TOKYO (Reuters) - U.N. atomic safety experts said Japan underestimated the threat from a killer wave to its crippled Fukushima power plant and urged sweeping changes to prevent a repeat of the crisis that triggered the world's worst nuclear disaster since Chernobyl.
In a report presented to Prime Minister Naoto Kan on Wednesday, an 18-member team from the International Atomic Energy Agency (IAEA) called for a rethink of the way nuclear facilities are built, run and regulated.
Officials have been criticised for failing to plan for a tsunami that would overrun the 5.7-metre (19 ft) wall at the plant in the northeast of the country, despite forecasts from the government and Tokyo Electric Power Co's scientists that such a risk was looming.
The wave that crashed into the plant after the 9.0-magnitude earthquake of March 11 has been estimated at around 14 metres (46 ft), nearly two-and-a-half times the height of the wall.
The IAEA report represented the first outside review of the crisis at Fukushima, which has still not been brought under control, and suggested that power companies in quake-prone countries may face higher hurdles in coming years to meet new international standards.
The report could also mark a new phase in a growing debate within Japan about how -- and whether -- the country's 49 remaining nuclear plants can return to operation.
"What we are trying to do is reflect on this and push for the highest safety standards worldwide," said Michael Weightman, Britain's top nuclear safety regulator and the leader of the IAEA inspection team.
The three-page IAEA report urged Japan to overhaul its system of nuclear regulation in order to make officials overseeing safety independent of the ministry that promotes nuclear power.
In 2007, the IAEA was ignored when it called on Japan to create a more powerful and independent nuclear regulator and clarify the responsibilities of the four government agencies with some responsibility for plant safety.
Goshi Hosono, a Kan aide who received the IAEA report, said a set of post-disaster reforms due to be announced as soon as next week would take up the question of how to make its nuclear regulatory agency independent.
"I think that the way nuclear plants are regulated will be taken up as one of the problem areas," Hosono told reporters.
Workers and officials have described a scene of near chaos when the quake and tsunami hit Fukushima on March 11. Key safety systems, including gauges and vents, were disabled and it was not clear in the crucial first hours who was responsible for decision making, according to accounts.
The IAEA said workers at Fukushima had been "dedicated" and "determined" but urged Japan to build "hardened" command centres to allow for a better response in future emergencies.
The Fukushima crisis has prompted a rethink of energy policy in Japan and around the world. Germany's government has vowed to abandon nuclear power.
Japan has pulled back from a commitment to build new reactors, but it remains uncertain how quickly existing plants will be allowed to restart.
Currently Japan is operating only 19 of its pre-Fukushima tally of 54 reactors. Unless local officials can be convinced that Tokyo has a plan to make the others resistant to the kind of blackout that plunged Fukushima into meltdown, more plants will drop off-line for maintenance.
In the worst case, all of Japan's reactors could be shut down by the middle of 2012. That would take out 30 percent of the nation's electricity generation and raise the risk of deeper, near-permanent power rationing, officials say.
Weightman said nuclear plants could be made safe in Japan, but said it would take a line of new defences against tsunamis and a commitment to rethink safety standards over time.
"You can't predict when a natural disaster will occur, but you can say, 'let me predict the consequences'," he said.
The IAEA report said Japanese officials would need to show a longer-term plan for cleaning radiation from around Fukushima so that the more than 80,000 residents evacuated from the area could return. It urged stepped-up and long-term health monitoring for residents and workers.
Opposition parties seeking to topple Kan's government with a no-confidence vote say the government bungled the early response to Fukushima. Environmental critics, including Greenpeace, charge Japan with ignoring a growing risk to radiation spilling into sea and groundwater.
The crisis has also diverted resources from rebuilding after the quake and tsunami that killed about 24,000 people in northern coastal Japan.
Experts say the IAEA report represents a starting point in the debate over what needs to be done to make nuclear plants safe in a country where the risks of earthquakes are still imperfectly understood.
"There are aspects of the planning for the safety of the Fukushima plant which are, in retrospect, very stupid, and show a lack of imagination," said Kim Kearfott, a University of Michigan nuclear safety expert who toured Japan on her own this week. "The nuclear industry can do better than this."
Editing by John Chalmers