International Atomic Energy Agency (IAEA) chief Rafael Grossi speaks to media in Zaporizhzhia on September 1, 2022. AFP
The shutdown came as a team of UN experts was en route to inspect the plant -- Europe's largest atomic facility -- that is located on the frontline in an area repeatedly hit by shelling.
The plant's proximity to the fighting has stoked global fears of a nuclear disaster.
Energoatom, which operates the Zaporizhzhia nuclear power plant (ZNPP), said the emergency protection system kicked in shortly before 5:00 am (0200 GMT) "due to another (Russian) mortar shelling" and that "operating power unit five was shut down".
In a posting on Telegram, it said the plant's backup power supply for in-house needs "was damaged" in the attack, causing a blackout.
"This is the second time in the last 10 days that the criminal actions of the Russian military have caused the shutdown of the unit and the power plant blackout," it said.
After the power plant was captured by Russian forces on March 4, Energoatom initially shut off two of the reactors, followed by a third after a shelling incident on August 5.
With a fourth reactor being repaired, that left only reactors five and six working, although last week, they were both shut down then restarted.
With Thursday's shutdown, only reactor six is currently functioning.
"Power unit six keeps operating in the Ukraine power system and at the same time powers the ZNPP's in-house needs. The power plant's Ukrainian workers are doing their best to prevent damage to its infrastructure," it said.