05000499/LER-2015-001
| South Texas Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat |
| 4992015001R00 - NRC Website | |
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I. Description of reportable event
A. Reportable event classification
This event is reportable under 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by Technical Specifications and 10 CFR 50.73(a)(2)(v)(B) as an event that could have prevented the fulfillment of the safety function of systems that are needed to remove residual heat.
B. Plant operating conditions prior to event
On March 4, 2015 Unit 2 was operating in Mode 1 at 100% power.
C. Status of structures, systems, and components (SSCs) that were inoperable at the start of the event and that contributed to the event There were no SSCs that were inoperable at the start of the event that contributed to the event.
D. Narrative summary of the event
turbine-driven auxiliary feedwater (AFW) pump 24. AFW pump 24 was declared inoperable for the surveillance at 1507 hours0.0174 days <br />0.419 hours <br />0.00249 weeks <br />5.734135e-4 months <br />. During the test, the as-found pump discharge pressure was recorded by the STP Nuclear Operating Company (STPNOC) licensed Reactor Operator (RO) as 1430 pounds per square inch gauge (psig). The recorded pressure was below the surveillance acceptance criteria of greater than or equal to 1519 psig. The pump was declared operable again at 1608 hours0.0186 days <br />0.447 hours <br />0.00266 weeks <br />6.11844e-4 months <br />. At 1646 hours0.0191 days <br />0.457 hours <br />0.00272 weeks <br />6.26303e-4 months <br />, the RO logged the surveillance as completed satisfactorily and forwarded the surveillance to the Unit Supervisor (US) for review. The US, who is a STPNOC licensed Senior Reactor Operator (SRO), performed his review prior to the end of shift at 1800 hours0.0208 days <br />0.5 hours <br />0.00298 weeks <br />6.849e-4 months <br /> and indicated on the surveillance package cover sheet that the test data was within acceptance criteria.
On March 11, 2015 the Operations Surveillance Coordinator, who is a STPNOC licensed SRO, performed a subsequent review of the March 4, 2015 AFW pump 24 surveillance and discovered that the acceptance criteria for the as-found discharge pressure had not been met.
The Unit 2 Shift Manager was contacted and at 1631 hours0.0189 days <br />0.453 hours <br />0.0027 weeks <br />6.205955e-4 months <br /> AFW pump 24 was declared inoperable. A follow-up surveillance was performed at 1957 hours0.0227 days <br />0.544 hours <br />0.00324 weeks <br />7.446385e-4 months <br /> and the as-found discharge pressure was 1747 psig. Although this value is within the acceptance criteria, it was higher than expected. A prompt operability determination (POD) concluded that even though the March 11, 2015 surveillance acceptance criteria was met, the results indicated a degrading trend and there was no reasonable assurance that the acceptance-criteria would be met during subsequent surveillance testing. The POD was accepted by the Unit 2 Shift Manager on March 12, 2015 resulting in AFW pump 24 being declared inoperable beginning on March 4, 2015 at 1417 hours0.0164 days <br />0.394 hours <br />0.00234 weeks <br />5.391685e-4 months <br />.
The AFW pump 24 governor and servo actuator were replaced on March 12, 2015. Following a satisfactory surveillance test, AFW pump 24 was declared operable on March 14, 2015 at 0315 hours0.00365 days <br />0.0875 hours <br />5.208333e-4 weeks <br />1.198575e-4 months <br />. The total duration of AFW pump 24 being inoperable was approximately 229 hours0.00265 days <br />0.0636 hours <br />3.786376e-4 weeks <br />8.71345e-5 months <br /> which exceeds the Technical Specification Allowed Outage Time of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />; additionally, the Configuration Risk Management Program was not applied. This condition is reportable under 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by Technical Specifications.
A review of equipment availability during the period of AFW pump 24 inoperability was conducted. On March 9, 2015 at 0300 hours0.00347 days <br />0.0833 hours <br />4.960317e-4 weeks <br />1.1415e-4 months <br />, one of the Unit 2 motor-driven auxiliary feedwater pumps (AFW pump 21) was declared inoperable for planned maintenance and was returned to operable status on March 9, 2015 at 1634 hours0.0189 days <br />0.454 hours <br />0.0027 weeks <br />6.21737e-4 months <br /> (duration of approximately 13.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />). Although plant Technical Specifications allow two auxiliary feedwater pumps to be inoperable for up to 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, this period of inoperability occurred after AFW pump 24 had exceeded its Allowed Outage Time of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. This condition is also reportable under 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by Technical Specifications.
The STP Updated Final Safety Analysis Report (UFSAR) Chapter 15 analysis for the Condition II loss of normal feedwater event assumes that operator action is performed within fifteen minutes after a reactor trip to start a third AFW pump to deliver flow to a third steam generator. During the AFW pump 21 maintenance on March 9, 2015 the pump motor's breaker was racked out from 0348 hours0.00403 days <br />0.0967 hours <br />5.753968e-4 weeks <br />1.32414e-4 months <br /> until 1006 hours0.0116 days <br />0.279 hours <br />0.00166 weeks <br />3.82783e-4 months <br />. During this period of time, the two remaining operable AFW trains would not have been capable of mitigating a loss of normal feedwater flow event as analyzed in UFSAR Chapter 15. This condition is reportable pursuant to 10 CFR 50.73(a)(2)(v)(B) as a condition that could have prevented the fulfillment of the safety function of systems to remove residual heat.
Prior to being declared inoperable on March 4, 2015, AFW pump 24 was previously demonstrated operable via surveillance testing on February 5, 2015.
E. Method of discovery
The AFW pump 24 surveillance acceptance criteria not being met was discovered during a post-surveillance review by the Operations Surveillance Coordinator. The condition that could have prevented the fulfillment of the safety function of systems to remove residual heat was discovered during a review of equipment availability during the period of AFW pump 24 inoperability. Note that for this event, the discovery date is different than the event date because the inoperable component was discovered on March 11, 2015 a week after the surveillance test was performed. Therefore, the event date is March 4, 2015 and the discovery date is March 11, 2015.
II. Component failures
A. Failure mode, mechanism, and effects of failed component
The AFW pump 24 failure mode was the inability to reliably maintain adequate pump discharge pressure. Degradation of the AFW pump 24 governor was the failure mechanism leading to the inoperable condition. The effect of the degraded component was inoperability of AFW Pump 24; no other dependent systems or components were rendered inoperable or degraded. During the period of inoperability, there were three redundant motor-driven AFW pumps operable and available except for a 13.5 hour5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> period on March 9, 2015 when only two redundant motor-driven AFW pumps were operable and available. During these time periods, no event occurred that would have required an AFW actuation.
B. Cause of component failure
Preliminary investigation by an offsite vendor identified that the AFW pump 24 governor was degraded. A combination of as-found compensation needle valve position being too low and blemishes found on the pilot valve plunger likely contributed to the variability in governor speed response.
C. Systems or secondary functions that were affected by failure of components with multiple functions As stated in the narrative summary above, during the 13.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> that two AFW pumps were inoperable, the UFSAR described operator action to start a third AFW pump would not have been able to be performed following a loss of normal feedwater event.
D. Failed component information (Energy Industry Identification System (EIIS) designators provided in {brackets}) Auxiliary feedwater system {BA} Turbine-drive auxiliary feedwater pump: {P} Manufacturer: Sulzer Bingham Pumps, Inc.
Model: 4X6X9C-MSD Turbine-driven auxiliary feedwater pump governor: {65} Manufacturer: Woodward Governor Company Model: PGA
III. Analysis of the event
A. Safety system responses that occurred
There were no safety systems that responded during this event.
B. Duration of safety system inoperability
AFW pump 24 was declared inoperable for 228 hours0.00264 days <br />0.0633 hours <br />3.769841e-4 weeks <br />8.6754e-5 months <br /> and 58 minutes.
C. Safety consequences and implications
AFW pump 24 was demonstrated operable per surveillance testing on February 5, 2015 and was declared inoperable on March 11, 2015. Since the AFW pump 24 failure could have occurred at any time between February 5, 2015 and March 4, 2015, the actual failure duration cannot be conclusively determined. The STP Probabilistic Risk Assessment (PRA) was used to estimate the Incremental Core Damage Probability (ICDP) and Incremental Large Early Release Probability (ILERP) associated with this AFW pump 24 condition. The ICDP and ILERP were determined to be less than 1E-06 and 1E-07 respectively, indicating a very small increase in safety risk.
An actual demand for AFW did not occur during the period of inoperability; therefore, there was no adverse effect on the health and safety of the public.
IV. Cause of the event
On March 4, 2015, AFW pump 24 was not recognized as inoperable due to human error. The two Control Room operators involved used ineffective verification practices of surveillance acceptance criteria. There were no other extenuating circumstances (e.g., time or situational pressures) surrounding the failure to recognize that the acceptance criteria was not met.
V. Corrective actions
The RO and SRO involved in the event were coached and counseled and remediated by Operations Management. A lessons learned bulletin was issued to all operations crew for dissemination and review.
After AFW pump 24 was declared inoperable, corrective maintenance was performed to replace the turbine governor and servo actuator. Post-maintenance surveillance testing restored AFW pump 24 to operable status.
VI. Previous similar events
There are no previous STP LERs with a cause similar to failing to recognize that surveillance acceptance criteria were not met.