A new finding is raising questions about the explanation first offered for what was happening inside damaged reactors at the Fukushima Daiichi nuclear plant in 2011.
Nuclear fuel in 3 reactor cores melted following the earthquake and tsunami that March.
But Tokyo Electric Power Company officials failed to describe these as meltdowns.
They said there were no grounds for reaching that conclusion.
But 2 months later the utility formally admitted all 3 had melted down.
NHK has learned that the firm's own manual says a meltdown has occurred if at least 5 percent of a core has melted. Nuclear fuel is housed in the core.
The operator told NHK it discovered this definition in the course of responding to a request from a Niigata Prefectural Government panel investigating the accident.
An NHK reporter says this would suggest TEPCO did not understand the precise definition of a meltdown until nearly 5 years after the accident.
The utility says it will continue to investigate why it didn't use the word meltdown soon after the crisis began.