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.

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The head of the International Atomic Energy Agency is warning that the risk of a nuclear accident at the Zaporizhzhia nuclear power plant has "significantly increased," following ongoing fighting around the site.

"Let me be clear, the shelling around Zaporizhzhia nuclear plant must stop," IAEA Director General Rafael Mariano Grossi said in a brief recorded statement released on Friday.

Grossi also warned that the continued fighting might require the plant to shut down its last operating reactor. That would set into motion a chain of events that could intensify the current nuclear crisis. Here's how.

The Zaporizhzhia nuclear plant is the largest in Europe, capable of producing thousands of megawatts of electricity. But the plant also needs power from the same electricity grid it feeds.

The power is used to run the various parts of the plant, including its safety and cooling systems. Specifically, nuclear power plants require water to be pumped constantly through their cores in order to function safely, and the pumps need electricity.

At Zaporizhzhia, the power is normally supplied by four high-voltage lines, which connect the nuclear complex to Ukraine's electricity grid, but the conflict has seen those lines systematically cut. The last 750kV line was severed on September 3, according to the IAEA.

A backup line was disconnected two days later due to a fire on the site. In a press conference shortly after returning from Zaporizhzhia, Grossi told reporters that he believed the power lines were being deliberately targeted:

"It is clear that those who have these military aims know very well... to hit where it hurts so that the plant becomes very, very problematic," he said.

Since losing its last connection to the grid on Sept. 5, the nuclear plant has been powering itself in so-called "islanding operation mode." Under this setup, the Unit 6 reactor has been producing low levels of electricity that are running the rest of the facility.

The reactors at Zaporizhzhia are designed to operate in this mode during startup, according to a nuclear engineer who worked directly with the reactors when the plant began operations in the 1980s, but who was not authorized to speak publicly by his current employer.

"It's not good, it cannot be done for a long time," he says. The problem is less to do with the reactor itself than the turbine, generators and other systems–all of which are designed to run at significantly higher power levels than islanding operation mode provides.

Adding to the problem, Grossi said in his statement, is the increasing strain on the plant's Ukrainian operators. Many of the plant's current staff of just under 1,000 live in the nearby town of Enerhodar. Its water, sewage and electrical supplies have all been disrupted in recent days by the same fighting that's damaged the lines around the plant.

"The shelling is putting in danger operators and their families, making it difficult to adequately staff the plant," Grossi says.

With conditions deteriorating, it seems more likely that Ukrainian authorities will decide to power down the last reactor. But in the short term, that could exacerbate the crisis.

That's because nuclear reactors are more like charcoal grills than gas stoves. Even after they're shut off, they remain hot for a long period of time. Water must still circulate in the cores to prevent a meltdown.

With its reactors shut down, Zaporizhzhia will switch to backup emergency diesel generators to keep the reactors cool. The emergency generators themselves are a tried-and-true method for cooling a nuclear reactor. In fact, the Nuclear Regulatory Commission requires U.S. plants to switch to emergency diesel generators immediately, bypassing the "islanding operation mode" used in Zaporizhzhia.

"We don't want to go on the diesel generators, but it's a situation you can abide by for awhile," says Steven Nesbit, a nuclear engineer and member of the American Nuclear Society's rapid response taskforce, which is tracking the current crisis. For example, after losing power during Hurricane Andrew in 1992, the Turkey Point Nuclear Plant in Florida operated for days on emergency diesel power.

According to the IAEA, the Zaporizhzhia plant has more than a dozen emergency generators standing by. Normally, the plant holds a 10-day reserve of diesel fuel, the agency says, and currently has approximately 2,250 tonnes of fuel available.

If that fuel is depleted, or the generators are damaged in further fighting, it could trigger a meltdown.

But Nesbit says that doesn't necessarily mean there would be a Chernobyl-like catastrophe. The meltdown at Chernobyl was due to a unique mix of design flaws and operator error that would be essentially impossible to replicate at Zaporizhzhia.

And unlike the Fukushima nuclear disaster in 2011, some of the reactors at Zaporizhzhia have already been shut down for a while, allowing the nuclear fuel to cool somewhat, Nesbit says.

Even in the worst case scenario, the reactors at Zaporizhzhia are a modern design surrounded by a heavy "containment" building, Nesbit says. "It's reinforced concrete, typically about three to four feet of that; it's designed to withstand very high internal pressures."

That could allow it to hold in any radioactive material.

But the world's nuclear agency doesn't want to test any of this. And for that reason, Grossi is calling on all sides to implement a safety zone immediately.

"There is no time to waste," he says.

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.

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.

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. Transcript provided by NPR, Copyright NPR.

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