Japan switched on the first nuclear reactor in nearly two years following a country-wide shutdown in the wake of the 2011 Fukushima disaster.
Kyushu Electric Power began the restart of the No. 1 reactor at its Sendai plant at about 1.30am GMT today, a spokesman said, but the reactor will take about 12 hours to go critical and a few days to reach full power.
While two reactors were allowed to restart for one fuelling cycle under the old standards in 2012, the whole sector has been shut down since September 2013, forcing Japan to import record amounts of expensive liquefied natural gas.
Prime Minister Shinzo Abe and much of Japanese industry want reactors to be restarted to cut rising power costs and they are seeking to reassure the public that tougher standards mean the sector is now safe.
However, opinion polls show a majority of the public oppose the move after the nuclear crisis triggered by the earthquake and tsunami in March 2011 that caused meltdowns at the Fukushima Daiichi plant, releasing radioactive material and forcing 160,000 from their homes.
Following the disaster, tougher standards were introduced for the nuclear industry, forcing utilities to have their reactors relicensed, refitted and vetted by regulators. Of Japan's 25 reactors at 15 plants for which operators have applied for permission to restart, only five at three plants have been cleared so far.
Rebooting the industry safely will help cut energy costs, but it is also crucial for Abe's plans to export nuclear technology, said Malcolm Grimston, a senior research fellow at Imperial College in London.
"Japan also has to rehabilitate itself with the rest of the world's nuclear industry," said Grimston.
The head of Japan's atomic watchdog said that the new safety regime meant a repeat of the Fukushima disaster would not happen, but 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.