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updated 4/5/2011 1:18:34 PM ET 2011-04-05T17:18:34

As Japan battles to regain control of the nuclear power plants crippled by Friday's massive earthquake and killer tsunami, experts say as bad as the situation is, it's no Chernobyl.

The April 26, 1986, accident at the Chernobyl Nuclear Power Plant in what is now Ukraine remains the worst nuclear accident in history and the only one to be classified as a Level 7 on the International Nuclear Event Scale, which runs from Level 0 to 7. The crisis in Japan is classified as a Level 4, indicating a reactor accident with local consequences.

Even without nuclear reactors operating, it takes about 10 days for heat from the decay of radioactive materials used to produce power to taper off to safe levels, said Dwight Williams, a researcher with the Massachusetts Institute of Technology's nuclear and science engineering department.

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Japan's nuclear power plants shut down as designed when the 8.9-magnitude earthquake, one of the largest ever in recorded history, rocked the country's east coast. Diesel-fueled generators kept cooling water circulating around the containers holding the radioactive fuels for about an hour. When the generators ran out of gas, battery-powered backups kicked in.

Eventually, plant operators resorted to pumping seawater into three of the reactors in an attempt to keep heat to manageable levels.

"Normally, you don't pump seawater into a reactor. Water in a reactor is really, really purified and the seawater is not what you want to have going in there. The cost of cleaning out the seawater is not economic. So if you see seawater going into a reactor, that means at some point this reactor is going to be put out of service prior to its design life," Williams told Discovery News.

"They made a decision: 'Hey look, we're not going to worry about the economics of this. We're just trying to get this thing under control,'" he said.

Events such as the two explosions at the beleaguered Fukushima nuclear power plant, located 160 miles northeast of Tokyo, show that the situation remains dynamic. The explosions were due to the buildup of hydrogen gas and were not expected to leak significant amounts of radioactive materials.

"They are making it through this unstable, transient situation," Williams said. "We're less than a week into this, so there's considerable time to go before things just die down on their own. But hopefully with the seawater and with them being able to get additional battery power then they can at least stop the nuclear reactions from generating the heat uncontrollably."

On Monday, the Nuclear Regulatory Commission said it had dispatched two boiling-water reactor experts to Japan as part of a team of U.S. experts. The commission also said it expects no levels of harmful radiation to reach the United States from Japan's damaged nuclear power plants.

The situation in Chernobyl quickly spiraled into disaster because the reactors didn't shut down, leading to explosions that ripped apart the reactor and spread radioactive contaminants throughout the Ukraine, Russia and Europe.

"Chernobyl was as severe as you can get," Williams said. "This is, in my mind, a very local situation. But until we're able to get to the point of a stable equilibrium, there is always the risk that things could spiral out of control and we end up with the bad situation that we had at Chernobyl. I would say that at this point that's not only unlikely, it's highly unlikely."

Yukiya Amano, head of the International Atomic Energy Agency, said that the reactor vessels of nuclear power plants impacted by the disaster are intact and, so far, the amount of radiation that had been released was limited.

"The Japanese authorities are working as hard as they can, under extremely difficult circumstances, to stabilize the nuclear power plants and ensure safety," Amano said in a statement.

© 2012 Discovery Channel

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