By Ikuko Kao
TOKYO, March 22 (Reuters) - Japan's largest utility, Tokyo Electric Power Co. (TEPCO) (9501.T), said on Thursday there was a "criticality" incident at one of its nuclear power plants in 1978 that could have lasted for up to 7-½ hours.
TEPCO's admission came a week after another Japanese utility said it had concealed a similar incident in 1999.
There were no records of the incident at the No.3 unit at TEPCO's Fukushima Daiichi plant in Fukushima, northern Japan.
It came to light during interviews with two former workers at the plant, both now aged around 70, a news briefing was told by Nagao Suzuki, general manager at TEPCO's nuclear power plant management unit.
TEPCO said it was still investigating but was unaware of any injuries or radiation leak as a result of the incident.
"One of them had a quite clear memory about the incident. He took it quite seriously," Suzuki said.
Fuel rods fell in a reactor core and may have caused self-sustaining nuclear fission, or criticality.
It took about 7 ½ hours to place the rods into the proper positions, he said.
Technical data which should have recorded the incident was abandoned after 10 years in line with regulations that governed nuclear power plant management in the 1970s. Toshiba Corp. (6502.T)., the maker of the unit, provided some data to back up what the worker said about the incident, Suzuki said.
It was unclear if the workers had intended to conceal the incident, he said.
TEPCO has found five past cases of mishandling of fuel rods, including the one at the Fukushima plant.
The other four, which did not lead to criticality, also involved Toshiba units.
Utilities are now reporting such incidents to the government after the Trade Ministry ordered them in November to investigate power plant records and report the findings by the end of March, a process the ministry said would help improve the industry's safety controls.
TEPCO's admission comes after Hokuriku Electric Power Co. (9505.T) said last week it had covered up an incident in 1999 that caused criticality that lasted for 15 minutes.
(Additional reporting by Osamu Tsukimori)