THE firm that ran the Fukushima nuclear plant has apologised for its “cover up” when it delayed disclosure of meltdowns at three reactors.

Tokyo Electric Power Company (Tepco) president Naomi Hirose’s apology followed the revelation last week that an investigation had found his predecessor instructed officials during the 2011 disaster to avoid using the word “meltdown”.

“I would say it was a cover up,” Hirose told a news conference. “It’s extremely regrettable.”

Tepco instead described the reactors’ condition as less serious “core damage” for two months after the earthquake and tsunami on March 11, 2011, wrecked the plant, even though utility officials knew and computer simulations suggested meltdowns had occurred.

An investigative report released last week by three company-appointed lawyers said Tepco’s then-president Masataka Shimizu instructed officials to use the milder description under alleged pressure from the prime minister’s office, though the investigators found no proof.

Former officials at the prime minister’s office have denied the allegation.

Tepco has been accused of softening its language to cover up the seriousness of the disaster, though the investigation found Tepco’s delayed acknowledgement did not break any law.

Hirose said he will take a 10 per cent pay cut, and another executive will take a 30 per cent cut, for one month each to take responsibility.

The report found that Shimizu’s instruction to avoid using the term “meltdown” delayed full disclosure of the plant’s status to the public, even as people who lived near the plant were forced to leave their homes, with some of them permanently unable to return.

Tepco finally used “meltdown” in May, 2011, in its public statements after another computer simulation showed fuel in one reactor had almost entirely melted and fallen to the bottom of the primary containment chamber, and that the two other reactor cores had melted significantly.

The issue surfaced earlier this year in a separate investigation in which Tepco acknowledged that a company manual had been overlooked during the crisis. The acknowledgement reversed its earlier position that it had no internal criteria to guide how it characterised a meltdown to the public.

The report said Tepco officials, who had suggested possible meltdowns, stopped using the description after March 14, 2011, when Shimizu’s instruction was delivered to the company’s vice president at the time, Sakae Muto, in a memo at a televised news conference.

Shimizu had a company official show Muto his memo and tell him the prime minister’s office has banned the specific words.

Government officials also softened their language on the reactor conditions around the same time, the report said.

Then-top government spokesman Yukio Edano, now secretary general of the main opposition Democratic Party, criticised the report as “inadequate”, raising suspicion over the report by the lawyers seen close to the ruling party ahead of an upcoming Upper House election.

Tepco has been accused of a series of cover-ups in the disaster, though the report found that the company’s delayed meltdown acknowledgement was not illegal.