Europe’s largest nuclear power plant has faced a series of crises since Russian forces seized it more than a year ago amid a blaze of gunfire. Last summer, it was subject to repeated shelling and on at least one occasion artillery hit an area where spent nuclear fuel is stored.

Control of the plant, which is on the eastern bank of the Dnipro River, has given Moscow a degree of leverage over Ukraine’s energy production. It has not produced electricity for the grid since last year but even so it requires power from outside for safety purposes.

A single high-voltage cable has been bringing electricity in for much of the war. It has repeatedly been severed by shelling, leading to seven blackouts and the reliance on diesel generators, whose capacity can be measured in days, to keep safety systems running. On each occasion the external power has been quickly restored.

The risk that Ukraine faces at the nuclear plant stands apart, even amid the destruction of Russia’s full scale invasion, which has led to the occupation of parts of the country and the flight into exile as refugees of more than 8 million people. The United Nations says it has confirmed the deaths of about 9,000 civilians, but that it believes “the actual figures are considerably higher.”

It is the first time that a nuclear power plant has found itself in an active battle zone, according to Mr. Grossi, and concern over a potential accident is particularly acute in a country that experienced the worst civilian nuclear disaster in history, at the Chernobyl plant in 1986.