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Following a period of public consultation, UKAEA Dounreay has chosen grout as its preferred method of isolating the 65-metre deep facility from the surrounding groundwater.
Subject to regulatory approvals, a 10 metre-wide band of rock around the shaft will be sealed by injecting grout into the fissures to form a deep containment barrier that stops groundwater flowing into the shaft and becoming contaminated with radioactivity.
Most of the groundwater entering the shaft is pumped out, checked and discharged to sea but a small amount is known to have flowed through the shaft since the 1950s, transporting radioactivity that has contaminated the rock on the seaward side. Surveys indicate this is equivalent to three per cent of the activity routinely pumped out and discharged to sea.
Hydraulic isolation of the shaft will stop this and reduce the amount of activity that needs to be discharged. It will also eliminate any lingering doubts that the shaft could be a continuing source of particles found in the marine environment near Dounreay.
The grout “curtain” will envelop the shaft, stabilising its environment for waste retrieval and eliminating the risk of major leakage as an environmental hazard.
A series of exploratory boreholes and grouting trials are scheduled over the next 12 months before work to isolate the shaft begins in Autumn 2005. Between 350 and 400 boreholes will be drilled in an oval-shaped ring around the shaft and these will be injected with grout in an operation expected to take 2-4 years to complete.
The programme of work includes the reinforcement of a plug at the base of the shaft with high-strength concrete to ensure it can withstand the changes in water pressure that are expected to occur when waste retrieval begins.
UKAEA consulted panels of stakeholders and the public about the Best Practicable Environmental Option for cleaning up the rock after the waste has been retrieved. Natural attenuation emerged as the preferred option, with some stakeholders recommending immobilisation for additional reassurance.
UKAEA recognises that a final decision cannot be taken until after the waste has been retrieved. Grouting will immobilise the activity in a 10m-wide band of rock.
Norman Harrison, director of UKAEA Dounreay, said: “Today’s announcement means we are starting to decommission the shaft four years earlier than previously programmed.
“The main environmental hazard associated with the shaft at present comes from the risk of a major leak, albeit this is remote. Containing the shaft with a thick wall of grout mitigates this, and underlines our determination to tackle the hazards at Dounreay on the earliest timescale possible.”
UKAEA geologist Warren Jones said: “We recognised that a decision on the degree of remediation of the rock can be taken only after the shaft has been emptied. However, it was important to obtain the views of a wide range of stakeholders on the acceptability of the options and this has enabled us to choose grout as the method of isolation. Grouting will immobilise any radioactivity within a 10-metre band of rock but this does not preclude its excavation at a later date.”
PUBLIC PARTICIPATION IN END
STATE OF WASTE SHAFT 21 November 03
UKAEA is today inviting members of the public to participate in the choice of Best Practicable Environmental Option for remediation of the rock around the waste shaft at Dounreay.
Options for remediating the rock once the waste has been removed range from natural decay of the radioactivity to quarrying large quantities of rock from beneath the seabed for disposal as low level waste.
UKAEA is consulting the public now because the agreed “end state” for decommissioning the shaft will be an important factor in choosing the most appropriate techniques for its hydraulic isolation and retrieval of the waste.
Dounreay director Norman Harrison said: “The safe decommissioning of the shaft and its legacy of waste dating back to the 1950s promises to be a major feat of engineering that can bring global recognition of the skills and expertise now being developed in the Highlands.
“I believe it is important that stakeholders are not only kept fully informed about this work but also have the opportunity to help us make decisions that are publicly acceptable and value for money. The consultation period we are announcing today is just such an opportunity, and I look forward to learning of people’s views about the options open to us.”
Groundwater has diffused a small amount of the radioactivity into the rock surrounding the shaft since the first solid waste was authorised for disposal in 1958. It is equivalent to three per cent of the activity that is pumped from the shaft each year and discharged to sea in accordance with an authorisation issued by the Scottish Environment Protection Agency.
Technical assessments have been
undertaken of a range of options for remediating the rock once the bulk
waste has been removed from the shaft. The options include:
The costs of the options range from nil for natural attenuation to £300 million for excavation and disposal of the rock.
Two panels of stakeholders facilitated independently have met and reviewed the technical assessments. The opportunity to register views is being extended today to all stakeholders and members of the public.
Their findings, which are being published today along with a summary paper for consultation and technical reports, point to natural attenuation with some form of immobilisation in-situ.
UKAEA geologist Warren Jones said: “The hydrogeological work that has been carried out confirms the need to isolate hydraulically the shaft before we can begin to retrieve the waste. Two of the most promising techniques for diverting the natural flow of groundwater around the shaft are temporary freezing of the rock and injecting with grout, which is permanent.
“The purpose of this BPEO study is to help us decide how far we should go to remediate the rock once the shaft has been emptied. Once we know that, we can proceed with a method of hydraulic isolation that is compatible with the agreed end state.”
The closing date for the consultation period is 7 February 2004.
In 1998, the Government announced that it had accepted the advice of UKAEA that the best practicable environmental option to decommission the shaft would be to retrieve the waste.
UKAEA is now developing proposals for waste retrieval, treatment and storage facilities to enable the emptying of the shaft to take place.
A summary consultation paper is being issued to more than 800 stakeholders who have registered an interest in the Dounreay Site Restoration Plan. Anyone can register an interest via the UKAEA website at http://www.ukaea.org.uk/news/dsrp.htm
UKAEA is responsible for the decommissioning and environmental restoration of Dounreay and its other former research and development sites. The Dounreay Site Restoration Plan was published in October 2000 and can be viewed at http://www.ukaea.org.uk/dounreay/rplan.htm
UKAEA is currently spending £140-150 million per annum at Dounreay. This worth approximately £75 million a year to the economy of the Highlands.