The operator of Japan's damaged Fukushima nuclear plant acknowledged Thursday it failed for two months to announce that meltdowns had occurred in the cores of three of the reactors.

Tokyo Electric Power Co. said its officials were unaware of a company emergency manual that defined a meltdown as damage exceeding five per cent of a reactor's fuel.

Instead, TEPCO described the condition of the reactors as less serious "core damage" for two months after the plant was hit by a massive earthquake and tsunami in March 2011, despite early damage estimates ranging from 25 to 55 per cent.

TEPCO has been accused of softening its language to cover up the seriousness of the disaster. The Nuclear and Industrial Safety Agency, Japan's nuclear regulatory unit at the time of the accident, was also reluctant to use the word "meltdown" and replaced a spokesman after he suggested one day after the disaster that one had occurred. His successors further softened their description, saying there was only external damage to the fuel cladding.

TEPCO said its initial wording may have been misleading, but didn't affect its response.

"Core damage or meltdown, it didn't make any difference in how we responded to the emergency, which was to cool the cores no matter what," TEPCO spokesman Shinichi Nakakuki said. He said the company promptly reported the estimated damage percentage to the government as required by law.

In May 2011, TEPCO finally adopted the term "meltdown" after a computer simulation showed fuel in one reactor had almost entirely melted and fallen to the bottom of the primary containment chamber, and that two other reactor cores had melted significantly.

Experts are developing remote-controlled robots to locate and assess the melted fuel — key to a successful decommissioning of the plant, which will take decades.

TEPCO and government officials in charge of the cleanup of the Fukushima Dai-ichi plant say they are unaware of any internationally accepted definition of what constitutes a meltdown, and don't know where the company's five per cent benchmark came from when it was set in 2003.

TEPCO said it discovered the emergency manual this month and promised an investigation into why it was overlooked.