In a handout photo taken from video released by the Russian Defense Ministry Press Service on September 2, 2022, International Atomic Energy Agency (IAEA) director Rafael Grossi, center, in a white helmet, and other IAEA officials walk while inspecting the Zaporizhzhia Nuclear Power Plant in Energodar, Ukraine, September 1, 2022 | RUSSIAN DEFENSE MINISTRY PRESS SERVICE/AP

A small group of United Nations experts was at Europe's largest nuclear plant for a second day on Friday to assess damage to the facility that is caught on the front lines of Russia's war in Ukraine. A convoy of inspectors arrived at the Russian-occupied Zaporizhzhia Nuclear Power Plant on Thursday, but it's unclear how much access they were given to critical areas of the sprawling compound.

CBS News correspondent Debora Patta says massive explosions near the plant threatened to derail the International Atomic Energy Agency inspectors' mission even before it began on Thursday, but IAEA director Rafael Grossi and his team braved the shelling and finally managed to start their inspection of the plant after six months of war.

"I will continue to be worried about the plant until we have a situation which is more stable, which is more predictable," Grossi said.

Nuclear plants require a constant supply of electricity to keep spent nuclear fuel cool, and the reactors at Zaporizhzhia have already had to rely on back-up generators on several occasions as a result of the fighting around the plant.

Shells have even landed inside the compound in recent weeks as Ukraine and Russia accuse each other of putting the residents around it, and the entire region, at risk of a possible atomic accident.

"When it goes on to back-up power, we're in the last chance saloon, as it were," nuclear expert Hamish de Bretton Gordon told CBS News. "There is nothing after that."

"If this was a power station in the U.S. or the U.K.," he said, engineers might have to rely on back-up power to keep vital systems running "maybe once in a generation." The Ukrainian technicians operating the Zaporizhzhia facility — reportedly under intense pressure from the occupying Russian forces — have been forced to switch on the plant's generators twice within a week.

If the back-up were to fail, de Bretton Gordon says, a chain reaction would be set in motion.

"If that is not cooled on a regular basis, it then creates what we call a meltdown," he told Patta. Potentially, a nuclear catastrophe.

That's why Grossi has left a team of at least five inspectors at the plant, who will continue to work there until the weekend with a singular goal: To prevent a global disaster.

Grossi has said that his organization won't be leaving the plant even after this weekend, as they intend to establish a constant presence there.

In what appeared to be Moscow's first confirmation of any firm position on the notion of a lasting IAEA mission at Zaporizhzhia, a Russian ambassador in Austria said Friday that his country was happy for two inspectors to remain at the plant permanently.

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