Reactor No. 4 was shut down at 6:46 a.m. local time by a safety control system after protection equipment in the reactor's steam generator malfunctioned, the state nuclear Energoatom company said in a statement.
Plant workers are repairing the defect and expect to restart the reactor by early Friday, it said.
The radiation level at all of the country's atomic power plants was normal, Energoatom said.
The Zaporizhia nuclear power plant is Europe's largest, operating six nuclear reactors. Only four of them are currently active.
Ukraine operates 13 nuclear reactors at four atomic power plants, but some reactors are frequently halted due to malfunctions or scheduled repairs.
Ukraine was the site of world's worst nuclear disaster after 1986 explosion at the Chernobyl atomic power plant, which was closed for good in December 2000.
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