Dounreay waste shaft work begins
WORK has started to isolate the notorious waste shaft at Dounreay to allow its cocktail of radioactive and chemical materials to be cleaned out.
Emptying the shaft is seen as one of the world's most challenging nuclear clean-up jobs. It will require the building of a new treatment and storage facility, bringing the total cost of the 20-year project to 180 million.
Planning permission has been granted to build a raised platform over the shaft to allow up to 400 boreholes to be drilled to a depth of 80 metres in an oval ring. These will then be filled with grout which will solidify into a 360-degree "curtain" to seal off the shaft from groundwater.
The work, costing 16 million, is part of a project to completely isolate the shaft by mid-2008 with an overall cost of about 27 million. Retrieving the shaft's contents using specially-designed robots is due to start in 2019 and take six years to complete. The new plant to treat and store the waste will cost 150 million.
The grout curtain will form a solid protection ten metres wide. As well as providing reassurance about leakage, it is hoped it will also eliminate doubts about the shaft being the source of radioactive particles which have been found on nearby beaches and the seabed.
Throughout Dounreay's history, groundwater getting into the shaft has become contaminated with radioactivity. Most of this is discharged into the sea, but a small amount has gone through the shaft and contaminated rock. The grout wall will divert the natural flow of groundwater away from the shaft while retrieval of the contents goes ahead.
Simon Middlemas, Dounreay's deputy director, said: "Being granted planning permission to isolate the shaft is a major step forward for the decommissioning of the Dounreay site. The shaft is one of the major nuclear decommissioning challenges in the world. The decision by Highland Council to grant planning permission allows us to move forward with our programme to decommission the site as planned."
Cleaning out the waste shaft is also a significant part of the 2.6 billion programme to return the 140-acre complex to a near greenfield site by 2036.
The shaft, which was built 200ft down into rocks, was sunk in the 1950s during construction of Dounreay's effluent tunnel. It was later authorised by the Scottish Office for waste disposal.
It was taken out of routine operation in 1971 but was still used for specially authorised dumping until 1977 when an explosion in the gas space above the water line blew off its heavy concrete lid.
In 1998, the government announced that it had accepted a recommendation by the UK Atomic Energy Authority that the best practicable environmental option to decommission the shaft would be to retrieve the waste.
About 750 cubic metres of waste has accumulated in the shaft, including radioactive and chemical material, containers and a large range of equipment and machinery. An inventory in 1998 showed a total of 16,348 disposals in the shaft's lifetime, with everything from paper tissues and rubber gloves, to vacuum cleaners and fire doors.
However, efforts to record the contents have been hampered by previously poor record-keeping. Staff found some disposals during night shifts went unrecorded while other records were disposed of.
The maximum fissile content is estimated at 4kg of plutonium and 98.6kg of uranium-235 in a total of 1,165kg of uranium.
Since November 1983, over 900 radioactive particles have been found on the seabed and another 238 on the enclosed beach at Dounreay. A further 57 have been found on the public beach at nearby Sandside. Dounreay has spent 10 million in research in recent years but experts are still not sure how the particles got into the sea or on to beaches.
Warren Jones, the technical manager of the shaft isolation project, said: "UKAEA does not believe the shaft is the source of the ongoing particles issue. But hydraulically isolating the shaft by installing a grout curtain will seal it off and means it will no longer be a possible source."
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Wednesday 19 June 2013
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