TOKYO — Japan admitted Tuesday that 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 written 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.