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And in Japan today the government raised its assessment of the crisis at the trouble nuclear power plant to the highest possible level. The rating was bumped up from 5 to 7 on an international scale used to evaluate the seriousness of nuclear incidents. The move was based on new data on the amount of radiation released in the early days of the crisis, not on any recent change in the plant's status.

The accident in Japan is now in the same category as the Chernobyl disaster in the Soviet Union. But the Japanese government emphasizes that the Fukushima plant has released only one-tenth the amount of radiation released at Chernobyl, meaning the lasting effects on human health in Japan are expected to be far less severe. NPR's Richard Harris has this report on the cleanup that lies ahead.

RICHARD HARRIS: Even though we don't know many of the details about what's happening at the reactors, you can conceptualize the fight that's being waged. Lake Barrett, who coordinated cleanup at Pennsylvania's Three Mile Island reactor back in 1979, takes it all the way back to the basic elements of ancient Greece. They had fire, air, water and earth. At Fukushima it's pretty much the same: energy, air, water and solids.

Mr. LAKE BARRETT (Nuclear engineer): So if you go back to those four basic principles, what the engineers are doing in Fukushima is first they have to deal with energy dissipation. That is the cooling of the decay products in the core and keep the core cool.

HARRIS: Over the past month, they've managed to do that pretty well, but at a price. They are cooling with copious amounts of water. That's led to continuing venting of sometimes radioactive steam. It's nowhere as bad as it was in the early days of the crisis, he says, but gaseous releases will continue to be a problem until they can shift to a better cooling system. So that's energy and gas. They also have a watery mess on their hands.

Dr. BARRETT: In the case of Three-Mile Island, we had about a little more over half a million gallons of very highly radioactive water in the basement of the containment building. It was about 10 feet deep. They're facing the same situation in Fukushima, but they have three of these cores that have severe damage to them. And so they probably have tens of millions of gallons of the same highly radioactive water that they're dealing with.

HARRIS: A huge challenge, to be sure, but Barrett says they were able to decontaminate the radioactive wastewater at Three Mile Island.

Dr. BARRETT: The very first systems we had operating within 10 days, and then we had a better system operating in a month. And then we had a better yet system operating in about a year to handle the most highly contaminated water. That water was all cleaned up at Three Mile Island and it was safely discharged.

HARRIS: Fukushima Daiichi may or may not have any functioning systems to clean up its water. And there's no telling how much more radioactive water is going to be produced during the continuing operations there. But on the plus side, Barrett says there are large tanks for storing contaminated water and more are on the way. And once energy, gas and water are under control, what's left are the solids. Those are the most radioactive materials, and even getting to them is a problem.

Three Mile Island was still intact following its accident, but the overhead cranes that usually do all the work were damaged by fire.

Dr. BARRETT: So it took us years to refurbish the cranes to get access down to the reactor itself. Then we had to lift the reactor head off.

HARRIS: In fact, it took five years after the accident until they could finally look inside the reactor. And they discovered that 30 percent of the nuclear core had melted. Still, engineers on the scene figured out how to deal with that.

Dr. BARRETT: They worked down through 20 feet of water for shielding with long tools, and started picking up the pieces of broken fuel in the core and placing it in special canisters with vents and filters on them, and put those canisters in a transport cask and shipped them to Idaho.

HARRIS: They'll try to do the same basic thing at Fukushima Daiichi, and its three damaged cores.

Leo Lessard, at the French nuclear company Areva, says just getting to the cores is going to be much more difficult than it was at Three Mile Island. For starters, the tops of two buildings have collapsed, so that debris will have to be cleared.

Mr. LEO LESSARD (Nuclear Engineer, AREVA NP Inc.): A lot of that material is probably very radioactive, so theyll have to be shielding and precautions incorporated in order to protect workers who have to do some manual operations in those areas.

HARRIS: Robots may be useful, he says, but don't assume that they can pick through all that debris. And if it turns out that some of the nuclear fuel has managed to leak through a hole in the reactor vessel, as some officials have speculated, it will be quite a challenge to scoop it up and put it into a shielded container.

Mr. LESSARD: In that case, you've a tremendously high radiation field as opposed to having it underwater, where the water is providing complete shielding.

HARRIS: He expects the cleanup to take more than a decade. And Lake Barrett says, count on it costing $10 billion. Engineers can break it down to the basics, and they know how to do each individual step, but nobody has ever tried a nuclear clean up on this scale before.

Richard Harris, NPR News.

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