Only two of 54 reactors are still operating amid safety concerns after the March 11 earthquake and tsunami triggered a radiation crisis at Tokyo Electric Power Co's Fukushima Daiichi nuclear plant, leading to widespread contamination and mass evacuations.
Tokyo Electric's last active reactor is set to go into maintenance on March 26, while Hokkaido Electric's Tomari No. 3 unit is scheduled to be closed for routine checks in late April or early May.
No reactors shut for routine maintenance have restarted as they need to meet new safety checks and receive clearance from both the central and local governments.
"A tight supply-demand balance (of electricity) does not affect our judgment on nuclear safety and we are in the process of making that judgment," Trade Minister Yukio Edano, who holds the energy portfolio, told parliament.
"It is a quite possible that no reactors will resume operations towards the summer."
It was the clearest statement yet by Edano that all the reactors might well be offline in the summer. Edano said in January that Japan could avoid the mandatory power cuts seen last summer even if all reactors were down.
But some manufacturers such as steel makers are pressing for restarts, fearing that potential power shortages and more costly electricity will squeeze profits already hit by a strong yen.
Last summer, car and electronics makers coped with power cuts by shifting work schedules to off-peak hours -- a big logistical headache that required negotiations with unions -- while big steel makers met most needs with in-house electricity.
No reactor can restart until it passes computer-simulated "stress tests" to confirm it can withstand earthquakes and tsunamis on the scale that wrecked Tepco's Fukushima Daiichi plant. Then cabinet ministers need to sign off and local governments, by custom although not by law, need to agree.
This month, the government nuclear safety watchdog approved first-stage stress tests on an initial pair of idled reactors, but the government has yet to make clear if second-stage stress tests -- simulations in which a nuclear plant is exposed to several severe events at the same time -- are also necessary.
After Fukushima destroyed the myth that nuclear power was safe, even some pro-nuclear local authorities want new safety guidelines. Others say no reactors should resume operation until an official probe of the Fukushima accident is completed and the causes made clear -- a step not likely until May or June.
The dwindling share of nuclear power -- which before the crisis accounted for 30 percent of electricity demand -- has forced Japan to import more oil and liquefied natural gas even as global energy prices soar amid growing tension between Iran and the West.
Copyright Thomson Reuters 2012