The latest phase of offshore clean-up recovered 429 fuel fragments from the seabed off Dounreay during August, September and October.
They were detected and retrieved by a remotely-operated vehicle that spent 37 days systematically searching an area of seabed equivalent in size to 22 international football pitches.
It takes the total number of particles detected on the seabed and nearby coastline since 1983 to almost 2000.
The contract was awarded to Land and Marine to carry out this latest phase of the offshore clean-up.
Its 60m-long barge LM Constructor, with a crew of 22, was stationed off Dounreay from August 7 until October 17 and covered 160,510m2 of seabed.
It retrieved 429 fragments, of which 81 were above the threshold for being classed as “significant”, as defined by Dounreay Particles Advisory Group in its assessment of potential health effects. The most radioactive fragment measured 100 million becquerels of Caesium-137.
The other 348 were categorised as “relevant” and “minor”..
This year’s work takes the total number of fragments recovered from the seabed to 1533.
Divers recovered 930 between 1997 and 2007. The site proved the viability of ROV technology the same year and in 2008 recovered 55 using this method. Another 115 were found by ROV last year.
Ongoing monitoring of nearby beaches meanwhile has yielded 433 finds since 1983.
Bill Thomson, the particles senior specialist at Dounreay Site Restoration Ltd, said the increase in number of particles retrieved this year was due to improved detection capability of the machinery deployed by Land and Marine and a stable working platform which allowed 24 hour operation.
“Their initial target was 12.5 hectares. They achieved more than 16 hectares by the time the weather closed in and the barge pulled out,” he said.
“We’re compiling a detailed report of the work and will present this next month to the Particles Retrieval Advisory Group, the body of independent experts set up to advise us and the Scottish Environment Protection Agency.