日本工程师奋战令人担忧的核电站

LIANE HANSEN, host: The nuclear cliffhanger in Japan continues today as engineers struggle to avert meltdowns at several damaged nuclear reactors. The most serious situation is in Fukushima Prefecture（福岛县）. Several reactors there lost power after the earthquake and tsunami. At one of them, an explosion ripped off the top of the building surrounding the reactor. Some radiation has been released into the air. While safety officials say the levels are not dangerous, about 170,000 people have been evacuated from the area.

NPR science correspondent Christopher Joyce is in the studio. And, Chris, this nuclear crisis is going into its third day now. How bad is it?

CHRISTOPHER JOYCE: It's still very serious. It's a bit hard to get all the details. There've been rather muddled reports from Japan's safety agency. But here's the situation.

At the Daiichi nuclear power complex（福岛第一核电厂）, there are six reactors. And after the quake, they lost electricity to the whole area. Three of the reactors were already shut down. They're okay. But reactor units one, two and three are all having problems cooling their reactor cores without that electric power.

Now, yesterday, there was an explosion at unit number one, and that was probably due to hydrogen that builds up in the cooling system. What happens is that the cooling water in the reactor gets too hot and hydrogen gets stripped out of it, and that hydrogen is explosive. And engineers were venting some of that hydrogen into the building that the reactor sits in. And apparently it exploded. And it didn't damage the reactor itself, just the building it sits in.

As well as that, now they've had to bring in seawater to cool that reactor because the freshwater system just is not working well enough.

HANSEN: And Japanese authorities now say there may have been a partial meltdown inside the reactor core?

JOYCE: Yes. There's no way to know for sure because you can't get inside, it's so hot into the reactor vessel. But they found a signature of what could be a partial meltdown, which is Cesium-137（铯-137，其放射性同位素）. It's an element, radioactive element. They found it in the air. And that suggests that some of the fuel inside the reactor - it's supposed to be in tubes that are sealed - may have been broken or melted.

HANSEN: What about the other two operating reactors at Daiichi?

JOYCE: They're having trouble cooling, as well. Officials say that unit three may have had some partial fuel melting, as well. And now apparently they're going to use seawater on all three to keep them cool.

HANSEN: Does venting that vapor release radiation into the air outside the complex?

JOYCE: It does. Authorities say that the levels around the plant had been at several points of time - several times higher than they should be. Medical teams are testing people. They're reporting some exposure to workers and residents, but nothing terribly dangerous.

HANSEN: So what has to happen to avoid a total meltdown and catastrophe here?

JOYCE: Well, they need - first of all, the most important thing is to get the AC electric power back. They've been using backup power, battery power just to make sure that the cooling water doesn't evaporate; because if it does, and exposes thee fuel bundles for any significant period of time, they will start to melt.

This is what happened at Three Mile Island in 1979 in Pennsylvania(三里岛, 美国宾夕法尼亚州米德尔敦附近一岛，该地核电站曾在1979年发生灾难性事故）. There was a partial melt down that was contained and stopped before it released radiation, serious amounts of radiation.

At Chernobyl（切尔诺贝利）, of course, it wasn't contained. It wasn't stopped. The fuel melted. It melted right down through the reactor, through the building, caught on fire and spread radiation all over Europe.

NPR's Chris Joyce. Chris, thank you.

JOYCE: You're welcome.