Shelling in Ukraine once again threatened the largest nuclear power plant in Europe on Thursday, damaging equipment on the grounds, as Russian and Ukrainian forces blamed each other for creating the risk of a catastrophic nuclear accident in the middle of a war zone.
That danger prompted a meeting Thursday of the United Nations Security Council, where Rafael M. Grossi, the head of the International Atomic Energy Agency, the U.N. nuclear watchdog, said that although experts had assessed there was “no immediate threat” to nuclear safety at the Zaporizhzhia nuclear plant, “this could change at any moment.”
A single errant shell could be disastrous, but amid the combat and conflicting claims, it has been hard for independent monitors to judge what is happening. As alarm grew this week, the U.N. secretary general warned that any attack on the facility was “suicidal,” and the State Department said the United States supported a demilitarized zone around the nuclear plant.
“Any military action jeopardizing nuclear safety or nuclear security must stop immediately,” Mr. Grossi said. “These military actions taking place near such a large nuclear facility could lead to very serious consequences.”
Russian forces seized control of the plant, the largest nuclear generator in Europe, soon after invading Ukraine in February, but have kept the Ukrainian workers there operating the plant. Ukrainian officials have accused Russia of using the facility as a shield for bombarding the city of Nikopol and its surroundings on the opposite bank of the Dnipro River, where local officials said at least 13 people were killed in shelling on Wednesday.
Ukrainians say they do not want to cause a nuclear crisis by shooting back at the Russian occupiers of the plant. But in fact, it has been hit — though there have been no reports so far of damage to the reactor buildings — driving worldwide expressions of fear for an accident and conflicting accusations of responsibility. Last week, shelling damaged a spent fuel storage area at the complex, though there has been no indication of a resulting radiation leak.
Mr. Grossi said that recent explosions near an electrical switchboard caused the shutdown of a transformer and two backup transformers. Another round of shelling was in the area of the plant’s nitrogen-oxygen station, he said, causing a fire that was put out. And at least one staff member working in the area where dry spent nuclear fuel is stored was injured in yet another episode of shelling.
The situation, he said, has been “deteriorating rapidly to the point of becoming very alarming.”
The most serious threat to the plant is a strike, either directly on a reactor or on vital supporting gear, that causes a breakdown of its cooling system, said Edwin Lyman, a nuclear power expert at the Union of Concerned Scientists, a private group in Cambridge, Mass. He noted that the loss of coolant during the Fukushima Daiichi accident in 2011 resulted in three reactors undergoing some degree of core meltdown.
If the cooling is interrupted, Dr. Lyman explained, in a matter of hours the nuclear fuel can become hot enough to melt. Eventually, it can melt through the steel reactor vessel and even the outer containment structure, releasing radioactive material.
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Mr. Grossi, who has led two missions to Ukrainian nuclear facilities since the war began, urged Russia and Ukraine to cooperate enough to grant him and a team of experts access to the Zaporizhzhia plant, where they could assess the damage and ensure the plant’s safety and continued monitoring.
He said a mission on the ground was critical because although the agency had received information about the state of the facility and operation from both nations, their statements were “frequently contradictory.” The I.A.E.A. could only corroborate “some very important facts,” such as the damage to the facilities and the state of backup systems, by visiting with a team, he said.
“This is a serious hour, a grave hour, and the I.A.E.A. must be allowed to conduct its mission to Zaporizhzhia as soon as possible,” he said.
Russia’s ambassador to the United Nations, Vasily Nebenzya, said Russian forces were ready to assist a trip by the nuclear experts. “We believe it justified for I.A.E.A. representatives to go to the Zaporizhzhia power plant as quickly as possible,” potentially before the end of August, he said.
But he also said that the U.N. officials needed to tell Ukraine, which he blamed for artillery and missile attacks against the plant, that it must immediately stop military actions in the area.
Ukraine’s national atomic energy company, Energoatom, claimed in a statement that it was Russian forces that had hit “very close to the first power unit” on Thursday and had “damaged the domestic sewage pumping station.”
“The situation is getting worse, because there are radiation sources nearby and several radiation sensors are damaged,” it added.
Mr. Nebenzya, the Russian ambassador, said it was absurd to think that his country’s troops were attacking a site they have controlled for months, when they would be the ones most at risk from a radioactive calamity.
“Elementary logic would suggest that our soldiers would have no reason to shell either the plant or the town or themselves even,” he said.
Ukraine’s ambassador, Sergiy Kyslytsya, accused Russia of trying to divert attention away from strikes that killed Ukrainian civilians in the area, saying it had resorted to dangerous “staged shelling” of the power plant itself.
Russian forces, he said, were known for their “deceit, sabotage and cover-ups like the one we witnessed today.”
He said that Ukraine also supported a U.N. mission to the site, but that Russia’s continued shelling against Ukrainian-controlled towns and cities, about five miles away, was preventing it. “If Russia stops shelling these cities, what in the world will prevent the mission to cover these five miles?” he said.
What no one disputes is that shelling around the site risks one of the worst nuclear incidents in history — a prospect that is anything but abstract to Ukrainians. The Chernobyl plant, site of the world’s worst nuclear accident in 1986, is also in Ukraine, and was also seized by Russian troops in this war, though they later withdrew.
“For years absolutely no one could imagine that Europe could be threatened with a new disaster at a nuclear plant not because someone would violate some safety standards, but because some state would deliberately use a nuclear plant for terror,” President Volodymyr Zelensky of Ukraine said on Thursday, speaking to a meeting of northern European defense ministers.
“This is what Russia is doing now,” he said. “And if the Soviet authorities tried to conceal the Chernobyl disaster and its full consequences, the Russian authorities are much more cynical and dangerous — they themselves do everything to maximize the risk of a nuclear disaster, and lie to the whole world that someone else is allegedly to blame.”
Ukrainian officials have accused Russian forces of subjecting the plant’s staff to harsh interrogations, even torture, and of holding some of them at the site against their will.
The United Nations secretary general, António Guterres, said on Thursday that he was “gravely concerned” about increasing hostilities in and around the Zaporizhzhia nuclear complex.
“Over the past several days, there have been reports of further deeply worrying incidents that could, if they continue, lead to disaster,” Mr. Guterres said in a statement. “I am calling on the military forces of the Russian Federation and Ukraine to immediately cease all military activities in the immediate vicinity of the plant and not to target its facilities or surroundings.”
A State Department spokesman on Thursday said the United States supported a demilitarized zone around the nuclear plant and called on Russia to cease military operations on its grounds or nearby.
William J. Broad and Matthew Mpoke Bigg contributed reporting.