Onagawa and Fukushima Daiichi site elevation decisions

The Fukushima Daiichi site was a natural bluff about 35 m above sea level. In 1967 TEPCO removed about 25 m of it, which put the reactor buildings at about 10 m. The reasons given were lower running costs for the seawater cooling pumps, reaching solid bedrock, which helps against earthquakes, and easier transport of equipment with lower construction costs. TEPCO engineer Hiroshi Kaburaki wrote in January 1969 that the decision compared construction costs with the operating costs of the circulation water pumps.

At Onagawa, run by Tohoku Electric, the site height was set at 14.8 m above sea level, with the first floors of the main buildings and key outdoor structures at 15.0 m. Press accounts credit Yanosuke Hirai (1902–1986), Tohoku Electric's vice president from 1960 to 1975, as the one member of the planning committee who argued for 14.8 m while colleagues thought about 12 m was enough. According to those accounts he cited the 869 Jogan tsunami and argued for an intake design that kept cooling water available when the sea drew back. Tohoku Electric's own 2024 paper credits an in-house committee of experts and does not name Hirai.

The committee for the Onagawa site height was formed in 1968 and the decision was made in the late 1960s; no exact date has been published. Construction started in 1980, and Unit 1, a 524 MW boiling water reactor, entered commercial operation in June 1984. Tohoku Electric says the tsunami assumed at the time was only about 3 m.

The 2011 tsunami was about 13 m at the Onagawa plant, according to Tohoku Electric and the Japan Atomic Industrial Forum (JAIF). The earthquake also lowered the land by about 1 m, bringing the Onagawa site to about 13.8 m, so the remaining margin was under 1 m. Other publications give different figures: The Bulletin of the Atomic Scientists gives 14.3 m at Onagawa and 13.1 m at Daiichi, and Wikipedia gives 15 m at Onagawa fishing port, which is the town and not the plant.