Hitachi identified a programming error in the software used for measuring the impact of earth quakes on pipes at atomic power stations, after being in use for about 28 years.
A Hitachi programmer who developed the software, about three decades ago, made a mistake that allows the software to underestimate the quake impact on steel pipes associated with eight nuclear reactors owned by six utilities including Tokyo Electric Power Co.
Many people had lost confidence in the safety of Japan's nuclear power plants after a 6.8 magnitude earthquake caused fire and radiation leaks at a Tokyo Electric facility last year in July.
One of the eight plants was built by General Electric Co., which has a partnership with Hitachi, while others just employ Hitachi technology.
Owners of the nuclear reactors include Tohuku Electric Co., Chubu Electric Power Co., Hokuriku Electric Power Co., Chugoku Electric Power Co., and Japan Atomic Power Co.
Hitachi is Japan's third largest builder of nuclear reactors.