Isolation Of Dounreay's Shaft Commences
26th January 2007
Dounreay has started drilling up to 400 boreholes around the site's waste shaft in the biggest step so far towards its eventual clean-out.
Grout will be injected through the boreholes to seal fissures in the rock around the 65 metre deep shaft and so create a giant containment barrier in the shape of a boot around the shaft that will isolate the radioactive waste from groundwater.
The project - the first of its kind in the world - will prevent large volumes of groundwater flowing into the shaft during waste retrieval and becoming contaminated. It will also reduce the risk of leakage from the shaft in the interim.
The work is being carried out by UKAEA on behalf of the Nuclear Decommissioning Authority and follows agreement by regulatory authorities.
UKAEA's Warren Jones, shaft isolation senior project manager, said: "Decommissioning the waste shaft is one of the biggest clean-up challenges in the world today, so I am delighted that we have now commenced this important phase of work to clean it out."
The boreholes will be drilled in stages in a boot shape around the shaft, with constant monitoring to provide assurance that each stage has been completed successfully.
The change from an oval grout curtain to a boot shape evolved last year following trials by the project team at Dounreay. It was realised a boot shape would be more efficient because it limits the internal surface area of the grout curtain and so helps to minimise the volume of water entering the shaft after construction.
Prior to the construction of the grout curtain, 12,000 tonnes of concrete was poured to create a raised working platform at the top of the shaft. A plug at the base of the shaft, which separates it from a sub-sea tunnel carrying the site effluent discharge pipe, was reinforced and a section of the tunnel infilled.
Isolation of the shaft has commenced four years earlier than previously planned and is expected to take between two to four years to complete. The £16 million isolation contract was awarded to Ritchies, the specialist geotechnical division of Edmund Nuttall Ltd, and the total cost of this phase of the shaft decommissioning is £27 million.
Once complete, isolation is expected to improve safety and environmental conditions during waste retrieval operations. The timescale for this is the subject of discussion with regulators and the NDA about funding availability.
A video telling the history of the shaft, its isolation and showing how it could be emptied was screened for the first time to members of the Dounreay Stakeholder Group on Wednesday 24 January 2007.
The DVD is also available on the UKAEA website and copies are available from the site.
See http://www.ukaea.org.uk/sites/dounreay_site_video.htm
Retrieving the waste from the shaft is one of the most complex decommissioning jobs. Concept designs for removing the waste are still being developed, evaluating what techniques can be utilised, what equipment is needed and how this is operated to successfully retrieve the waste from the shaft and silo. This project is to provide retrieval equipment, a treatment and packaging plant and interim intermediate level waste storage. The footage gives an indication of the plans being proposed.
Steve Efemey, UKAEA's shaft retrieval project manager, said: "We are still in the concept design phase of this project and some significant alterations have been made since the DVD was produced, however it does give some insight into how we intend to approach this incredibly complex project."
During construction of the subsea effluent tunnel at Dounreay in the 1950s, a 65-metre vertical shaft was excavated to remove the spoil. Following the completion of the tunnel, the shaft was plugged at the base and authorised by the Scottish Office for the disposal of solid intermediate-level waste. The last disposal took place in 1977 prior to an explosion.
A detailed independent assessment of this work was carried out by the Scottish Environment Protection Agency. SEPA's agreement to proceed followed UKAEA's response to recommendations in the report, which can be found at
http://www.sepa.org.uk/pdf/radioactivity/dounreay/Dounreay_shaft_technical_assessment.pdf
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