A circuit breaker deemed quake-prone in 1978 remained installed at the Fukushima No. 1 nuclear plant for more than 30 years, contributing to its total loss of power after the devastating earthquake and tsunami in March last year, sources said.
The circuit breaker at the plant, run by Tokyo Electric Power Co., collapsed in the magnitude 9.0 quake, the sources said.
According to a report compiled by a research institution on electric equipment in October 1978, that variety of breaker was not resistant enough to quakes.
After it fell, the subsequent tsunami submerged power distribution boards and other devices at the six-reactor complex, triggering the dreaded "station blackout." Since the high waves also flooded most of its emergency diesel power generators, the plant had no way to keep the fuel cores in reactors 1 to 4 cool.
The 1978 report by members of a panel at the Electric Technology Research Association said that the structure supporting the circuit breaker in question was not strong enough and pointed to many cases of damage caused by past temblors.
The members, including officials of Tepco and the Natural Resources and Energy Agency as well as experts, recommended that utilities install a more quakeproof device, which housed major equipment in a tank.
The Nuclear and Industrial Safety Agency says most of the nuclear plants in Japan have introduced the tank-shaped device.