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

FAQ on Radiation Risk From Tsunami-Damaged Nuclear Plants

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

Three of the six buildings housing Fukushima Daiichi's six 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.  The third blast at unit 2 may have caused a small breach in the inner containment vessel, as radioactivity around the plant shot up to dangerous levels before going down.

All this water being pumped through the reactors becomes radioactive. Some of it is extremely radioactive, and has badly injured two workers. As of March 28, there were fears that the accumulating water would escape tunnels below the facility and pour into the ocean. While this would be an ecological disaster for the local area, the radiation would rapidly be diluted by the Pacific Ocean. However, seafood from the area would be unsafe to eat.

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 with each release of the high-pressure steam.

Japan's nuclear energy agency reported on March 14 that measures of radiation outside the plant are higher than legal limits, but that they were not dangerously high. That changed on the morning of March 15, when radiation levels shot to 400 millisieverts per hour -- well above the danger zone. Radiation levels then dropped to about 0.6 millisieverts per hour.

By comparison, a chest X-ray is 0.02 to 0.67 millisieverts. In a year, the typical U.S. resident is exposed to 3 millisieverts. A person who receives a short-term dose of 1,000 millisieverts will experience radiation sickness but probably will survive. Short-term doses of 2,000 to 10,000 millisieverts have an increasing probability of causing a fatal cancer.

Radiation levels in Tokyo are reported to be 20 times above normal but still not in the danger zone. However, a no-fly zone has been imposed for an 18-mile radius around the nuclear plant.

On March 14, officials ordered the evacuation of all residents living within 20 kilometers (about 12.5 miles) of the plant. On March 15 they advised people living between 20 kilometers and 30 kilometers (about 18.6 miles) to remain indoors. News reports indicate many of these residents are fleeing the area instead. Some 100,000 people are reported to be in the area.

Another radiation issue is in reactor 4, which had been shut down before the earthquake. Spent fuel rods still were cooling in the plant's rooftop pool.  That cooling system appears to be in trouble, as a fire was reported in the plant. Should these fuel rods become exposed, a large amount of radiation will be released. As of March 18, plant officials had used helicopter drops and water cannons to try refill the pool, which may be leaking.

For now, the chief radiation danger is to plant workers desperately trying to mitigate the disaster. Only a skeleton crew remains on site, threatened by radiation, fire, and hydrogen explosions.

One of the reactors, reactor 3, is said to be running on mixed-oxide (MOX) fuel. MOX fuel contains plutonium in far greater amounts than regular nuclear fuel and would be more toxic if released into the environment.


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