Earthquake in Japan: What's in the radioactive vapors?

Answers to your questions about the news.
March 12 2011 1:17 PM

Me, Myself, and Iodine

What's in the radioactive vapors leaking from the damaged Japanese nuclear power plant, and how dangerous is it?

A South Korean passenger watches TV showing Japan's Fukushima No. 1 atomic plant spewing fumes. Click image to expand.

Japanese officials evacuated residents within a 12-mile radius of a damaged nuclear power plant Saturday after an explosion blew the roof off a building at the site. While the danger of a total meltdown at the reactor appears to be very low, radioactive materials have been detected leaking into the atmosphere along with steam from the plant. What exactly is in this vapor?

All sorts of radioactive particles, a few of which can be deadly in large quantities. These materials are known as "radioisotopes"—versions of an element that, because they have an abnormal number of neutrons, frequently decay and release radiation. They occur as a result of nuclear fission, when a uranium atom splits into lighter elements and releases energy. Among the most common ingredients in the water steam leaking from the Japanese reactor are iodine-131, cesium-137, xenon-133, xenon-135, and krypton-85. (Often elements have many different varieties of isotope, some more stable than others. The number after the element name identifies the specific isotope.)

Of these, the most troubling is iodine-131, which can be absorbed by the thyroid when inhaled, causing thyroid cancer  and leukemia. Gases like krypton-85 and xenon-133 don't interact with bones or tissue, but since they are highly unstable, they decay in bursts of radiation that can prove harmful to other bodily systems. But the body tolerates a certain amount of radiation every day, from cosmic rays to watching TV, and it's only in much larger quantities that the byproducts of a nuclear power plant become dangerous. While radiation spiked to 1,000 times normal levels in one reactor control room, Japanese officials insist that exposure levels outside the plant are not highly hazardous. Even so, area residents have been advised to drink bottled water, stay indoors, and hold washcloths over their noses and mouths. As a precaution against iodine-131, officials have also announced plans to distribute potassium iodide pills, which saturate the thyroid with a stable form of iodine before the more dangerous isotope can be absorbed. They only work, however, if swallowed pre-emptively.

Explainer thanks Dr. Dave Lochbaum, Director of the Nuclear Safety Project, and Neil Sheehan of the U.S. Nuclear Regulatory Commission.

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AP video of the Japanese nuclear plant explosion.

Katy Waldman is a Slate staff writer.