Posted on: 23 April 2012
On February 9, 2012, during a refueling outage, loss of off-site power (LOOP) occurred and emergency diesel generator (EDG) 'B' failed to start while EDG 'A' was out of service for scheduled maintenance, resulting in a station blackout (SBO). Off-site power was restored 12 minutes after the SBO condition began. The LOOP was caused by a human error during a protective relay test of the main generator. The EDG 'B' failing to start was caused by the failure of the EDG air start system. Further investigation revealed that the utility did not exercise proper control of electrical distribution configuration to ensure the availability of the Station Auxiliary Transformer (SAT) while conducting test on the Unit Auxiliary Transformer (UAT).After restoring off-site power through the SAT, the operators eventually recovered shutdown cooling by restoring power to a residual heat removal pump. During the loss of shutdown cooling for 19 minutes, the reactor coolant maximum temperature in the hot leg increased from 37℃ to 58.3℃ (approximately 21.3℃ rise), and the spent fuel pool temperature slightly increased from 21℃ to 21.5℃.There was no adverse effect on the plant safety as a result of this event, no radiation exposure to the workers, and no release of radioactive materials to the environment. However, inconsisent with the requirements, the licensee did not report the SBO event to the regulatory body in a timely manner and did not declare the "alert" status of the event in accordance with the plant emergency plan. The licensee reported this event to the regulatory body about a month after the event had occurred.