The Southern Storage Area (SSA) at Harwell in the UK was used as an ammunition store by the Royal Air Force until 1945. After the Second World War the Harwell site was taken over by the newly formed Atomic Energy Research Establishment (AERE), which used the SSA for the storage and handling of low-level radioactive substances and chemical materials.

In its new role, begun in 1996, the United Kingdom Atomic Energy Authority (UKAEA) has inherited the clean-up programmes of the former nuclear research plants at Winfrith in Dorset, Windscale in Cumbria, Dounreay in Caithness, Scotland, and Harwell in Oxfordshire. The site remediation project of the SSA was one of UKAEA’s largest decommissioning tasks at Harwell, and has been one of the most challenging land remediation programmes in the UK.

Work to restore the 18-acre site has been ongoing since the 1980s and the final phase of work involved the return of the land to a safe and clean condition suitable for unrestricted public access. The restoration of the SSA was also a requirement before a development for 275 houses adjacent to the SSA could go ahead.

Best option

To determine the “Best Practicable Environmental Option” for restoration of the SSA, UKAEA worked with the environmental consultants Dames & Moore International and the National Radiological Protection Board (NRPB). The options were carefully considered using a public environmental impact assessment process. The local council led this process and Oxfordshire County Council, the Environment Agency and the Health and Safety Executive were involved.

Although public safety is always a key priority of decommissioning projects, the presence of the Chilton School right next to the SSA highlighted the issue further. Each option was analysed in detail to demonstrate that the work could be completed safely, without risk to public health and the environment. UKAEA considered the most appropriate method was to remove all the wastes from the SSA and to carefully dig over the site to ensure no harmful material was missed. With advice from the NRPB, a clean-up target for each waste material that could be present on the site was established, and these targets were agreed with the regulators.

All safety cases were made public, and a transparent and open approach was used during the project. UKAEA ensured that the local community was kept informed at all times by holding several public meetings, publishing and distributing newsletters, and setting up an e-mail network. Pressure groups were also allowed onto the site to conduct their own assessments.

Phased clean-up

Buildings and surface wastes were removed in the 1980s. In the 1990s extensive surveys were completed to plan final restoration of the site. This included surveying all the pits, and carrying out air and soil surveys to provide a background baseline. The main liabilities of the SSA were:

• Six unlined chemical pits (totalling about 1000m3) that contained laboratory wastes, including low levels of radioactivity, and which were responsible for historical low-level chemical groundwater pollution.

• Five beryllium pits (totalling about 5000m3) that contained beryllium-contaminated materials, poly-chlorinated biphenyl materials (PCBs) and mercury wastes.

• The rest of the SSA required thorough checking before it could be declared clean.

• An additional hazard at the former munitions storage site was the likelihood of the presence of explosive devices.

A groundwater treatment plant had been set up on the site in 1995, and groundwater samples were regularly monitored (via boreholes). The physical works began about two years ago and were carried out by about 40 contractor personnel from RWE Nukem and VHE Construction under a small UKAEA project management team.

Excavations at the former “bomb dump” (as the SSA is referred to by the local population) uncovered around 1100 small live practice bombs, 12,000 rounds of small arms munitions, several 500kg and 750kg bomb casings and other devices that included flares, fuses, a depth charge and several mines. Twenty five controlled explosions were carried out during the project.

For each contaminated area, samples were taken and hotspots removed, before a 300mm layer was removed and the process repeated. Excavation of the pit wastes took place within tented enclosures and used methods that prevent dust generation. All the excavated waste was examined, classified and disposed of accordingly. About 250,000m3 of soil at the SSA was sorted through, of which 250m3 was classified as low-level radioactive waste. About 12,000m3 of other waste was classified as exempt radioactive, and therefore did not have to be sent to a radwaste facility. In total, 23 ISO containers were required, costing around £40,000 per container for characterisation and disposal at a licensed site.

The principal hazard to members of the public was the accidental release of hazardous dusts from the site. The process of removing the wastes from the pits and digging over the rest of the site to search for other wastes was carefully monitored and subject to independent regulation.

The final use of the site has not yet been decided. Work has begun on re-landscaping, and this will be followed by a programme of grass and tree planting. The land has been restored – without any release of hazardous materials during the project – to a condition where it could be used as a public recreation area.