Despite the projection by its own safety engineers that the older assumptions might be mistaken, Tokyo Electric was not breaking any Japanese nuclear safety regulation by its failure to use its new research to fortify Fukushima Daiichi, which was built on the rural Pacific coast to give it quick access to sea water and keep it away from population centers.
"There are no legal requirements to re-evaluate site related (safety) features periodically," the Japanese government said in a response to questions from the United Nations nuclear watchdog, the International Atomic Energy Agency, in 2008.
In fact, in safety guidelines issued over the past 20 years, Japanese nuclear safety regulators had all but written off the risk of a severe accident that would test the vaunted safety standards of one of their 55 nuclear reactors, a key pillar of the nation's energy and export policies.
That has left planning for a strategy to head off runaway meltdown in the worst case scenarios to Tokyo Electric in the belief that the utility was best placed to handle any such crisis, according to published regulations.
In December 2010, for example, Japan's Nuclear Safety Commission said the risk for a severe accident was "extremely low" at reactors like those in operation at Fukushima. The question of how to prepare for those scenarios would be left to utilities, the commission said.
A 1992 policy guideline by the NSC also concluded core damage at one of Japan's reactors severe enough to release radiation would be an event with a probability of once in 185 years. So with such a limited risk of happening, the best policy, the guidelines say, is to leave emergency response planning to Tokyo electric and other plant operators.
PREVENTION NOT CURE
Over the past 20 years, nuclear operators and regulators in Europe and the United States have taken a new approach to managing risk. Rather than simple defenses against failures, researchers have examined worst-case outcomes to test their assumptions, and then required plants to make changes.
They have looked especially at the chance that a single calamity could wipe out an operator's main defense and its backup, just as the earthquake and tsunami did when the double disaster took out the main power and backup electricity to Fukushima Daiichi.
Japanese nuclear safety regulators have been slow to embrace those changes.
Japan's Nuclear and Industrial Safety Agency (NISA), one of three government bodies with responsibility for safety policy and inspections, had published guidelines in 2005 and 2006 based on the advances in regulation elsewhere but did not insist on their application.
"Since, in Japanese safety regulation, the application of risk information is scarce in experience (the) guidelines are in trial use," the NISA said.
Japanese regulators and Tokyo Electric instead put more emphasis on regular maintenance and programs designed to catch flaws in the components of their aging plants.
That was the thinking behind extending the life of the No. 1 reactor at Fukushima Daiichi, which had been scheduled to go out of commission in February after a 40-year run.
But shutting down the reactor would have made it much more difficult for Japan to reach its target of deriving half of its total generation of electricity from nuclear power by 2030 -- or almost double its share in 2007.
The Ministry of Economy, Trade and Industry (METI) figured it could reach the target by building at least 14 new nuclear plants, and running existing plants harder and longer. Fukushima's No. 1 reactor was given a 10-year extension after Tokyo Electric submitted a maintenance plan.
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