Backup generator deployed to prevent Ukraine nuclear disaster

Zelensky said the plant's Ukrainian staff prevented disaster by quickly switching on the backup diesel generators.

"If our station staff had not reacted after the blackout, then we would have already been forced to overcome the consequences of a radiation accident," he said.

Ukrainian officials told the International Atomic Energy Agency (IAEA), the UN's nuclear watchdog, that Europe's largest nuclear power plant lost connection to its primary power source "at least twice" on Thursday.

Its backup generator can power its cooling systems, but its two operating reactor units remain disconnected from the electricity grid.

If emergency power to the plant's security systems is disrupted, water used to cool down spent fuel rods will heat up and evaporate. Without coolant, the fuel rods become vulnerable to catching fire, causing explosions which would release radiation into the atmosphere.

According to the Ukraine state nuclear company Energoatom, this was the first complete disconnection in the plant, which was captured by Russian forces in March but is still operated by Ukrainian technicians.

The facility normally has four external power lines, but three of them were lost earlier during the conflict.

"Russia has put Ukraine and all Europeans in a situation one step away from a radiation disaster," Zelensky said, urging that UN officials be given access to the site.

IAEA director general Rafael Mariano Grossi said he would personally lead a visit to the site in the "next few days."

"Almost every day, there is a new incident at or near [the Zaporizhzhia facility]", he tweeted on Thursday. "We can't afford to lose any more time."

"Russia should agree to the demilitarised zone around the plant and agree to allow an International Atomic Energy Agency visit as soon as possible," White House press secretary Karine Jean-Pierre told press.

But writing on Telegram, Vladimir Rogov, a Russian-appointed official in the occupied town of Enerhodar near the plant, denied any wrongdoing on Russia's part and said that the blackout was caused "as a result of provocations by Zelenskiy's fighters."