A Japanese government agency that spent several years evaluating the Fukushima Daiichi nuclear plant declared the facility safe after dismissing concerns from a member of its own expert panel that a tsunami could jeopardize its reactors.

Yukinobu Okamura, a prominent seismologist, warned of a debilitating tsunami in June 2009 at one of a series of meetings held by the Nuclear and Industrial Safety Agency to evaluate the readiness of Daiichi, as well as Japan’s 16 other nuclear power plants, to withstand a massive natural disaster. But in the discussion about Daiichi, Okamura was rebuffed by an executive from the Tokyo Electric Power Co., which operates the plant, because the utility and the government believed that earthquakes posed a greater threat.

That conclusion left Daiichi vulnerable to what unfolded on March 11, when a 9.0-magnitude earthquake struck off Japan’s northeast coast. Experts now say that Daiichi, as designed, withstood the quake. It was the ensuing tsunami, with waves more than 20 feet high, that knocked out the facility’s critical backup power supply and triggered a nuclear emergency, resulting in widespread releases of radiation.

The disaster highlights the government’s miscalculation in prioritizing one natural disaster over another and casts scrutiny on a review that more often reaffirmed NISA’s and Tepco’s standards than challenged them.

“Now I regret that I didn’t stress this more strongly, to push them to research this,” said Okamura, a director at a government-funded research institution.

The triple catastrophe, Japan’s greatest crisis since World War II, has left more than 23,000 people dead or missing and caused more than $300 billion in damage, according to a government estimate. The consequences from the nuclear crisis, though, are likely to have the broadest and longest-lasting implications, as nations reexamine their own nuclear safety standards and their reliance on nuclear energy.

No tsunami expert sought

In earthquake-prone Japan, an island nation that depends on its 54 reactors at 17 power plants for 30 percent of its energy supply, the disastrous 6.9-magnitude Kobe quake in 1995 prompted the government to require improved nuclear safeguards and construction standards. The new guidelines tailored standards for each plant based on historical seismic activity in its region.

In 2008, NISA appointed a panel of engineers, geologists and seismologists to review the safeguards and suggest revisions. Tepco officials were not on the panel but attended the meetings.

The experts were assigned to examine each nuclear power plant, but what they focused on was largely predetermined by NISA, based on such factors as geography and the historical record, according to a member of the group. For example, at the Hamaoka facility in Shizuoka prefecture, to the southwest of Tokyo, the reviewers were asked to look closely at the risks posed by both earthquakes and tsunamis. That power plant is located along a major fault line.

But at Fukushima Daiichi, along the northeast coast, the review panel was instructed to focus on earthquakes because a major tsunami was considered unlikely, said Takashi Azuma, a panel member who studies earthquake fault lines at the National Institute of Advanced Industrial Science and Technology.

Of the seven panel members assigned to study Daiichi, none was a tsunami expert, Azuma said. From April 2008 through June 2009, the group met 22 times, he said, talking mostly about the earthquake dangers posed by the fault line closest to the plant. The risk of a tsunami “never came up,” Azuma said.

The Daiichi panel wrapped up its review and, on June 24, 2009, presented its findings to a larger working group of 40, which included just two tsunami experts. It was there that Okamura, who also works at the science and technology institute, first raised the idea that a tsunami could be as risky as an earthquake.

In A.D. 869, Okamura told the panel, a massive quake struck off the coast of Sendai, in northeastern Japan, sending a tsunami wave more than two miles inland. Only in recent years had a handful of Japan’s tsunami experts concluded that the disaster was more than allegorical, based on evidence collected in geological layers and sediment deposits.

“Research results are out, but there is no mention of that [tsunami] here, and I would like to ask why,” Okamura asked a Tepco official at the meeting, according to a transcript Azuma provided to The Washington Post.

Initially, the Tepco official downplayed the danger, saying that the guidelines for Fukushima had instead factored in a far more recent earthquake, whose magnitude measured 7.9. Okamura pressed on, pointing out that the so-called Jogan earthquake of 869 knocked down a castle.

“As you know, it is a historic earthquake,” the Tepco official said, dismissing its relevance.

“I don’t know how that conclusion can be drawn,” Okamura said. “To have no mention of that, to me, leaves me unsatisfied.”

According to the transcript, a NISA official ended the debate by promising to follow up. At the next meeting, the working group approved the Daiichi safety report that declared the complex’s safeguards sufficient.

Review was in progress

Tepco’s defenders say that the power company made a good-faith effort last year to learn more. Japanese tsunami expert Kenji Satake said that company executives consulted with him last year, asking about the 869 disaster. “They were in the midst of analysis when this earthquake hit,” Satake said.

Masaru Kobayashi, of NISA’s seismic safety office, described the panel’s work as part of a mid-term report and said NISA and Tepco were building on it with more research on tsunamis, landslides and other risk factors.

“We were about to start moving on to the next check and this disaster occurred,” Kobayashi said. “It is now too late to say that we wish we checked earlier.”

Yoshimi Hitosugi, a Tepco spokesman, said there was little reason to predict a quake the size of March 11’s, noting that scientists believe the Jogan earthquake had a magnitude of 8.4.

The Fukushima Daiichi plant had been built with retaining walls to withstand a 20-foot-tall wave, according to panel members and Japan’s nuclear agency. Tepco officials now believe a wave reached well above the retainer wall and flooded the low-lying backup generators.

The tsunami overwhelmed the facility, drowning the generators and shutting down the cooling system essential in preventing spent fuel rods and reactor cores from overheating.

“The diesels were in a very low area,” said Ken Brockman, former director of nuclear installation safety at the U.N.-backed International Atomic Energy Agency. “That would make them very susceptible to a tsunami or even an internal flooding event.”

The resulting nuclear emergency raises questions: To what degree must regulators design expensive safeguards against once-a-millennium disasters, particularly as researchers learn more about the world’s rarest ancient catastrophes?

“This is a question that addresses very much the political will of the country,” Brockman said. “The engineers will say, ‘You tell me what you want, we’ll protect it to that level.’ It’s just an issue of raising the elevation, building the retainer walls. The engineering can be done. You just have to give them the criteria.”

Special correspondents Kyoko Tanaka and Akiko Yamamoto in Tokyo contributed to this report.