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Tests underway to retrieve underground waste

September 24, 2009

The superstructure above the underground silo is dismantled in the latest phase of its decommissioning

One of the most demanding clean-up jobs ever undertaken in the history of nuclear energy is gathering pace.

It involves the recovery and packaging of more than 1500 tonnes of radioactive waste that has lain submerged for up to 50 years in the ground beneath Britain’s abandoned experiment with fast reactors at Dounreay.

Tests are underway in Scotland, Germany and the USA to develop the heavily-shielded plant that will sift and package the hazardous debris once it is brought back to the surface.

Later this year, work is expected to begin on a scheme design for the robotic machinery that will descend up to 65 metres below the surface to recover the waste from its watery resting place.

Detailed designs will be ready by 2012, when construction companies worldwide are expected to bid for the contract to build the major plant needed to empty the two separate vaults.

“We’re on course to start construction in 2013, with the first waste due to come to the surface in 2017,” said Steve Efemey, project manager at Dounreay Site Restoration Ltd, the clean-up company behind Britain’s biggest nuclear decommissioning project. 

The mobilisation of the design team later this year heralds a step-change in the 25-year project to empty the UK’s first and only authorised repository for intermediate-level radioactive waste.

The oldest of the two facilities is an unlined shaft, 65 metres deep. It was first used in 1958. A second vault  – a 750m3 concrete box set just beneath the surface and known as the wet silo – was used from 1971.

They are thought to have been used on almost 30,000 separate occasions to sink radioactive waste from Britain’s early experiments at becoming self-sufficient in nuclear energy.

Since 1988, when the UK Government decided to clean out both facilities, work is complete on the first phases of their decommissioning. These included a £27 million hydro-geological project that wrapped the entire shaft with grout to divert the flow of groundwater, demolition of disused headworks and other redundant buildings, and ongoing ground services preparation costing £6 million.

Ten miles from Dounreay, at the T3UK industrial test site run by a local company, JGC, heavy plant is being assembled from around the world to create a full-scale mock-up of the waste processing line.

The remotely-operated line includes cranes, grabs, power washing, feed conveyors, metal shredding, rotary screens, a trommel to separate solid and liquids, water decontamination, sludge treatment and drum filling. In Germany and the United States, tests are being conducted on an X-ray system to scan and characterise each piece of waste.

The assembled process line will be tested to its limits, before the final version is installed at Dounreay. It will be housed near the top of the shaft and silo in sealed cells to protect operators from the radiological and chemical dangers in the waste emerging from underground.

Waste minimisation is an important factor in the project. Five years ago, the estimated number of 500-litre intermediate-level waste drums needed to contain the debris after processing was 9300. Today, it is less than 2000.

“We’ve looked across every industry using similar plant, such as waste water, mining and aviation security, and assembled a process where virtually every part is proven already in other fields,” said Steve Efemey.

Innovation in the hydraulic isolation of the shaft eliminated £200 million in future water treatment costs. And the adoption of process plant tried and tested by other industry has kept the cost of this current phase of the project to less than £1.5m a year.

The project team is in the final stages of the approval process for tendering the scheme design contract for the waste retrieval system. Within 12 months, upwards of 80 engineers could be working on this phase of the project.

Dounreay belongs to the Nuclear Decommissioning Authority. Its regional director Randall Bargelt said: "I am delighted that DSRL are progressing with reducing hazard at Dounreay, using world-class innovative solutions to remotely retrieve and package the wastes from the shaft and silo for future safe storage."

The total estimated cost of decommissioning the shaft and silo, excluding the cost of managing the recovered waste in the longer term, is approximately £290 million.

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