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MICHELE NORRIS, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Michele Norris.

ROBERT SIEGEL, host:

And I'm Robert Siegel.

Problems at three nuclear reactors near Fukushima, Japan, have led to news reports that range from reassuring to terrifying.

NPR's Jon Hamilton has been talking with nuclear accident experts, and they say at the moment, there is plenty of cause for concern, but not alarm.

JON HAMILTON: The benchmarks for nuclear mishaps were set by two incidents: one at Three Mile Island in Pennsylvania in 1979, the other at Chernobyl in the former Soviet Union in 1986. Most experts think Fukushima will end up somewhere between those two. The trouble is that leaves a lot of wiggle room.

David Brenner is from the Center for Radiological Research at Columbia University.

Professor DAVID BRENNER (Director, Center for Radiological Research, Columbia University): They were really very, very different. Just to put those - put it in perspective, Chernobyl was roughly the equivalent of a million Three Mile Islands.

HAMILTON: In terms of the amount of radiation released. The amount at Three Mile Island wasn't enough to cause any detectable health problems. Radiation from Chernobyl killed dozens and put many thousands at increased risk for cancer.

Brenner says it's unlikely Fukushima will be that bad.

Prof. BRENNER: I think in almost any situation, we're much closer to Three Mile Island than Chernobyl. It's simply impossible to imagine anything like Chernobyl happening in these reactors, despite all the problems they're having.

HAMILTON: There have been powerful explosions at two different reactors in Fukushima. That's drawn comparisons to Chernobyl, where a blast sent huge amounts of radiation into the atmosphere. But the explosions at the Fukushima plant didn't do that. One reason is that in Chernobyl, the nuclear reaction itself was out of control.

Brenner says in Japan, all three reactors shut down the moment the earthquake struck.

Prof. BRENNER: What has happened is that the core of the reactor continues to stay very hot and needs to be cooled down, and that's typically done with what's called secondary cooling. And it was the secondary cooling that failed.

HAMILTON: That's not good, though. The failure has allowed at least two reactors to become hot enough to produce large amounts of hydrogen. Some of that hydrogen got out of the cores and caused the explosions.

But the explosions occurred outside the so-called containment vessels that surround each reactor, so the reactors themselves stayed intact. That makes the problems in Fukushima sound more like those in Three Mile Island.

But Kenneth Bergeron, a physicist who has worked on simulations of nuclear accidents, says there are still reasons to worry. For one thing, he says, scientists have detected radioactive cesium and iodine outside the power plants.

Dr. KENNETH BERGERON (Physicist): That is a telltale sign that the fuel rods have become overheated and that the cladding on those fuel rods has ruptured, a very bad sign. Not quite the same thing as a meltdown, but it's the precursor to a meltdown.

HAMILTON: Japanese officials say at least two of the reactors at Fukushima probably have experienced some melting at the core.

But Dale Klein, the former chair of the U.S. Nuclear Regulatory Commission, says that doesn't tell you much.

Dr. DALE KLEIN (Former Chairman, Nuclear Regulatory Commission): What we call a core melt, it's like an automobile accident. You can have all the way from a fender bender to a massive collision. And so when we talk about fuel melting, you could end up just having a few fuel elements or you could just have the top of them.

HAMILTON: About half of the fuel melted in the reactor at Three Mile Island. In a complete meltdown, though, the nuclear fuel ends up in a molten mass hot enough to burn its way right through the steel pressure chamber surrounding the core.

Klein says once that happens, the last defense is a containment structure made of thick steel and concrete.

Dr. KLEIN: If you look at Chernobyl, they did not have a containment. And so once they had an accident, they had massive releases of radioactive materials. In the case of Japan, as long as the containment and the reactor vessel remain intact, it's not likely they will have massive radiation releases.

HAMILTON: But scientists say the containment vessels in Fukushima are not as large or as strong as the one at Three Mile Island.

Jon Hamilton, NPR News.

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