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Nuclear Meltdown in Japan: What's the Risk of Radiation?

FAQ on Radiation Risk From Tsunami-Damaged Nuclear Plants
WebMD Health News
Reviewed by Laura J. Martin, MD

March 18, 2011 -- Experts now say it will be weeks until the emergency situation at the earthquake/tsunami-damaged nuclear power plants in Japan are brought under control.

Damage and failed cooling systems have raised the fears of a total meltdown in at least one of the six nuclear reactors at the Fukushima Daiichi facility. At least one reactor appears to have suffered a partial meltdown. Moreover, a protective pool in which spent nuclear fuel is stored has been leaking water, resulting in the release of radioactivity.

There's hope that a major nuclear disaster can be averted. But what are the risks from the radiation that already has been released -- and from the radiation that could be released if containment efforts fail?

Here's WebMD's FAQ.

What does it mean for a nuclear energy plant to melt down?

"Meltdown" is not a technical term, but it vividly describes the worst-case scenario for a nuclear reactor.

Nuclear reactors generate power via controlled nuclear fission, which occurs when enough radioactive material is gathered into a critical mass. Control rods can separate the radioactive material, thus ending the nuclear reaction.

That's exactly what happened when the March 11 earthquake hit Japan. Control rods effectively halted the nuclear reactions.

This means there is no danger of a nuclear blast from Japan's damaged nuclear plants, even if fuel rods melt inside one of the containment vessels and pool into critical mass. 

But the nuclear materials inside the reactors remain radioactive, which means they give off a great deal of heat. It takes a long time to cool these materials down, and spent fuel rods must be kept submerged in a cooling bath until their radioactivity decays and their intense temperature goes down.

If not cooled, these materials will melt. In a worst-case scenario, they could possibly melt right through the thick metal shield that contains the reactor, spilling highly radioactive materials into the environment. That could happen if the melted fuel pools to critical mass and restarts a fission reaction. This would not mean a nuclear explosion, but it would generate a great deal more radioactivity and heat.

When the tsunami hit Japan's Fukushima Daiichi nuclear plant, three active reactors lost power. That meant serious trouble, as the pumps needed to cool the overheated reactors with water stopped running when their battery backups were exhausted. While power has been restored, workers are still fighting to keep the fuel rods submerged.

Nuclear reactors have two main containers that keep radioactive materials from spreading outside the plant. One container is the thick-walled building surrounding each reactor. The other is a thick metal vessel that makes up the outer wall of the reactor itself.

As water cools the fuel rods in the innermost container, steam is created. The intense heat also releases hydrogen from the water. If the pressure inside the container gets too high, the steam has to be vented. This releases some radioactivity. It also releases hydrogen, which can build up inside the building and explode.

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