EXPLAINER-'Close call' at Ukrainian nuclear plant

Ukraine's Zaporizhzhia nuclear power plant, which is under Russian control, was rocked by shelling on Sunday, drawing condemnation from the U.N. nuclear watchdog which said such attacks risked a major disaster. Repeated shelling of the plant in southern Ukraine has raised concern about the potential for a grave accident just 500 km (300 miles) from the site of the world's worst nuclear accident, the 1986 Chornobyl disaster.

Here is what we know about the incident: WHAT HAPPENED?

The head of the International Atomic Energy Agency (IAEA) said the plant came under the most intense shelling of recent months on Saturday, shortly after 6 p.m. local time, and on Sunday at 9:15 a.m. There were more than a dozen blasts within 40 minutes, according to the IAEA.

IAEA experts reported "damage in several places, including a radioactive waste and storage building, cooling pond sprinkler systems, an electrical cable to one of the reactors, condensate storage tanks, and to a bridge between another reactor and its auxiliary buildings". External power supplies were not affected and radiation levels at the plant remained normal, the IAEA said.

The biggest risk is from overheating nuclear fuel, which could happen if the power that drives the cooling systems was cut. Shelling has repeatedly cut power lines. Besides the reactors, there is also a dry spent fuel storage facility at the site for used nuclear fuel assemblies, and spent fuel pools at each reactor site that are used to cool down the used nuclear fuel.

'CLOSE CALL' IAEA Director General Rafael Grossi said it was a "close call".

"We were fortunate that a potentially serious nuclear incident did not happen," Grossi said. "Next time, we may not be so lucky. We must do everything in our power to make sure there is no next time." Grossi wants a nuclear safety and security protection zone around the plant.

"Even though there was no direct impact on key nuclear safety and security systems at the plant, the shelling came dangerously close to them. We are talking metres, not kilometres. "Whoever is shelling at the Zaporizhzhya Nuclear Power Plant, is taking huge risks and gambling with many people's lives," Grossi said.

WHO SHELLED THE PLANT? Russia and Ukraine blamed each other for shelling the Russian-controlled plant. Reuters was unable to independently verify who was telling the truth.

Russia's defence ministry said Ukraine's armed forces fired 11 large calibre shells at the plant on Nov. 19 and 12 large caliber shells from 9:15-9:45 a.m. local time on Sunday and then two more at power lines. Russia said the shelling was conducted from Marhanets in the Dnipropetrovsk region.

"The regime in Kyiv does not cease provocations aimed at creating a threat of a disaster at the Zaporizhzhia nuclear power plant," the Russian defence ministry said. Ukraine's nuclear energy firm Energoatom said the Russian military shelled the plant. It said there had been at least 12 hits on the plant on Sunday.

"The nature of the damaged equipment at the Zaporizhzhia nuclear power plant shows that the attackers aimed at, and disabled, precisely the infrastructure that was necessary for the start-up of reactors 5 and 6," Energoatom said. "The Rashysty [a portmanteau of Russian and fascists] once again engaged in nuclear blackmail and thus endanger the whole world with their actions!" it said.

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