Japan shut down all of its 54 reactors after the 2011 Fukushima disaster, but has since brought 12 of 33 still operable units online -- although none in eastern and northern regions.

Unit number two at the Onagawa plant in the northeastern Miyagi region, next to Fukushima prefecture, was to become the 13th on Tuesday, according to Tohoku Electric Power Company.

Japan has been turning back to nuclear power in order to cut emissions, reduce expensive imports of fossil fuels and meet energy demand for data centres for artificial intelligence (AI).

"Nuclear power, along with renewable energy, is an important decarbonised power source, and our policy is to make maximum use of it on condition that safety is ensured", top government spokesman Yoshimasa Hayashi told reporters on Tuesday.

The 2011 earthquake and tsunami that killed around 18,000 people cut power lines and flooded backup generators at Fukushima Daiichi, sending three reactors into meltdown.

Safety and regulatory standards have been tightened since, and the Onagawa plant -- cleared in 2020 to re-start -- has increased the height of its anti-tsunami wall to 29 metres (95 feet) above sea level, one of the highest in Japan.

The reboot also marks the first time a boiling water reactor (BWR) -- the same model used at Fukushima -- will be brought back online since the meltdown.

"The importance of restarting (nuclear reactors) is growing from the perspective of our nation's economic growth driven by decarbonised power sources", Hayashi said.

Under its current plan, Japan aims for nuclear power to account for 20-22 percent of its electricity by 2030, up from well under 10 percent now.

It wants to increase the share of renewables to 36-38 percent from around 20 percent and cut fossil fuels to 41 percent from around two-thirds now.

The E3G think-tank ranks Japan in last place -- by some distance -- among Group of Seven industrialised nations on decarbonising their power systems.