KYIV, Ukraine — Shelling once again threatened the safe operation of the Zaporizhzhia nuclear power plant, even as United Nations officials expressed cautious optimism that a permanent presence of its inspectors at the plant was helping to lower the risk of a nuclear disaster.
The plant lost the connection with its last remaining main external power line after shelling on Friday evening, forcing engineers to rely on a lower-voltage reserve line to power the cooling equipment needed to prevent meltdowns, the International Atomic Energy Agency said in a statement.
Nuclear power plants must sometimes rely on external power drawn from the grid to cool the reactor cores and spent fuel. If the external power lines are cut, the Zaporizhzhia plant must turn to backup diesel generators, and if those malfunctions or run out of fuel, a meltdown becomes possible.
After visiting the plant, Rafael Mariano Grossi, the director of the agency told a news conference on Friday that his biggest concern regarding the physical safety of the facility itself was related to a reliable connection to external power.
That connection has been severed at least twice in recent weeks. When the main power lines and the reserve line were damaged by shelling and fires on Aug. 25, there was a blackout at the plant, and diesel generators had to be relied on to prevent a disaster. The I.A.E.A. said it was informed by Ukraine that it had happened again on Friday night after renewed shelling in the area.
Fighting has raged across southern and eastern Ukraine, and the plant lies precariously close to the front lines of some of the most intense fighting.
Mr. Grossi, who has avoided placing blame for the shelling on either the Russians or the Ukrainians, said Friday that it appeared the power supply to the plant was being deliberately targeted.
“It is clear that those who have these military aims, know very well that the way to cripple or to do more damage is not to look into the reactors which are enormously sturdy and robust,” he said. Instead, it is being hit where it hurts — the critical power lines essential for running the plant.
Without external power, the cooling system of the plant can cease functioning, nuclear experts say. Power is needed to pump water past the nuclear core and carry heat away. While the chain reaction in the core can be shut off quickly, the radioactive material there will continue to give off heat for a long time and can eventually damage the radioactive fuel or the reactor.
On Saturday, Mr. Grossi said that the presence of the agency’s inspectors, who were able to confirm the damage to the external power line, was already proving valuable. He called it a “game changer.”
“Our team on the ground received direct, fast, and reliable information about the latest significant development affecting the plant’s external power situation, as well as the operational status of the reactors,” he said.
One of the six reactors at the plant is currently operational, the agency said and is producing electricity both for cooling and other essential safety functions at the site and for Ukrainian households and factories.
Najmedin Meshkati, a professor of civil and environmental engineering at the University of Southern California, said the presence of monitors at the Zaporizhzhia plant was a good thing, but he warned that only time would tell if it changes the behavior of the Russian forces in control of the facility.
Even if it is operated safely, the plant can be used as leverage by Moscow. It is a vital piece of infrastructure that can power four million Ukrainian homes. Russian forces could wait for international focus to shift and then — when Ukrainians need the power the most this winter — “pull the plug on this plant,” he said.
As fighting raged in Ukraine this week near Europe’s biggest nuclear power plant, the United Nations’ nuclear watchdog agency managed to inspect the site and, perhaps more important, station two inspectors there to monitor its safety.
Putting independent nuclear experts at the Zaporizhzhia power plant to give unbiased reports on conditions there was a victory for the International Atomic Energy Agency and its director general, Rafael Mariano Grossi.
For weeks, Russia and Ukraine have blamed each other for shelling at the plant that Mr. Grossi said on Friday had raised the possibility of a nuclear catastrophe. The danger has only grown as Ukraine has stepped up a counteroffensive in the region.
“Now, when there is an allegation that something has happened at the plant, you can turn to us,” he said, rather than weighing the conflicting claims of Russia and Ukraine. “That’s the difference.”
The drama over the I.A.E.A.’s visit to the plant has dominated the news out of Ukraine for days. On Thursday, Mr. Grossi and 13 experts made it through the front lines after weeks of negotiation and inspected the site for about four hours.
Six members of the team stayed behind to continue working through the weekend. Mr. Grossi said that two of them would stay indefinitely.
Mr. Grossi said he expected to produce a report “early next week, as soon as we have the full picture of the situation by the end of the weekend, more or less.” He said he would brief the United Nations Security Council on Tuesday.
Though the U.N. team had success in its initial mission to inspect the plant, the agency has little power to do more than sound alarms. It cannot impose a cease-fire or establish a demilitarized zone near the plant, which many experts see as the only way to ease the danger.
On Saturday, President Recep Tayyip Erdogan of Turkey offered in a phone call with President Vladimir V. Putin of Russia to help mediate on matters about the plant, according to a statement from his office.
The plant has been repeatedly bombarded over the last month, and at a news conference in Vienna on Friday, Mr. Grossi said that physical damage from ordnance was the biggest threat. “The physical integrity of the facility has been violated not once, but several, several times,” he said.
More than once, reactors have been shut down or disconnected from the grid because critical power lines that are needed to keep reactor cores cool were damaged. Just hours before the U.N. team arrived on Thursday, shelling forced one of the reactors to shut down and switch to diesel generators.
BERLIN — European officials have expressed confidence that they can endure winter with limited Russian energy, as Moscow postponed restarting the flow of natural gas to Germany through a closely watched pipeline.
The European Union has been preparing for the possibility that Russia may cut gas deliveries in retaliation for European opposition to Moscow’s invasion of Ukraine. Under the long tenure of President Vladimir V. Putin, Russia has wielded its energy supply in myriad ways for foreign policy gains, often in efforts to seek leverage over European policies by turning off the gas spigot in the wintertime.
The E.U.’s economy commissioner, Paolo Gentiloni, said Saturday that the bloc was “well prepared to resist Russia’s extreme use of the gas weapon,” according to Reuters.
“We are not afraid of Putin’s decisions, we are asking the Russians to respect contracts, but if they don’t, we are ready to react,” he said on the sidelines of an economic forum in Italy.
Germany, in particular, has imposed tough energy-saving measures.
“Even if things get really tight again with deliveries from Russia, we’ll most likely get through the winter,” Olaf Scholz, the German chancellor, said in an interview with the WAZ, a regional daily, that was published on Friday and that he posted to his Twitter account on Saturday morning.
The German ministry overseeing gas deliveries noted that Germany’s gas storage is already nearly 85 percent full, a target set for the beginning of October.
And while Germany was getting 55 percent of its natural gas from Russia in February, when Russia first attacked Ukraine, Russian gas accounted for around 10 percent of Germany’s gas mix on Tuesday — the last full day when gas flowed through the Nord Stream 1 pipeline — thanks to months of sourcing gas from other countries. Currently, Germany receives the bulk of its natural gas from Norway, the Netherlands, and Belgium.
“We have noted Russia’s unreliability in recent weeks and accordingly we have continued — undeterred and consistently — with our measures to strengthen our independence from Russian energy imports,” a spokeswoman for the German ministry responsible for energy said in a statement on Friday. “As a result, we are now much better equipped than we were a few months ago.”
Among the host of energy-saving rules mandated by the government to prepare are regulations that came into force Sept. 1 and which state that most public buildings can only be heated to 66 degrees Fahrenheit and cannot be externally lit after 10 p.m. But officials note that the situation is still tense and that gas savings are very much required.
“I don’t want to be misunderstood; this is not yet the ‘all clear signal,” Robert Habeck, the energy minister, said on Wednesday.
Gazprom, the Russian-owned energy giant, had been expected to resume the flow of gas through the Nord Stream 1 pipeline on Saturday after three days of maintenance. But hours before, it said that it had found oil leaks around a turbine used to pressurize the pipeline, forcing it to call off the restart. It did not give a timeline for restarting.
The gas giant said on Saturday that Siemens Energy, the German company that built the turbine, was going to help repair the broken equipment. But Robin Zimmermann, a spokesman for the company, said that as of Friday night it had not received any such request.
Siemens also does not believe that the claimed leak would be enough to force a full shutdown of the turbine, the company said. “From our technical understanding as the manufacturer of the turbine, what was found yesterday is no reason to let the turbine stand still,” Mr. Zimmerman said.