The Onagawa Nuclear Power Plant sits on 432 acres of coastline in Miyagi Prefecture, straddling the border of Onagawa town and Ishinomaki city. Managed by Tohoku Electric Power Company, it was the most quickly constructed nuclear power plant in the world when its first reactor came online. All three boiling water reactors were built by Toshiba. But the plant's most important engineering feature was never the reactors -- it was the seawall. When the 2011 Tohoku earthquake struck, Onagawa was the closest nuclear plant to the magnitude 9.0 epicenter, less than half the distance of the stricken Fukushima Daiichi facility. The plant experienced some of the strongest ground shaking of any facility affected by the earthquake. Yet its 14.8-meter seawall blocked the worst of the tsunami. At Fukushima, the seawall stood just 5.7 meters high. That difference -- those extra nine meters championed by Yanosuke Hirai decades earlier -- defined two utterly different outcomes.

What happened next at Onagawa defied every assumption about nuclear plants and disaster. Two to three hundred residents of the town, their homes destroyed by the tsunami, evacuated to the only safe structure left standing in the vicinity: the nuclear plant's gymnasium. The reactor operators -- the very people the public might expect to flee -- instead opened their doors, providing food and blankets to their displaced neighbors. It was a scene unimaginable at Fukushima, where 150,000 people were evacuating away from the plant. Reuters noted at the time that Onagawa might prove nuclear facilities can withstand the greatest natural disasters and retain public trust. Indeed, while the Fukushima disaster transformed Japan's relationship with nuclear power, Tohoku Electric preserved much of its pre-disaster goodwill in the Onagawa area.

Onagawa was not entirely unscathed. A fire broke out in the turbine hall -- a building separate from the reactors -- but was quickly extinguished. On March 13, two days after the earthquake, radiation levels on site briefly spiked to 21 microsieverts per hour, triggering a mandatory low-level state of emergency. Within ten minutes the readings dropped to 10 microsieverts per hour, and the IAEA soon confirmed levels had returned to normal background. Japanese authorities determined the brief spike came from radiation drifting from Fukushima, not from Onagawa itself. A subsequent IAEA inspection in 2012 called the plant's structural performance 'remarkably undamaged given the magnitude of ground motion experienced.' Still, closer examination revealed the No. 2 reactor building's third floor had lost about 70 percent of its structural rigidity, and first floors had lost 25 percent compared to their original construction.

All three reactors entered cold shutdown after the earthquake and stayed there for over a decade. In 2013, Tohoku Electric applied to restart Unit 2 under Japan's newly strengthened Nuclear Regulation Authority standards. Approval came in 2019, but additional safety upgrades and local consultations pushed the timeline further. Unit 1, the oldest reactor, was slated for decommissioning in 2018. Finally, on October 29, 2024, Reactor 2 came back online -- thirteen years after the earthquake silenced it. The restart made Onagawa one of the handful of Japanese nuclear plants to resume operations in the post-Fukushima era, a testament both to the plant's original engineering and to the years of reinforcement work that followed.

Yanosuke Hirai never witnessed the vindication of his engineering judgment. He died 25 years before the tsunami that proved him right. Beyond the taller seawall, Hirai had also insisted the plant's water intake cooling system be designed to function even if the sea withdrew before a tsunami -- anticipating the exact behavior that crippled Fukushima's cooling systems. His foresight was not merely technical; it was an act of imagination, envisioning a disaster his colleagues considered impossible. Today the Onagawa plant stands as one of nuclear engineering's clearest parables: that margins of safety are defined not by consensus, but by those willing to argue for the extra meters.