The committee, which convened six times to review findings gathered during inspections of the No. 1 unit at the power station, found that Tokyo Electric had met all required protections from earthquakes. Inspectors, however, had spent just three days inspecting the No. 1 unit, a period that industry experts say was far too brief because assessing the earthquake risk to a nuclear plant is one of the most complex engineering problems in the world.

Despite these doubts, the committee recommended that Tokyo Electric be given permission to run the No. 1 unit, which was built by General Electric and began operating in 1971, for an additional decade. During the approval process, the company claimed that the reactor was capable of running for 60 years.

Mitsuhiko Tanaka, an engineer who worked on the design of the reactors at the Fukushima Daiichi plant, said the reactors there were outdated, particularly their small suppression chambers, which increased the risk that pressure would build up within the reactor, a fault eliminated in newer reactors. Since the tsunami, officials at Fukushima Daiichi have tried to relieve rising pressure inside the reactors, several times resorting to releasing radioactive steam into the atmosphere, a measure that in turn has contributed to the contamination of food and water in the area.

“It was about time the reactor was replaced,” Mr. Tanaka said. “The tsunami would have caused great damage, regardless. But the pipes, the machinery, the computers, the entire reactors — they are just old, and that did not help.” Somewhat younger reactors, Nos. 2, 3, and 4, also suffered extensive damage.

Regulators approved the 10-year extension even though aging reactors at Tokyo Electric, as well as those at other power companies, had suffered a series of problems as far back as a decade ago. Attempts to cover them up and manipulate data, particularly by Tokyo Electric, the country’s biggest utility, underscored not only the problems of the nuclear industry but also Japan’s weakness in regulating it. The company has admitted wrongdoing.

A Tokyo Electric spokesman, Naoki Tsunoda, said: “We are committed to carrying out proper inspections in the future. We will study why this has happened and endeavor to inform the public.”