Combat at Ukraine Nuclear Plant Adds Radioactive Dangers to Russian Invasion

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 not 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.