The report by the private Rebuild Japan Initiative Foundation also paints a picture of confusion during the days immediately after the accident. It says the U.S. government was frustrated by the scattered information provided by Japan and was skeptical whether it was true.
The U.S. advised Americans to leave an area within 50 miles (80 kilometers) of the plant, far bigger than the 12-mile (20-kilometer) Japanese evacuation area, because of concerns that the accident was worse than Japan was reporting.
The misunderstandings were gradually cleared up after a bilateral committee was set up on March 22 and began regular meetings, according to the 400-page report.
The report, compiled by scholars, lawyers and other experts, credits then-Prime Minister Naoto Kan for ordering Tokyo Electric Power Co., the utility running the plant, not to withdraw its staff and to keep fighting to bring it under control.
TEPCO's president at the time, Masataka Shimizu, called Kan on March 15 and said he wanted to abandon the plant and have all 600 TEPCO staff flee, the report said. That would have allowed the situation to spiral out of control, resulting in a much larger release of radiation.
A group of about 50 workers was eventually able to bring the plant under control.
TEPCO, which declined to take part in the investigation, has denied it planned to abandon Fukushima Dai-ichi. The report notes the denial, but says Kan and other officials had the clear understanding that TEPCO had asked to leave.
But the report criticizes Kan for attempting to micromanage the disaster and for not releasing critical information on radiation leaks, thereby creating widespread distrust of the authorities among Japanese.
Kan's office did not immediately respond to requests for comment on the report.
Kan acknowledged in a recent interview with The Associated Press that the release of information was sometimes slow and at times wrong. He blamed a lack of reliable data at the time and denied the government hid such information from the public.
It will take decades to fully decommission Fukushima Dai-ichi. Although one of the damaged reactor buildings has been repaired, others remain in shambles. A group of journalists, including a reporter from The Associated Press, were given a tour of the plant on Tuesday.
Workers have used tape to mend cracks caused by freezing weather in plastic hoses on temporary equipment installed to cool the hobbled reactors.
"I have to acknowledge that they are still rather fragile," plant chief Takeshi Takahashi said of the safety measures.
The area is still contaminated with radiation, complicating the work. It already has involved hundreds of thousands of workers, who have to quit when they reach the maximum allowed radiation exposure of 100 millisieverts a year.
The report includes a document describing a worst-case scenario that Kan and the chief of the Japan Atomic Energy Commission secretly discussed two weeks after the disaster.
That scenario involved the possibility of more nuclear fuel rods burning, causing the release of more radiation and requiring the evacuation of a much wider region, including Tokyo.
The report also concludes that government oversight of nuclear plant safety had been inadequate, ignoring the risk of tsunami and the need for plant design renovations, and instead clinging to a "myth of safety."
"The idea of upgrading a plant was taboo," said Koichi Kitazawa, a scholar who heads the commission that prepared the report. "We were just lucky that Japan was able to avoid the worst-case scenario. But there is no guarantee this kind of luck will prevail next time."" Go to http://www.boston.com/business/articles/2012/02/28/probe_finds_japan_withheld_risks_of_nuke_disaster/ Also see "comments":