Japanese nuclear regulators failed to review and approve steps taken after 2002 to protect against tsunamis at the Fukushima plant and these proved insufficient to prevent the disaster three months ago, a UN report showed.
A detailed assessment by an expert team from the International Atomic Energy Agency - the first outside review of Japan's nuclear crisis - suggested several shortcomings both before and after a tidal wave crippled the power station.
But it also praised the way workers on the ground dealt with the situation at Fukushima Daiichi after the massive earthquake and huge tsunami devastated its reactors on March 11, triggering the world's worst nuclear accident in a quarter of a century.
"The operators were faced with a catastrophic, unprecedented emergency scenario with no power, reactor control or instrumentation," said the 160-page report, prepared for a ministerial nuclear safety meeting in Vienna next week.
It "has at times required exceptional levels of leadership and dedication by workers on the sites and elsewhere".
A three-page summary was issued at the end of the 18 member team's May 24-June 2 inspector mission to Japan. It said the country underestimated the threat from tsunamis to the Fukushima plant and urged sweeping changes to its regulatory system.
Officials in Japan had earlier been criticised for failing to plan for a tsunami that would surge over the 5.7m wall at the nuclear power station in the country's northeast, despite forecasts that such a risk was looming.
The wave that crashed into the complex after the 9.0 magnitude earthquake was about 14m high.
In a new setback to efforts to restore control over the quake-stricken plant, a rise in radiation halted the clean-up of radioactive water at Fukushima on Saturday only hours after it got under way.
The full IAEA report said there had been "insufficient defence-in-depth provisions" for tsunami hazards, even though they had been considered in the design and siting of the plant operated by Tokyo Electric Power Company, or Tepco.
Extra protective steps were taken as a result of an evaluation after 2002 - the projected tsunami height was increased - but they were not enough "to cope with the high tsunami run-up values and all associated hazardous phenomena".
"Moreover, those additional protective measures were not reviewed and approved by the regulatory authority," said the report. It added: "Severe accident management provisions were not adequate to cope with multiple plant failures."
The document, obtained by Reuters, was submitted to IAEA member states on Friday but has not yet been made public.
At the June 20-24 IAEA-hosted meeting, some 150 nations will begin charting a strategy on boosting global nuclear safety, but differences over how much international action is needed may hamper follow-up efforts, diplomats say.
Japan's crisis has prompted a rethink of energy policy 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.
Three reactors at the Japanese complex went into meltdown when power and cooling functions failed, causing radiation leakage and forcing the evacuation of some 80,000 people.
Japanese officials have come under fire for their handling of the emergency and the authorities have admitted that lax standards and poor oversight contributed to the accident.
In 2007, the IAEA was ignored when it called on Japan to create a more powerful and independent nuclear regulator, and the report underlined the need for greater regulatory control.
Japan has a well organised emergency preparedness and response system but "complicated structures and organisations can result in delays in urgent decision making", it added.
The report also listed wider lessons for improving nuclear safety worldwide and help avert any repeat of the disaster, saying reactors should be built so that they can withstand rare and "complex combinations" of external threats.