But Dounreay Particles Advisory Group (DPAG) remains satisfied that the contamination does not pose a significant risk to the public.

In its fourth and final report, it also welcomes the launch of a multi-million pound clean-up of the seabed immediately off the plant.

The area – which is part of a fishing exclusion zone – contains a cache of 1500 or so potentially lethal particles.

DPAG was set up seven years ago to provide independent advice to the Government about the extent and risk posed by the off-site pollution.

In the conclusions to its final report, DPAG said there is no evidence of a year-by-year change in the pattern of contamination of the privately owned Sandside Beach.

It attributes increased finds to significant recent improvements in the monitoring system deployed.

The data over the past couple of years has led them to conclude that the number of so-called relevant particles being washed ashore is four times the previous estimate.

Relevant particles are those with an activity between 100,000 and 1,000,000 becquerels (Bq), whose potential health effects are adjudged as likely to be transient, such as temporary skin blistering.

Previously, DPAG rated the risk of someone coming into prolonged skin contact with such a particle at Sandside at one in 80 million. It has revised this to one in 20 million.

The new assessment would suggest that about two relevant particles are arriving at Sandside every month.

DPAG has meanwhile increased the chances of someone coming into contact with higher-active particles from one in 80 million to one in 60 million.

These are rated as significant both in terms of both their activity and their health effects. They are clustered on the seabed immediately off the site's old liquid discharge system, strongly suspected as the source of the historic leaks.

Most of the finds at Sandside have been below 100,000 Bq with the most active recovered so far being 500,000 Bq.

DPAG renews its previous recommendation that Sandside be monitored fortnightly, rather than monthly as at present. It also calls for quarterly scans of the beaches at Brims Ness, Crosskirk and Dunnet and annual monitoring of the beaches at Melvich, Murkle, Peedie, Thurso and Scrabster.

It concludes: "Provided that monitoring and recovery of particles occurs at the frequencies recommended, DPAG considers that the continuing rates of detection and level of activity of particles on these beaches do not pose a significant risk to the public."

It further presses for continuing efforts to be made to seal the defunct subsea diffusion chamber, which remains an ongoing potential source of particles.

DPAG welcomed Dounreay Site Restoration Ltd's launch in August of a bid to retrieve the most active particles on the seabed.

DSRL managing director Simon Middlemas, who attended the launch of DPAG's report in Edinburgh on Wednesday, said it would be acting on all the recommendations.

"The expert work carried out by DPAG over several years has made an enormous contribution to our understanding of this problem," he stated.

"Without their expert scrutiny advice we would not have been able to begin cleaning up the seabed and so begin recovering the remnants of past practices."

He said the revision of the number of particles coming ashore at Sandside and the associated risks should not be cause for alarm.

"It's not a horrendous increase and it's still nowhere near a significant risk."

Phil Cartwright, who is in charge of the seabed clean-up, said: "This report is the culmination of many years of work.

"It has defined the limited extent of the area where the highest activity particles remain and made possible the definition of a clean-up project.

"It assists validation of the technical basis of our clean-up programme, increasing our confidence that we are able to remediate the impact of this legacy insofar as is practicable," he said.

"We support all the recommendations and will continue to focus our efforts on implementing the clean-up in co-operation and partnership with others."

The Scottish Environment Protection Agency said good progress is being made in tackling Dounreay's off-site pollution.

Radioactive Substances Unit manager Byron Tilly said: "This report makes a further important contribution to our understanding of the situation at Dounreay.

"It follows on from the good work done previously by DPAG and will enable us to improve our knowledge of the sources of particles and how they behave.

"This will inform the future arrangements for retrieval of particles, monitoring of the environment around Dounreay and decommissioning of the old diffuser through which effluent was historically discharged."

iain-grant@ukf.net

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