An emergency cooling system for the No.1 reactor at the Fukushima Daiichi nuclear plant was set at a mode that was difficult to start for nearly 30 years until 2010.
The No.1 reactor was the first of the 3 reactors at the plant to melt down in the 2011 accident.
The isolation condenser system was automatically activated after the massive earthquake 6 years ago, and operators used it to cool down the reactor.
However, they failed to make full use of it, and misjudged its operating status after power was lost in the tsunami.
The subsequent meltdown of the No.1 reactor caused a hydrogen explosion.
NHK interviewed officials from Tokyo Electric Power Company and requested the disclosure of information. NHK found that the setting of the emergency cooling system was changed in 1981 to make it difficult to start.
The isolation condenser is supposed to switch on automatically when the pressure inside the reactor rises for some reason. But its settings were altered so that another device for reducing internal pressure would start first.
There is no record of the isolation condenser being used for nearly 30 years, even when problems occurred.
Safety measures were reviewed the year before the 2011 accident, and the cooling system was reset to make it easy to start.
However, it was never actually tested before the 2011 accident.
The utility says it cannot confirm why the setting was changed in 1981 as there are no records, and it was not tested because there was a risk of a radioactive leak if the system became damaged.
The company says employees were told about the isolation condenser in their training courses.
Hosei University Professor Hiroshi Miyano says people cannot use such a device without experience, and this may have been a factor behind the scale of the accident. He says safety equipment testing and training should be reviewed at other nuclear plants.