Studies at Dounreay on the repopulation of offshore sediments by hot particles

Abstract

Since 2000, much of the effort of diving surveys offshore of Dounreay to locate thepresence of radioactive particles in the seabed sediments has been directed to aprogramme of repopulation studies, in which selected areas of the seabed have beensurveyed a number of times and cleared of identified particles on each occasion.This work has led to an understanding of the distribution of particles within the seabed offDounreay. The two-population model originally proposed by Atkinson (2001 UKAEADocument reference 000052) and further refined into three populations by Clayton andAtkinson (2002 UKAEA Document PSG Issue Note (02)33) has been substantiallyconfirmed by the extended data set now available.It is apparent that the upper layers of the seabed sediments, containing a population ofparticles, are essentially mobile. These sediments migrate over the seabed driven by tidalwave and surge induced seabed currents, recontaminating areas which have been previouslycleared of particles. The number of particles present in this layer at any given location hasnot been effectively reduced by the removal of particles over the years, nor has thedistribution of activity within this population varied significantly. The highestconcentration of particles, and the most active, reside close to and to the northeast of theeffluent diffuser outfall. Particle numbers and their activity decrease with distance from thediffuser, and the rate of decrease is significantly greater to the southwest compared to thenortheast.By contrast, there is evidence that the population of particles retained in thedeeper sediments has changed significantly as a result of the repopulation surveys.Close to the diffuser, the population of particles identified at depth during initialsurveys is high and contains significantly more highly active particles than arefound in the surface sediments. It is also evident that once the deeper sedimentsare cleared of particles, the level of repopulation of these sediments over time issubstantially reduced. There is little mixing of the more deeply buried sediments.Following a period of stormy weather over the winter of 2004–05, repopulation surveys wereinitiated in March 2005 to see if the effect of these storm events could be detected in thedistribution of particles in the sediments. Little evidence for this was found, andwas limited to the recovery of a single particle of high activity in the surfacesediments of Repop 12 located over 2 km to the northeast of the effluent diffuser.The justification for continuation of the programme of repopulation surveys is questioned,particularly on health and safety grounds. The programme of surveys may eventuallyexhaust the mobile particle population such that repopulation rates decrease significantlyalthough there is no evidence as to when this may occur. It is suggested that little furtherbenefit will be obtained from the continued retrieval of additional and similar information.