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Radiation and the Oceans

by Woods Hole Oceanographic Institution on 9 Apr 2011
Human sources of radiation released into the atmosphere over the past 60 years Woods Hole Oceanographic Institution (WHOI) ©
On March 11, 2011 a magnitude 9.0 earthquake—one of the largest ever recorded, occurred 80 miles off the coast of Japan. The earthquake created a series of tsunamis, the largest estimated to be over 30-feet, that swept ashore along the northeast coast of the main island, Honshu. In addition to killing more than 9,000 people, the earthquake and tsunamis badly damaged the Fukushima Daiichi nuclear power plant, eventually causing four of the six reactors there to release radiation into the atmosphere and ocean.

What is being released from the Fukushima reactors and how dangerous is it?
So far, we know that releases from the Fukushima reactors have been primarily composed of two radioactive substances: iodine-131 and cesium-137. In large doses, both of these isotopes or radionuclides, as they are called, can cause long-term health problems. So far, however, only those working at the plant face the most serious exposure.

More about iodine-131 and cesium-137

Are there different types of radiation?
In general, there are two types of radiation, ionizing and non-ionizing. Non-ionizing radiation includes visible light and radio waves—things that, as the name implies, do not have the ability to form charged ions in other materials. Ionizing radiation, however, can and as a result presents a serious health threat because it can alter the atomic structure of living tissue. Ionizing radiation also comes in several different types, including alpha, beta, and gamma radiation, all with different degrees of concern and health impacts.

More about types of radiation

How long is the radiation from these substances a risk to humans and the environment?
Radioactive materials are, by their very nature, unstable and decline in strength over time. This change is measured in half-lives—the length of time it takes for the radiation to decrease by one-half. Every radioactive substance has a different half-life, ranging from fractions of a second to billions of years. Those with longer half-lives are potentially more difficult to deal with because they remain radioactive for longer periods of time. Cesium-137, for example, has a half-life of 30 years and so is a potentially serious health threat for decades or centuries. Iodine-131, on the other hand, has a half-life of just 8 days and so loses much of its potency after just days and effectively disappears after one to two months.

More about half-lives

How far can radiation travel?
Ionizing radiation itself cannot travel very far through the air. Typically, dust and other particles, seawater and other liquids, or even gases become radioactive due to exposure to radionuclides and are then transported great distances. In the months and years after the explosion at the Chernobyl nuclear power plant in Ukraine scientists were able to track the spread of radioactive material in the atmosphere and the ocean around the globe. Within a week after the explosions at the Fukushima plant, there were reports of very small increases in the continental U.S.

More about mapping and monitoring radiation from Japan

What is the normal background level of radiation?
The normal background level of radiation is different for different places on the planet. Radiation in some places is higher because these receive less of the natural protection offered by Earth’s atmosphere or because they are in places where the surrounding rocks contain more radioactive substances, such as radon. In the ocean, the largest source of radiation comes from naturally occurring substances such as potassium-40 and uranium-238, which are found at levels 1,000 to 10,000 times higher than any human sources of radiation (see illustration). The largest human release of radionuclides was the result of atmospheric nuclear weapons tests carried out by the U.S., French and British during the 1950s and 60s. Despite even the high concentration of nuclear fallout in the Pacific caused by U.S. tests on the Marshall Islands, there is no known adverse health effect associated with eating seafood from the Pacific.

More about natural background radiation

If there are warnings in Japan about eating certain products contaminated by radiation, why is it safe to eat the seafood?
Except for the vicinity of the reactors, seafood and other products taken from the sea should be safe for human consumption. Radiation levels in seafood should continue to be monitored, of course, but radiation in the ocean will very quickly become diluted and should not be a problem beyond the coast of Japan. The same is true of radiation carried by winds around the globe. However, crops and other vegetation near the reactor site (including grass that cows eat to produce milk) that receive fallout from the atmosphere build up radioactivity can remain contaminated even if washed. When these foods are consumed, a person receives much of this dose internally, often a more severe pathway to receive radiation than by external exposure.

More about radiation and food safety

How does radiation released from the Japanese reactors compare to the accident at Chernobyl?
We still don’t know exactly how much radiation was released at Fukushima or how much will ultimately be released before the reactors are fully contained. The Chernobyl accident was much more violent and resulted in a complete breach of the reactor vessel. The event also started a very hot graphite fire that released large amounts of radioactive material into the atmosphere equivalent to between 3 and 5 percent of the total reactor inventory. Winds carried the radioactive fallout first to the north and eventually into the Black Sea to the south. Radiation in the Black Sea and Baltic Sea, though elevated, remained well below EPA guidelines for radiation in drinking water.

More about the after-effects of Chernobyl

How will the radioactive material released in Japan affect humans?
It’s still too early to tell, but unless we learn that the type or amount of material released is larger than reported or changes dramatically it will likely have significant long-term impacts only within a few miles or tens of miles from the plant. This is because the further the radioactive material travels, the more dispersed (and the less harmful) it becomes. The effects of Chernobyl were felt well beyond Ukraine in part because the amount of radioactive material released was large and because it also included substances such as plutonium that have very long half-lives. That being said, people who live near the plants would be wise to follow the minimum safe distance restrictions and other precautions recommended by the Japanese government and at-risk individuals should take suggested extra precautions such as taking potassium iodide to avoid thyroid problems.

More about radiation in the environment

Is there any danger to people in other parts of the world?
Prevailing winds over from Japan blow east towards North America; ocean currents in the region also flow generally east into the North Pacific, though much slower than winds. However, radioactive materials carried by winds or currents will be quickly diluted until the radiation falls below background levels. Unless radioactivity from Fukushima finds its way directly to another part of the world through food or other commercial products, it should become sufficiently dispersed over time that it will not prove to be a serious health threat elsewhere. Over time, the radioactivity associated with the Fukushima plant should continue to decline even further. In particular, radiation from iodine-131 will decay very quickly, but even the effects of the much longer-lived cesium-137 will decline in strength. Today, people who eat seafood from the Black Sea, which received a considerable amount of fallout from Chernobyl (see map), consume a dose of cesium-137 that is 100 times below the one provided by a naturally occurring radionuclide, polonium-210, that is not considered harmful to animals or humans.

More about the environmental health effects of radiation

Why is this event of interest to oceanographers?
Oceanographers use substances called tracers to study the path and rate of ocean currents and of processes such as mixing that are important parts of the global ocean and climate systems. There are many different radionuclides that scientists use as 'clocks' to measure how fast the ocean mixes and sediment accumulates on the seafloor. Some of these substances are natural, but many are the result of human activity, such as the Chernobyl accident or nuclear weapons testing, and now releases at Fukushima.

More about radioactive tracers in the Woods Hole Oceanographic Institution

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