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

March 14, 2011 -- Among the casualties of Japan's earthquake/tsunami disaster were several nuclear power plants. Damage and failed cooling systems have raised the fears of a meltdown happening.

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.

But the nuclear materials inside the reactors remain radioactive -- which means they give off a great deal of heat. It takes days to cool these materials 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.

When the tsunami hit Japan's Fukushima Daiichi nuclear plant, three 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.

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.

At least two of the three buildings housing the Fukushima reactors had hydrogen explosions. One, on March 14, injured 11 workers and could be felt for miles. However, the internal containment walls appear to remain intact.

How much radiation has escaped Japan's damaged nuclear plants?

One of the emergency measures being taken to prevent a meltdown is the release of steam from the reactors. This means some radiation is released into the environment.

Japan's nuclear energy agency reports that measures of radiation outside the plant are higher than legal limits, but that they were not dangerously high as of March 14.

Nevertheless, officials have evacuated all residents living within 20 kilometers (about 12.5 miles) of the plant.

If the Japanese nuclear reactors melt down, how dangerous would it be?

Some of the workers trying to prevent a meltdown have already suffered radiation sickness and injury from explosions related to hydrogen buildup outside the reactor core.

"The scariest outcome from such releases of radiation are the immediate effects, which are going to be only felt by personnel who have to go into the building and shut it down," radiation biologist Jacqueline P. Williams, PhD, tells WebMD. Williams is a researcher in the department of radiation oncology at the University of Rochester, N.Y.

The long-term effects of radiation exposure, Williams says, are the various cancers that can occur.

The most cancers most commonly associated with radiation are leukemia and cancers of the thyroid, lung, and breast.

The worst nuclear disaster ever was in 1986 at the Chernobyl nuclear plant in northern Ukraine. This wasn't the same kind of disaster as in Japan, as it was caused by a series of human and mechanical failures. The result was a series of explosions that shot a plume of radioactive materials into the air.

Fallout from Chernobyl fell heavily on Belarus, Russia, Ukraine, Finland, Norway, Sweden, Austria, and Bulgaria.

"The Chernobyl cloud technically covered a huge area. It did follow the jet stream and everything else," Williams says. "But radioactive particles have substance, they have weight. The bigger the particle, the quicker it falls out of the cloud. So the contamination area where you have risks from contamination are relatively close to the disaster site. As far as I am aware, the cancers from Chernobyl radiation occurred in and around Chernobyl itself."

However, radioactive iodine from the Chernobyl cloud fell on fields where it was absorbed by grass, eaten by cows, and drunk as milk by children. Until 1998, there was a significant increase in thyroid cancer among children in the affected areas.

A meltdown in Japan would be devastating to the local environment. Should there be a release of radiation, and should winds blow in the wrong direction, residents of Japan would be affected to some degree. But the effects almost certainly will not go far beyond the borders of Japan.

Could radiation from Japan's nuclear plants affect the U.S.?

No radiation from the Japan disaster is expected to hit the U.S., say experts from the Oregon State University department of radiation health physics.

"Any radioactive contaminants released will end up raining out of the atmosphere into the Pacific Ocean, where they will be diluted and absorbed, or in the very near vicinity of the plants," Kathryn Higley, PhD, says in a news release. "This is not Chernobyl."