Dounreay Site Restoration Ltd -Seabed

Seabed

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.

Map of offshore diver finds

Report of 2007 diving surveys

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.

The independent Dounreay Particles Advisory Group estimated in 2008 that some 5000-6000 particles may be present in 60 hectares of the seabed near the historic discharge outlet. Of these, it was estimated that 1350 particles would be large enough to cause significant health effects. This is in the vicinity of the 2km radius of the fisheries exclusion zone.

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 was a remotely-operated vehicle carrying radiation detection equipment and a suction system that could detect and retrieve significant particles buried up to 50 cm in the sediment. Each time a particle was detected, the device extracted a “cone” of sediment and this passed through a filter to an on-board storage tank. When a number of particles has been collected on-board, the device was lifted to a surface vessel where the particle was segregated and the sand returned to the seabed. The particles were returned to Dounreay.

In 2010, a new contract was won by Land and Marine. It deployed a larger ROV with a wider version of the Groundhog radiation detection system developed by Nuvia.

A team of 22 people work around the clock from the 60m-long barge LM Constructor during the periods of recovery work each summer. The on-board team consists of personnel from Land and Marine and Nuvia.

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 are also recovered.

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 clean-up started in August 2008 with a demonstration phase. Full0-scale recovery commenced the following summer and the initial phase is due to be complete in 2012. Weather conditions limit the period of operation to May-September each year.

Map of ROV finds

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 Particles Recovery 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 is reported by Dounreay Site Restoration Ltd to the Scottish Environment Protection Agency, the Particles Recovery Advisory Group and other interested parties, including land-owners and the site stakeholder group. The recovery 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 initial phase, depending on the results.

PARTICLES RECOVERED FROM SEABED (AS OF JULY 2011)
Significant	384
Relevant	506
Minor	994

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 continues through the offshore retrieval period, subject to access. 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.