Work to clean-up the seabed around the former nuclear plant at Dounreay is getting under way for the third year in a row.
Tiny radioactive particles lie in a "plume" on the seabed spread over an area equivalent in size to that of 600 Olympic swimming pools.
The fragments, spread over 60 hectares (148 acres) of seabed, are thought to be the source of nuclear material that have been found on other, nearby beaches.
Originating from the reprocessing of nuclear fuel, they were pumped into the sea up to 50 years ago, from a discharge point around 600m from the shoreline.
An operation to clean up the seabed will resume this month for the third consecutive year.
More than 1800 have already been recovered from the sea around Dounreay.
A 1500-tonne barge will be moved into position above the plume early in May and will work around the clock until late summer, weather permitting.
A remotely-operated vehicle will trawl the bottom of the sea at 1mph, removing the fragments of irradiated nuclear fuel buried in sediments.
Specialised equipment can detect radioactive fragments buried at least 50cm deep in the sediment.
- 'Significant' radioactive particle found on beach near Dounreay
- High-hazard radioactive waste found off Dounreay