The unit was shut down in October for inspections and maintenance under stricter rules brought in after a quake-sparked tsunami swamped the Fukushima plant in 2011, sending some reactors into meltdown and causing the worst nuclear disaster in a generation.
Utility Kyushu Electric said it switched on its No. 1 reactor at the Sendai plant in southern Kagoshima late yesterday evening, with commercial operations expected to begin early next month.
Its closure had left Japan with just two operating reactors, after dozens of units were switched off in the wake of the disaster.
The Sendai plant sits at the centre of a national debate over nuclear power.
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The region's top politician had earlier demanded that Kyushu Electric shut down the reactors completely, but the firm reached a compromise to carry out "special inspections" on top of regular maintenance work.
A spokeswoman for Kyushu Electric today said the company's review would continue after the restart to ensure "an even higher level of safety" for the reactor.
Also today, the Japanese government said the estimated cost of dealing with Fukushima have almost doubled from a previous forecast to nearly USD 190 billion.
The catastrophe forced the resource-poor country to turn to expensive fossil fuels to plug its energy gap, but fears about the safety of nuclear power and radiation exposure linger.
The two Sendai reactors were restarted last year under the Post-Fukushima safety regulations.
Another reactor at the Ikata plant in western Japan is also operating.
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