Dounreay nuclear waste dump

Dounreay is the most northerly and remote of the UK's nuclear sites. Just 32km from John O'Groats, it's perched on the coast above the stormy North Atlantic Ocean, looking directly at the North Pole. Temperatures are low most of the year, rain frequent, and wind almost constant.

Yet in these inhospitable surrounds, one of the most technically challenging nuclear decommissioning projects is currently underway. A few metres from the shoreline is a 60m-deep shaft, built during the construction of Dounreay, but now filled with nuclear waste and contaminated water. The UK Atomic Energy Authority (UKAEA), which runs the site, has the unenviable job of sealing off the shaft to prevent further water leaking in and removing the contents.

"There's nothing else like this in the world," says Warren Jones, UKAEA project manager at Dounreay.

Reactor trial

To understand what he means, a little history is necessary. Dounreay was built in the 1950s to trial the world's first electricity-generating 'Fast Breeder' reactor. It would subsequently become a key nuclear research centre, but the last reactor shut down in the 1990s and fuel reprocessing ceased in 2005.

When the original site was being constructed, a tunnel was driven out 600m under the seabed as part of the effluent treatment and disposal system (ETDS). To improve access during the construction work, a shaft was sunk just 10m from the coastline, 65m deep and 4.6m in diameter. The shaft became redundant following the construction of the tunnel, and the bottom was sealed off with a concrete plug, but it wasn't capped. And in 1959, it suddenly occurred to the site management that the shaft would make an ideal trashcan for nuclear waste.

The government apparently agreed, granting the UKAEA a licence to dispose of the radiologically contaminated material in the shaft. And so, over the next 18 years, everything from laboratory equipment to oil drums were dumped into the shaft.

A JCB is even rumoured to be down there. The shaft's use as a rubbish dump ended in 1977 after an explosion when sodium-potassium coolant reacted with water that had leaked into the shaft. But by then, the damage was done.

Today, with the decommissioning of Dounreay underway, the UKAEA is tackling the clean-up of the shaft. It is a massive challenge. Everything must be done by remote control, because of the contamination, while the waste itself is buried in 60m of contaminated water.

The first phase of the waste retrieval project is known as the 'shaft hydraulic isolation', which means drilling hundreds of boreholes around the shaft, and injecting grout at high pressure to seal fissures in the rock, thereby forming an impermeable barrier. This will stop further water leaking into the shaft, and becoming contaminated, once the waste starts to be lifted out. The cost of treating this water would run to an estimated £250m as it is, the shaft isolation will cost only £30m.

Leakage reduction

The contractor responsible for this is Edmund Nuttall subsidiary Ritchies. The geotechnical specialist normally tackles smaller projects, but this one is worth around £20m, comfortably the firm's biggest ever job. And as the sealing at Dounreay is required to dramatically reduce leakage from 350m3 per day to just 15m3 per day, it is a far tighter requirement than any previous grouting job Ritchies has handled.

Work commenced in summer 2004, with a comprehensive series of trial boreholes to investigate ground conditions and ensure the grouting system used would prove effective. These took place around 50m up the coastline, "close enough to replicate the site conditions, but not so close that contaminated water would leak into the trial boreholes," explains Jones.

A complex real-time system was developed by Ritchies and its suppliers to monitor the spread of grout through the bedrock using both underground pH detection and transient pressure analysis. All boreholes are cored. "It's more expensive than drilling, but there's less rock flour to dispose of," says Jones. Equipment used has been bought specifically for the job by Ritchies, at a cost of £850,000. "The borehole coring rigs are fairly conventional, off the shelf, but are all remote control," explains Ritchies geotechnical engineer Iain Robertson. "They will get through over 20km of coring here."

The grout plant, built by Colcrete, is bespoke and more sophisticated. It contains 34 pumps, 20 flow meters, 12 pressure transducers and two collodial mixers, all operated by computer control. Tests were also carried out to establish the most suitable grout for the job. The ultrafine cementious grout mix that Ritchies is using here is around five times finer than typical cement particles, but is still capable of penetrating rock fissures down to 25mu (micrometres) aperture to distances of 6m.

Raised platform

Once the testing phase was complete, in March 2006, the main works phase could begin. One obvious problem for Ritchies was the location of the shaft, just 10m from the high-tide mark. Therefore, it was decided to create a raised working platform on the foreshore around the top of the shaft. This was challenging in itself, involving the construction of a platform around 60m long by 40m wide using 12,000m3 of mass concrete and 1,700m3 of structural concrete. "We used fibre-reinforced concrete for the outside skin rather than rebar, which would have rusted and spoilt in these conditions," says Jones.

A concrete perimeter fence was also erected to provide further weather protection for the site crew.

Ahead of the main shaft isolation, Ritchies was required to infill with grout a section of the liquid effluent disposal tunnel (LEDT), because of its close proximity to the base of the shaft. Again, this required development of a grout mix that would meet strict requirements in terms of strength, viscosity, and permeability. The existing concrete tunnel plug was also reinforced as part of the operation, which was completed in September 2006.

Throughout the coring and grouting process, Ritchies is monitoring the ground movement using a system of sub-surface multi-level extensometers. Installed around the shaft ahead of the grouting operation in November 2006, these provide real-time data and the ability to identify movements in the rock mass, and have an accuracy of better than 0.1mm to depths of up to 100m.

Around 250 boreholes are being cored to depths of up to 90m or more, in two concentric rings. The first ring of 'blocker' holes will form an initial seal around the shaft, preventing grout injected later under higher pressure from leaking into the shaft. The second, primary ring, injected in three 'split spacing' passes, forms the hydraulic isolation. "This split spacing will ensure all the joints in the geology are picked up," explains Robertson. More than 500m3 of grout will be injected in all.

Grout curtain

When complete, the grout 'curtain' will form a boot shape around the shaft, with the toe of the boot covering the concrete plug in the LEDT. The boot design replaced the originally planned oval, and will reduce the internal surface area of the curtain and therefore minimise the volume of water leaking into the shaft.

Below the shaft, the grout is being injected at even higher pressures, the intention being that the boreholes will eventually join up and seal the base of the shaft.

Any water leaking into the shaft after the sealing operation will be pumped off to an existing treatment plant where any contaminants will be dealt with. The grouting work is scheduled to complete in summer 2008, with extensive testing to follow before the project finally closes in early 2009. Then will follow the most complex part of the shaft clean-up, the removal of the waste itself. Concept designs for this are still being developed.

In the meantime, the grouting technology developed here could mean more work for Ritchies elsewhere. "We've had visits from other nuclear sites, including Sellafield," says Jones, "but there's no reason why it couldn't be used in other applications, for example undersea tunnels."

Project fact file

Project: Dounreay Hydraulic Shaft Isolation

Client: UK Atomic Energy Authority

Contractor: Ritchies

Total project cost: £30m

Contractor value: £20m

Start date: September 2004

Scheduled end date: March 2009

Design engineer: Halcrow

Core drilling rigs: Boart Longyear

Grout plant: Colcrete Eurodrill