The latest investigation of the seabed where the Dounreay effluent discharge systems are located was completed on April 22.
During the seven-day diving operation, Fathoms UK Ltd carried out a number of activities which included the annual maintenance inspection of the new diffuser as well as continuing investigations into the integrity of the old diffuser and surrounding rock structure.
A survey last year confirmed the rock surrounding the old diffuser had been contaminated by the discharge of radioactive waste via the old system between the 1950s and 1998. Rock samples were collected by divers and brought to the surface, where monitoring showed that some of the pollution had rubbed off onto a diver’s suit. The suit and the rock samples are now being analysed at the site.
This investigation of the old diffuser, the outlet of which is on the seabed , 21 metres below the sea surface, was a continuation of the two-day survey carried out in 2007. During the 2007 survey one of the risers, or upstands, was accessed to a depth of 23m before a lead plug which had originally been inserted to block the riser at the seabed was found.
During the current survey, sixteen separate dives were carried out to check whether other risers were clear and assess whether access to the old diffusion chamber could be gained. Forward and side view cameras were used to view the inside of the riser and dosimetry equipment was deployed to record information which will help assessment of the options for remediation of the redundant system.
The last piece of work carried out involved the removal of rock samples from areas of high radiation detected in previous dives.
An unsuccessful attempt to remove the lodged lead plug was made with marine epoxy. As riser 1 has been confirmed as the only open riser a further attempt to remove the lodged plug is being considered.
Phil Cartwright, contaminated land and particles manager said: “The information gained from this survey is very important in improving our picture of the old diffuser system so that we can assess appropriate options for remediation. We have to be mindful that there are high-activity particles on the seabed in the vicinity of the old diffuser and while we cannot, at this stage, rule out the possibility of particles residing in the diffuser chamber, or its connecting pipework, early indications from the samples taken shows that the high levels of radioactivity detected in 2007 may be due to radioactive contamination in the rock structure, linked to the authorised discharge of liquid waste rather than actual particles. We will, however, continue to consider the possibility of particles being trapped in the rock mass above the diffuser chamber. “
The information gained from this work will be reviewed by the Scottish Environment Protection Agency (SEPA), Dounreay Particles Advisory Group (DPAG), and Committee on Medical Aspects of Radiation in the Environment (COMARE).