A power plant operator in southern Japan has restarted a reactor, the first to begin operating under new safety requirements following the Fukushima disaster.

Kyushu Electric Power Co. said Tuesday it had restarted the No. 1 reactor at its Sendai nuclear plant as planned. The restart marks Japan's return to nuclear energy four-and-half-years after the 2011 meltdowns at the Fukushima Dai-ichi nuclear power plant in northeastern Japan following an earthquake and tsunami.

The national broadcaster NHK showed plant workers in the control room as they turned the reactor back on. Tomomitsu Sakata, a spokesman for Kyushu Electric Power, said the reactor was put back online as planned without any problems.

The disaster displaced more than 100,000 people due to radioactive contamination in the area and spurred a national debate over this resource-scarce country's reliance on nuclear power.

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.

All of Japan's nuclear plants were shut down for the past two years. 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.