A lead plug is inside the chamber of the diffuser
Divers have begun an investigation into the condition of a system used to discharge radioactive waste from a nuclear site into the sea.
The subsea diffuser at Dounreay in Caithness was used until 1998, before it was shut down and plugged with lead.
Contaminated land and particles manager Phil Cartwright said the device may contain radioactive particles.
The probe of the diffuser is part of a wider clean-up of rogue particles on the seabed.
Mr Cartwright said: "We are currently proposing to target particle clean-up on the seabed over a three-year period and then tackle the remediation of the old diffuser, followed by a further three years of re-monitoring and recovery."