By Kiyoshi Takenaka and Yoko Kubota
TOKYO (Reuters) - Japan's nuclear safety agency said on Friday it could not estimate how long will it need to review safety tests of nuclear plants, keeping industry and the public guessing about the potential impact of the tests on power supplies.
Last week, the government announced plans to order "stress tests" for the nation's nuclear reactors hoping to bolster public confidence shaken by the March 11 earthquake and tsunami, which triggered meltdowns at the Fukushima nuclear plant and set off a radiation crisis.
But the initial confusion over how and when the tests would be carried out and how binding their results would be for utilities willing to restart reactors fanned fears of power outages if idled reactors stayed off line.
Tomoho Yamada, a director at the Nuclear and Industrial Safety Agency (NISA), told reporters that the agency did not want its review of stress tests to drag on, but would not give any specific estimates.
"We cannot assume how comprehensive the reports from the utilities will be," Yamada told a news briefing on Friday after the agency submitted plans to the Nuclear Safety Commission on how to conduct the stress tests.
"If they are compiled well, it will take a short time ... It is difficult to say at this point how long it will take for us to make the assessments ."
Currently 35 out of Japan's 54 reactors remain shut, some due to the March disaster but about half due to regular maintenance and inspections.
Power utilities have been offering assurances that they should be able to supply enough electricity to, combined with voluntary and government-ordered savings by power users, avoid blackouts during the peak summer season.
Still, there is concern about electricity supply over the longer run amid fears that lengthy stress tests -- simulations to check nuclear plants' ability to withstand extreme events -- could lead to a total nuclear shutdown by next spring.
The government has said the tests would be split into two stages: the first would focus on reactors that remain off line for routine maintenance, in order to bring those that meet safety requirements back on line.
In the second stage, which will apply to all of Japan's reactors except for the crisis-hit Fukushima facility, utilities will go beyond existing figures and calculate the limits at which reactors would start to see serious damage to their fuel cores.
The safety agency said it would expect plant operators to submit reports from the second round of tests before the end of this year.
Goshi Hosono, a cabinet minister appointed to oversee Japan's response to the nuclear crisis, said that while NISA and the Nuclear Safety Commission will assess reports from the utilities, the final call on whether reactors are safe to operate would be made by politicians.
"We will have to make the decisions," he told a news conference, adding that ministers including the prime minister would make the call.
He also told Reuters in an interview that stress tests will not only determine the restart of reactors but also how long the operating life of existing reactors can be extended.
"We will continue the stress tests. That's going to be necessary to make a judgment about how we define safe nuclear power," he said.
Hosono's comment differed from that of NISA's Yamada, who suggested that the tests need to take place only once.
The Nuclear Safety Commission will decide as early as next week whether NISA's plan on the stress tests is acceptable.
Japanese companies, consumers and power utilities lack clarity over how long it may take before the nuclear power industry will recover its full capacity. Nuclear power accounted for nearly one-third of Japan's power supply before March 11 but that ratio slipped to less than one-fifth in June.
There is just as much confusion about Japan's long-term energy policy.
Earlier this week, Kan said the Fukushima crisis had convinced him that Japan should wean itself off nuclear power and eventually become nuclear-free, but ministers in his cabinet quickly dismissed that as his personal view.
(Writing by Tomasz Janowski, Additional reporting by Kevin Krolicki; Editing by Daniel Magnowski)