日本震后福岛核电站发生爆炸

LINDA WERTHEIMER, host: This is WEEKEND EDITION from NPR News. I'm Linda Wertheimer. Scott Simon is away.

We begin this hour with the latest from Japan after an explosion at a nuclear power station today created fears of a meltdown.

Japanese officials have announced a last ditch effort  to cool off the nuclear reactor which was damaged in yesterday's earthquake and tsunami. Officials say the explosion was not a nuclear explosion.

In a moment we'll hear more about the ongoing rescue and recovery efforts in Japan, but NPR's Jon Hamilton joins us now with more details on the failed nuclear power plant. Jon, what did explode?

JON HAMILTON: Well, information is really pretty sketch, but you can see certainly in the news videos that there was a very strong blast that damaged something, a building at the power plant, and it looked like the building that actually houses the reactor that's been having all this trouble.

And it apparently occurred, too, as workers were trying to add water to keep this overheating nuclear core from getting any hotter. And when you do that you can produce hydrogen and that's a reason - something that could have caused the explosion.

WERTHEIMER: The hydrogen, which is highly flammable just poof, went...

HAMILTON: It's highly explosive.

WERTHEIMER: The reactor's been in trouble since the earthquake, hasn't it?

HAMILTON: Yeah. It's had a lot of problems with the cooling system. First, the earthquake cut off the main power to the cooling systems. It needs pumps to circulate water through.

Then, backup systems failed, they were running on batteries at one point, and eventually nothing they did seemed to be enough to keep the temperatures in the core under control .

WERTHEIMER: Radiation, has any radiation leaked out?

HAMILTON: Well, government officials are now saying that yes, it's probably not from the explosion, mind you. What it probably came from was as pressure built up, they have several times, it appears, vented steam, and that steam has been near the core, so it's a little bit radioactive. So there has been a release of some radioactive material.

And there also have been reports of higher radiation levels inside and outside the plant. And there's been talk about whether there was some cesium, which is something you get in a nuclear reactor.

The other sign is that the government has extended the evacuation area around this plant now to 12 miles away.

WERTHEIMER: Jon, the steam - the steam that comes from water that's been in contact with the reactor's core, presumably that would - that we would be much more concerned about that if the building, which ought to be containing all that is gone.

HAMILTON: Right. Well, the - it appears that what went in the explosion is the outermost building, which is really just a shell housing. There is what is known as the containment structure inside that. That did not go in the explosion, and all the radioactive material is still inside that containment structure.

WERTHEIMER: Okay. So what is the last ditch effort then?

HAMILTON: Well, what they say this morning is that they are going to flood the core of this reactor with sea water. It's built on the coast so there's lot of water there, and they will put sea water in. So what the sea water will do is keep it cool. They've been having trouble circulating enough water to keep it cool. The sea water will certainly do that, although it will do bad damage to the system as it does it.

They're also going to put in boric acid. And boric acid is really useful in these cases because it helps to - it's known as a sort of poison to a nuclear reaction. It suppresses the radioactive materials, keeps them from reacting as quickly.

WERTHEIMER: So do people think that's gonna work, sea water?

HAMILTON: Well, there's not a lot of experience to go on here. In fact, I'm not aware of another instance in which this has happened. I'm not saying it hasn't, but I couldn't find any evidence that this has been tried before.

There's also a really important outstanding question, and that is, what is going on in the core? Are they flooding it just because it's hot, or has it actually started to melt, you know, this thing we talk about as the meltdown. If the core is actually melting, it means that everything inside this containment structure we mentioned could become highly radioactive. And once that happens inside a containment structure, you pretty much just have to seal it up with concrete. You sort of entomb it.

WERTHEIMER: NPR's Jon Hamilton is here in our Washington D.C. studios. Jon, thank you.

HAMILTON: You're welcome.