Transuranium radionuclide pollution in the waters of the La Maddalena National Marine Park

摘要

Following the grounding and subsequent explosion, in October 2003, of a nuclear submarine in the waters of the La Maddalena National Marine Park, fears arose of possible radioactive leakages. However, isotopic analyses on algae showed that the gamma-ray emitting artificial radionuclides that one might expect to leak from a damaged nuclear reactor (such as U-235, I-131, Cs-137) were absent, and that U-238/U-234 activities were in equilibrium with values typical of sea water; this excluded any direct anthropogenic contamination as a result of the accident. We used alpha autoradiographic techniques to detect possible traces of transuranium radionuclides; 160 samples of algae, granites, sea urchins, gastropods, limpets, cuttlefish and jellyfish were collected from the area, as well as from other Mediterranean coastlines and the Baltic Sea. All samples were autoradiographed, and selected samples further analysed by alpha spectrometry. There were no alpha track concentrations above background levels in our control Mediterranean specimens. In the samples from the La Maddalena and Baltic areas two different track distributions were observed: 1. those homogeneously distributed over the surfaces examined; 2. groups (10 to over 500) of radially distributed alpha tracks (forming "star" bursts, or "hot spots") emanating from point sources. By comparing radionuclide activities measured by alpha spectroscopy with alpha track densities, we extrapolated Pu activities for all samples. About 74% of algae had Pu activities of less than 1 Bq/kg and 0.25 Bq/kg, 16% had accumulated Pu to levels between 1 and 2 Bq/kg, and a very few specimens had concentrations between 2 and 6 Bq/kg. Plots showed that alpha tracks and stars concentrate around the northern and eastern margins of the Rada (Basin) di Santo Stefano, sites facing the nuclear submarine base on the eastern shore of the island of Santo Stefano. What is the source of these nuclides: last century's atmospheric nuclear testing, Chernobyl or a local source? Their concentrated, extremely localised occurrence seems difficult to explain in terms of left-over worldwide nuclear pollution. A local source seems more plausible.