Fukushima nuclear power plant

Security guards dressed in hazmat suits approach a car at the main gate of the Fukushima nuclear power plant. (Steven Herman / April 13, 2011)

Japan acknowledged Tuesday that it was unprepared for a severe nuclear accident like the tsunami-generated Fukushima disaster and said damage to the reactors and radiation leakage were worse than it previously thought.

In a report being submitted to the United Nations' nuclear watchdog agency, the government also acknowledged reactor design inadequacies and a need for greater independence for the country's nuclear regulators.

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

Photos: Earthquake in Japan

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

The report says 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.

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 neighborhoods around the plant.