The Zaporizhzhia nuclear power plant in the middle of the fighting in Ukraine was temporarily cut off from the electrical grid Thursday because of fire damage, causing a blackout in the region and heightening fears of a catastrophe in a country haunted by the Chernobyl disaster.

The plant, Europe’s largest, has been occupied by Russian forces since the early days of the war. The government in Kyiv alleges Russia is essentially holding the plant hostage, storing weapons there and launching attacks from around it, while Moscow accuses Ukraine of recklessly firing on the facility.

On Thursday, the plant was cut off from the grid for the first time after fires damaged a transmission line, according to Ukraine’s nuclear power operator. The damaged line apparently carried outgoing electricity — and thus the region lost power, according to Yevgeny Balitsky, the Russia-installed governor. As a result of the damage, the two reactors still in use went offline, he said, but one was quickly restored, as was electricity to the area.

The line that was apparently affected is different from the one that carries power to run cooling systems essential for the safe operation of the reactors. A loss of power in those supply lines is a major concern of experts warily watching the fighting.

Still, Thursday’s cutoff underscored concerns about the battles around around the plant.

“Anybody who understands nuclear safety issues has been trembling for the last six months,” Mycle Schneider, an independent policy consultant and coordinator of the World Nuclear Industry Status Report, said before the latest incident at the plant.

Ukraine cannot simply shut down its nuclear plants during the war because it is heavily reliant on them, and its 15 reactors at four stations provide about half of its electricity. Still, an ongoing conflict near a working atomic plant is troubling for many experts who fear that a damaged facility could lead to a disaster.

That fear is palpable just across the Dnieper River in Nikopol, where residents have been under nearly constant Russian shelling since July 12, with eight people killed, 850 buildings damaged and over the half the population of 100,000 fleeing the city.

Liudmyla Shyshkina, a 74-year-old widow who lived within sight of the Zaporizhzhia plant before her apartment was bombarded and her husband killed, said she believes the Russians are capable of intentionally causing a nuclear disaster.

Fighting in early March caused a brief fire at the plant’s training complex that officials said did not result in the release of any radiation. Ukrainian President Volodymyr Zelenskyy says Russia’s military actions there amount to “nuclear blackmail.”

No civilian nuclear plant is designed for a wartime situation, although the buildings housing Zaporizhzhia’s six reactors are protected by reinforced concrete that could withstand an errant shell, experts say.

The more immediate concern is that a disruption of electricity supply to the plant could knock out cooling systems that are essential for the safe operation of the reactors, and emergency diesel generators are sometimes unreliable. The pools where spent fuel rods are kept to be cooled also are vulnerable to shelling, which could cause the release of radioactive material.

Kyiv told the International Atomic Energy Agency, the U.N.’s nuclear watchdog, that shelling earlier this week damaged transformers at a nearby conventional power plant, disrupting electricity supplies to the Zaporizhzhia plant for several hours.

The atomic agency’s head, Rafael Mariano Grossi, said Thursday he hopes to send a mission to the plant within “days.”

Negotiations over how the mission would access the plant are complicated but advancing, he said on France-24 television after meeting in Paris with French President Emmanuel Macron, who pressed Russian President Vladimir Putin in a phone call last week to allow the U.N. agency to visit the site.

“Kyiv accepts it. Moscow accepts it. So we need to go there,” Grossi said.

At a U.N. Security Council meeting Tuesday, U.N. political chief Rosemary DiCarlo urged the withdrawal of all military personnel and equipment from the plant and an agreement on a demilitarized zone around it.

Currently only one of the four lines supplying the plant with power from outside is operational, the U.N. atomic agency said. External power is essential not just to cool the two reactors still in operation but also the spent radioactive fuel stored in special facilities onsite.

“If we lose the last one, we are at the total mercy of emergency power generators,” said Najmedin Meshkati, a professor of civil and environmental engineering at the University of Southern California.

He and Schneider expressed concern that the occupation of the plant by Russian forces is also hampering safety inspections and the replacement of critical parts, and is putting severe strain on hundreds of Ukrainian staff who operate the facility.

“Human error probability will be increased manifold by fatigue,” said Meshkati, who was part of a committee appointed by the U.S. National Academy of Sciences to identify lessons from the 2011 nuclear disaster at Japan’s Fukushima nuclear plant. “Fatigue and stress are unfortunately two big safety factors.”

If an incident at the Zaporizhzhia plant were to release significant amounts of radiation, the scale and location of the contamination would be determined largely by the weather, said Paul Dorfman, a nuclear safety expert at the University of Sussex who has advised the British and Irish governments.

The massive earthquake and tsunami that hit the Fukushima plant destroyed cooling systems that triggered meltdowns in three of its reactors. Much of the contaminated material was blown out to sea, limiting the damage.

The April 26, 1986, explosion and fire at one of four reactors at the Chernobyl nuclear plant north of Kyiv sent a cloud of radioactive material across a wide swath of Europe and beyond. In addition to fueling anti-nuclear sentiment in many countries, the disaster left deep psychological scars on Ukrainians.

Zaporizhzhia’s reactors are of a different model from those at Chernobyl, but unfavorable winds could still spread radioactive contamination in any direction, Dorfman said.

“If something really went wrong, then we have a full-scale radiological catastrophe that could reach Europe, go as far as the Middle East, and certainly could reach Russia, but the most significant contamination would be in the immediate area,” he said.

That’s why Nikopol’s emergency services department takes radiation measurements every hour since the Russian invasion began. Before that, it was every four hours.