The Ikata nuclear plant began generating electricity Monday, ahead of its planned reconnection to the grid on Sept. 7, operator Shikoku Electric Power Co. said.
The plant in Ehime Prefecture is one of only two nuclear power stations currently operating in Japan, despite the government seeking to plug back into the energy source more than five years after the Fukushima crisis.
The plant's No. 3 reactor was reactivated Friday after it cleared safety checks drafted in the wake of the catastrophe at the Fukushima No. 1 nuclear plant.
The reactor is the sole operating unit running on plutonium-uranium mixed oxide fuel, which contains plutonium extracted from reprocessing spent fuel.
MOX is a central component of Japan's fuel cycle and is also important as a way to reduce the nation's stockpile of weapons-grade plutonium.
But MOX fuel is more radioactive than ordinary uranium fuel, and this meant it was controversial even before the Fukushima crisis.
The 890-megawatt reactor will shift to commercial operation after final checks by the Nuclear Regulation Authority.
The unit was taken offline in April 2011 for scheduled maintenance and inspections and remained in that state until last week.