Transport and accumulation of radionuclides in north Wales and Scottish loch sediments

UoM administered thesis: Phd

  • Authors:
  • Hamza Al-Qasmi

Abstract

During operations at the Sellafield nuclear fuel reprocessing complex, artificial radionuclides are discharged to the Irish Sea under authorisation, where they are dispersed. This has led to the labelling of nearby sediments with a range of radionuclides. A proportion of the historical discharges has been retained in offshore sediments which act as a long-term secondary source of both soluble and particle-associated artificial radionuclides. These radionuclides are of interest both in assessing possible environmental impacts and as tracers for coastal processes. In this project, the distribution and transport of Sellafield-derived radionuclides have been investigated. Results are presented here for a study of the geochemistry of artificial and natural radionuclides and their accumulation in sediment cores from different environmental settings. The data are interpreted in the context of the historical radioactive discharges to the Irish Sea and biogeochemical processes in marine and coastal sediments.The two northern sediment cores are from the Lower and Upper Basins in Loch Etive, a sea-loch on the west coast of Scotland. The activity profiles of Cs-137 showed broad peaks in both cores which can be related to the maximum Sellafield discharges of Cs-137 in the mid-1970s and suggests that Cs-137 was mainly transported in solution to Loch Etive during that period. Enrichments of Mn, Fe and Mo in sediments and porewaters in both basins of the loch result from biogeochemical redox processes. Enrichments of U-238 and U-234 in the Lower Basin could be a result of the cycling of natural uranium. By contrast, the Sellafield-derived artificial isotope U-236 does not seem to be affected by redox-driven reactions in the Lower Basin. The Pu-238/ Pu-239,240 ratios suggest contributions from both historical Sellafield discharges and global fallout Pu. The uniform sediment distributions of Pu and Am, which do not reflect Sellafield historical discharges, suggest the existence of a homogenous secondary source. This could be the offshore 'mud patch' in the vicinity of Sellafield, from which the supply of radionuclides primarily reflects the time-integrated Sellafield discharges. This source could also account for the continuing supply of Cs-137, even after substantial reductions in discharge.The southern two cores are a soil core from the riverbank of the Afon Goch and an intertidal sediment core from its estuary, Dulas Bay in Anglesey, North Wales. Particulate input to the Afon Goch is dominant for all artificial radionuclides (including the more soluble Cs-137 and U-236) with an estimated lag time of about a decade. Preferential northward seawater movement in the NE Irish Sea limits solution input of Cs-137 and U-236 to the areas south of Sellafield. The relatively long lag time reflects both the water circulation pattern and distance between the study site in north Wales and the source point in Cumbria. The intertidal sediment core in Dulas Bay showed a slow accumulation rate with relatively low activities of Sellafield-derived radionuclides, in contrast to the Afon Goch soil core. This could reflect the regular tidal cycle in Dulas Bay which disturbs and washes the sediment regularly and thus limits accumulation of particulates with the associated radionuclides. Two redox active zones are observed in the top and the bottom of the Afon Goch core, although there is no evidence for any redistribution of Pu and natural uranium by these redox processes. However, U-236 showed variable distribution in the core. This could be a potential response to the geochemical conditions, showing that U-236 may be a promising tracer for the environmental processes and a signature of the Sellafield historical discharges of irradiated uranium.

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Original languageEnglish
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Award date31 Dec 2016