Radionuclides in the marine environment - uses and concerns.
Radioactive contamination of the marine environment is again in the news, predominantly because of recently-reported dumping of radioactive waste, ship reactors, and submarines by the former Soviet Union. This article attempts to place these activities in the broader context of radiological protection and the application of radionuclides (radioactive isotopes) in marine science. It also serves to illustrate the range of radionuclide research and monitoring being conducted at the Bedford Institute of Oceanography.
Radionuclides in the marine environment are derived from both natural and anthropogenic sources. The naturally-derived nuclides include uranium and thorium decay series nuclides that, because of their long half-lives, are found ubiquitously throughout the environment and cosmogenic radionuclides, such as carbon-14 and tritium (3H), which are produced in the upper atmosphere.
The largest anthropogenic contributions of radionuclides into the marine environment occurred during the 1950s and 1960s as a result of atmospheric nuclear weapons tests. Radioactive fallout included activation products such as plutonium-239 and the longer-lived fission-products caesium-137 and strontium-90. A major input of plutonium-238 has also occurred as a result of the burn-up of a satellite radiothermal generator, SNAP 9A, in 1964.
On a local scale, the spillage of plutonium from four nuclear weapons following an American B-52 bomber crash near Thule, Greenland, in 1968 was also a significant anthropogenic source.
The other major influxes of artificial radionuclides to the ocean have been the result of discharges from nuclear fuel reprocessing plants such as those at
Sellafield in the United Kingdom and at Cap de la Hague in France. Cooling water streams from nuclear generating stations, which generally contain relatively low levels of radionuclides, constitute insignificant sources on a large scale.
Although the ocean has been used for the disposal of solidified radioactive waste, most notably in the eastern North Atlantic under the auspices of the Nuclear Energy Agency, the amounts of activity associated with such deliberate disposals are again relatively small.
The primary purpose of regulatory attention to radionuclides in the environment is the protection of human health. Some natural nuclides, such as polonium-210, can constitute a significant proportion of the natural radiation dose |1~ in seafood, but do not generally represent risks as large as those associated with extreme exposures to natural radon in the air. The greatest concerns tend to focus on human exposure to artificial radionuclides, such as plutonium, 90Sr and 137Cs, released into the marine environment from accidents, dumping or industrial sources. Only relatively recently has there developed a concern for the effects of radiation exposure on organisms other than humans.
Canada has been something of a leader in the field of environmental concerns. In 1976, an environmental monitoring program was established for the first coastally-sited nuclear generating station in Canada at Point Lepreau, NB.
The Atlantic Environmental Radiation Unit (AERU) was established in 1977 as a section within the Marine Chemistry Division at the Bedford Institute of Oceanography to conduct radionuclide research and to implement the Point Lepreau Environmental Monitoring Program (PLEMP) pursuant to an interdepartmental agreement on its objectives and purpose. AERU now conducts a wide range of research and monitoring focusing on the presence and behavior of radionuclides in the marine environment. In addition, it conducts research on geochemical, geochronology and biological applications of radionuclides.
PLEMP is one of the most comprehensive off-site monitoring programs for nuclear power generating stations in the world. It has now been in operation for 16 years. It has demonstrated that not only are radionuclide releases (excepting tritium) from CANDU reactors minor, but also a capacity to detect anomalous releases of radionuclides from both the generating station and other sources. For example, the PLEMP atmospheric monitoring network is sufficiently sensitive to have detected fallout from the Chinese nuclear weapons tests in 1980.
Furthermore, approximately 10 days after the Chernobyl accident in 1986, Iodine-131 was detected both in particulate and gaseous forms at air monitoring stations in New Brunswick and Nova Scotia. From the ratios of cesium and ruthenium isotopes it was inferred that the initial impingement of Chernobyl radioactivity on Eastern Canada came from an airborne plume travelling across the Arctic Ocean.
Sea dumping of radioactive waste has taken place since soon after the second world war. It has been carried out by a number of countries including Belgium, the Netherlands, Switzerland, the United Kingdom and the United States. Since 1975, this practice has been regulated under the provisions of the London Convention, 1972: the International Atomic Energy Agency (IAEA), as the competent international agency for radioactive matters under this Convention, periodically defines high-level radioactive waste, for which dumping is proscribed under the London Convention, and issues recommendations regarding the procedures for sea dumping of low-level radioactive waste, most recently in 1986 |2~. In these, and a number of ancillary IAEA activities, Canadian scientists have been involved.
Canada has also played an important role in the assessment of risks associated with sea disposal of solid radioactive waste in the Northeast Atlantic through participation in the Coordinated Research and Environmental Surveillance Program (CRESP) of the Nuclear Energy Agency (NEA). The NEA has also carried out comprehensive safety assessments, most recently in 1985 |3~, in connection with the re-evaluation of the suitability of the Northeast Atlantic Dumpsite used by European countries under the NEA.
Applications of radionuclides
Naturally-occurring radionuclides have been extensively used as tracers in oceanography to determine the nature and rates of a wide range of processes. Initial tracer applications included the use of tritium (as tritiated water) for water circulation and mixing and 14C for the marine carbon cycle. More recent work has focused on the use of particle-reactive radionuclides, such as 234Th, as tracers for the bio-geochemical cycling of metals through the oceans. Lead-210 has been widely used for determining sedimentation rates and vertical mixing or bioturbation rates in marine and lacustrine sediments. Methods have been recently developed in AERU for determining the age of fish through measurements of the isotope pair, 210Pb/226Ra, in the fish otolith or ear bone and for the ageing of crabs and other crustaceans through measurements of 228Th/228Ra in their carapaces. Artificial radionuclides such as 239,240Pu have also been extensively used to establish dating horizons in sediments while 137Cs has been applied to examining vertical advection and mixing rates in surface and intermediate waters of the ocean basins.
Relatively recently, it has come to light that some dumping of radioactive waste in the Barents and Kara seas was carried out by the former Soviet Union. Dumped material comprised liquid and solid radioactive wastes including nuclear reactor assemblies and nuclear-powered submarines some of which still contain their nuclear fuel. According to the recommendations of the IAEA, sea dumping of radioactive waste carried out under the provisions of the London Convention, 1972, is restricted to the deep ocean in areas of depth greater than 4000 metres and between latitudes 50 |degrees~ N and 50 |degrees~ S.
The alleged dumping in northern Russian waters is at shallow depths (20-100 metres) and relatively close to the shores of the island of Novaya Zemlya. This makes the dumping of greater a priori health and environmental concern than that at the Northeast Atlantic dumpsite for which radiological risks have been assessed in considerable detail |3~.
The IAEA is planning to conduct a detailed assessment of the nature, release rates and effects of radionuclides dumped in the Kara and Barents seas and any other areas in which radioactive waste has been dumped by the former Soviet Union. Other organizations are expressing similar concerns about the contamination of land areas by the military weapons industry, the resultant risks to human health, and the potential mobilization of nuclides through river systems to the marine environment.
AERU has long been active in radioactivity investigations in the Arctic Ocean. These studies have included measurements of the transport of 137Cs from the Sellafield reprocessing plant into the Arctic Ocean through Fram Strait and the vertical cycling of 210Pb, 210Po and 226Ra in the Central Arctic Ocean.
In 1984, a joint Canadian-Danish investigation was undertaken of the dispersion of plutonium spilled from nuclear weapons at Thule, Greenland. Minimal dispersion of plutonium has occurred on a radial axis directed away from the B-52 crash site. Significant burial of plutonium has occurred as a result of sedimentation and bioturbation or mixing. The plutonium released from the nuclear weapons is in the extremely refractory oxide form which these studies have shown to be relatively immobile in sedimentary regimes in Thule harbor. These results are very relevant to investigations of the radioactive waste dumping on the Russian Continental Shelves.
Most recently, AERU has been conducting studies of marine sediments in the Barents Sea through collaborative investigations with Norwegian and Russian scientists. The most interesting of these samples were collected off the western coast of Novaya Zemlya, the site of much of the atmospheric and underground nuclear weapons testing previously conducted by the USSR. These samples are presently being analyzed in the AERU laboratory for fission and activation products to determine whether any contamination from dumped material can be detected in nearby sediments.
1. Individual and collective dose rates from naturally-occurring radionuclides in seafood, In Radiation Protection - and Practice, Proceedings of the Fourth International Symposium, Malvern, U.K., June 1989, HMSO, 1989.
2. Definition and Recommendations for the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter, 1972; 1986 Edition, Safety Series No. 78, International Atomic Energy Agency, Vienna, 1986.
3. Review of the continued suitability of the dumping site for radioactive waste in the North-East Atlantic, Nuclear Energy Agency, Organization for Economic Cooperation and Development, Paris, 1985.
J.N. Smith and J.M. Bewers are with the Marine Chemistry Division, Physical and Chemical Sciences, Department of Fisheries and Oceans, Bedford Institute of Oceanography, Dartmouth, NS.
|Printer friendly Cite/link Email Feedback|
|Author:||Smith, J.N.; Bewers, J.M.|
|Publication:||Canadian Chemical News|
|Date:||Oct 1, 1993|
|Previous Article:||Reasons for scepticism about greenhouse warming.|
|Next Article:||The Titanic: from metals to minerals.|