Radioactive materials may enter the sea from two main sources: from weapon testing via atmospheric fallout and from atomic power industries. The main contaminants are strontium-90, caesium-137 and plutonium-239. In the UK, a major source of radiation pollution has been via the discharge of cooling water from Sellafield Nuclear Power Station. The level of discharge has been considerably reduced in recent years but caesium-137 (which does not occur naturally) remains in sufficient quantities for it to be used as a tracer for ocean currents around Scotland and into the North Sea.
Some highly dangerous radioactive wastes are also disposed of by dumping into the deep oceans in sealed containers. The containers will eventually corrode and the fate of the residues is uncertain. Although there are few links between the food chains of the deep ocean and the shallow waters used for commercial fishing, it is known that some deep-water currents surface in the Antarctic. There are hopes that incorporation of radioactive wastes into solid glasses from which nothing can leach or leak may prove a safe method of disposal.
The fallout from the explosion of the Chernobyl nuclear reactor in the Soviet Union in 1986 greatly increased the radioactive load of the world's oceans. There is currently great concern that some of the atolls in the South Pacific used for underground testing of atomic devices may collapse and release huge quantities of radioactivity.
Seaweeds can concentrate radioiodine with great rapidity and fish absorb a variety of radioactive substances. In addition radioactive substances can bioaccumulate in marine animals in a similar way to heavy metals. The effects on marine organisms are not fully understood but may include genetic disturbances and increased mortality both in young stages and in adults. Interestingly, many marine invertebrates can withstand radiation doses that would kill people. Some deep-water marine shrimps, exposed to doses of natural radiation sufficient to debilitate people, remain unharmed. A variety of cancers in humans, such as childhood leukaemia, is linked to radiation exposure.
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