A device described by its operators as a hi-tech worm has been used to probe the condition of a pipeline once used to discharge radioactive effluent.
The £100,000 pipe crawler has beamed back images from inside the system at the Dounreay plant in Caithness.
The underground pipeline to the sea was in use from 1957 to 1992.
Dounreay Site Restoration Limited (DSRL) said the worm spent five days in the system, sending back video images and radioactivity readings.
The experimental nuclear power complex on the Caithness coast is being decommissioned.
Martin Howse, project manager with DSRL, said: "The tunnel system has been flooded for more than half a century and the pipeline itself was taken out of routine use in 1992, so we don't know what condition the pipeline is in or if it safe to leave in place."
He added: "We are looking for signs of structural degradation, trapped debris and radioactive contamination."
Radioactive particles have been found on beaches near Dounreay in recent years.
The metallic fragments of reprocessed reactor fuel are linked to a historic discharge from the plant into the sea.