The behaviour of 236 U in the North Atlantic Ocean assessed from numerical modelling: A new evaluation of the input function into the Arctic.

A numerical model, previously validated with other radionuclides, was applied to simulate the dispersion of 236 U released from European nuclear fuel reprocessing plants in the North Atlantic and Shelf Seas using a published reconstruction of Sellafield and La Hague releases. Model results are in better agreement with observations if the lowest estimation of such releases are used. This implies that approximately 40kg of 236 U has been discharged from Sellafield. It was found that adsorption of 236 U on bed sediments of the shallow European Shelf Seas plays an essential role in its dispersion patterns. This contrasts strongly with the more conservative behaviour of 129 I in the same area. This has two important implications in the use of 236 U as oceanographic tracer; i) special care must be taken in coastal areas, as sediments might act as sinks and sources of 236 U; ii) the annual input function of 236 U into the Arctic is not directly controlled by the annual discharges from Sellafield and La Hague, since sediments from the Irish, Celtic and North Sea modulate and smooth the signal. Only 52% of the total releases enter into the Arctic Ocean.