NEW YORK - The head of the U.N.'s Atomic Energy Agency said Monday the Japanese nuclear emergency is not yet as severe as the Three Mile Island accident in 1979. To understand what went wrong at Fukushima, and what dangers still exist there, you need to know how a reactor works - and what didn't work on Friday.
Inside a massive concrete containment dome is a reactor full of radioactive uranium - heating up in rods through a process called fission.
"It would be like if you had a tough guy go in a room with a bunch of other tough guys and they start hitting and beating on each other and it just spreads," Univ. of Georgia professor Cham Dallas told CBS Evening News anchor Katie Couric. "And then you have a riot."
The thing that keeps the riot from leaving the rods is water - which circulates, heats up and produces steam to power a turbine - making energy.
As soon as the earthquake tremors began, Japanese energy officials shut down 11 reactors by interrupting the fission process.
Dallas likened the process to shutting off a stove. "If you go to your stove and you want to turn your stove off, you turn the knob, you turn off the stove. The way you turn off a nuclear reactor is somebody turns a knob and they put control rods next to the uranium rods."
And water must continue to flow around the rods to cool them down. At three reactors though, power failures kept that from happening.
"Your backup system should have kicked in. They have all these generators - didn't happen," Dallas said. "Apparently they failed and so they had to start putting sea water in. That's like the Hail Mary pass - where you're just really desperate now."
Even with the two Fukushima explosions, so far this is nothing like Chernobyl. In 1986, the control rods malfunctioned and the fuel rods melted down. A subsequent explosion catapulted tons of radioactive material into the atmosphere.
"One hundred times as much radioactivity as Hiroshima and Nagasaki atomic bombs combined went up into the air at Chernobyl," Dallas said.
Twenty-five years later, a dead zone with a 16 mile radius still surrounds Chernobyl. At least 11,000 children have developed thyroid cancer there.
Unlike Chernobyl, the explosions at Fukushima have come from the containment dome - not the radioactive core.
"The containment vessel only had a little bit of radioactivity in it," Dallas said. "But it's still nothing next to what you would get, or you would have to get in order to get sick."
There are 104 nuclear reactors here in the United States. Could this happen here?
"I would say based upon our preparedness and on expertise and equipment, personnel, no, it won't happen here," he said. "But when you take a look at the simple things going wrong, over there in Japan, like a generator - that's disturbing."