Recognition…Jim Huylebroek for the New York Times

Kyiv, Ukraine — Europe’s largest nuclear power plant was disconnected from the country’s power grid Monday after renewed shelling nearby, Ukrainian energy officials said, putting critical cooling systems once again at risk of relying solely on backup power.

Herman Galushchenko, Ukraine’s energy minister, said a fire resulting from the shelling severed the Zaporizhia power plant’s last connection to a back-up line, which was its only source of external power.

Reactor No. 6, the plant’s only functioning reactor, was still producing electricity for the plant itself, and as of Monday evening, engineers had not turned on any diesel generators, according to an official from Energoatom, the Ukrainian company responsible for running the facility.

Mr Galushchenko said it was another precarious moment made even more ominous by the fact that fire crews were unable to reach the scene of the fire.

“Repairs on the lines are now impossible,” he said. “There’s fighting all around the station.”

An International Atomic Energy Agency inspection team that had been at the facility left behind two monitors hoping they would witness unfolding events and the tensions at the facility, which was being held by Russian forces but still operated by Ukrainian engineers will, could alleviate . The greater hope had been that the shelling would stop.

The agency said that according to Ukrainian officials, the reserve line was “deliberately disconnected to put out a fire.”

“The line itself is not damaged and will be reconnected once the fire is out,” said the organization, which is part of the United Nations.

Edwin Lyman, a nuclear energy expert at the Union of Concerned Scientists, a private group in Cambridge, Mass., said the current situation – with the plant relying on one of its own reactors to power cooling systems – is ” not unique, but it is not common practice.”

He pointed out that the International Atomic Energy Agency, which sets reactor safety standards for nuclear power plants, released a technical document in 2018 detailing the backup procedure.

“Some existing nuclear power plant technologies have this capability,” says the IAEA document, “while others do not.” Even plants that do have the capability could face “a time limit of generally a few hours” for back-up power be.

Najmedin Meshkati, a professor of civil and environmental engineering at the University of Southern California, said the external power outage — which has happened at least twice at the Zaporizhia plant in recent weeks — is “one of the most horrific events that could happen at a nuclear power plant.” .”

dr Meshkati, a member of the committee appointed by the United States National Academy of Sciences to learn lessons from the disaster at Japan’s Fukushima nuclear power plant, said there was no point in running the reactor.

An engineer in contact with people at the facility and in the satellite city of Enerhodar said Monday her colleagues had reported heavy shelling in the area over the past three days.

“Dwelling houses were damaged and many more people were injured and killed than was reported in the Ukrainian media,” said the engineer, who spoke on condition of anonymity because she feared reprisals against her friends and family. “People continue to leave the city, including workers at the plant.”

Ukrainian officials tried to keep up pressure on the International Atomic Energy Agency to propose a robust assessment of both the conditions at the plant and the challenges faced by Ukrainian engineers charged with its safe operation.

Repeated shelling over the past month has damaged all of the facility’s connections to four external high-voltage power lines, forcing it to use a lower-voltage backup line to power the cooling equipment needed to avoid core meltdowns. It was this reserve line that was cut Monday.

When the main power lines and backup line were damaged by gunfire and fires on August 25, a power outage at the facility forced reliance on diesel generators to prevent a disaster.

Rafael Mariano Grossi, the director of the International Atomic Energy Agency, told a news conference on Friday that his main concern for the facility’s physical security is related to a reliable connection to external power supply.

William J. Broad contributed reporting from Brunswick, Maine.