GUY RAZ, host:
And as we just heard, the biggest worry now is over those nuclear power plants.
NPR's science correspondent Nell Greenfieldboyce is following that part of the story.
And, Nell, what is the latest right now?
NELL GREENFIELDBOYCE: Well, there hasn't been a lot of information coming out, but I can tell you what we do know, mainly from reports out by the International Atomic Energy Agency, which is the main international agency that coordinates safe and peaceful use of nuclear technologies.
And so all the biggest problems have been taking place at the Fukushima Daiichi power plant. And there are three reactors there that are having trouble.
Reactor number one had that explosion that ripped off the roof and the wall of the building, and that was caused by a buildup of hydrogen. And they've been pumping seawater in to try to cool that reactor down.
And this is a pretty extreme measure to take because seawater is corrosive, and it means the reactor is pretty well ruined.
So now, Japanese officials are currently pumping seawater in to try to cool another reactor, reactor number three at the same power plant. And there's concern that hydrogen is building up there and that there could be another hydrogen explosion.
And all that hydrogen comes from water getting too hot in the reactor, and so the hydrogen is basically being stripped out of the water.
But Japan's main government spokesperson has said that for both of these reactors, reactor number one and reactor number three, they're assuming the possibility that a partial meltdown has occurred.
RAZ: Hmm. What does that mean?
GREENFIELDBOYCE: Well, a partial meltdown would mean that some portion of the reactor core itself has become uncovered with coolant and then exposed to air. And that would let it get hot enough - it would sort of heat up to the point where the rods that contain the nuclear material would begin to overheat and even melt.
So the important thing to know is that even though if there could be a partial meltdown, officials say the steel containment vessels that surround the reactor cores are still intact. And if this remains the case, that should limit what's released into the environment.
RAZ: Mm-hmm. Nell, there's another nuclear power plant that's having some problems as well, right?
GREENFIELDBOYCE: Right. There's a plant up the coast that said it's detected increased radiation levels in the area around the plant. That's the Onagawa plant. But officials have told the International Atomic Energy Agency that the reactors there are under control.
So they have to investigate the source of those increased levels of radiation, and it could just be from radioactive substances that were sort of scattered into the air by the earlier explosion.
RAZ: Give us a sense, Nell, of what the possible effects could be on all of the people around these power plants, many of whom have not been evacuated yet.
GREENFIELDBOYCE: Well, they did call for evacuation of a large area. I believe they were calling for around 200,000 people to be evacuated. And obviously, you know, people take concerns about nuclear power plants, you know, very seriously.
But so far, experts, nuclear experts, are stressing that you would expect limited effects on the environment, and that really with all of what Japan has to deal with right now, this is perhaps not as big an issue as some may fear.
RAZ: That's NPR's science correspondent Nell Greenfieldboyce.
Nell, thank you.
GREENFIELDBOYCE: Thank you.
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