Divers took to the sea on April 15 to investigate the old subsea diffuser system that was used to discharge radioactive effluent from Dounreay until 1998.
The work will produce information about the rock above the old diffuser chamber and contamination levels to permit assessment on how to decommission the structure.
Investigations began last summer when divers from Fathoms UK Ltd sent a video camera 22 metres down one of the risers before it was stopped by a lead plug installed after the diffuser was taken out of service.
The work is expected to take place over a four-day period when divers will take more images inside the diffuser risers and check radiation levels. The divers will obtain marine samples, seek access via one of the other risers, inspect the new diffuser located nearby and collect rock samples.
Phil Cartwright, contaminated land and particles manager said: "The information will help us understand the conditions from the seabed to the diffuser chamber. Before a final decision is taken on how to remediate the old diffuser we need to understand the potential effect remediation of the discharge system may have on the marine environment. We cannot rule out the possibility that there are particles in the old diffuser system and the options for remediation will take this into account. This project is integral to the clean-up of particles on the seabed which we intend to start this summer. 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."