Japan's new nuclear regulator will impose tighter safety standards for atomic plants, its head said.
Shuichi Tanaka also said his new body would have the authority to restart reactors idled since last year's Fukushima disaster once new safety standards were in place and met. Restarting such units is a key point in reducing the import bill for fossil fuels to produce electricity.
Tanaka, head of the Nuclear Regulation Authority (NRA), said previous standards would prove irrelevant if a plant was struck by an earthquake or tsunami stronger than anticipated.
The March 2011 calamity that wrecked the Fukushima station had highlighted the need to take full account of the latest geological data.
"The existing safety standards fall short of international levels," Tanaka said, adding they in particular lacked severe accident management and disaster prevention measures. "We've aimed to make new ones comparable internationally and also come up with good ones taking into account Japan's geological characteristics."
Tanaka said utilities had pointed to "stress tests" conducted since the disaster which they said showed a reactor could withstand an external force, for example, triple its designed capacity.
"But our argument is: what if there were an external force five times as much?"
The NRA started operations last month, replacing two regulators that had been widely criticised for being too close to the industry they were supposed to monitor, contributing to the catastrophe and proving inept in tackling it.
The operator of the Fukushima plant, Tokyo Electric Power Co (TEPCO) acknowledged for the first time last week that it had failed to anticipate and deal with the disaster, in which three reactors suffered meltdowns.
The new safety standards are to be in place by next July and Tanaka has promised never to allow a repeat of Fukushima.
Speaking amid tight security in his city-centre office, he said the authority would be able to give the green light to restarts of reactors, though it had only limited power to order a shutdown of units already in operation.
All 50 reactors in Japan were shut down after the disaster.
Two were restarted earlier this year at the Ohi power station on Japan's northwestern coast on orders of the prime minister and three senior cabinet colleagues despite warnings by some geologists of dangerous fault lines running beneath the plant.
Bringing eight reactors online from the current two would save about 240 billion yen ($3bn) in fuel costs by power utilities in the year to next March, according to an estimate by Institute of Energy Economics of Japan.
"So far, we understand the Ohi plant is not exposed to any imminent danger," Tanaka said. "But the more we move on, the more cases we will clarify which do not meet (the new standards). Then, we'll order utilities to make changes necessary to comply."
NRA-appointed experts will visit Ohi next month, including one specialist who has warned of dangers with the site.