Nearly five years after the nation’s worst nuclear accident, Tokyo Electric Power Co. has admitted that its staff failed to follow damage assessment guidelines, according to which they should have reported the meltdowns almost immediately.

A Tepco spokesman on Wednesday said the company’s Disaster Management Manual requires a reactor to be declared “in meltdown” if 5 percent or more of its fuel rods are determined to be “damaged.”

Tepco knew the extent of the damage early on. As of March 14, 2011, it estimated that 55 percent of the fuel rod assemblies of the reactor No. 1 and 25 percent of those at reactor No. 3 were “damaged,” based on the levels of radiation detected, Tepco spokesperson Yukako Handa told The Japan Times by phone.

Yet, despite widespread public skepticism at that time, the company refused to use the word “meltdown” for a period of about two months.

This led to widespread public speculation about a cover-up and failure to admit the extent of the damage. The sudden removal of a nuclear regulator spokesman fueled this.

Handa said a meltdown would have been declared if the guidelines had been followed correctly. But she said Tepco reported its estimates of damage to the government immediately — as required by law — and its failure to describe the situation as one of meltdown did not break regulations.

“Executives in charge of public relations at the time of the accident were not aware of the assessment criteria written in the Disaster Management Manual,” Handa said.

“They believed there was no clear definition of a ‘meltdown,’ so they didn’t make any clear remarks about one,’ ” she said.

Handa said Tepco will investigate why it failed to follow the assessment manual.

Wednesday’s announcement by Tepco was the first confirmation that such a manual even exists. NHK broke the news earlier in the day.

Whether to admit a “meltdown” was taking place at the plant was a sensitive topic for both the central government and Tepco from the start.

On March 12, one day after the tsunami knocked out power and cooling facilities, Koichiro Nakamura, a senior official at the now-defunct Nuclear and Industry Safety Agency, told a news conference that a “meltdown of a reactor’s core” may be taking place at the Fukushima plant, given the radiation levels detected.

Nakamura was promptly removed from a PR position at the agency, sparking speculation of a government cover-up of something critical underway at the site.