|Fig. 1: Distribution of sea dumping sites used for disposal of low- level radioactive waste. (Source: A. Jones, after Calmet. )|
Before 1993 there were no international laws preventing countries from dumping nuclear waste in the ocean. In the period from 1946 to 1993, thirteen countries, including the US, used ocean dumping to dispose of their nuclear waste.  We can see the distribution of these known dumping sites in Fig. 1. Throughout this period international organizations have held conventions and developed treaties to address the problem.
In 1946, "the first dumping operation took place at a site in the North East Pacific Ocean, about 80 kilometers off the coast of California."  In 1957 the International Atomic Energy Agency (IAEA) held its first Advisory Group Meeting on Radioactive Waste Disposal into the Sea. The following year, the UN held its first Conference on the Law of the Sea (UNCLOS I). These two meetings laid the groundwork for international law regarding the ocean dumping of nuclear waste. 
In 1972, many countries adopted the London Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter. The convention entered into force in 1975, prohibiting the dumping of high level radioactive waste.  High level radioactive waste are the highly radioactive byproducts of nuclear reactions. As of 1986, the formal definition of high level waste is as follows:
Irradiated reactor fuel or the liquid and solid wastes from the reprocessing of irradiated reactor fuel
Any other waste matter of activity concentration exceeding:
5 ×10-5 TBq/kg for alpha emitters
2 ×10-2 TBq/kg for beta/gamma emitters with half-lives greater than one year (excluding tritium)
3 TBq/kg for tritium and beta/gamma emitters with half-lives of one year or less 
Note that under this convention the dumping of low level waste is still allowed, but only after a special permit has been issued. 
In 1983 the decision was made to adopt a moratorium on low level waste dumping. In 1985, the Contracting Parties to the London Convention introduced this voluntary moratorium.  In 1993 the Contracting Parties to the Convention adopted a resolution which prohibited the sea disposal of radioactive wastes and matter.  This resolution came into force Feb. 20, 1994, totally prohibiting radioactive waste disposal at sea.
Several expeditions and surveys have been conducted to examine old ocean dumping sites.
The Northeast Atlantic dumping site was last surveyed in 1996. They analyzed samples collected above the sea bed for anthropogenic radionuclides (radioactive nuclides resulting from human activity). The analysis showed measurable but negligible enhancement of radioactivity at this site. 
Radiological surveys of the Northeast Pacific and Northwest Atlantic Ocean sites are conducted from time to time by the Environmental Protection Agency and the National Oceanic and Atmospheric Administration. Samples of sea water, sediments, and deep sea organisms collected at these sites have not shown excess levels of radionuclides, except for some samples of sediment taken close to the disposed containers. 
From 1992 to 1994, joint Russian and Norwegian expeditions visited four dumping sites in the Kara Sea. Samples were taken of water, sediment, and organisms. Again, radioactive contamination was negligible except for sediment samples taken close to the waste containers.
Overall the surveys indicate that dumping had negligible but measurable effects on radioactivity at old dumping sites. It is good that we curbed ocean dumping before it became a major problem. Although the findings of the surveys indicate that it would take a lot of dumping to make the ocean unsafe, one can only imagine what a catastrophe it would be if we did make the ocean radioactive.
© Aaron Jones. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 "Inventory of Radioactive Waste Disposals at Sea," International Atomic Energy Agency, TECDOC-1105, August 1999.
 D. P. Calmet, "Ocean Disposal of Radioactive Waste: Status Report," International Atomic Energy Agency, IAEA Bull. 31, No. 4, 47 (April 1989).