At Ukraine's Zaporizhzhia Nuclear Power Plant, risk of an accident has increased The head of the world's atomic watchdog warned that the reactors at Zaporizhzhia might have to be shut down. That would start a clock ticking at the site.

Here's why the risk of a nuclear accident in Ukraine has 'significantly increased'

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AILSA CHANG, HOST:

We start this hour in Ukraine, where a crisis at a nuclear plant appears to be escalating. Earlier today, the head of the International Atomic Energy Agency put out a statement saying the risk of an accident at the Zaporizhzhia nuclear power plant had, quote, "significantly increased." He called for an immediate nuclear safety zone around the plant.

(SOUNDBITE OF ARCHIVED RECORDING)

RAFAEL GROSSI: Let me be clear. The shelling around Zaporizhzhia nuclear power plant must stop.

CHANG: NPR's Geoff Brumfiel has been tracking these developments and joins us now. Hi, Jeff.

GEOFF BRUMFIEL, BYLINE: Hi there, Ailsa.

CHANG: OK. So can you just catch us up on the latest? What exactly is happening right now?

BRUMFIEL: Right. So this is a Ukrainian plant. It's been occupied by Russia since March, but Ukrainians continue to operate it. And it's been supplying power to both Russian- and Ukrainian-held territory. The current situation really started in August. There was this big uptick in shelling. Both sides blame each other for that. But at the start of this month, that shelling led the last main power line connecting the plant to the grid to go down. And about four days after that, a backup line went down. That means the entire plant has being cut off from the electricity grid for about four days. And that's not good because nuclear plants need power.

CHANG: And explain why that is.

BRUMFIEL: Right. So these plants obviously produce electricity, but they also require it to operate all their safety systems and, most importantly, their cooling systems. Pumps to keep water moving through the cores and keep them from overheating need to keep running. If they stop, a meltdown is possible.

CHANG: OK. But you said that they've been without power from the grid for four days now. Do we know how they've been keeping the plant safe during that time?

BRUMFIEL: Yeah. Interestingly, this type of reactor is able to run in something called islanding operation mode. That basically means that they keep the reactor on or keep one of the reactors on but turn it way down so it's not producing a lot of power. It's a pretty cool trick. And it can power the rest of the plant, but it can't go on forever because the other equipment just isn't designed to run at low power like this. And Grossi also says the workers are a factor. They live in a nearby town that's lost power, water and sewage. He's concerned that the staff will have to leave for their own safety. And that's another reason that the plant's Ukrainian owners are discussing whether to shut it down.

CHANG: So if they do shut down the last reactor at the plant, does that mean this crisis will be over?

BRUMFIEL: So unfortunately not. It actually makes things a little bit worse in the short term. I mean, if you think of a nuclear plant like cooking on a stove, you might think it's like a stove. You can turn down the stove, and it just turns off. But it's actually more like cooking on charcoal. So even when you're done, those coals stay hot.

CHANG: Right.

BRUMFIEL: And that means water needs to keep going to the cores. I spoke to a nuclear engineer named Steve Nesbit with the American Nuclear Society. He says all plants are prepared for this kind of emergency. They have backup generators to keep the water pumping.

STEVE NESBIT: We don't want to go on the diesel generators, but it's a situation you can abide by for a while.

BRUMFIEL: And in the case of Zaporizhzhia, the IAEA says they normally have about 10 days of fuel on site, but it might be a little less 'cause we know they've had to run those generators a little bit.

CHANG: OK. So if they shut the reactor down, the clock starts ticking. They'll need to get more fuel to the site for those generators. I don't want to speculate too much here, but what would be the worst-case scenario at that point?

BRUMFIEL: Well, the worst-case scenario is the generators run out of fuel, the reactors heat up, and there might be a meltdown. But just before we go, I want to say, this won't be a Chernobyl-like crisis. These are much newer reactors. They're safer. They have containment buildings that could potentially help. The IAEA doesn't want to test any of this stuff. And for that reason, they're calling on all sides to cut it out, knock it off right now.

CHANG: That is NPR's Geoff Brumfiel. Thank you so much, Geoff.

BRUMFIEL: Thank you.

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