By Fredrik Dahl
VIENNA (Reuters) - Nuclear power plants should be designed and located so that they can withstand rare and "complex combinations" of external threats, U.N. experts said in a report drawing wider lessons from Japan's atomic crisis.
Japan's nuclear accident has created a "unique opportunity" to seek to learn and improve safety worldwide, an 18-member team from the International Atomic Energy Agency said in the report that marked the first outside review of the Fukushima disaster.
The 160-page document, obtained by Reuters on Friday, was prepared for a major international meeting next week that will launch a push to strengthen reactor standards in the face of mounting public concern about the risks posed by nuclear safety.
A three-page summary was issued at the end of the team's review mission to Japan earlier this month, saying the country underestimated the threat from a killer wave to the Fukushima plant and urging sweeping changes to its regulatory system.
The full report listed 16 lessons from the world's worst nuclear accident since Chernobyl a quarter of a century ago on how to better prepare and deal with these kinds of extreme events.
They included measures both to help prevent any repeat of the Fukushima catastrophe -- caused by a massive earthquake followed by a huge tsunami on March 11 -- as well as action on emergency response and preparedness.
Japanese officials have been criticized for failing to plan for a tsunami that would overrun the 5.7 meter (19 ft) wall at the plant in the country's northeast, despite warnings that such a risk was looming.
"There were insufficient defense-in-depth provisions for tsunami hazards," the IAEA report said.
"There is a need to ensure that in considering external natural hazards ... the siting and design of nuclear plants should include sufficient protection against infrequent and complex combinations of external events."
Any changes in such hazards should be regularly reviewed for their impact on plant configuration, it said, adding that an active tsunami warning system should be established "with the provision for immediate operator action."
The Fukushima crisis has prompted a rethink of energy policy in Japan and around the world, underlined by Germany's decision to shut down all its reactors by 2022 and an Italian vote to ban nuclear power for decades.
Japanese officials from Prime Minister Naoto Kan on down have been widely criticized for their handling of the disaster and the authorities have admitted that lax standards and poor oversight contributed to the accident.
Three reactors at the complex went into meltdown when power and cooling functions at the complex failed, causing radiation leakage and forcing the evacuation of some 80,000 people.
The IAEA report said that for severe situations such as the total loss of off-site power, "simple alternative sources for these functions including any necessary equipment ... should be provided" and located at a safe place.
It called for the "physical separation and diversity of critical safety systems" and said nuclear sites should have "adequate on-site seismically robust, suitably shielded, ventilated and well equipped buildings to house the Emergency Response Centers."
In 2007, the IAEA was ignored when it called on Japan to create a more powerful and independent nuclear regulator, and the report stressed the importance of this.
"Nuclear regulatory systems should ensure that regulatory independence and clarity of roles are preserved in all circumstances in line with IAEA Safety Standards," it said.
At the June 20-24 IAEA-hosted meeting, some 150 nations will begin mapping out a strategy on boosting nuclear safety but differences on how much international action is needed may hamper follow-up efforts, diplomats say.
(Additional reporting by Sylvia Westall; editing by Alison Williams)