TOKYO - The Japanese nuclear power plant that was closest to the epicenter of last year’s earthquake suffered more ground shaking than Fukushima but was largely undamaged because it was designed with enough safety margins, nuclear inspectors said Friday.

The Onagawa plant in northern Japan recorded temblors that exceeded its design capacity and the basement of one of its reactor buildings flooded. But the plant maintained its cooling capacity, its reactors shut down without damage to their cores and there were no signs of major damage to crucial safety systems.

The U.N. nuclear watchdog’s inspectors found the Onagawa plant managed to avoid a catastrophe like Fukushima because its safety systems “successful functioned,” said Sujit Samaddar, who led the 19-member International Atomic Energy Agency mission.

“With the earthquake of this magnitude, we would have expected the plant to have more damages, and that was not the case,” Samaddar said. “This indicated there were significant margins in the designs.”

In contrast, the 9.0-magnitude earthquake on March 11, 2011, knocked out a power line at the Fukushima Dai-ichi plant and generated a large tsunami that flooded its emergency generators, destroying the plant’s cooling systems. Catastrophic meltdowns occurred in three reactors, releasing radiation that has tainted the surrounding environment.

The plant has since stabilized but more than 100,000 people still can’t go home due to radiation fears, while work to decommission the plant will take about 40 years. It was the world’s worst atomic accident since Chernobyl.

The Onagawa plant was about 70 kilometers (44 miles) from the quake’s epicenter, while Fukushima Dai-ichi was nearly 180 kilometers (112 miles) from the epicenter. Onagawa is about 120 kilometers (74 miles) north of Fukushima Dai-ichi.

The tsunami was more than 13 meters (43 feet) high at both nuclear plants. Fukushima Dai-ichi’s seawall was built to withstand a tsunami of up 5.7 meters (18.7 feet). Onagawa’s seawall was nearly 14 meters (46 feet) high and survived the tsunami. It has since been extended to nearly 17 meters (56 feet) above sea level.

Investigations by the government, the parliament and private groups have found that Tokyo Electric Power Co. underestimated the earthquake and tsunami risks faced by its plant at Fukushima despite a history of quakes in the region. The investigations have also criticized TEPCO and government regulators, which have developed cozy relationships, for ignoring safety standards and recommendations, including those by IAEA and other groups.

The IAEA visit July 30-Aug. 9 visit was its first to the Onagawa plant since the disaster. The group inspected the plant and interviewed dozens of workers and officials to assess how its structure, systems and components responded to the quake and its violent shakings.

Samaddar said he hoped to make similar inspections at other plants in Japan to share information and improve safety at nuclear power across the world. No visits are scheduled yet and would only take place at Japan’s invitation.

Five nuclear plants total suffered some level of damage from the earthquake and tsunami, but all but Fukushima Dai-ichi were shut down safely.
In May, the last of Japan’s 50 working reactors were turned off as safety checks were carried out, but two are now back online and generating power. Despite public protests, the government is eager to restart reactors because of the ballooning cost of fuel imports to keep the power supply running.