By Linda Sieg and Aaron Sheldrick
TOKYO (Reuters) - Japan's government, buffeted by conflicting pressure from anti-nuclear voters and pro-nuclear business interests, is stressing the negative impact of a speedy exit from atomic energy as it nears a decision on a new energy mix.
Japan is rethinking its whole energy policy after an earthquake and tsunami damaged the Fukushima nuclear power plant in March last year, triggering the world's worst nuclear crisis in 25 years.
Defining nuclear power's role has become a hot button issue for Prime Minister Yoshihiko Noda, whose unpopular Democratic Party faces an election expected within months.
Signs the government, worried about a growing anti-nuclear movement, was leaning toward a target of abandoning nuclear power by 2030 have, experts say, galvanized a push-back by utilities and their business and bureaucratic supporters.
Among the numbers that the government floated this week was a forecast that household energy bills would by 2030 rise by nearly double 2010 levels if Japan abandoned nuclear power.
But that has been disputed.
"It is wrong and clearly designed to frighten the population to continue using nuclear power plants," said Arnie Gunderson, a veteran U.S. nuclear engineer and director of Fairewinds Energy Education Corp, a non-profit organization.
"What will dramatically increase electric bills is the true cost to clean up after the (Fukushima) Daiichi disaster," he said by email.
Predictions that power bills would double fail to take into account people's efforts to cut energy use, other experts said.
"Our estimate is that households will use 60 to 70 percent less electricity by 2030," Hiroshi Komiyama, chairman of Mitsubishi Research Institute, told Reuters.
"Our calculation is that households would pay less than half of the current payments by 2030."
The government has also forecast that 50 trillion yen ($638 billion) would be needed to boost the share of power from renewable sources such as solar to offset the loss of nuclear, and double that would be needed to cut consumption through conservation and better efficiency.
Some experts said those figures were not only too high but also underestimated the positive economic impact of investment in renewable energy and conservation.
"It's not very reasonable because it assumes quite expensive renewable costs," said renewable energy guru Tetsunari Iida, head of the Institute for Sustainable Energy Policies.
"Also, such investment would stimulate the economy, but they are assuming that it is a burden," added Iida, who last month startled pro-nuclear interests by coming in a respectable second in an election for governor of a conservative, rural prefecture in western Japan.
The government is expected to decide soon which of three options for nuclear power's share of electricity by 2030 it will select for a medium-to-long-term energy mix: zero, 15 percent or 20-25 percent.
More than half of voters want the government to abandon nuclear power sooner or later, surveys show, and weekly protests near parliament and Noda's office have grown since he approved the restart of two reactors this summer.
All of Japan's reactors were shut for checks and maintenance in the months after the Fukushima disaster. Only the two have been restarted.
Business lobbies are strongly opposed to substantially reducing nuclear power's share, which before Fukushima was almost 30 percent, arguing higher electricity rates would push production and jobs overseas.
In addition, regional utility monopolies would bear a heavy financial burden if they had to write off reactors early.
"Many reactors are under-depreciated and there are not enough reserves set aside for decommissioning," Keio University economics profess Masaru Kaneko told reporters recently.
"If they can't be fully utilized, the remaining book value will turn to losses."
Given the deep divisions on the issue, political parties are eager to defuse it, most likely with vague promises, analysts said. In a draft election manifesto released on Wednesday, the Democrats said they hoped to end reliance on nuclear energy and called for an "energy revolution", but gave no details.
"Vested interests, on the one hand, and the more unorganized public opinion are going in different directions," said Koichi Nakano, a professor at Sophia University. "All they want to do is ... avoid making it a central issue of the election."
($1 = 78.3900 yen)
(Additional reporting by Risa Maeda, Kentao Hamada and Tetsushi Kajimoto; Editing by Robert Birsel)