The last reactor at Europe's largest nuclear power plant has stopped

STEVE INSKEEP, HOST:

We're also following developments of the Zaporizhzhia nuclear power plant in southern Ukraine. You may recall Russian troops occupy it and that fighting continues nearby. The plant's Ukrainian operators now say they have powered down the last working reactor. NPR's Geoff Brumfiel has been following this story. Geoff, good morning.

GEOFF BRUMFIEL, BYLINE: Good morning.

INSKEEP: What does it mean to power down a reactor?

BRUMFIEL: Well, basically, what it means is that you put in the control rods. You stop the nuclear reactions. And you sort of switch it off so it can't produce electricity anymore and take it off the grid.

INSKEEP: OK. So is it safe now?

BRUMFIEL: Not exactly. So it turns out that...

INSKEEP: (Laughter) OK.

BRUMFIEL: ...Nuclear reactors use heat to make electricity. And it's kind of like cooking with charcoal. So these reactor cores stay hot even after they're turned off. You have to keep water moving through the cores to keep everything cool. And so even in this moment, although everything's switched off, there is still a need to keep water circulating through the reactors.

INSKEEP: And of course, the continued worry is there about some kind of shell landing in the wrong place, doing the wrong thing, something going terribly wrong, which does raise a question. This has been going on for months. Why did the Ukrainians not shut this reactor down before now?

BRUMFIEL: Well, they actually did shut down several other reactors at the site. But of course, they had the same problem. These reactors still had warm cores, still had nuclear reactions going on, heating water in the cores. And so they needed water moving through them and they kept losing power to the site. Shelling kept knocking out power lines repeatedly throughout August and early September. So what they did was they took one of the reactors and they started something called island mode. Basically, this is a cool trick where they can use the reactor at very low power to keep the lights on at the plant and keep water moving through its core and all the other cores as well.

INSKEEP: OK. So that's why one reactor was still on. But then, why switch it off now?

BRUMFIEL: Right. Right. I mean, now it's kind of the opposite question because it seems like that's pretty essential to safety. But the problem is island mode can only work for so long. The turbines and pumps and things on these reactors are designed to operate at much higher power outputs. And they get damaged the longer island mode goes on. On top of that, you have staffing issues. They've had trouble with the staff in the nearby town of Enerhodar, where they've had power and water and sewage problems. And of course, there's ongoing shelling. So on Sunday, they got power restored to the plant from the grid. And I think they really saw an opportunity here to shut down that last reactor safely while they were connected to external electricity. They took it. And they brought that final reactor offline. So now it is in the process of cooling down like all the others.

INSKEEP: All right. What happens if the electricity goes out again? We might even say when it goes out again, given that it is a war zone.

BRUMFIEL: Yeah, exactly. I mean, this is why the IAEA continues to describe the situation at the plant as precarious. These reactors need to be cooled for years, honestly, I'm hearing...

INSKEEP: Wow.

BRUMFIEL: ...From some nuclear experts. So they're going to have to rely on emergency diesel generators at the site if the light goes that - if the lights go out again. Now, fortunately, the atomic agency says that there are 20 generators available. But they only have about 10 days of fuel. So if something happens over a long period, they're going to need extra fuel. And that's why the agency wants to have this permanent safe zone around the plant, to make sure that the power is secure.

INSKEEP: Geoff, thanks for the update.

BRUMFIEL: Thank you, Steve.

INSKEEP: That's NPR's Geoff Brumfiel. Transcript provided by NPR, Copyright NPR.