Tokyo Electric Power Co, the operator of the tsunami-crippled Fukushima power plant, shut its last operating nuclear reactor on Monday for regular maintenance, leaving just one running reactor supplying Japan's creaking power sector.
Japan has 54 reactors, but since the tsunami last March triggered the world's worst nuclear crisis in 25 years at the Fukushima plant, it has been unable to restart any reactors that have undergone maintenance due to public safety concerns.
Tepco said it shutdown the No.6 reactor at its Kashiwazaki Kariwa plant, the world's biggest nuclear power plant, raising concerns about a power crunch this summer when electricity demand peaks due to hot weather.
We are likely to be able to provide stable electricity supply at the moment, but we would like to ask customers to continue conserving power, Tepco President Toshio Nishizawa said in a statement released on Sunday.
We are currently closely studying the summer power supply situation. We will do our utmost to operate in a stable way and maintain our facilities, he added.
Out of the 17 reactors owned by Tepco, which provides electricity to some 45 million people in the Tokyo area, all six at its devastated Fukushima Daiichi plant, 240 km (150 miles) northeast of Tokyo, are off line, as well as four at its neighbouring Fukushima Daini plant.
At its Kashiwazaki Kariwa plant, 230 km northwest of Tokyo, three remain offline after a magnitude 6.8 earthquake struck the area in July 2007 and small fires followed. Four others are under maintenance.
Japan's last running reactor, Hokkaido Electric's Tomari No.3, is set to go off line on May 5 for maintenance.
Greenpeace Japan's Executive Director Junichi Sato said that the country could survive without rushing to restart its nuclear sector.
Japan is practically nuclear free, and the impact on daily life is invisible, Sato said in a statement
With proper demand management, energy efficiency measures, and more than sufficient backup generation in place, there is no excuse for shortages in the coming months, and absolutely no need to rush restarts of nuclear plants.
To avoid blackouts, utilities have restarted old fossil fuel plants and have called for power conservation, but some analysts warn of power shortages in the summer, especially given ageing fossil fuel plants could be less reliable.
The process to restart halted reactors is unclear. Japan's nuclear safety watchdog and another experts' panel are currently reviewing stress test results submitted by utilities that gauge how reactors can withstand extreme events like a huge tsunami.
Once they give approval, ministers including Prime Minister Yoshihiko Noda can give the green-light for the restarts, but only after they deem there is enough local and public support, and surveys show this may not be easy.
(Reporting by Yoko Kubota; Editing by Ed Davies)