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 inadequacies 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 task force, 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 "inadequate" basic reactor design _ the Mark-1 model developed by General Electric _ 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.
GE declined to comment on the specific conclusions of the report. Catherine Stengel, a spokeswoman for GE's nuclear division, said in a statement that the company is committed to being part of the "analysis, learnings and evolution of the industry" in the wake of the nuclear crisis in Japan.
"We believe lessons learned from the events in Japan are going to make the industry safer," Stengel said.
GE says 32 Mark-1 reactors, designed 40 years ago and upgraded since, still operate around the world. Twenty-three of them operate in the United States.
So-called hard venting systems in the Mark-1 reactors were added long after the reactors had been in service in response to a 1989 recommendation by the U.S. Nuclear Regulatory Commission. GE helped the operators establish general requirements for the vents, the company says, but the vents themselves were designed and installed by nuclear operators in the U.S. and Japan.
Japan's report also noted that the six-reactor Fukushima plant paired up two reactors to share some facilities and equipment, also delaying the accident responses.
The report said the vents lacked filtering capability, causing contamination of the air, and the vent line interfered 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 report also noted the outermost containment buildings should have had vents to prevent a series of explosions at three units.
The melted cores and radiation leaks have irradiated workers, including two control room operators whose exposures have exceeded the government limit.
Earlier Tuesday, the Health and Labor Ministry inspectors visited the plant to investigate if TEPCO used adequate caution.
Lack of protection for plant workers early in the crisis and inadequate information about radiation leaks were also a problem, nuclear crisis task force director Goshi Hosono said.
Hundreds of plant workers are scrambling to bring the crippled reactors to a "cold shutdown" by early next year and end the crisis. The accident has forced more than 80,000 residents to evacuate from the plant's neighborhoods.
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. Bureaucracy and division of responsibility by several government agencies also delayed decision-making, the report said.
The report also said accident management measures, which are left up to operators' voluntary effort, should be made legally binding. Accident management guidelines have not been reviewed or improved since being introduced in 1992, it said.
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."
Also Tuesday, a 10-member government-appointed panel of experts launched a separate investigation into the worst nuclear accident since Chernobyl in 1986. The panel is expected to look into a wide range of areas, including TEPCO's corporate culture, Japan's nuclear safety myth, government regulatory functions, and the accident's effect on the economy and food safety.
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.
AP Energy Writer Jonathan Fahey contributed to this report from New York.