GROUT CURTAIN hydraulic isolation will safeguard the Dounreay nuclear waste shaft as part of a £16M isolation contract awarded by the United Kingdom Atomic Energy Authority (UKAEA), it was announced last month.
The move is being taken in advance of a tricky operation to extract nuclear waste, including plutonium and uranium, for treatment and safe storage, from the experimental reactor site near John O' Groats on the north eastern tip of Scotland.
UKAEA Dounreay site director Norman Harrison said: 'Hydraulic isolation of the waste shaft is the first phase of one of the major nuclear decommissioning challenges in the world today.'
Scottish contractor Ritchies will install the 10m wide shield around the heavily contaminated unlined structure to prevent radioactive leachate from escaping. The Kilsyth-based division of Edmund Nuttall will drill 350 to 400 boreholes to depths of 80m in an oval shaped ring so grout can be injected into rock fissures.
It will also reinforce a concrete plug at the bottom of the 65m deep shaft. Once complete, the scheme will allow waste retrieval at a later date.
Designers considered secant piles or a major ground freeze as possible solutions to enclose the site because of doubts over grout's ability to form a perfect seal. But a UKAEA spokesman said recent advances in mix design and pumping technologies meant a grout curtain was now a viable and affordable alternative.
The decision to use grouting follows consultation with local residents and environmental groups.
Isolation and emptying of the unlined shaft's toxic contents is being carried out to stop the flow of contaminated groundwater into the highly fractured surrounding rock.
The 4.5m diameter shaft was constructed in 1953 as a mucking adit during building of a subsea outfall pipe and was used as a repository for nuclear waste from 1959. But in 1977 an explosion in the shaft put an end to further disposal, although about 750m 3of radioactive waste had already accumulated in the shaft.
Dounreay was Britain's centre of fast reactor research and development from 1955 until 1994. During this time, three nuclear reactors, fuel reprocessing and other associated nuclear facilities were built and operated on the 57ha site.
Trial boreholes will be drilled over the next 12 months. Isolation of the shaft is due to begin in autumn 2005. The project is expected to take between two and four years to complete.