Tokyo Electric Power Co. said Wednesday that the company could have determined that core meltdown occurred in reactors at the disaster-hit Fukushima Daiichi nuclear plant earlier than it did.
"We could have made a judgment that core meltdown occurred at an earlier stage" than the utility admitted after the March 2011 tsunami disaster crippled the plant, a company official said at a press conference.
The utility known as TEPCO initially said core damage took place before it admitted in May 2011 that the reactor trouble was core meltdown.
The plant operator had explained shortly after the disaster struck that there was no basis to determine reactor core meltdown.
But TEPCO discovered this month that its internal operation manual defines core meltdown as more than 5 percent of a reactor's core being damaged.
The power company said it will launch an in-house investigation involving third party experts to look into why the reference to meltdown in the manual was overlooked.