TOKYO - Japan admitted Tuesday it had been 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 acknowledges reactor design flaws and a need for greater independence for the country's nuclear regulators.

The report says 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 power and cooling systems of Tokyo Electric Power Co.'s Fukushima Dai-ichi nuclear plant.

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 is to be submitted to the IAEA.

"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 says. It also recommends a national debate on nuclear power.

The report says the flaws in basic reactor design - the Mark-1 model developed by General Electric - include 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.

According to GE's website, 32 Mark-1 reactors, designed 40 years ago and upgraded since, still operate around the world.

Japan's report also notes that the six-reactor Fukushima plant pairs up two reactors to share some facilities and equipment, delaying responses to the accident.

The report says the vents lacked filtering capability, causing contamination of the air. 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 that 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.

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.

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.