KYIV, Ukraine (AP) — Ukraine’s Zaporizhzhia Nuclear Power Plant, Europe’s largest atomic power station, spent hours operating on emergency diesel generators Monday after losing its external power supply for the seventh time since Russia’s full-scale invasionof its neighbor, the head of the U.N. nuclear watchdog said.

“The nuclear safety situation at the plant (is) extremely vulnerable,” Rafael Grossi, head of the International Atomic Energy Agency, said in a tweet.

Hours later, national energy company Ukrenergo said on Telegram that it had restored the power line that feeds the plant.

But for Grossi, it was another reminder of what’s at stake at the Russian-occupied plant which has seen shelling close by.

“We must agree to protect (the) plant now; this situation cannot continue,” Grossi said, in his latest appeal for the area to be spared from the fighting between Ukrainian and Russian forces. IAEA staff are deployed at the plant, which is occupied by Russian troops.

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The plant’s six nuclear reactors, which are protected by a reinforced shelter able to withstand an errant shell or rocket, have been shut down. But a disruption in the electrical supply could disable cooling systems that are essential for the reactors’ safety even when they are shut down. Emergency diesel generators, which officials say can keep the plant operational for 10 days, can be unreliable.

Fighting, especially artillery fire, around the plant has fueled fears of a disaster like the one at Chernobyl, in northern Ukraine, in 1986. Then, a reactor exploded and spewed deadly radiation, contaminating a vast area in the world’s worst nuclear catastrophe.

Energoatom, Ukraine’s state nuclear company, blamed Russian shelling for the loss of the last high-voltage transmission line to the plant in Russian-occupied southern Ukraine, about 500 kilometers (300 miles) from Kyiv. It was not possible to independently verify that claim.

The facility is “on the verge of a nuclear and radiation accident,” Energoatom warned. Once the power line was restored, Energoatom described the situation as “stabilized.”

Grossi said it was the seventh time the plant had lost its outside power supply since Russia’s full-scale invasion in February 2022