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At 10:43 p.m. CDT on September 22, 2003, with the plant operating at approximately 78 percent power, an automatic reactor scram occurred during scheduled testing of the main turbine control valves. The scram signal originated from reactor steam pressure instruments following a malfunction of the main turbine control system which caused the control valves to move toward the closed position.

A containment isolation signal initiated due to the expected reactor low water level alarm, which caused the isolation of the suppression pool cooling system, as designed. This event is being reported in accordance with 10CFR50.73(a)(2)(iv) as a valid actuation of the reactor protection system and the containment isolation logic circuitry. Modifications are being considered to prevent recurrence of this condition.

Turbine control valve testing has been suspended pending further corrective actions.

This event was of very low safety significance, as the response of the plant to the scram signal was bounded by the safety analysis.

On October 19, 2006, at approximately 5:57 p.m. CDT, an automatic reactor scram occurred in response to a low water level signal (Level 3) in the reactor vessel. This condition was the result of the inadvertent closure of the motor-operated isolation valves in the main feedwater headers supplying the reactor. These valves closed when part of a chart recorder was accidentally dropped on their control switches. The high pressure core spray system automatically actuated as designed when its reactor water level (Level 2) initiation setpoint was reached. The reactor core isolation cooling system was out of service for planned maintenance. Reactor steam pressure began to decrease as expected, and when pressure reached 849 psig approximately three minutes after the scram, the main steam isolation valves (MSIVs) automatically closed.

The MSIVs closed because the reactor mode switch was not promptly re-positioned as required by scram response procedures. This event is being reported in accordance with 10CFR50.73(a)(2)(iv) as an automatic actuation of the reactor protection system and the high pressure core system (including the Division 3 diesel generator). Also, primary containment isolation signals actuated as a result of the Level 2 condition and the low reactor steam pressure signal to the MSIVs.

At 2:34 p.m. CST on January 20, 2011, while the plant was in a refueling outage, standby service water (SSW) pump "C" started automatically during system realignment. The Division 1 SSW subsystem (pumps "A" and "C") was being started to facilitate maintenance on the normal service water system.

When the "A" pump was manually started, the pressure transient caused by the realignment of the motor-operated valves in the system caused a momentary low system pressure, actuating SSW pump "C" automatically. This event resulted from a weakness in the operating procedure, in that the intended system configuration for this operation exceeded the flow capacity for one pump. Actions are being taken to strengthen this and other similar procedures to prevent recurrence. This event is being reported in accordance with 10CFR50.73(a)(2)(iv)(A) as a condition that resulted in the automatic actuation of the "C" SSW pump.

On December 23, 2011, at approximately 6:10 a.m. CST, the main turbine tripped unexpectedly, resulting in a reactor scram. The plant was stable at 100 percent power at the time of the event, and no safety-related systems were out of service. Operitors implemented the appropriate response procedures, and began to stabilize reactor vessel pressure and water level. The closure of the turbine control valves resulted in the actuation of at least fifteen of sixteen main steam safety relief valves. A subsequent high reactor water level caused a trip of all three reactor feedwater pumps. As reactor water level lowered back through the normal operating range, operators attempted to restart a feedwater pump, but component malfunctions were encountered on "B" and "C" pumps. The reactor core isolation cooling (RCIC) system was manually actuated approximately nine minutes after the scram and injected water into the reactor for approximately two minutes.

The "A" feedwater pump was restored to service approximately one minute after RCIC was initiated. The cause of the turbine trip was a spurious backup over-speed trip resulting from an electrical discharge from the turbine shaft in the vicinity of the EHC turbine speed pickup probe. The cause of the electrical discharge was due to a failure of the shaft grounding system. The plant responded as designed, and no emergency core cooling system actuation setpoints were exceeded. This event is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) as a condition that resulted in the automatic actuation of the reactor protection system (RPS).