The Nuclear Decommissioning Authority

 

Retrieving waste from the shaft at Dounreay is a major decommissioning and clean-up project which posed challenges for UKAEA and its contractors. Thanks to highly innovative approaches in dealing with those challenges, substantial progress has been made in just a few months.

The 65 metre deep Shaft was originally excavated in the 1950s to provide a route for removing rock during the construction of an under sea tunnel that was to be used for discharging low level liquid effluent to sea.

When the tunnel was finished the shaft was plugged with concrete and allowed to fill with groundwater. In 1958 the Scottish Office allowed UKAEA to use the shaft for disposing of radioactive waste. A wide variety of radioactive contaminated material was dumped in it over the years until 1977, when it was closed after a hydrogen explosion.

 

Recently under tightened environmental legislation, UKAEA has been required to remove the waste from the shaft. Following comprehensive stakeholder consultation in 2004 UKAEA selected grouting as its preferred method of isolating the Shaft from the surrounding groundwater. This would enable retrieval of the waste. UKAEA awarded the £16 million contract for the work to Ritchies, the specialist geotechnical division of Edmund Nuttall Ltd, in October 2004.

The Highland Council granted UKAEA planning permission in March 2006 for the first phase of shaft isolation.

To isolate the Shaft a raised working platform, big enough to take all the drilling equipment and a shaft retrieval headworks, had to be built on the sea shore up to the level of the cliff. 400 boreholes will need to be drilled around the shaft and fine grout injected into both these rock fractures and the liquid effluent discharge tunnel. This will prevent water from entering the Shaft.

The project has changed the shoreline at Dounreay. The platform (consisting of 11,000 cubes of concrete) is built on the shoreline and is subject to the elements. A major challenge was how to avoid the problem of corrosion of the reinforcing bar. To overcome the problem the outer layers were made up of 1,700 cubic metres of fast drying cement reinforced with plastic fibres which eliminated the potential for corrosion.

In addition to this up to 12,000 cubic metres of mass infill concrete was used to build the core of the platform. All of the concrete was supplied by local contractor J Gunn & Sons.

 

The platform was built to allow the boreholes to be drilled to a depth of 80 metres in a ring around the Shaft. The fine grout injected under pressure into the boreholes will penetrate the existing rock fractures and once solidified will form a barrier reducing the volume of groundwater flowing into it. This will mean that less water will have to be dealt with as contaminated waste, and ensure that nothing can escape from the Shaft into the surrounding environment.

Construction of the raised platform has been completed and the borehole drilling to reinforce the plug in the tunnel started in late August 2006.

This was a major achievement in itself as only a relatively short working window was available each day due to the tides. Every morning the area had to be cleared of debris and seaweed washed up by the sea and the area had to be thoroughly cleaned.

Borehole Drilling

 

The borehole drilling and grouting work is expected to take between two and four years to complete. Then the major challenge will be removal of the waste which will be done remotely. The situation is further complicated by the quantity and variety of the waste as well as the depth at which the work needs to be undertaken. Metallic waste will have corroded over the last 50 years but the degree to which this has happened is unknown. There will also be 60 metres of contaminated water in the base of the Shaft.

Concept Designs

Concept designs have been developed for the waste retrieval, treatment and storage. The waste will be conditioned and made safe for long-term storage or disposal. It will be retained at Dounreay pending the national policy for the management of Intermediate Level Waste.

 

The NDA is making sure the knowledge gained on the shaft isolation is shared, and to that end a team from B38 at Sellafield will be travelling to Dounreay to meet UKAEA’s Project Manager, Warren Jones and looking at possible use of the innovative isolation technology. This is being facilitated by Keith Riding from the NDA's Engineering Validation Unit.

Region 4 are also planning that sludge recovery and treatment knowledge from a project at Hunterston A is shared with the shaft recovery team.