The shutdown leaves Japan without nuclear power for the first time since 1970 and has put electricity producers on the defensive. Public opposition to nuclear power could become more deeply entrenched if non-nuclear generation proves enough to meet Japan's needs in the peak-demand summer months.
"Can it be the end of nuclear power? It could be," said Andrew DeWit, a professor at Rikkyo University in Tokyo who studies energy policy. "That's one reason why people are fighting it to the death."
Japan managed to get through the summer last year without any blackouts by imposing curbs on use in the immediate aftermath of the earthquake and tsunami. Factories operated at night and during weekends to avoid putting too much stress on the country's power grids. A similar success this year would weaken the argument of proponents of nuclear power.
"They don't have the polls on their side," said DeWit. "Once they go through the summer without reactors, how will they fire them up? They know that, so they will try their darndest but I don't see how."
Japan has 54 nuclear power reactors, including the four at Tokyo Electric's Daiichi plant in Fukushima that were damaged in the earthquake and tsunami, culminating in three meltdowns and radiation leaks for the worst civilian nuclear disaster since Chernobyl in 1986.
One by one the country's nuclear plants have been shut for scheduled maintenance and prevented from restarting because of public concern about their safety.
The last one running, the No3 Tomari reactor of Hokkaido Electric Power Co in northern Japan, is scheduled to shut down early on Sunday. Anti-nuclear activists will celebrate with demonstrations over the weekend.
COSTLY OPTION
The last time Japan went without nuclear power was in May 1970, when the country's only two reactors operating at that time were shut for maintenance, the Federation of Electric Power Companies of Japan says.
Nuclear power provided almost 30 percent of the electricity to keep the $5 trillion economy going before the March 11, 2011 disaster that killed almost 16,000 people and left more than 3,000 missing.
A year on, the level of public concern about the safety of the industry is such that the government is still struggling to come up with a long-term energy policy, a delay having a profound impact on the economy and underlining just how costly it will be to contemplate a nuclear-power-free future.
Having boomed in recent decades on the exports prowess of big brands like Sony, Toyota and Canon, the economy suffered its first trade deficit in more than three decades in 2011 as power producers spent billions of dollars on oil-and-gas imports to fuel extra generation capacity.
At the time of the Fukushima crisis, then Prime Minister Naoto Kan called on Japan to wean itself off of nuclear power. Up to that point, Japan had been planning to lift the share of nuclear generation to over 50 percent by 2030 from about 30 percent.
The government of current Prime Minister Yoshihiko Noda has softened Kan's call. Noda says Japan cannot afford to be nuclear free, although he still holds that as an ideal.
But the government has no clear timetable for getting nuclear power back up and running as it tries to navigate the public opposition -- rare in Japan -- and the demands of business that wants a stable supply of power.
Cabinet ministers last month rushed to try to win over the public to allow the restart of two nuclear power reactors at Kansai Electric Power Co's Ohi plant in western Japan, in what experts said was a recognition of the implications of a nuclear-free summer.
The public remained unconvinced. A poll by Kyodo news agency last weekend showed about 60 percent of the public opposed to restarting the two reactors.
Most mayors and governors whose communities host nuclear plants want safety assurances beyond government-imposed stress tests before agreeing to restarts, a Reuters poll showed in March.
To overcome the opposition, some politicians have been more forceful. Yoshito Sengoku, the acting president of the ruling Democratic Party of Japan, on April 16 called an abandonment of nuclear energy the equivalent of "mass suicide," Kyodo news reported. His comment was criticized by Chief Cabinet Secretary Osamu Fujimura, indicating internal divisions over how to handle the issue.
Trade Minister Yukio Edano - the government's point man for energy policy - walks a fine line, saying both that safety must come first while trying to win the support of local communities for restarts.
Kansai Electric Power Co, the utility most reliant on nuclear power, and some other electricity producers have warned of power shortages this summer but have largely avoided lobbying publicly for restarts for fear of a backlash.
GLOBAL SHIFT
Ultimately, some argue Japan's economy, already weakened by years of deflation, would suffer if reactors are not restarted.
"It's not an option Japan should take. There will be less employment and the economy will be on a shrinking trend," said Takeo Kikkawa, a professor at Hitotsubashi University.
Japan's liquefied natural gas imports climbed 18 percent in volume and 52 percent in value to 5.4 trillion yen ($67 billion) in the year through March.
Renewable energy, although given emphasis in energy policies being formulated, is not expected to be much of an immediate salve. Energy from renewable sources account for about 10 percent of Japan's power generation, most of that from hydroelectric dams. Wind and solar together contribute about 1 percent.
Worldwide, there has been a shift with Germany, Italy and Switzerland moving away from atomic energy, prompting the International Atomic Energy Agency to revise down its forecast for growth in the industry.
The United States, China and India are still planning to increase the number of reactors.
In Japan, a delay in setting up a new, more independent Nuclear Regulatory Agency due to deadlock in a divided parliament is further clouding the outlook.
Some analysts say the government is not going to turn public opinion unless it admits that nuclear power is never going to be absolutely safe.
"The debate needs to be recast," said Bob Geller, a professor of geophysics at Tokyo University. "They have to come clean, and say, in effect - look we know they're not perfectly safe but we've made a careful evaluation of the risks, which we'll make public." ($1=80.165 yen)
(Additional reporting by Yoko Kubota, Linda Sieg, Osamu Tsukimori and Risa Maeda in Tokyo: Editing by Edwina Gibbs and Neil Fullick)
Reuters