamination disposed of within a purpose built near surface disposal facility
Uncertainty in the understanding of the nature and extent of contamination and in the pattern of groundwater movement prevented progress to a thorough Best Practicable Environmental Option (BPEO) Study stage and a number of recommendations were put forward to help formulate a programme of characterisation work called the Sellafield Contaminated Land Study (SCLS). The SCLS project investigated the Sellafield site outside of the Separation Area, together with adjacent offsite areas, primarily to the west. The work consisted of a drilling programme of over 70 boreholes with associated sampling and analysis of soils and groundwater, followed by the installation of permanent groundwater monitoring wells. The drilling work was supported by geophysical surveys and evaluation of the existing monitoring boreholes. The results did not identify any major sources of contamination but indicated the presence of some radioactive contaminants in groundwater that were interpreted to have originated from inside of Separation Area (SCLS Contamination Report, SCSL Geological Report, SCLS Hydrogeological Report).
Work to investigate the Separation Area where the majority of the sources of contamination are known to occur has been taken forward in the Sellafield Contaminated Land and Groundwater Management Project (SCL&GMP) which started in April 2007 and ran until the end of 2009. The drilling programme included the drilling of 145 boreholes to investigate ground conditions, acquire soil and groundwater samples for analysis and to allow long term groundwater monitoring in selected locations. More information is available in 'Phase 2 of the Sellafield Contaminated Land Management Programme'
