More than a hundred fragments of spent nuclear fuel were removed from the seabed in the latest phase of work to clean up and shut down the former nuclear research site at Dounreay.
The particles were detected and retrieved by a remotely-operated vehicle that systematically scanned an area of seabed equivalent in size to more than 10 football pitches.
The robotic system recovered 115 particles during the two-month operation. Of these, 29 were in the higher hazard category defined by independent experts as a "significant" threat to health.
Another 16 suspected fragments detected by the ROV also gave readings in this category but were not retrieved. Six could not be targeted accurately for retrieval and 10 were buried deeper in the sediment than the 45cm reach of the ROV retrieval system.
Monitoring of local beaches was carried out during the seabed clean-up. A total of 15 particles, none of them "significant", were detected and retrieved onshore since seabed clean-up started on June 1. Of these, 14 were removed from local beaches.
Two small areas of elevated radioactivity were detected during a survey of land adjacent to the site and zoned for development as a low-level waste disposal facility. One of these contained a "minor" particle. Both areas were excavated and this survey is continuing.
Dounreay clean-up contractor DSRL reported all the finds to the Scottish Environment Protection Agency. The results will also be considered by the Particles Retrieval Advisory Group, a body of independent experts set up to advise DSRL and SEPA.
The offshore clean-up is the implementation phase of a major study carried out with public consultation to identify the best practicable environmental option to address a legacy of discharges from the former research site.
This summer's work focused on an area at the west of the main particle plume on the seabed. The plume maps were produced by Dounreay Particles Advisory Group, a body of independent experts, and based on the results of studies and detailed statistical analysis. The maps indicate that the most radioactive particle remnants are located in this area, where fishing is prohibited.
Phil Cartwright, particles and contaminated land manager at DSRL, said: "This was the first full phase of seabed clean-up following a demonstration last autumn of the detection and retrieval technique.
"The area targeted was discussed with the Scottish Environment Protection Agency and Particles Retrieval Advisory Group and provided a range of challenges to the detection equipment. The information we gained will assist DSRL to develop the best way forward for this project and by SEPA and PRAG to consider the effectiveness of the retrieval technique and if the plume map needs to be changed. The retrieval of 115 particles, with total radioactivity of 500 million becquerels (of ceasium-137) is a positive start to the seabed clean-up."