While Zaporizhzhia appeared secure on Friday, Russia’s seizure of the vast energy production site and potential targeting of another nuclear plant created risks of an accident.
LVIV, Ukraine — In darkness, Russia captured Europe’s largest nuclear power plant on Friday in Ukraine, prompting questions about the reasons it invaded the sprawling reactor site as well as the health risks to Ukrainians fighting desperately for their lives and freedom.
And it’s not the only power plant in Ukraine that could face attack by Russian forces. Some troops already appear to be marching toward another facility west of the Zaporizhzhia power plant, a Ukrainian energy official said.
For the moment, the Zaporizhzhia nuclear complex appears safe, with the plant’s array of sensitive detectors finding no releases of radioactivity above the usual background levels.
The vast site, on the Dnieper River roughly a hundred miles north of Crimea, hosts not only six reactors whose cores are full of highly radioactive fuel but also many acres of open ponds of water where spent fuel rods are submerged to cool off, typically for years. Experts fear that an errant shell or missile might set off an environmental disaster and, if a fire broke out, release clouds of radioactive particles that get carried by the wind around Europe.
“The thing you worry about is an ignorant soldier who is scared and fires off a rocket or a mortar that causes a calamity,” said David Albright, a physicist and president of the Institute for Science and International Security, a private group in Washington that tracks nuclear arms and matériel.
An accident of that nature could pose enormous dangers to Ukrainians and people in surrounding countries, including Russia.
“If you have a cloud with nuclear isotopes which can be formed in this accident at Z.N.P.P. then only God knows where it will go,” Petro Kotin, the acting president of Energoatom, the state company that runs Ukraine’s nuclear energy facilities, said, referring to Zaporizhzhia.
A threat of that kind emerged during Friday’s assault on facilities adjacent to the nuclear reactors at Zaporizhzhia.
For days before the attack, social media reports had detailed how residents of Enerhodar, a city adjacent to complex, had set up a giant barrier of tires, vehicles and metal barricades to try to block a Russian advance into the city and the reactor site. The barricade was so large that it could be seen from outer space by orbiting satellites.
On Wednesday, Ukraine’s minister of energy, Herman Galushchenko, gave an address in which he praised the city’s heroism. “You are currently going through a difficult ordeal,” he said. “But knowing many of you personally, I am confident in your every action and devotion to our state.”
All week, the website of the Zaporizhzhia complex had noted that the Ukrainian military unit 3042, which guards the complex, was “in combat readiness.”
When the Russian forces broke through, fighting resulted in exchanges of munitions and a fire, although it was contained to a training facility on the perimeter of the complex. Mr. Kotin said a Russian shell hit the No. 1 reactor, but its thick walls gave it the strength to survive the blow.
International observers and Ukrainian officials said that as of 6 a.m. on Friday, the complex was able to function safely. Mr. Kotin said that the Russian forces had allowed the nuclear plant’s operations to continue at a normal pace since they took over the complex.
“The personnel itself is working normally in the unit, if we can say ‘like normal,’” said Mr. Kotin, who served as the Zaporizhzhia facility’s general manager and was closely involved in its construction before leading Energoatom.
Rafael M. Grossi, director general of the International Atomic Energy Agency, an arm of the United Nations that sets safety standards for the world’s nuclear reactors, said on Friday that three of the site’s six reactors had been shut down and that the remaining three were operating at less than full capacity.
Mr. Kotin said the Russian forces did not include nuclear energy specialists from Russia, only soldiers who were not familiar with the operations of a complex power facility. But the Russians had allowed one shift of workers to be replaced by another, which may be crucial to keeping the plant operating safely.
“The most dangerous thing for the plant is when people do not go on rotation,” said Olena Pareniuk, a nuclear safety expert at the National Academy of Sciences of Ukraine. “They get tired. But to work at the nuclear plant is like to be a surgeon, it is important for people to be rested and not stressed to avoid mistakes.”
But Dr. Pareniuk warned that living under military occupation would put a strain on the thousands of Ukrainians who work at Zaporizhzhia. Mr. Kotin warned of a humanitarian crisis in Enerhodar, which he said was cut off from deliveries of food and other supplies. He said that Russian soldiers were seizing food from the town’s shops. The city was working to restore heat, water and electricity.
Should an accident occur in which the reactor fuel overheats and melts the reactor’s core or shielding, Frank N. von Hippel, a physicist who advised the Clinton administration and now teaches at Princeton, said it could be a repeat of the meltdowns that happened in 1979 at Three Mile Island near Harrisburg, Penn.; in 1986 at Chernobyl in Ukraine; and in 2011 at the Fukushima Daiichi plant in Japan.
Further combat at Zaporizhzhia could pose an additional danger involving the site’s spent fuel rods, which contain up to roughly 30 tons of plutonium, according to an estimate by Mr. Albright and two colleagues. Plutonium is a highly toxic metal that, if inhaled or ingested in minute quantities, can cause death by cancer.
Mr. Grossi said that he was “extremely concerned” about the situation and that the main priority at this point was to ensure the safety and security of the plant, its power supply and the people who run it. “Firing shells in the area of a nuclear power plant,” he said, “violates the fundamental principle” that seeks to maintain a nuclear site’s physical integrity at all times.
Speaking at an emergency U.N. Security Council meeting on Friday, Russia’s ambassador, Vasily Nebenzya, denied his country had purposely targeted the Zaporizhzhia nuclear complex early Friday, and challenged Ukrainian officials’ account that Russian troops attacked the plant and that the fire started as a result of the gun battle. He maintained that Russian forces patrolling outside of the nuclear plant had come under fire from Ukrainian militants inside a training building and had returned fire. He said the Ukrainians had then set the building on fire.
According to the I.A.E.A., Ukraine gets a bit more than half its electricity from its 15 reactors — an unusually high fraction globally. In comparison, nuclear power in the United States generates about 20 percent of the nation’s electricity.
Russia-Ukraine War: Key Things to Know
Nuclear plant seized. Russian troops seized control of Zaporizhzhia’s nuclear power plant, the largest in Europe, located in southeastern Ukraine. A nearby fire had been extinguished, and there was no immediate sign that radiation had leaked.
When running at full power, the six reactors of the Zaporizhzhia complex produce a total of 6,000 megawatts of electric power, according to the I.A.E.A. In comparison, the Chernobyl plant in northern Ukraine, now shut down, produced 3,800 megawatts — roughly a third less.
Western experts said a wider possible repercussion from the Russian takeover of the Zaporizhzhia complex was the slow degradation of Ukraine’s power grid, which could throw nuclear power plants offline and result in cascading blackouts. In addition, Moscow might use its takeover as a weapon to threaten the shutdown of the entire Ukrainian power grid.
“The Russians understand that energy is a massive tool of power,” said R. Scott Kemp, a professor of nuclear science at M.I.T. “It’s a point of tremendous leverage.”
Intentional changes in the power output of the Zaporizhzhia complex, he said, “can essentially cause the whole country to lose its clean water, the pumping of gas, the refrigeration of food and electrical power need for the communications of the military and the government. It’s a serious vulnerability.”
Mr. Kotin said he expected additional Russian threats to other nuclear power facilities in Ukraine, particularly the South Ukrainian nuclear power plant.
That plant generates about half as much power as Zaporizhzhia and is further west in the city of Yuzhnoukrainisk. Mr. Kotin said Russian troops were about 20 miles from the South Ukrainian plant but were already fighting with Ukrainian forces on the way to it.
Mr. Kotin says the Russian military’s purpose in occupying the nuclear stations could be to warn Ukrainians that they will be cut off from electrical power if they do not acquiesce to the invasion. “If we do no like them,” he said, they’ll threaten “to destroy our nuclear objects.” He added that another possibility was that capturing the energy plants would aid a Russian plan to divide the country into manageable pieces: By controlling power production in the south, they would be able to control the south, he suggested.
A darker possibility relates to nuclear weapons manufacture.
On Wednesday, Mr. Grossi of the I.A.E.A. dismissed Russian claims that Ukraine was seeking to acquire atomic weapons, saying that his agency’s oversight of the nation showed that Kyiv’s nuclear program was entirely peaceful. But Russia may be motivated to seize Ukraine’s nuclear energy plants in part to gather atomic material for itself or to close off an unlikely and difficult path for Kyiv to acquire a nuclear weapon.
Plutonium is one of two main fuels used in the cores of atom bombs. Mr. Albright of the Institute for Science and International Security said that the spent fuel at the Zaporizhzhia plant could in theory, if suitably processed, yield fuel for up to 3,000 warheads.
While plutonium from reactors is not considered high-grade weapon fuel, the United States during the Cold War conducted studies and tests that showed it was usable. In 1962, it went so far as to successfully test a nuclear bomb made out of reactor-grade plutonium.
The Russians, too, have extracted bomb material from reactor plutonium throughout the nuclear age, and experts say they fear that the Ukrainians could in theory learn the dark art one day. Such a scenario would require a major failing of the I.A.E.A., which is heavily focused on ensuring that no operator of a peaceful nuclear plant anywhere on the planet seeks to covertly extract plutonium from spent fuel for nuclear arms.
Valerie Hopkins reported from Lviv, Ukraine, and William J. Broad reported from New York. Maria Varenikova, Farnaz Fassihi, Marc Santora and Christoph Koettl contributed reporting.
