Robot speeds up recovery of undersea radioactive particles

A new underwater robot is helping shave years off a nuclear clean-up operation taking place off the north coast of Scotland.

The remotely operated vehicle (ROV) built by UK-based engineering firm Land & Marine (L&M) recovered more than 400 radioactive particles from the seabed around Dounreay between August and October this year.

This compares with 930 particles recovered by human divers between 1997 and 2007 as part of the initial survey and clean-up operation.

The company responsible for cleaning up the area’s former nuclear power station, Dounreay Site Restoration (DSR), hopes the ROV will allow it to meet its completion criteria by the end of next year.

‘This has been much more efficient and a much quicker operation,’ Bill Thomson, DSR’s senior specialist in particle remediation, told The Engineer.

‘If we were doing it with divers it would take years and years. In fact, I couldn’t even estimate how long.’

The 8.5-ton ROV was designed specifically for the Dounreay clean-up operations and cost around £800,000. It spent 37 days systematically searching an area of seabed equivalent to 22 football pitches for up to 24 hours a day.

The vehicle crawls along the seabed with a 2m-wide array of radiation detectors supplied by Nuvia, which also operates similar detectors along Dounreay’s beaches.

When it locates radioactive particles, which are usually the size of a grain of sand, it uses an auger and targeting detectors to suck up and filter the surrounding sand and water.

L&M, which has been contracted to complete the operation, had to design a lift mechanism, as well as the ROV itself, to deploy and recover the vehicle several times a day without putting undue pressure on it.

‘In standard North Sea technology, you’ll deploy an ROV and it will do an extended spell sub-sea and it’s done its job,’ said L&M’s head of marine and the Dounreay project manager, Steve Goodwin.

‘We were working on recovering the vehicle maybe six times a day, pulling it through the most risk-prone area that many times.’

Rather than using an A-frame or crane to deploy the ROV, L&M created a lift that the vehicle could be driven onto to prevent it smashing into the boat as it was lowered.

‘The radiation technology was existing technology that works on the land that had to be marinised to work under pressure in a sub-sea environment,’ added Goodwin. ‘Our job was interfacing and bringing all those bits together into an operational system.’

The radioactive particles are mostly spent fuel pin fragments of caesium 137 that were washed into the sea from the power station in the 1960s. They tend to be between 150 micrometres and 5mm in diameter and give off gamma radiation.

The exact number of particles deposited in the sea is uncertain but is thought to be several thousand, revised down from initial estimates of tens of thousands, in an area of around 60 hectares up to 2km from the shore.

A total of 1,533 fragments have been recovered so far using the current and previous ROVs, as well as by the diving operation.

DSR is awaiting criteria from the Dounreay Particles Recovery Advisory Group specifying how many particles it must remove to achieve adequate safety levels, as it would be impossible to say for certain that all fragments had been recovered.

‘The quicker we can get the offshore particles, the quicker we can argue for a reduction in the amount of beach monitoring we do,’ said Thomson.

Once the site for the UK’s experimental fast reactor programme, Dounreay is now the location for one of the world’s largest, trickiest, nuclear clean-up jobs. Click here to read more.