(原文1)Automatic Reactor Trip.doc

Event Title: Automatic Reactor Trip Due To Unstable Steam Generator Level Control During Power Ascension (PEN45931 updated with OE31556 and LER 2010-002-00) Event Summary: On May 17, 2010, Millstone Unit 3 tripped while at 17% power due to unstable control of steam generator levels. The root cause of this event is inadequate design of the steam generator water level control system at low power. Title: Reactor Trip Due To Unstable Steam Generator Level Control During Power Ascension Event Date: May 17, 2010 Station Name/Unit Number: Millstone Power Station Unit 3 Significance: Unstable level control resulted in a reactor trip. Lessons Learned for the Industry: Operation of the steam generator level control system at low power had historically been problematic. Over time, this condition had become accepted. Applicability: Feedwater System Engineers, Control Engineers, AOV Component Engineers. Description: On May 17, 2010, reactor power was being increased from 9% to 17%. The steam generator feedwater regulating bypass valves (FRBV) were being operated in automatic. Steam generator levels were being maintained within the 45% - 55% band specified by plant procedures. About 1-1/4 hours before the reactor trip. The FRBVs for the A D steam generators were taken to manual to reduce steam generator level oscillations. These actions were successful. At this time the oscillations in the B C steam generators were considered acceptable. By about 1 hour before the reactor trip, the oscillations of the B C steam generator levels were oscillating between 40% to 60%. The FRBVs were placed in manual, as directed by startup procedure, prior to adjusting power range nuclear instruments. At this time the level in the C steam generator was at a peak and feed flow was at a minimum. Shortly after, the operator began to increase feed flow to the C steam generator. Level had decreased to 25% before it began to increase. As level increased, he pushed the down button for ab