Marine ecosystem uptake of nuclear reprocessing derived radiocarbon (14C)

• Main Author: Tierney, Kieran Michael
• Published: University of Glasgow 2017
• Subjects: 363.72; Q Science (General)
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.726724

The nuclear energy industry produces radioactive waste at various stages of the fuel cycle. In the United Kingdom, spent fuel is reprocessed at the Sellafield facility (formerly known as Windscale), in Cumbria on the north-west coast of England. Waste generated at the site comprises a wide range of radionuclides, including radiocarbon (14C) which, due to its long half-life (5730 years), environmental mobility and high bio-availability, is the largest contributor to collective dose commitment from the entire nuclear industry. 14C is disposed of in various forms, one of which is as highly soluble inorganic carbon within the low-level liquid radioactive effluent, via pipelines, into the Irish Sea. The discharged 14C is then rapidly incorporated into the dissolved inorganic carbon (DIC) pool. This project aimed to better understand the fate of Sellafield discharges of 14C to the marine environment. Investigations of intertidal sites in the Irish Sea and West of Scotland found 14C enrichment above ambient background levels in shell material, including at the most northerly site, located 265 km north of Sellafield. Dissolved inorganic 14C is readily utilised during shell formation by calcifying organisms and mussel shell 14C activities at sites closer to Sellafield appear to be varying in response to the total Sellafield 14C discharge activity over the preceding 5 years. Due to subsequent erosion of this material, 14C is transferred to finer fractions of intertidal sediments where it is accumulating. During photosynthesis, primary producing organisms also utilise carbon derived from the DIC pool. This uptake and the trophic-level transfer of 14C within the Irish Sea and West of Scotland marine environments were examined. The 14C activities of Irish Sea DIC and biota in the east and west basins were enriched and highly variable. A general decrease in 14C activity with distance from Sellafield was observed, although, enriched activities were also found in the West of Scotland where the activities were more homogenous. Organic sediments were significantly less enhanced than associated benthic organisms. This could be due to rapid scavenging of labile, 14C-enriched organic material by organisms and mixing to depth with older, 14C depleted material. Commercially important species were 14C enriched; however, the radiation dose from their consumption is extremely low and radiologically insignificant. To evaluate the transfer of 14C to top predators in the UK marine environment, 14C activities were examined in stranded marine mammals. All Irish Sea samples were enriched, as were most from the West of Scotland, although the 14C activities were lower. In demonstrating transfer of enriched 14C to apex predators for the first time, this study also showed that marine mammal activities correlated significantly with distance from Sellafield and Sellafield 14C discharge activities for 24 months prior to stranding. These measurements also provided some insight into harbour porpoise (Phocoena phocoena) ecology, indicating high foraging fidelity and suggesting the animals stranded on the West of Scotland did not forage in the Irish Sea. The studies in this thesis examined the dispersion of Sellafield-derived 14C to both near- and far-field sites and the subsequent ecosystem uptake and trophic transfer at these locations. However, it is important to attempt to understand the fate of 14C discharges beyond the limitations and scope of analytical investigations. To do so, the first spatial-temporal ecosystem model to predict the ecological fate of Sellafield-derived 14C was developed. The observed trends in 14C activities between different species were predicted by the model which illustrated the integration of 14C in species at higher trophic levels through time.