The Zaporizhia Nuclear Power Plant said on its website that radiation levels in the area remained normal and that the station's other five reactors continued to operate without any problems.
It said the plant's sixth reactor was "disconnected from the grid by the generator's internal defence mechanism" early Sunday.
A short circuit in the same plant's third reactor contributed to power shortages in Ukraine at the start of the month.
The Zaporizhia station, in the southeast of the country, is Europe's largest and fifth-most powerful in the world, AFP reported.
It generates 40 percent of Ukraine's nuclear power and switched on its first reactor in 1984.