On Tuesday, Tomomitsu Sakata, a spokesman for Kyushu Electric Power, said the reactor was put back online as planned without any problems.
The Fukushima disaster displaced more than 100,000 people due to radioactive contamination and spurred a national debate over this resource-scarce country's reliance on nuclear power.
A majority of Japanese oppose the return to nuclear energy. Dozens of protesters, including ex-Prime Minister Naoto Kan, who was in office at the time of the disaster and has become an outspoken critic of nuclear power, were gathered outside the plant as police stood guard.
The Nuclear Regulation Authority affirmed the safety of the Sendai reactor and another one at the plant last September under stricter safety rules imposed after the 2011 accident, the worst since the 1986 Chernobyl explosion. The second Sendai reactor is due to restart in October.
Koichi Miyazawa, Japan's industry minister, said Tuesday that the government would “put safety first” in resuming use of nuclear power.
Protesters outside the Sendai plant are not convinced.
“You will need to change where you evacuate to depending on the direction of the wind. The current evacuation plan is nonsense,” said Shouhei Nomura, a 79-year-old former worker at a nuclear plant equipment maker, who now opposes atomic energy and is living in a protest camp near the plant.
All of Japan's 43 workable reactors were idled for the past two years pending safety checks. To offset the shortfall in power output, the country ramped up imports of oil and gas and fired up more thermal power plants, slowing progress toward reducing its emissions of greenhouse gases.
Prime Minister Shinzo Abe has sought to have the reactors restarted as soon as possible to help reduce costly reliance on imported oil and gas and alleviate the financial burden on utilities of maintaining the idled plants.
Utilities are seeking approvals to restart 23 reactors, including the other Sendai reactor.