Radionuclides in the oceans

In the different chapters of this book, radionuclides are studied in geographical areas of different size, and varying physical and biological features. Therefore, sorne presentations deal with oceans taken as a whole (Atlantic, Pacifie and Indian oceans). Sorne sources of radionuclides, such as atmospheric fallout, both before and after Chernobyl, have a large scale impact, whereas others, such as marine dumping sites

Others presentations focus on of particular interest seas from the point of view of specifie radionuclide sources and the particular processes which govern radionuclide behaviour. The Channel, the North Sea and the Irish Sea are quite shallow and receive, directly or indirectly a small fraction of the wastes released by reprocessing plant wastes. The Baltic Sea has been strongly affected by Chernobyl, and is subject to a high salinity pressure gradient. The Black Sea has also been marked by Chernobyl and is strongly anoxie. The Mediterranean Sea receives an input of radionuclides from the Rhone River outflow into the western basin and has exchanges with the Atlantic Ocean. Dumped radioactive wastes, radionuclide inputs from the Ob and Yenisei river basins, radionuclides from European reprocessing plants transported by ocean currents, and radionuclide transport by ice must be considered in the Arctic, Barents, Kara and Laptev Seas.

The knowledge of the distribution of radionuclides in the ocean is gradually being updated through a continuous flow of data, ranging from the periodical monitoring records issued by national authorities to the results of local and ocean scale scientific cruises, carried out within the framework of large international programs. This paper tries to summarize and to update the state of our knowledge regarding the Atlantic Ocean, by scaling sorne prominent features. We consider successively the ocean scale distribution of radionuclides from bomb testing fallout, compared with the natural back­ ground, the regional inputs from land-based sources and from the adjacent seas. Finally, the controlled or uncontrolled dumping practice and the accidents like the Chemobyl case are also taken into account to estimate the collective doses to the world population.

Anthropogenic radionuclides have been introduced to the Atlantic Ocean from a variety of sources since the advent of the nuclear age. The sources include fallout from atmospheric nuclear weapon's tests, nuclear waste dumping/ disposai activities and accidents, in particular, the Chemobyl accident in April1986. The temporal and spatial nature on these inputs has affected greatly the past, present and future distributions of the various radionuclides involved. The natural radionuclides resulting from the non­nuclear industry (e.g., the fertilisers plants) should also be considered in comparison with those of primordial or of cosmogenic origin. However, the natural radionuclides are ubiquitous in the ocean in significant concentrations, and the anthropogenic contribution to the natural stock is not expected to be comparatively relevant.

a) All ocean inputs: this applies chiefly to fallout from atmospheric nuclear weapon's tests which took place in the late 50's and early 60's in both the North and South Atlantic. About 3/4 of the Atlantic input was to the North Atlantic and 1/4 to the South Atlantic. In each hemisphere, the delivery was maximal at mid-latitudes and minimal at the equator and at the poles. The input of 238Pu from the burn-up of the U.S. SNAP 9A satellite over the Southern Hemisphere in 1964 was also widely introduced to the Atlantic but, in this case, the latitudinal distribution was reversed, with the large fraction of the input going into the South Atlantic.

c) Dumping and accidents: these include the planned dumping of packaged low level nuclear waste at a variety of sites and by severa! countries, the accidentallosses in deep water of nuclear powered submarines, as well as the disposai of reactors with their fuel in the shallow water of the Northem Seas by the Former Soviet Union (FSU). In ali these cases, any releases which have occurred have lead to extremely localized contamination to date.

For each of these categories, the type of radionuclides released, their isotopie ratios and the ratio between species can be used to determine their origin. The ratios also provide new opportunities to gain an understanding of processes as the large scale transport associated with the global ocean circulation, the vertical mixing, the biological accumulation and the short-circuit mechanisms. The wide spectrum of half-lives of radionuclides allows to use them for a variety of scientific applications, from the local study of the sediment and the processes in the seabed to the changes in the ocean scale cirçulation over decades. Sorne of these issues have been addressed in the pro­ ceedings of a previous symposium on the use of radionuclides in the study of marine processes (Kershaw et al., 1991). The aim of the present paper is to review and update the actuallevel of knowledge by putting emphasis on sorne prominent features associated with the three categories of inputs indicated above.