Ukrainians are once again concerned about the fate of a nuclear power plant in a country that was the site of the world’s worst nuclear accident in 1986, Chernobyl.

The Zaporizhzhia nuclear power plant, Europe’s largest, has been occupied by Russian forces since the beginning of the conflict, and ongoing fighting near the facility has heightened fears of a disaster that could affect nearby towns in southern Ukraine — or potentially an even larger region.

The Ukrainian government claims that Russia is essentially holding the Soviet-era nuclear plant hostage, storing weapons there and launching attacks from around it, while Moscow accuses Ukraine of recklessly firing on the facility in Enerhodar.

“Anyone who understands nuclear safety issues has been trembling for the last six months,” said Mycle Schneider, an independent policy consultant and the World Nuclear Industry Status Report’s coordinator.

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 supply roughly half of the country’s electricity. Even so, many experts are concerned that an ongoing conflict near a functioning nuclear plant could lead to a disaster.

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

Liudmyla Shyshkina, a 74-year-old widow who lived near the Zaporizhzhia plant before her apartment was bombed and her husband was killed, believes the Russians are capable of causing a nuclear disaster on purpose.

Fighting caused a brief fire at the plant’s training complex in early March, which officials said did not result in the release of any radiation. Ukraine’s President, Volodymyr Zelenskyy, has called Russia’s military actions in the country “nuclear blackmail.”

The more immediate concern is that a disruption in the plant’s electricity supply could knock out cooling systems that are critical to the reactors’ safe operation, and emergency diesel generators are sometimes unreliable. Pools where spent fuel rods are stored to cool are also vulnerable to shelling, which could result in the release of radioactive material.

The International Atomic Energy Agency, the United Nations’ nuclear watchdog, was informed by Kyiv that shelling earlier this week damaged transformers at a nearby conventional power plant, causing electricity supplies to the Zaporizhzhia plant to be disrupted for several hours.

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

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

According to the agency, only one of the plant’s four power lines connecting it to the grid is currently operational. External power is required not only to cool the two reactors that are still operational, but also to cool the spent radioactive fuel stored in special facilities on-site.

He and Schneider expressed concern that Russian forces’ occupation of the plant is impeding safety inspections and the replacement of critical parts, as well as putting hundreds of Ukrainian workers under extreme stress.

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

The massive earthquake and tsunami that struck the Fukushima nuclear power plant destroyed cooling systems, causing meltdowns in three of its reactors. Much of the contaminated material was blown out to sea, which helped to limit the damage.

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