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

Since 2000, much of the effort of diving surveys offshore of Dounreay to locate the presence of radioactive particles in the seabed sediments has been directed to a programme of repopulation studies, in which selected areas of the seabed have been surveyed 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 off Dounreay. The two-population model originally proposed by Atkinson (2001 UKAEA Document reference 000052) and further refined into three populations by Clayton and Atkinson (2002 UKAEA Document PSG Issue Note (02)33) has been substantially confirmed by the extended data set now available. It is apparent that the upper layers of the seabed sediments, containing a population of particles, are essentially mobile. These sediments migrate over the seabed driven by tidal wave and surge induced seabed currents, recontaminating areas which have been previously cleared of particles. The number of particles present in this layer at any given location has not been effectively reduced by the removal of particles over the years, nor has the distribution of activity within this population varied significantly. The highest concentration of particles, and the most active, reside close to and to the northeast of the effluent diffuser outfall. Particle numbers and their activity decrease with distance from the diffuser, and the rate of decrease is significantly greater to the southwest compared to the northeast. By contrast, there is evidence that the population of particles retained in the deeper sediments has changed significantly as a result of the repopulation surveys. Close to the diffuser, the population of particles identified at depth during initial surveys is high and contains significantly more highly active particles than are found in the surface sediments. It is also evident that once the deeper sediments are cleared of particles, the level of repopulation of these sediments over time is substantially 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 were initiated in March 2005 to see if the effect of these storm events could be detected in the distribution of particles in the sediments. Little evidence for this was found, and was limited to the recovery of a single particle of high activity in the surface sediments 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 eventually exhaust the mobile particle population such that repopulation rates decrease significantly although there is no evidence as to when this may occur. It is suggested that little further benefit will be obtained from the continued retrieval of additional and similar information.