Tokyo, Mar 15: A Japanese power company today admitted that it had covered up a 1999 incident in which mishandling of nuclear fuel rods led to an unintended self-sustaining nuclear fission chain reaction for 15 minutes.

Hokuriku Electric Power Co said there had been no radiation leak as a result of the mistake, which caused the company's Shiga No. 1 nuclear unit in central Japan to go into a ''critical state'' for 15 minutes. The unit was shut down manually after an automatic shut-down function failed.

''Criticality'' occurs when a nuclear fission chain reaction becomes self-sustaining.

The company apologised at a briefing for not reporting the incident, which occurred during a test while the unit was off line for a planned inspection.

The incident, which was not recorded in the firm's operational diaries, came to light after Hokuriku conducted an internal investigation, an official at the Ministry of Economy, Trade and Industry (METI), told a news conference.

In September 1999, hundreds of residents near a nuclear plant in Tokaimura, 140 km northeast of Tokyo, as well as plant and rescue workers, were exposed to radiation when workers put nearly eight times the normal amount of uranium into a container, causing a criticality accident that took 20 hours to bring under control.

METI said it would order Hokuriku Electric to shut down the No 1 unit and conduct a thorough check of safety measures there.

Hokuriku has two nuclear power generation units at its sole Shiga plant. The other No. 2 unit, which has been shut since July 5 for inspections, has been undergoing planned inspections, with the restart of the unit expected in the first half of May.

The Hokuriku Electric incident tops a growing list of cases of Japanese utilities uncovering past lapses of nuclear power management and reporting.

Reuters

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