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A damaged nuclear power plant can release a range of radioactive materials: some relatively harmless, others more dangerous.
The process that splits uranium using high heat creates more than 100 new chemicals, said Joseph Mangano, executive director of nonprofit Radiation and Public Health Project, which researches the effects of radiation on public health.
In a total meltdown, several radioactive gases are released on the less-toxic end of the spectrum, including nitrogen-16, tritium and krypton. Each of these gases is light and tends to dissipate quickly, posing little danger to humans, said Kirby Kemper, a nuclear physicist at Florida State University.
Nitrogen-16 decays quickly and becomes stable oxygen. Krypton gas also is very light and would get released into the atmosphere and dissipate quickly.
Tritium gas is low-energy and has a half-life of 12 years. When it decays, it becomes a stable form of helium. Half-life is the time it takes one-half of the atoms of a radioactive material to disintegrate.
The most dangerous emissions in a meltdown enter the body through inhalation, ingestion or absorption through the skin. If the core of a nuclear reactor melts down completely, many of the extremely dangerous radioactive substances -- such as uranium and other heavy metals -- would drop to the bottom of the containment vessel and not be spewed into the air. They would have to be cleaned up by nuclear-hazard crews.
Far more worrying are several radioactive chemicals released as particles about one-quarter the size of a grain of sand. These include iodine-131, strontium-90 and cesium-137. Some of these chemicals are especially dangerous to humans because they mimic substances the body naturally uses, such as iodine, and are readily incorporated into the body's tissues.
According to the U.S. Environmental Protection Agency, radioactive iodine can disperse rapidly in air and water under the right conditions. However, iodine-131 has a short half-life of eight days, which means it will decay away completely in the environment in a matter of months.
Radioactive iodine can enter the body by ingestion or inhalation. It can settle on grass where cows might eat it and pass it to humans through their milk. It may also settle on leafy vegetables or become concentrated in marine and freshwater fish, which people will then consume, the EPA says.
Strontium-90 has a half-life of 29.1 years. Because it behaves chemically like calcium, it tends to concentrate in the bones and teeth.
Strontium-90 mainly enters the body through food and water, and its ingestion has been linked to bone cancer, cancer of the soft tissue near the bone, and leukemia.
Cesium-137, with a half-life of 30 years, is another dangerous substance emitted in a full-scale nuclear meltdown. It may be ingested with food and water, or inhaled as dust. Exposure increases the risk of cancer.
Despite these dangers, the specter of a scenario as bad as Chernobyl is unlikely in Japan, Dr. Kemper said.
The nuclear core in many newer reactors, including the ones in Japan, is enclosed by a steel containment vessel.
Gautam Naik / Avery Johnson
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