Dose distribution measurements and calculations for Dounreay hot particles

Abstract

Discrete fragments of irradiated nuclear fuel have been discovered on the foreshore at the Dounreay nuclear site in Scotland, offshore on the seabed and at nearby beaches which have public access. The fragments contain mainly 137Cs and 90Sr/90Y and for particles recovered to date, 137Cs activities are within the range of 103 to 108 Bq. The most active particles found at Sandside Beach contain ∼3×105 Bq137Cs. Direct measurements of the spatial dose distributions from 37 fuel fragments were measured in detail for the first time using radiochromic dye film as part of a national evaluation of the associated potential radiological hazard. Monte Carlo code calculations of the doses are in good agreement with measurements, taking into account variations to be expected due to differences in shape and the increasing importance of self-absorption for the larger, more active fragments. Dose measurements provide little evidence for wide variations in the 137Cs: 90Sr/90Y ratio between fragments. Specific attention is given to the evaluation of skin dose, averaged over an area of 1 cm137 at a depth of 0.07 mm, since this is of major radiological concern. There is no obvious dependence of skin dose on the site of origin of the fragments (foreshore, seabed or beaches) for a given 137Cs activity level. A dose rate survey instrument (SmartION) was shown to provide a rapid and convenient method for skin dose assessment from fuel fragments in the 137Cs activity range measured (2×105 to 2×107 Bq). A conversion factor multiplier of 240 can be applied to the open window SmartION scale reading to estimate the skin dose rate within ±25%. © The Author 2007. Published by Oxford University Press. All rights reserved.