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Friday, 25 July 2014

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Mini submarines offer a helping hand at Sellafield

MINI submarines designed and built in Barrow are helping to remove radioactive sludge at Sellafield.

Remotely operated mini submarines have been used at the Sellafield nuclear site to demonstrate just how radioactive sludge could be removed from the bottom of a historic fuel storage pond.

One of the biggest decommissioning challenges at Sellafield is the First Generation Magnox Storage Pond which handled 27,000 tonnes of nuclear fuel over its lifetime. It is estimated that there is up to 1500 cubic metres of radioactive sludge left in the 60-year-old pond – the equivalent of more than half an Olympic swimming pool.

The 1950s pond has thick reinforced concrete walls however it was built with no roof and is open to the elements, so sludge has been accumulating at the bottom of the pond just like in any garden pond.

The sludge is made up of algae, corrosion products and wind-blown material but is radioactive and therefore needs careful handling.

Head of the facility Martin Leafe said: “We’ve taken technology used in the hazardous deep sea conditions and applied it at Sellafield in our own hazardous nuclear fuel storage ponds.

“The mini submarines can explore, survey and carry out remote operations at the bottom of the pond, while the workers remain safe and dry outside the pond.

“We are building up our knowledge of the radioactive sludge characteristics to help develop equipment for bulk sludge retrievals, which will have to cope with sludge up to one-metre-deep in places. The greater our understanding of the sludge the more opportunities we will have to accelerate decommissioning of this priority project.

“Significant savings will be made using mini submarines rather then designing and building complex bespoke equipment which was our original plan. We now have the ability to clear space in this hugely congested storage pond to help with the recovery of nuclear fuel, contaminated waste and radioactive sludge as part of our work to empty this legacy facility.”

Sellafield Ltd has already built a Sludge Packaging Plant alongside the historic pond and inactive commissioning of the new plant is being carried out.

Before it can start operating, some sludge has to be sucked up from the bottom of the seven-metre-deep pond to test it and that’s where the remotely operated mini submarines come in thanks to Rovtech Systems in Barrow. 

The facility manager Dave Skilbeck said: “We are extending our Remotely Operated Vehicle capability all the time to help empty this historic nuclear pond. Our experienced pilots can ‘fly’ the machines within the pond to survey the fuel and sludge, take sludge samples for analysis, and pick up and consolidate nuclear fuel into fuel containers for export out of the pond.

”Our latest success has been attaching an eductor to the ROV which has proved that we can lift the sludge from the bottom of the pond. The eductor device is like a powerful jet pump that creates a negative pressure differential that draws the sludge up through a thick hose.

“We need about 50 cubic metres of sludge to commission and test the new Sludge Processing Plant later this year and we’ve now proved the technology to get the job done. We can safely get the radioactive sludge off the bottom of the pond whilst leaving behind the water which is a significant step forward.”

Radioactive sludge to a depth of 30 centimetres was successfully removed from a small area of the pond floor right down to the concrete base slab. This capability will be used to help pond floor clearance and will provide access to the containers of nuclear fuel that needs to be retrieved and moved to modern storage.

The radioactive sludge is thick and although the eductor had been previously trialed using simulant materials, there was no guarantee that it would be effective when introduced into the FGMSP.

Video footage was taken by the ROV and it shows this grey sludge being pulled up through the hose and the yellow uranium is clearly visible.

Further trials are now being carried out to capture and temporarily store the sludge, before it is turned into a consistency suitable to start pumping it across to the new SSP1 at the end of 2014.

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