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Dounreay waste shaft gets grout curtain seal

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THE UK Atomic Energy Authority is to install a grout curtain around a nuclear waste filled shaft at its Dounreay experimental reactor site to prevent radioactivity leaching into surrounding ground.

The move is being taken in advance of a tricky operation to extract nuclear waste, including plutonium and uranium, for treatment and safe storage.

Dounreay shaft was excavated in 1953 as a mucking adit during construction of a subsea outfall pipe. The 4.5m diameter, 65m deep, unlined structure was approved as a repository for nuclear waste in 1959. But an explosion in the waste column in 1977 brought disposal to a halt.

Isolation and emptying of the shaft's toxic contents is being undertaken to halt the flow of contaminated groundwater out of the shaft into the highly fractured surrounding rock.

The decision to install a grout curtain follows consultation with local residents and environmental groups (NCE 20 November 2003).

A major ground freeze or installation of secant piled walls were initially preferred options.

This is because there were doubts about grout's ability to achieve a perfect seal.

But a UKAEA spokesman said that recent advances in grout mix design and pumping technologies had made a grout curtain technologically viable and affordable.

Contracts for the £16M job are due to be announced next month.

Work is expected to involve installation of 350 to 400 boreholes to depths of 80m.

Trial boreholes are to be drilled next spring with the main civils works starting the following autumn.

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