A probe has been launched into the discovery of suspected plutonium during checks at the Dounreay nuclear plant.
The ground around old effluent collection tanks was known to have been contaminated by historical leakage.
However staff checking the area in Caithness to finalise plans for a £4m clean-up detected higher than expected levels of radioactivity at a manhole.
Workers involved in taking samples from the site have been put on a programme of health checks as a precaution.
Further inspection of similar manholes has been put on hold while the system of work is reassessed.
Plutonium is a health risk if ingested or inhaled.