Reactor Trip and Safety Injection with Complications from Fire
Monday, April 19, 2010
On March 28, 2010, an electrical fault propagated by the failure of a bus-tie breaker to open resulted in a fire, and caused lowering voltage on 1 of 3 reactor coolant pumps (RCP), initiating a reactor trip. Due to the failure of the breaker to open to isolate the fault, power was lost to two buses. The subsequent cooldown resulted in an automatic safety injection initiation on low pressurizer pressure as operators focused on the fire. In following the procedure to restore the plant, operators attempted to reset the main generator lockout relay, which resulted in re-closing a separate breaker that had eventually opened to isolate the fault, re-energizing the fault and causing a second fire event in the switchgear. This resulted in grounds on the station's 2 DC buses. Further complications included a failure of the charging pump to swap suction sources from the Volume Control Tank to the Refueling Water Storage Tank on low level, and the closure of a Component Cooling Water (CCW) isolation valve to RCP thermal barrier heat exchanger which resulted in a temporary loss of CCW thermal barrier cooling for the RCP. The failure of the bus-tie breaker to open had been preceded by a loss of indicating lights for the control power associated with the breaker. The indicating lights had been unlit for months before the incident, but it was not recognized that this was indicative of an actual loss of control power to the breaker. The loss of control power resulted in the breaker not tripping which significantly complicated the incident. This event was originally rated a Level 1, however details that have come out during the NRC reactive inspection reviewing the incident, particularly regarding procedural deficiencies and inoperable equipment, have caused a re-evaluation of the incident as Level 2.
Huffman William (Bill) NRC Headquarters Operation Officer