TOKYO — Japan underestimated the danger of tsunamis and failed to prepare adequate back-up systems at the Fukushima Daiichi nuclear plant, a team of inspectors from an international nuclear watchdog said on Wednesday in a preliminary report on the nuclear crisis. But the report also praised the response of Japan, and particularly its plant workers, once the disaster struck.

The report, by the team from the Vienna-based International Atomic Energy Agency, also called for stronger regulatory oversight, saying that steps should be taken to ensure that “regulatory independence and clarity of roles are preserved in all circumstances.” This seemed to echo a widely held criticism in Japan that collusive ties between regulators and industry led to weak oversight and a failure to ensure adequate safety levels at the plant.

The report followed a weeklong inspection by the multinational team, led by Britain’s top nuclear safety official, Mike Weightman. Most the problems that it cited have already been well documented in the soul-searching here that has followed the nuclear accident, the world’s worst since Chernobyl in 1986.

The team said it visited three nuclear plants damaged by the March 11 earthquake and tsunami, including the radiation-spewing Fukushima Daiichi plant. The team said it released the three-page preliminary report to provide quick feedback to the Japanese government as well as lessons to the global nuclear industry.

It said it will release a longer version ahead of a nuclear safety conference that starts June 20 in Vienna.

Goshi Hosono, an adviser to Prime Minister Naoto Kan who serves as Japan’s point man on nuclear issues, said Tokyo accepted the team’s findings. He also said Japan would review its nuclear regulatory framework.

Wednesday’s report mixed praise with criticism. It spoke highly of Japan’s response to the crisis once it happened, calling the efforts of workers to regain control of the crippled reactors “exemplary.” It said their efforts had produced “the best approach to securing safety given the exceptional circumstances.”

The report also praised Tokyo’s steps to protect the population from radiation, calling its evacuations of surrounding areas “impressive and extremely well organized.”

The report’s strongest criticism was aimed at the failure to build adequate protections against large waves for the plant, which sits on Japan’s tsunami-prone northeastern coastline. While the plant was designed to withstand waves about 19 feet high, the tsunami was as high as 46 feet, the report said.

“The tsunami reached areas deep within the units causing the loss of all power sources except one emergency diesel generator,” the report said, adding that a blackout of the commercial power grid left the plant with “little hope of outside assistance.”

The report also said the disaster exposed the lack of varied and redundant backup systems at the plant. The huge tsunami, which struck 46 minutes after the magnitude-9.0 earthquake, destroyed the emergency diesel generators at four of the plant’s six reactors. This left them with no other power source beyond batteries that lasted only a few hours.

Once power was lost, critical functions such as the cooling system shut down, as did the instruments that told workers what was happening inside the reactors. Three of the reactors quickly overheated, causing meltdowns that eventually led to explosions, which hurled large amounts of radioactive material into the air.

The single surviving diesel generator allowed workers to maintain the cooling systems for reactors No. 5 and 6, which did not melt down. The No. 4 reactor was already safely shut down when the earthquake hit, but its cooling pool was damaged by the tsunami.

“The operators were faced with a catastrophic, unprecedented emergency scenario with no power, reactor control or instrumentation,” the report said. The tsunami also “severely affected communications systems both within and external to the site.”

“They had to work in darkness with almost no instrumentation and control systems,” the report said of plant workers.

The report called for better preparations against multiple disasters, including the construction of “hardened” emergency response shelters.

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