ZAPORIZHZHIA, Ukraine — A U.N. inspection team finally reached the embattled nuclear power plant in Russian-occupied southern Ukraine on Thursday, after an artillery bombardment forced operators to shut down one reactor and switch another to emergency power, highlighting the immense risks of combat around a site loaded with radioactive material.
The team of 14 experts with the International Atomic Energy Agency planned to gauge the damage to the Zaporizhzhia Nuclear Power Plant from weeks of shelling, its ability to operate safely and other risks in one of the most dangerous and complicated missions in the agency’s history. By evening, most of the team had headed back to Ukrainian-held territory, leaving behind five members who officials said would stay into the weekend.
“I worried, I worry and will continue to be worried” about the safety of the plant, Rafael Mariano Grossi, director general of the I.A.E.A., said after returning from the visit. He said he had spent about five hours at the plant, going to key locations there.
“It is obvious that the plant and physical integrity of the plant have been violated several times,” he said in brief comments to reporters on a roadside. “By chance or by deliberation? We don’t have the elements to assess that. But this cannot continue to happen.”
“Whatever you think about this war, this is something that cannot continue to happen,” he added.
The I.A.E.A., the United Nations’ nuclear watchdog group, had pleaded for months for access to the imperiled plant, and as recently as Wednesday, comments by Russian officials cast doubt on their willingness to cooperate.
The agency’s team piled into nine armored S.U.V.s Thursday morning to make the nerve-wracking trip into an active war zone, across the front lines and through multiple checkpoints, in a drive that was forced to detour and lasted several hours longer than planned because of shelling near its intended route.
There was little indication Thursday of what the experts had found. An I.A.E.A. spokesman earlier had said the team intended to present its findings back at its headquarters in Vienna by the end of the week. The agency hopes to establish a permanent presence at the plant.
As the U.N. experts set off on Thursday morning from Ukrainian-held territory toward the dangerous buffer zone separating the two armies, mortar shells struck the plant, the Ukrainian nuclear power company, Energoatom said. It blamed Russian forces for the attack, which caused equipment failures that forced the reactor shutdown and switch to backup generators. Ukrainian officials said strikes killed civilians Thursday near the plant, but they were unable to ascertain how many.
Russia’s Ministry of Defense said it was Ukraine that had shelled the nuclear complex and the inspectors’ route, and that “four shells exploded 400 meters from the first unit” of the plant. In a statement, the ministry claimed that Russian forces had intercepted two groups of Ukrainian commandos, up to 60 troops combined, who crossed the Dnipro River in boats to sabotage or seize the plant.
The extent of damage to the plant, Europe’s largest nuclear power station, was not immediately clear, and there were no reports of heightened radiation levels around the facility. But weeks of repeated strikes in and around the plant, which is controlled by Russian forces but operated by Ukrainian engineers, have raised fears of a nuclear catastrophe.
The urgency of the threat prompted the U.N. team to make the last-minute decision to proceed to the plant even as the thud of artillery was heard in the parking lot of their hotel, 30 miles away. Before setting off, Mr. Grossi, who led the mission, said that the Ukrainian military had briefed him on the combat overnight near the planned route.
“We are aware of the current situation; there has been increased military activity,” he said.
“But weighing the pros and cons, and having come so far, we are not stopping,” he added. “We consider that we have the minimum conditions to move, accepting the risks are very, very high.”
Foreign journalists were not permitted to accompany the inspectors, who were among the few international personnel who have crossed the front line since the war began in February.
The I.A.E.A. had said that at Zaporizhzhia its team would check on safety systems, assess damage to the plant and evaluate the working conditions of the staff, who have endured a harsh occupation. Among the main concerns is that fires or other damage could cause cooling systems to fail and lead to a nuclear meltdown.
In the hours before the mission departed, Ukrainian officials reported barrages by rocket artillery, fire from howitzers and the flight of attack helicopters in the town of Enerhodar, which lies along the route the nuclear monitors traveled. Videos posted online showed one round shrieking into the town and exploding in a courtyard.
The town’s mayor, Dmytro Orlov, said it was Russian shelling that had hit the town, and that “machine gun fire is heard,” as well. There were civilian casualties, he said, but it was unclear how many.
The Ukrainian authorities said the Russian military was firing chaotically in the area of the plant to raise the specter of a nuclear accident that would be blamed on Ukraine, and to delay the I.A.E.A. team, cutting into the time allotted for its inspection of the sprawling plant.
Vladimir Rogov, an official in the Russian army’s occupation administration in the region surrounding the plant, accused Ukraine of firing on Thursday morning. Russia’s defense ministry confirmed the deployment of helicopters, but said they were attacking Ukrainian commandos.
Ukraine’s military intelligence agency issued a statement saying the Russian military had further elevated the risk of an accident at the station on Thursday by preventing some employees, including firefighters, from entering the plant during the monitors’ visit. It was not possible to independently verify the claim.
On Wednesday, one Russian occupation official in the region said the I.A.E.A. inspectors would have just one day to see the plant, though the agency called for at least three. Another official said the team would not be waved through checkpoints along their route, but would have to wait in line with civilian traffic, potentially consuming much of that day.
The I.A.E.A. has inspected nuclear sites in Iran, Iraq and North Korea, but its mission on Thursday was its first into a battlefield. Earlier in the conflict, Russian troops occupied the Chernobyl nuclear disaster zone for a few weeks, stirring up radioactive soil and terrifying the power plant’s workers, but the main radiation risk they created was to themselves.
Solemn-faced, the monitors heaved on blue, armored vests in a hotel parking lot as dawn broke on Thursday.
It was unclear what benefits the perilous visit would bring.
The I.A.E.A., which is largely toothless in enforcing its recommendations, can offer an impartial glimpse of goings-on inside a plant the Russians seized six months ago, said Oleksandr Sukhodolya, an energy analyst at the National Institute of Strategic Studies in Kyiv. But that could mean little more than reinforcing the obvious, that fighting around the plant has emerged as among the gravest risks of environmental and health hazards of the war.
The agency can also help draw more attention to dangers that have already alarmed governments around the world, which might spur new sanctions against Russia, he said.
The mission will assess whether the so-called seven pillars of civilian nuclear safety — a list that includes the full functioning of backup systems and the guaranteed physical security of a power plant — are being met, said Ivan Plachkov, a former Ukrainian minister of energy. But the I.A.E.A. has no authority to order a cease-fire or require that Russia withdraw its forces from the station — the two steps outside experts say could actually solve the problem.
“The I.A.E.A. provides only information,” Mr. Plachkov said.
Marc Santora contributed reporting from Kyiv, Ukraine, and Oleksandr Chubko from Zaporizhzhia, Ukraine.