Seabed clean-up
The largest and most hazardous fragments are believed to be located close to the old discharge outlet on the seabed. Natural disturbance of the sediment and corrosion over time is believed to cause their fragmentation, creating smaller, lighter and less hazardous particles that are mobilised like grains of sand by the complex tidal movements and carried further afield to places such as Sandside beach.
Since 1997, divers have surveyed areas of seabed near Dounreay, retrieving particles they encountered. The results of these surveys and other research allowed experts to produce a 3-D model of the pollution.
The independent Dounreay Particles Advisory Group estimates fifteen hundred particles large enough to cause significant health effects are still located in the sediment over an area of approximately 60 hectares around the old outlet. A further six thousand smaller, less hazardous particles may also be present in this area. This is in the vicinity of the 2km radius of the fisheries exclusion zone.
The largest public consultation exercise ever carried out at Dounreay progressively narrowed down a range of possible clean-up options until a preferred option emerged with public support.
The consultation process recognised that recovery of every particle was impractical but that it is feasible to seek recovery of larger, more hazardous particles that pose a significant health risk and which have settled in the sediment close to Dounreay.
Following a competitive tendering exercise and seabed trials in 2007 and 2008, a contract to carry out this work was awarded to Wick-based Fathoms Ltd.
The system selected is a remotely-operated vehicle carrying radiation detection equipment and a suction system that can detect and retrieve significant particles buried up to 50 cm in the sediment. Each time a particle is detected, the device extracts a “cone” of sediment and this passes through a filter to an on-board storage tank. When a number of particles have been collected on-board, the device is lifted to a surface vessel where the particle is segregated and the sand returned to the seabed. The particles are returned to Dounreay.
The clean-up is targeted at larger particles of concern to public health but any smaller particles in the “minor” and “relevant” categories that are detected will be recovered this way as well.
Each time a significant particle is recovered in the 60-hectare area, this removes a potential future source of particles found on local beaches.
The maximum operating speed of the ROV is 0.6 metres per second. Weather permitting, this gives a potential maximum coverage of 4000 sq metres per day but factors such as time to retrieve particles will influence this.
The clean-up started in August 2008 with a demonstration phase and is expected to continue for a further three years. Weather conditions limit the period of operation to May-September each year.
Trials of the retrieval system indicated that disturbance on the seabed is minimal and is likely to be much less than natural forces during storm conditions. However, following discussions with Dounreay Particles Advisory Group, it was decided to monitor an area of seabed on the edge of the fishing exclusion zone. This allows the effect of the seabed recovery to be checked in advance of seeing an increase or decrease in particle numbers or activities at Sandside. A similar area to the east of the retrieval operations has not been planned at present, but this will be kept under review.
The effectiveness of the clean-up will be reported by Dounreay Site Restoration Ltd to the Scottish Environment Protection Agency, Dounreay Particles Advisory Group and other interested parties, including land-owners and the site stakeholder group. The strategy will be kept under regular review and adjusted if necessary in light of the results and feedback.
Further seabed clean-up may be needed at the end of the three years.
Onshore monitoring continues during retrieval work. The Dounreay foreshore, being closest to the area where the offshore work will happen, is likely to be the first area to see a change in particle arrival rate, although particle finds on the Dounreay foreshore do not seem to follow a pattern and may be linked to the sea conditions.
Monitoring on Sandside beach through the offshore retrieval period is planned. In addition, monitoring of an offshore area across the plume towards Sandside will provide an early indication of a change in the particle population.
The offshore retrieval work will initially be undertaken to the east of the original discharge point. High activity particles disturbed by the offshore work are likely to remain close to their original location as more significant disturbance is likely to occur during winter storms, but this does not cause these higher activity particles to move quickly. The only place where particles of significant activity have come ashore is on the Dounreay foreshore.
A remotely operated vehicle replaced divers in recent years to carry out survey work on the seabed
Divers surveyed the seabed since 1997, mapping the extent of pollution and retrieving particles
Monitoring of beaches will continue during the clean-up, subject to access
The remotely operated vehicle being used to clean up the seabed
The remotely operated vehicle being deployed by Fathoms for seabed clean-up
