TOKYO—When Tokyo Electric Power Co. broke ground on the now defunct Fukushima Daiichi nuclear-power station 44 years ago, the utility made a fateful construction decision that raised the plant's vulnerability to the tsunami that ultimately crippled its reactors.

In 1967, Tepco chopped 25 meters off the 35-meter natural seawall where the reactors were to be located, according to documents filed at the time with Japanese authorities. That little-noticed action was taken to make it easier to ferry equipment to the site and pump...