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On November 25, 2017 Comanche Peak, Unit 2 received alarms indicating a trip of both main feedwater pumps. After confirming a decreasing water level in all four steam generators, the control room initiated a manual reactor trip. All safety systems responded as designed including the automatic start of the auxiliary feedwater system. The cause of the trip of both main feedwater pumps could not be positively identified. Causal analysis indicates that a prior plant modification maintained power to abandoned relays in the Solid State Protection System that may have caused both main feedwater pumps to trip. Subsequent actions were taken to remove the fuses that provided power to the abandoned relays on both Unit 1 and Unit 2 to eliminate recurrence from this possible source. Additional corrective actions have been entered into the Comanche Peak Corrective Action Program.

All times below are in Central Standard Time (CST).

At 1005 on June 9, 2016, during routine periodic predictive maintenance thermography activities, the 'A' phase connection to the Unit 1, Train A Safety Chiller was identified to be at an elevated temperature. Inspection found three of the six electrical connections associated with the chiller to be un-torqued. Subsequent investigation found the affected connections should have been torqued on May 10, 2016 as part of restoration actions following planned preventive maintenance. On June 22, 2016 an evaluation was completed that determined the Unit 1, Train A Safety Chiller was inoperable from May 28, 2016 to June 9, 2016.

The cause of this event was the restoration and post work activities by Maintenance personnel did not ensure that the Unit 1, Train A Safety Chiller was properly configured per procedure and ready to be turned over to Operations. Corrective actions included replacing the Phase 'A' cable and dashpot relay and re-torqueing the Phase 'B' and 'C' dashpot relay terminations. The applicable procedure will be revised before the next scheduled performance to have the equipment specific thermography inspection performed as a part of the maintenance restoration/post work activities.

All times in this report are approximate and Central Time unless noted otherwise.

On February 4, 2014, Luminant Power recognized Operating Experience at another station to be potentially applicable to Comanche Peak Nuclear Power Plant (CPNPP). An NRC inspection report described a failure to comply with reactor coolant system (RCS) Pressure/Temperature (P/T) Limits when RCS pressure decreases below 0 pounds per square inch gauge (PSIG). At the time of discovery, CPNPP's Pressure Temperature Limit Report (PTLR) only described pressures equal to or greater than 0 PSIG. However, since early 1996, CPNPP's analyses and procedures allowed drawing a vacuum during RCS refill following refuelings. Therefore, Luminant Power is conservatively reporting RCS pressure below 0 PSIG as a violation of Technical Specification 3.4.3 "RCS Pressure and Temperature (P/T) Limits.

The coordination of the change in RCS operating conditions was less than adequate, in that the associated affected documents were not revised as a result of failing to correctly identify the documents and/or involve all applicable parties, as well as the subtlety of the PTLR pressure value discrepancy. The apparent cause of not maintaining RCS pressure within the limits described in the PTLR was a legacy design implementation error. The apparent mental model by licensed operations personnel was not aligned with the literal applicability of the PTLR curves, in that the curves are applicable during RCS vacuum fill conditions, and not limited to only heat-up, cool-down, and inservice leak/hydrostatic testing conditions. Luminant Power interpreted the PTLR to only apply to RCS pressure conditions above atmospheric pressure, and therefore did not consider it in conflict with the adoption of the vacuum refill analysis and procedure. Corrective actions included revising the PTLR to specify negative 14.7 pounds per square inch gage (PSIG) as the lower limit. The explicit PTLR compliance error during vacuum refill did not adversely affect the health and safety of the public or station personnel.

All times in this report are approximate and Central Daylight Time unless noted otherwise.

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On January 20, 2010, Comanche Peak Nuclear Power Plant (CPNPP) Unit 1 was in Mode 3 during a planned outage and Unit 2 was in Mode 1 operating at 100% power. At 1827 hours, during review of an Oconee INPO OE report, parallels were discovered between the Oconee design and the CPNPP design. Further investigation determined that the required completion times for TS 3.3.2, Condition J, may not have been completed in the past when Units 1 and 2 were operating at low power with only one MFWP providing MFW flow.

The cause analysis of this event determined that the plant design and original operating philosophy was not compatible with the NRC's clarification of the intent of TS 3.3.2, Function 6.g. Corrective actions include the completion of procedure changes on Unit 1 to ensure compliance with TS. Since Unit 2 has been operating at 100% power with both MFW pumps in service since discovery of this condition, corresponding changes to the Unit 2 procedures will be completed prior to removing one MFW pump from service during the next shutdown of Unit 2 or prior to startup of Unit 2 from any event which results in an unplanned trip of Unit 2.

All times in this report are approximate and Central Standard Time unless noted otherwise.