A Russian soldier guards part of the sprawling Zaporizhzhia Nuclear Power Station complex in territory under Russian military control in southeast Ukraine, May 1, 2022 | AP

Ukrainian officials say the United Nations' atomic energy watchdog agency is expected to send a team to visit the massive Zaporizhzhia nuclear power plant next week, after it was temporarily cut off from the Ukrainian power grid for the first time ever on Thursday. Ukrainian officials said Friday that residential areas near the plant were hit yet again by Russian shelling overnight, but that operations were resuming at the facility's two working reactors.

Fire damage to a power line at the nuclear plant, which is the largest in Europe, cut electricity to the whole region Thursday and stoked fears of a potential atomic catastrophe in a country that still suffers from collective nightmares over the 1986 Chernobyl disaster.

The Zaporizhzhia plant sits on the southeast bank of the Dnieper River, right on the edge of Russian-occupied territory in southeast Ukraine, putting the sprawling complex squarely on the front line of some of the most intense battles raging between Ukrainian and Russian forces.

President Biden spoke Thursday with Ukraine's President Volodymyr Zelenskyy and both men urged Russia to withdraw its troops from the facility, which they've occupied since early in the invasion ordered by Vladimir Putin on February 24.

As CBS News correspondent Debora Patta reported, the plant had never been completely cut off from Ukraine's power grid before, and since a constant electricity supply is critical to cooling spent fuel and avoiding a disastrous meltdown at any nuclear plant, the temporary disconnection did nothing to allay the rising concern among outside experts and Ukrainians living in the shadows of Zaporizhzhia's reactors.

"If the power goes off, we're then reliant on fairly elderly diesel generators to run the safety systems," nuclear expert Hamish de Bretton Gordon explained to CBS News, referring to the backup power systems that Ukrainian engineers at the plant were able to put into action on Thursday. He stressed that while the backup generators are a functional stopgap, they're not intended to serve as a primary power supply for long.

"Once you lose the main power supply, you're almost in a two-engine airplane which loses one engine, and then you're in a bad position," he said.

Zelenskyy lauded the Ukrainian engineers — who've remained at the Zaporizhzhia facility and kept it running despite reportedly intense pressure from the Russian troops occupying it — for averting a disaster on Thursday by keeping the essential systems running.

Russia and Ukraine have accused each other of shelling the nuclear plant repeatedly since mid-July.

Patta says the Ukrainians living in the heavily bombarded towns and villages near the facility have been running safety drills, planning for a worst-case scenario. But the exercises in preparedness do little to put them at ease.

"Everyone will die if there's a nuclear disaster," declared resident Olena Sidoryakina. "Not just us in Ukraine, the whole world."

Zelenskyy issued a similarly dire warning on Thursday night, and he made it clear that no time should be wasted in the negotiations with Russia to get a team from the U.N.'s International Atomic Energy Agency (IAEA) access to the plant.

"Every minute that the Russian military stays at the nuclear plant is a risk of a global radiation disaster," he said after his phone call with Mr. Biden. Zelenskyy thanked the American leader for his "full support" in trying to get the sprawling nuclear plant "back under proper control of Ukraine and immediately provide access for the IAEA."

He said it was possible to make that happen before "the occupiers bring the situation to the point of no return," but warned that "it is easier to do it now than later, if the wind begins to carry radiation pollution to Europe."

"The IAEA and other international organizations must act much faster," Zelenskyy said.

In a tweet on Thursday, IAEA chief Rafael Mariano Grossi said the nuclear watchdog agency's visit was "imminent," but he didn't confirm remarks from Ukrainian officials that it was to happen next week. Grossi told a French radio station on Friday that the atomic agency might try to maintain a permanent presence at the Ukrainian nuclear facility.

Russia's representative to the IAEA, Mikhail Ulyanov, said Friday that "active preparations" for an IAEA visit to Zaporizhzhia were "underway," but he also declined to offer any confirmation of the precise timing of the mission.

Lana Zerkal, an adviser to Ukraine's energy minister, told Ukrainian media Thursday evening that the remaining logistical issues were being worked out for the IAEA team's visit to the nuclear plant, but she accused Russia of trying to derail the visit before plans for it were even nailed down.

"Despite the fact that the Russians agreed for the mission to travel through the territory of Ukraine, they are now artificially creating all the conditions for the mission not to reach the facility, given the situation around it," she said, likely referring to the ongoing shelling around the facility.

The regional Ukrainian governor said Friday that the already-battered city of Nikopol, just across the river from the Zaporizhzhia plant, was hit again by Russian shelling overnight, damaging 10 homes and a school but causing no casualties.

CBS News senior foreign correspondent Charlie D'Agata was in Nikopol and nearby Marhenets recently and saw the damage inflicted by Russian shells on civilian homes. Residents told him they could hear the rockets being fired nightly, and they said they were clearly coming from the Russian-held nuclear facility.

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