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Final curtain for Dounreay as clean up starts on site

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INSTALLATION OF a grout curtain to stabilise a nuclear waste-filled shaft at Dounreay, Scotland, started last week, paving the way for its decommissioning.

Contractor Ritchies has a £16M design and build contract to install an impermeable barrier around the 65m deep shaft, cutting it off from groundwater (NCE 26 May 2005).

The contract is one of the most 'delicate and challenging' geotechnical projects in the UK, said project manager for client UKAEA Warren Jones.

The shaft was filled with nuclear waste, including plutonium and uranium swarf, from the 1950s to 1977, when hydrogen produced by the decomposing waste exploded.

It is thought that the shaft is a source of radioactive pollution regularly found on nearby beaches.

An inner ring of 'blocker grout holes' is being installed first, said Jones. 'That will protect the shaft when we do main cut-off grouting.' Boreholes up to 80m deep are being drilled to create the blocker ring. They will be at 3m centres. The nearest will be 4m from the shaft.

Injection pressure will be relatively low to prevent grout spilling through ssures into the shaft itself, Jones said.

Ritchies will next create an outer ring of cut-off boreholes up to 90m deep and at 2m centres, into which grout will be injected at high pressure.

'We're going below the base of the shaft. By grouting at very high pressures we'll be able to cut off under the shaft, as well as around it, ' said Jones.

A specially designed mediumstrength thixotropic grout (a grout that swells on contact with water) is being used. It is a low-water mix to resist washing away by groundwater flows, but is able to penetrate ssures of as little as 15 microns, said Jones.

The grout is also a low shrinkage, low heat of hydration mix.

Boreholes have to be drilled to within 1% accuracy.

The shaft was built as a muck away route during construction of a sub-sea outfall tunnel for low level radioactive liquid waste. After completion a reinforced concrete plug was built in the adit connecting the shaft and outfall tunnel.

UKAEA decided to use the unlined shaft for solid radioactive waste disposal, rather than back l it. Last November Ritchies completed reinforcement of a concrete plug in the adit at the base of the shaft and back filled the adit with mediumstrength grout.

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