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...
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.
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.