KYIV, Ukraine — Russia’s sustained bombardment of Ukraine’s power grid is deepening concerns about the safety of the country’s nuclear facilities after a drone knocked out power for more than three hours to the site of the 1986 Chornobyl nuclear disaster and as Europe’s biggest atomic power plant remains disconnected from the grid, officials said.

Both Chornobyl and the Russian-occupied Zaporizhzhia Nuclear Power Plant are not working but they require a constant power supply to run crucial cooling systems for spent fuel rods in order to avoid a potential nuclear incident.

A blackout could also blind the radiation monitoring systems, installed to boost security at Chornobyl and operated by the UN nuclear watchdog, the International Atomic Energy Agency.

“Russia is deliberately creating t

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