The behaviour of 236U 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 ²³⁶U 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 ²³⁶U has been discharged from Sellafield. It was found that adsorption of ²³⁶U 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 ¹²⁹I in the same area. This has two important implications in the use of ²³⁶U as oceanographic tracer; i) special care must be taken in coastal areas, as sediments might act as sinks and sources of ²³⁶U; ii) the annual input function of ²³⁶U 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.

Natural and artificial radionuclides in a marine core. First results of 236U in North Atlantic Ocean sediments

There are very few data available of ²³⁶U in marine sediment cores. In this study we present the results from the first oceanic depth profile of ²³⁶U in a sediment core sampled in the North Atlantic Ocean, at the PAP site (4500 m depth, Porcupine Abyssal Plain (PAP) site, 49°0' N, 16°30' W). Additionally, the sediment core was radiologically characterized through the measurement of anthropogenic ¹³⁷Cs, ²³⁹Pu, ²⁴⁰Pu, ¹²⁹I and ¹⁴C and natural ²¹⁰Pb, ⁴⁰K and ²²⁶Ra. The measured ²³⁶U concentrations decrease from about 90·10⁶ at g^-1 at the seafloor down to 0.5·10⁶ at g^-1 at 6 cm depth. They are several orders of magnitude lower than the reported values for soils from the Northern Hemisphere solely influenced by global fallout (i.e. from 2700·10⁶ to 7500·10⁶ at g^-1). ²³⁶U/²³⁸U atom ratios measured are at least three orders of magnitude above the estimated level for the naturally occurring dissolved uranium. The obtained inventories are 1·10¹² at m^-2 for ²³⁶U, 80 Bq m^-2 for ¹³⁷Cs, 45 Bq m^-2 for ^239+240Pu and 2.6·10¹² at m^-2 for ¹²⁹I. Atomic ratios for ²³⁶U/²³⁹Pu, ¹³⁷Cs/²³⁶U and ¹²⁹I/²³⁶U, obtained from the inventories are 0.036, 0.11 and 2.5 respectively. Concentration profiles show mobilization probably due to bioturbation from the abundant detritivore holothurian species living at the PAP site sea-floor. The range of ²³⁶U, ¹³⁷Cs, ^239+240Pu and ¹²⁹I values, inventories and ratios of these anthropogenic radionuclides are more similar to the values due to fall-out than values from a contribution from the Nuclear Fuel Reprocessing Plants dispersed to the south-west of the North Atlantic Ocean. However, signs of an additional source are detected and might be associated to the nuclear wastes dumped on the Eastern North Atlantic Ocean.