Published: Jun 07, 2011 4:02 PM EDT
Updated: Jun 07, 2011 4:02 PM EDT

TOKYO (AP) - Japan admitted Tuesday it was unprepared for a severe nuclear accident like the tsunami-caused Fukushima disaster and said damage to the reactors and radiation leakage were worse than it previously thought.

In a report being submitted to the U.N. nuclear agency, the government also acknowledged reactor design flaws and a need for greater independence for the country's nuclear regulators.

The report said the nuclear fuel in three reactors likely melted through the inner containment vessels, not just the core, after the March 11 earthquake and tsunami knocked out the Fukushima Dai-ichi nuclear plant's power and cooling systems. Fuel in the Unit 1 reactor started melting hours earlier than previously estimated.

The 750-page report, compiled by Japan's nuclear emergency taskforce, factors in a preliminary evaluation by a team from the International Atomic Energy Agency and was to be submitted to the IAEA as requested.

"In light of the lessons learned from the accident, Japan has recognized that a fundamental revision of its nuclear safety preparedness and response is inevitable," the report said. It also recommended a national debate on nuclear power.

The report said the flaws in basic reactor design included the venting system for the containment vessels and the location of spent fuel cooling pools high in the buildings, which resulted in leaks of radioactive water that hampered repair work.

The report said the vents lacked filtering capability, causing contamination of the air, and the vent line interferred with connecting pipes.

Desperate attempts by plant workers to vent pressure to prevent the containment vessels from bursting repeatedly failed. Experts have said the delay in venting was a primary cause of explosions that further damaged the reactors and spewed huge amounts of radiation into the air.

The melted cores and radiation leaks have irradiated workers, including two control room operators whose exposures have exceeded the government limit.

Lack of protection for plant workers early in the crisis and inadequate information about radiation leaks were also a problem, nuclear crisis taskforce director Goshi Hosono said.

The report acknowledged a lack of independence for Japan's nuclear regulator, the Nuclear and Industrial Safety Agency, and pledged to improve safety oversight, as recommended in the IAEA report last week.

Trade and Industry Minister Banri Kaieda promised to share all available data about the accident and cooperate with the IAEA.

"Our country bears a serious responsibility to provide data to the international community with maximum transparency and actively contribute to nuclear safety," he said.

The report comes a day after NISA said twice as much radiation may have been released into the air as earlier estimated. That would be about one-sixth of the amount released at Chernobyl instead of the earlier estimate of one-tenth.

NISA said its analysis used a different method than had been employed by the plant's operator last month and is believed to "better reflect reality."

After Chernobyl, Japan stepped up nuclear safety measures but that effort did not last long, Hosono acknowledged.

"We should never repeat the same mistake," he said.

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