Ukraine Nuclear Plant Disconnected From Grid After Shelling

KYIV, Ukraine — A fire caused by renewed shelling near Europe’s largest nuclear power plant led to its disconnection from the national power grid on Monday, Ukrainian officials said, raising fears that despite the presence of U.N. inspectors, conditions at the Russian-occupied facility could deteriorate quickly and threaten a catastrophe.

The fire forced the staff to sever the plant’s last connection to a reserve line that was providing its only source of outside power, once again placing critical cooling systems at risk of relying solely on emergency backup power, Herman Galushchenko, Ukraine’s energy minister, said Monday.

Firefighting crews had not been able to reach the site of the blaze because of continued fighting around the plant, Mr. Galushchenko said.

Shelling, explosions and fires around the facility, the Zaporizhzhia nuclear plant in southern Ukraine, have raised fears for months about a possible disaster, but amid contradictory claims and persistent fighting, it has been difficult for international experts to gauge the danger from afar. Two members of a United Nations team, sent last week to inspect the facility, have remained at the plant in the hopes of ensuring its safety and perhaps reducing the fighting in the area.

The plant stands in an area bordering the Russian-occupied Kherson region, the site of Ukraine’s largest counteroffensive in months. On Monday, a Russian-installed official there briefly raised the possibility that fighting could delay a referendum that the Kremlin would use as a pretext to cement control.

The fire and shelling on Monday only showed again that the plant remains at serious risk — a sprawling facility where any number of factors could contribute to a potential nuclear catastrophe.

Fears have steadily increased since Russian soldiers took control of the plant, soon after they invaded in February and as shelling mounted in the area over the summer. Beyond that, Ukraine has warned that plant workers are laboring under exhausting, dangerous conditions.

Those concerns led the United Nations’ nuclear watchdog to send a small team through an active battlefield last week to inspect the plant.

The watchdog, the International Atomic Energy Agency, said Monday that the reserve line had been “deliberately disconnected in order to extinguish a fire.” The line itself was not damaged and will be reconnected once the fire is extinguished, the agency said.

The inspectors plan to release a report on the plant’s overall condition on Tuesday, the agency said.

Reactor No. 6, the only working reactor at the plant, was still producing power for the facility itself, and as of Monday evening, engineers had not switched on diesel generators, according to an official from Energoatom, the Ukrainian company responsible for operating the facility. Edwin Lyman, a nuclear power expert at the Union of Concerned Scientists, a private group in Cambridge, Mass., said it was “not unique, but it’s not standard practice” for a plant to rely on one of its own reactors to supply power to cooling systems.

In 2018, Mr. Lyman noted, the I.A.E.A. published a technical document detailing the backup procedure. The document said that even plants with this capacity may face “a time limit, generally of a few hours,” for the backup power.

Another expert, Najmedin Meshkati, a professor of civil and environmental engineering at the University of Southern California, warned that the loss of offsite power — which has happened at the Zaporizhzhia plant at least twice in the last few weeks — was “one of the most dreadful events that could happen to a nuclear plant.”

In Kherson Province, where Ukraine has been counterattacking, officials have shrouded details of its operation in secrecy, but on Monday evidence emerged that the military push may be disrupting plans by the province’s Russian-backed authorities to hold a referendum in Kherson on joining Russia.

On Monday, the deputy head of the Russian-appointed administration in Kherson, Kirill Stremousov, said that “events taking place” had forced the referendum to be delayed. He told Rossiya-1, a Russian state-run television network, that “this will be a practical decision, because we don’t jump the wagon.”

But several hours later, in an apparent contradiction of his earlier comments, he said: “There is no such pause. Everything is going according to plan.” He added that no date for the vote had been set.

Ukraine has made it a priority to stop any referendum that might bind captured lands closer to Russia, and its military intelligence unit said Monday that special forces had carried out strikes targeting Russian strongholds being used to prepare materials for referendums.

“The place where ballots were stored for the pseudo-referendum was destroyed,” the Ukrainian military said in a statement. “The warehouse was blown up by an explosion from inside the premises. All available printed materials were destroyed.”

The Ukrainians said that a Russian base charged with guarding the warehouse was also destroyed. And a day earlier, President Volodymyr Zelensky of Ukraine said the country’s forces had reclaimed two villages in the region.

It was not possible to independently verify the Ukrainian claims.

Almost as soon as Russian forces swept into the Kherson region early in the invasion, Moscow launched a campaign of indoctrination and intimidation as a prelude to the kind of referendum that it staged in Crimea in 2014, and that separatist forces held in parts eastern Ukraine.

Over the spring and summer, the occupation authorities arrested hundreds, replaced the currency with the ruble, handed out Russian passports, rerouted the internet to Russian internet servers and introduced a school curriculum approved by the Kremlin.

Residents in Kherson also reported that proxy officials had already started printing ballots for the referendum, and American officials warned that “sham” voting could begin any day.

Ukraine’s push to reclaim territory and stop referendums comes as it wages a separate campaign abroad to keep its European supporters unified — and sending cash and weapons — amid a deepening energy crisis set off by the Russian invasion.

The Kremlin has cut oil and gas supplies in retaliation for Western sanctions, leading to fears of a tough winter ahead for many around the continent and civil unrest. In response, Germany announced Monday that it would keep two of its three remaining nuclear plants operational, delaying the country’s plans to go nuclear-free in its energy.

For months, Ukraine has been building its arsenal and training soldiers to use weapons recently supplied by the West. Its offensive in Kherson is focused on isolating and attacking Russian forces on the western side of the Dnipro, the river that bisects the country and drains into the Black Sea.

By pounding Russian ammunition depots and attacking the four main river crossings, the Ukrainians hope to starve an estimated 15,000 to 25,000 Russian soldiers of munitions and supplies, forcing them to retreat, surrender or die.

The Ukrainian military has said that the campaign is a complex effort that features assaults on Russian positions, attacks on Russian forces behind enemy lines aided by Ukrainian partisans, and efforts to undermine the morale of Russian soldiers.

Ukraine’s military has imposed restrictions on journalists and urged people not to publicize details of the operations. Moscow tried to cast the offensive as a failure even before it began. But in recent days, some Russian military bloggers have noted Ukrainian advances.

More than half the local population is estimated to have fled since the Russian occupation began, but intensified fighting now threatens to cut off escape routes for civilians or trap them under shelling. Ukraine’s military high command said Monday that Russian occupation forces in Kherson had imposed a ban on the movement of all local residents.

Western military analysts cautioned against drawing firm conclusions about the counteroffensive, given the limited amount of clear information and the abundance of competing claims.

On Monday, the Ukrainian military southern command claimed that a regiment of Russia’s 1st Army Corp had refused to fight, in part because they had so little support and supplies, including water. Western officials have repeatedly described serious supply problems among Russian forces, but the Ukrainian claims Monday could not be independently verified.

Ukrainian soldiers fighting along the front have characterized the battles as slow and costly, with heavy losses on both sides.

In a sign of Ukrainian hunger for good news, social media accounts lit up Sunday night with videos and images of a soldier hoisting a Ukrainian flag on a rooftop. An official in the president’s office, Kyrylo Tymoshenko, said on Facebook that it had been taken that day in the village of Vysokopillia, about 90 miles from the capital of the southern Kherson region.

Mr. Zelensky appeared to nod to that report in a statement made after a meeting with military leaders on Sunday.

“Ukrainian flags are returning to the places where they should be,” the president said.

William J. Broad contributed reporting.