A reassessment of the dispersion properties of 99Tc in the North Sea and the Norwegian Sea

Abstract

The spatial and temporal evolution of the beta-emitting, anthropogenic radionuclide Technetium-99 ( 99Tc) in the North and Norwegian Seas have been simulated with a regional isopycnic Ocean General Circulation Model (OGCM), forced with realistic, daily averaged atmospheric forcing fields for the period 1975 to 2003. The main source of this soluble radionuclide is the reprocessing plants at Sellafield in the Irish Sea and La Hague on the French coast in the English Channel. The radioactive contaminants follow the general ocean circulation in the area, and are eventually transported northwards along the Norwegian coast heading towards the Arctic Ocean. Comparison with observational time series from two stations along the coast of Norway shows that the model fairly realistically captures both the amplitude and the temporal trend of 99Tc from Sellafield and La Hague. To isolate the effect of changes in the ocean circulation and mixing on the released tracers, idealized tracers with a clock attribution are included for both sources. These idealized tracers measure the age along the pathways of the tracers from the two sources. The age from the Sellafield-derived tracer at the island Hillesøy on the northern coast of Norway is found to be in the range 5–8 years, which is slightly older than previous estimates of the related concept of transit time. A complex pattern of variability is identified, with local and regional atmospheric forcing influencing both the northward tracer flux and the time elapsed since the tracer left the source.