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Field and model investigations of external gamma dose rates along the Cumbrian coast, NW England

A survey of the contribution to external dose from gamma rays originating from intertidal sediments in the vicinity of the British Nuclear Group Sellafield site showed that the major anthropogenic contributions were due to 137Cs and 60Co. At some sites, traces of other anthropogenic radionuclides were detected, namely 106Ru, 125Sb, and 154Eu. The proportions of fine grained material (<63 µm) were used to improve model predictions of dose contribution due to external exposure to gamma rays, using the CUMBRIA77/DOSE77 model. Model dose predictions were compared to those directly measured in the field. Using the new proportions of fine grained material (1–17.5%) in conjunction with field gamma-ray spectra, model predictions were improved considerably for most sites. Exceptions were at Drigg Barn Scar and Whitehaven Coal Sands sites, which had their own unique characteristics. The highest 60Co activity concentrations in this study were detected at Drigg Barn Scar. These relatively high activity concentrations of 60Co were due to the presence of 60Co in mussels and barnacles, hence upsetting the fine sediment relationships used in previous dose calculations. Whitehaven Coal Sands was unusual in that it contained higher levels of radionuclides than would be expected in sandy sediment. The mineralogy of these sediments was the controlling factor on 137Cs binding, rather than the proportion of fine grained material. By adjusting the effective fine grained sediment proportions for calculations involving 60Co and 137Cs at Drigg Barn Scar and Whitehaven Coal Sands respectively, the CUMBRIA77/DOSE77 model predictions could be improved upon significantly for these sites. This work highlights the influence of particle size and sediment composition on external dose rate calculations, as well as the potential for external dose contributions from biota.