Robot collects 300 radioactive particles from Dounreay seabed (only 400 to go)

The remotely operated vehicle (ROV), the size of a small bulldozer, was lowered from a barge anchored 550 yards off the Caithness complex at the beginning of August.

By yesterday it had recovered 279 particles, or hotspots, including 40 regarded as a "significant" risk to health.

The 24-hour operation, involving 22 workers, has covered almost all the 31 acres - an area the size of 17 football pitches - it targeted it the first of three summer campaigns. The search, which will continue this month, has so far cost 1.5 million on top of the 800,000 cost of the ROV.

A Dounreay spokesman said: "We know it's not realistic to expect every single particle ever released to be recovered. However we are making the best effort to recover as many of the more hazardous particles as we can as part of the clean-up of the site."

Last year a smaller operation recovered 115 particles from just over 18 acres of seabed, including 28 significant finds.

The particles have caused concern at Dounreay for more than quarter of a century. They are fragments of irradiated nuclear fuel discharged into the sea during the 1960s and 1970s.

The scale of the problem was only realised by the UK Atomic Energy Authority in the late 1990s. Particles were removed from beaches, but those on the seabed were only monitored. Divers were used to map the area of particles and remove them, but it was later realised this was too dangerous.

In 2007, after a two-year consultation, it was decided to remove the most hazardous offshore particles while continuing to recover those on beaches.

The ROV is now used to sieve the seabed off Dounreay while the specialist staff control its movements using an umbilical cable attached to the barge.

The tracked seabed crawler is based on technology developed for the offshore oil and gas industry.

It features a 7ft-wide detection system capable of finding particles buried up to 2ft deep in sediment.

The hotspots are collected in two on-board tanks, which are returned to the surface and emptied aboard the barge before being returned to the nuclear plant for analysis.

It is thought up to 700 particles may be found in the target area. Of these, more than 200 are thought to present a significant risk to health.

Before this year, more than 1,100 particles had been recovered from the seabed. Others have been transported to foreshores, with more than 400 found.

The work is part of the decommissioning of the Dounreay site being carried out over the next 15 years at a total cost of about 2.6 billion.