Operator error, faulty belt blamed in reactor problem

The two main factors of the 12-minute power loss inside the country’s oldest nuclear reactor last month were an operator’s carelessness in following stated instructions as well as a malfunctioning air supply belt inside the emergency diesel generator, according to the Nuclear Safety and Security Commission yesterday.

The safety inspection commission announced its first round of investigation results after it began its thorough investigation into the Gori-1 reactor last Tuesday. It plans on holding an additional inspection if necessary.

According to the commission, at the time when the Gori-1 reactor lost power on Feb. 9, the operator Korea Hydro and Nuclear Power Company was conducting a month-long safety check.

On the day the power was cut to the reactor, which can cause serious safety problems since no power reaches the storage zones for spent fuel and cooling water, the operator was testing the reactor’s protective relays.

Four minutes into the checkup, the three circuits that provide power to the reactor were all cut. Based on the stated instructions, when the operator conducts a safety checkup, at least one of the three circuits should be connected to provide power in case of emergency.

The operator had planned to finish checking the safety of the three circuits in order by Feb. 11 but the operator rushed it to move up the schedule and turned all three circuits off for testing.

“It was an insider’s fault,” said an official from the commission.

When all three circuits are disconnected from the reactor, the emergency diesel generator should work automatically to provide electricity inside within 10 seconds. There are two emergency generators for the Gori-1 reactor and at the time of the incident, one of the two was under safety check.

The other failed to work because of a flaw in the generator’s pressure valve that failed to provide air. There was an alternative, manual generator to provide power but the operators were not familiar with its operation. They instead chose to connect one of the three outside circuits quickly to restore the power in 12 minutes, the commission explained.

The commission also pointed out that the head of the Gori-1 reactor was notified about the power loss eight minutes after the blackout happened, and according to the safety manual the head and other high-level officials should have called for an emergency alert.

However, they decided not to report the incident to higher officials at the operator’s headquarters.

By Lee Eun-joo [angie@joongang.co.kr]