An important part of the work to close down Dounreay is to address a legacy of radioactive particles in the marine environment around the site.
These are fragments of irradiated nuclear fuel discharged to sea as a result of practices in reprocessing during the 1960s and 70s.
The most hazardous fragments are located close to an old discharge point on the seabed. Their disintegration is believe to be the source of smaller, less hazardous particles detected on local beaches since the early 1980s.
The discovery of particles in the seabed in the late 1990s lead to an extensive programme of research and consultation to identify the best practicable environmental option. This identified the retrieval of particles from the area of seabed where the highest activity particles have been found, and where the particle population density is at its highest, with on-shore monitoring specifically at Dounreay foreshore and Sandside beach, as the preferred way forward
Underwater clean-up started in August 2008, targeted at a 60-hectare area of seabed known as the "plume" where the most hazardous particles are located.
This work has continued each summer since.
By the end of 2012, more than 2200 particles had been removed from the seabed. Of these, 409 were deemed "significant" in terms of their potential health effects.
Some 60 hectares had been covered by the underwater detection and retrieval system, with some areas repeated. Total coverage was 90 hectares.
The programme is guided by the independent expert Particles Retrieval Advisory Group which advises both DSRL and the Scottish Environment Protection Agency.
Particles recovered from the seabed are returned to Dounreay
Main plume remediation area
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The start of the particle retrieval operation by remotely-operated vehicle heralded a positive step by Dounreay to deal with this major off-site legacy. It is undertaken as part of the Nuclear Decommissioning Authority’s site closure and clean-up programme at Dounreay.
At a glance
|Offshore clean-up area||60 hectares|
|Depth of water||Up to 30 metres|
|Estimate in 2008 of number of significant particles||1350|
|Maximum speed of clean-up vehicle||0.6 metres per second|
|Detection depth in sediment||50 cm|
|Coverage||Up to 4000 square metres per day|
|Initial length of clean-up||2008-2012|
|Number of public beaches monitored||7|
|Duration of monitoring||2020s|
|Total cost||£18-25 million|
Trials of the retrieval system indicated that disturbance on the seabed is minimal and much less than natural forces during storm conditions. However, following discussions with the Particles Retrieval Advisory Group, it was decided to include the monitoring of an area of seabed on the edge of the fishing exclusion zone, to allow the effect of the seabed recovery to be checked in advance of seeing an increase or decrease in particle numbers or activities at nearby Sandside beach. A similar area to the east of the retrieval operations has not been identified, but this will be kept under review.
The collection of evidence on the particle population and improved understanding of health effects enabled the definition of a fairly limited area of the seabed where the highest activity and hence the particles with the greatest health risk are located. Fishing has been prohibited in this area since 1997.
The development of detection equipment with retrieval capability means we are now able to systematically cover the seabed, retrieving particles as we progress.
The offshore retrieval work is accompanied by continued monitoring on-shore. 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 it is noted that 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 throughout the offshore retrieval period, subject to access. In addition, an offshore area on the plume towards Sandside will provide an early indication of a change in the particle population.
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..
Beaches monitoring and recovery work offshore is carried out for DSRL by a contractor, Nuvia, using its Groundhog detection system.
DSRL regularly updates a list of all particles detected around the site.
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Divers surveyed the seabed since 1997, mapping the extent of pollution and retrieving particles, before seabed clean-up started in 2008