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

Abstract

A survey of the contribution to external dose from gamma rays originatingfrom intertidal sediments in the vicinity of the British Nuclear GroupSellafield site showed that the major anthropogenic contributions were due to137Cs and 60Co. At some sites, traces of other anthropogenic radionuclides were detected, namely106Ru,125Sb, and154Eu. The proportions offine grained material (<63 µm) were used to improve model predictions of dose contribution due to external exposure togamma rays, using the CUMBRIA77/DOSE77 model. Model dose predictions werecompared to those directly measured in the field. Using the new proportions of fine grainedmaterial (1–17.5%) in conjunction with field gamma-ray spectra, model predictions wereimproved considerably for most sites. Exceptions were at Drigg Barn Scar andWhitehaven Coal Sands sites, which had their own unique characteristics. The highest60Co activityconcentrations in this study were detected at Drigg Barn Scar. These relatively high activity concentrations of60Co were due tothe presence of 60Co in mussels and barnacles, hence upsetting the fine sediment relationships usedin previous dose calculations. Whitehaven Coal Sands was unusual in that itcontained higher levels of radionuclides than would be expected in sandysediment. The mineralogy of these sediments was the controlling factor on137Cs binding, rather than the proportion of fine grained material. By adjustingthe effective fine grained sediment proportions for calculations involving60Co and 137Cs at Drigg Barn Scar and Whitehaven Coal Sands respectively, the CUMBRIA77/DOSE77model predictions could be improved upon significantly for these sites. This workhighlights the influence of particle size and sediment composition on external doserate calculations, as well as the potential for external dose contributions frombiota.