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At 1957 on November 07, 2017, VCSNS Unit 1 was operating. in Mode 1 at 100% reactor power when a turb'ne trip caused an automatic reactor trip. All systems responded as expected, with the exception of 'B' Steani Generator Feedwater Isolation Valve (FW1V) XVG1611B-FW. This valve did not appear to automatically close and was slow to indicate closed from the Main Control Board, however this did not complicate the response. All Control Rods fully inserted and all Emergency Feedwater (Mk') pumps started as required. The Operating crew stabilized the plant, which remained in Mode 3 with decay heat removal via the Steam Dump system to the Main Condenser.

The cause of the turbine trip has been determined to be a loss of Digital Control System (DCS) power to all three Main Feedwater Pumps (FWP), which was caused by the failure of Non-Safety Related Inverter XIT5905.

On August 28, 2017, at 0837, VCSNS Unit 1 automatically tripped due to a turbine trip. The turbine trip was caused by the Main Generator Differential Lockout due to a fault on the center phase, 230 kV lightning arrester, on the Main Transformer (XTF-1).

The plant trip response was normal. All control rods fully inserted. Balance of Plant (BOP) buses automatically transferred to their alternate power source, Emergency Auxiliary Transformers (XTF-31/32). Both Motor Driven (MD) Emergency Feedwater (EF) pumps and the Turbine Driven EF Pump started as designed.

The cause of this event was the failure of the center phase lightning arrester on XTF-1. The failed arrester, along with the other two lightning arresters that were in service on XTF-1 during the reactor trip, was replaced. The lightning arresters were sent to an independent lab, NEETRAC - Georgia Tech, for testing and evaluation.

The examination results indicate that the most probable cause of the arrester failure was an internal flashover of the metal oxide varistor blocks. The cause of the internal flashover is likely moisture ingress from the upper end seal.

On June 16, 2017, the station completed a past operability review and determined that an IsAnergency Feedwater Auto Start Actuation Signal was inoperable from November 12. 2016 until April 7. 2017. Technical Specification (TS) 3.3.2 Limiting Conditions for Operation (J,C.0) was entered due to having less than the minimum number of channels operable for Motor Driven Emergency Feedwater Pump (NIDE:MVP) actuation per TS Table 3.3-3 Functional IJnit 6.g.

'Ibis event was caused by the Low Pressure (LP) and High Pressure (HP) steam inlet valves not closing because the Secondary Operating Cylinder and associated Pilot Valve were corroded. Water intrusion into the 'C' Main Feedwater Punip (MFP) oil system had caused the corrosion of the carbon steel components within '1.1)P0022C such that the Secondary Operating Cylinder and Pilot Valve were not functional.

This condition is reportable under 10CFR50.73(a.)(2)(i)(B), an operation or condition which was prohibited by the plant's Technical Specifications.

On March 17, 2015 a past operability review determined that XVG03005A-SP, Outside Reactor Containment (ORC) Reactor Building (RB) Spray Sump Isolation Valve A, was inoperable due to failure to fully stroke open during a surveillance test. XVG03004A-SP, Inside Reactor Containment RB Spray Sump Isolation Valve A, and XVG03005A-SP automatically open on receipt of RWST Lo-Lo Level coincident with a SI signal to transfer XPP0038A, Reactor Building Spray Pump A, suction from the RWST to the RB Sumps.

XVG03004A-SP and XVG03005A-SP are also logically tied to XVG03001A-SP, RB Spray Pump A RWST Suction Header Valve.

The full-open limit switches on XVG03004A-SP and XVG03005A-SP initiate automatic closure of XVG03001A-SP. Since XVG03005A-SP did not stroke fully open, the automatic swapover sequence could not have been completed, rendering the "A" train of RB Spray inoperable.

The cause of XVG03005A-SP to fully stroke open was due to spring pack relaxation and associated torque switch setting tolerances which resulted in the torque switch opening prior to the valve reaching the 100% open limit switch.

The torque switch setting was adjusted from 1 to 1.5 to allow higher developed torque before the switch opens to trip the motor. This adjustment will also reduce the effects from spring pack relaxation.

1.0 ABSTRACT On April 14, 2014, while performing a surveillance test, the normally closed Component Cooling (CC) System emergency makeup valve (XVG09627B-CC) failed to stroke open. This surveillance test is performed each refueling outage and was last successfully tested during a mid cycle outage on March 30, 2013. The emergency makeup supply to the CC System is provided by the Service Water (SW) System. The SW System functions as a source of emergency makeup in the event of a complete loss of the normal makeup capability provided by the Demineralized Water System or if leakage exceeds the normal makeup capacity. The safety related function of XVG09627B-CC is to open to allow SW from the B Train to provide makeup to the B train CC system. By failing to open, this valve was unable to perform its design function without additional operator action.

Since the valve did not stroke open on April 14, 2014, the station has concluded the valve may not have operated if required during the remaining operating cycle after the successful test on March 30, 2013. While XVG09627B-CC was unavailable, it was discovered that XVG09627A-CC was also inoperable. A PRA risk evaluation determined the event of both valves being inoperable is of low safety significance. The cause of this event is believed to be low valve manipulation frequency, added frictional forces and possible spring degradation. XVG09627B-CC has been rebuilt with new closure and trip springs and new packing that has a lower friction resistance. The valve supports were adjusted to enhance the alignment of the operator and the pressure regulator closing force was reduced. This report is submitted in accordance with 10CFR50.73(a)(2)(i)(B) and 10CFR50.73(a)(2)(v)(D).

As a result of recent industry operating experience (OE 305419, EN 49411, EN 49419) regarding the impact of unfused Direct Current (DC) ammeter circuits in the Control Room, Virgil C. Summer Nuclear Station (VCSNS) performed a review of ammeter circuitry. On October 16, 2013, the review determined the described condition to be applicable to VCSNS resulting in an unanalyzed condition with respect to 10CFR50 Appendix R analysis requirements.

The wiring design for the ammeters contains a shunt in the current flow from each DC battery or charger. The ammeter wiring attached to the shunt does not contain fuses. It is postulated that a fire could cause one of the ammeter wires to hot short to ground. Concurrently, the fire could cause another DC wire from the opposite polarity on the same battery or the same battery charger to also short to ground. This would cause a ground loop through the unfused ammeter cable. The potential exists that the cable could heat up causing a secondary fire in the ammeter raceway. The secondary fire could adversely affect safe shutdown equipment and potentially cause the loss of the ability to safely shutdown per 10CFR50 Appendix R.

The apparent cause of this event is double short to ground faults of opposite polarity were not considered during the design process of the Battery System. A corrective action has been added to the National Fire Protection Association (NFPA) 805 implementation to provide a permanent solution to address this unanalyzed Appendix R condition. As an interim action, roving fire watches have been established for the affected areas.

On June 25, 2013 with the plant in Mode 1, the "A" Chiller (XHX0001A) shut down during a fast bus transfer of its 7.2 kV bus power supply due to the tripping of two molded case circuit breakers (MCCBs) located on the "A" Chiller skid. The "A" Chiller was running prior to a planned fast transfer of its 7.2 kV bus power supply from the normal power source (115kV) to the alternate source (230kV). Troubleshooting found both compressor motor MCCBs in the tripped condition.

The instantaneous trip calibration of the MCCBs was designed to trip the breakers with an incoming current greater than the nominal value of 2000A, and a current greater than this magnitude was experienced during this event. Trips of the MCCBs require local operator action to restart the chiller. A subsequent restart attempt resulted in both compressor motors not starting. The "A" Chiller has peen considered to be inoperable since being placed into service August 5, 2011 due to the inability of the chiller to respond to Engineered Safety Features (ESF) sequencer demand following a grid perturbation similar to the bus voltage transient that occurred with a "fast transfer" scenario. The Chilled Water System is an attendant cooling water system that supports the Control Room Emergency Filtration System (CREFS). VCSNS Technical Specifications (TS) 3.7.6 requires two trains of CREFS to be operable while in Mode one through four.

The MCCBs on the Chiller skid have been adjusted to trip at higher amperage. The instantaneous trip setting for the MCCBs has been changed from 2000A to an instantaneous trip range of 3063A to 3938A.

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On 06/14/2012, with the plant in Mode 1 at 100% power, it was determined that opening the code boundary valve between the safety related and seismically qualified Refueling Water Storage Tank (RWST) and the non-safety related and non-seismically qualified Spent Fuel Pool (SFP) Purification Loop in Modes 1-4 renders the RWST inoperable. This alignment was utilized for RWST water mixing in support of weekly surveillance sampling and for filtration of the RWST water prior to refueling outages. As a result, on multiple occasions the RWST was inoperable for a period longer than allowed by Technical Specifications (TS) 3.5.4, Emergency Core Cooling Systems - Refueling Water Storage Tank, Limiting Conditions for Operation (LCO).

The cause of this event is a result of regulatory requirements for the separation of seismically qualified and non-qualified systems, structures and components not being adequately incorporated into the Design Basis Document (DBD) and Updated Final Safety Analysis Report (UFSAR).

Immediate actions consisted of implementation of a Station Order (11-22), which indefinitely suspended this alignment, and submittal of a license amendment request (LAR) to revise TS 3.5.4 such that the non-seismically qualified piping of the SFP Purification System may be aligned to the RWST by operation of a seismically qualified manual ASME code boundary valve under administrative controls for performance of RWST surveillance requirements and pre-outage filtration. This change will only be applicable through the next two fuel cycles.

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On May 3, 2011 at 0514 hours, V.C. Summer Nuclear Station (VCSNS) determined the following:

During circuit analysis review for transitioning the Fire Protection Program to NFPA 805, VCSNS identified a violation of the Appendix R requirement to maintain one train of systems free of fire damage , which are necessary to achieve and maintain Hot Shutdown conditions. This violation applies to postulated fires in the Main Control Room (MCR) or Cable Spreading Room (CSR). Circuits were identified in the MCR and the CSR that impact a control power circuit that could result in the loss of ability to start the B Emergency Diesel Generator (EDG) using local controls.

A root cause analysis was conducted and determined that the root cause was a less than adequate design change/configuration management process. Specifically, a design modification caused the subject vulnerability and the associated Appendix R review (both performed by a vendor for VCSNS) did not ensure the issue was corrected.

Immediate corrective actions consisted of establishing roving fire watches and installing jumpers to defeat the subject control power start permissive contacts.

At 1444 on 11/09/06, after initial installation of the Alternate AC Power Supply, post modification testing was being performed in accordance with STP-125.021, "Periodic Testing of The Alternate AC Power Supply." During the test the 'B' Diesel Generator (DG) was in the standby mode and bus power was being supplied by the Alternate AC Power Supply.

As individual loads were manually loaded on the 1DB.bus, bus voltage momentarily decreased as each load was started.

When the 'B' Emergency Feedwater Pump was started, an undervoltage condition occurred on the bus that was sufficient to pick up the undervoltage (UV) relays, (27-1DB), starting the 'B' DG.

Design Engineering has determined that the bus voltages were sufficient for operation of the bus. The Alternate AC Power Supply was functional and met all test requirements. The 'B' DG was operable, and it auto-started on low voltage, according to design. No load breakers or MCC based contactors opened due to low voltage during the performance of the test. The Supplemental Instrument Air Compressor tripped during the low voltage dip from the Charging Pump start, and the 'B' train Hydrogen Analyzer trouble alarm came in. Both of these components have local controllers and are not supplied by MCC based contactors. Although both were affected by the low voltage on the bus they were not a contributing factor in the UV start of the DG.

STP-125.021 will be revised to align the DG to prevent an inadvertent start during testing. This action is scheduled for completion prior to the next scheduled test.

On September 8, 2003, at 1530 hours maintenance personnel opened the "A" train Control Room charcoal filter plenum to replace a charcoal filter that failed surveillance testing. On September 9, at 0730 hours, the shift supervisor asked if the system could perform its design function during an accident in the current configuration. At approximately the same time (0805 hours), maintenance was directed to close up the system (reinstall the plenum covers and gag the isolation dampers closed).

Engineering stated that the tag-out of the "A" train ventilation system inadvertently repositioned the isolation dampers for the emergency charcoal filter plenum and fan XFN 0030A open. The configuration would not allow maintaining the required positive 0.125 inch water column as specified in TS (3.7.6) and as verified by the intermittent control room low pressure alarms. This maintenance activity effectively breached the control room boundary and rendered the two emergency trains inoperable. This activity unknowingly placed the plant into Technical Specification Limiting Condition for Operability (LCO) 3.0.3, which requires 1 hour to prepare for plant shutdown and 6 hours to Hot Standby. This is being reported under 50.73(a)(2)(i)(B), a condition prohibited by Technical Specifications as the condition was undetected for 16 hours. Additionally, the event is reportable under 50.73 (a)(2)(v)(D) and 50.73(a)(2)(vii).

Replacement of the charcoal was suspended and the filter plenum was closed and isolated by manually gagging the isolation dampers while a plan was developed that would not degrade the ventilation system (charcoal replacement was completed on September 10, 2003). The NRC Emergency Operations Center was notified at approximately 1130 hours; event number EN# 40142.

A root cause evaluation has been completed and is attached to the corrective action document (CER 03-2819).

On April 22-23, 2003, steam propagation barrier door DRIB/107 was first discovered chained and strapped open and later found blocked open.

This event was caused by a lack of specific training for the Security Officers assigned to continuously monitor materials stored in the room behind the door. The door was being opened to provide cooling to the Security Officer on duty in the area. Upon first discovery, the chains and straps were removed and the door was secured.

Approximately 10 hours later, the door was found blocked open by the Security Officer on duty by physically standing with his back against the door. At this time the door was secured again.

Investigation revealed the event was reportable under 10 CFR 50.73(a)(2Xi)(B).

Opening door DRIB/107 beyond normal ingress and egress impacted both trains of control room ventilation as a result of breaching the pressure boundary encompassing all three Heating, Ventilation, and Cooling Water (HVAC) rooms. Loss of both trains of Control Room Ventilation is prohibited by Technical Specifications (TS) 3.7.6.

On June 1, 2002, at 1840, the reactor tripped automatically due to a spike on Nuclear Instrumentation (NI) Intermediate Range channel N-36. The reactor was being held steady at approximately 2% power and no significant plant evolutions were ongoing at the same time. The indicator for channel N-36 spiked to a value greater than the trip setpoint and the reactor protection system automatically tripped the plant. After the trip, spiking was still occurring on this channel. No other NI channel indicated any power increase during this event.

The cause was preliminarily determined to be noise resulting from a failed or degraded circuit card. Multiple circuit cards were replaced and the spiking problem did not reappear. The actual cause of the noise problem has not been determined. The vendor has indicated the power supply vintage has the potential to fail high and cause additional damage to circuit cards, although there is no evidence that a power supply failure occurred. The power supply was not replaced at this time. One circuit card (A-8) has the visual appearance of overheating. The card A8 will be sent off to the vendor for failure analysis.

A root cause investigation determined that a high voltage power supply problem was the cause. The power supplies have been replaced. The circuit cards and power supplies are being placed into the preventative maintenance program for routine replacement.

This is a voluntary report.

On January 25, 2002, diesel fuel was delivered onsite for the emergency diesels. Technical Specification surveillance requirement 4.8.1.1.2.d.2 for fuel analysis is implemented by Surveillance Test Procedure, STP 606.001. This procedure directs that a sample be obtained from each truck delivering diesel fuel oil and selected analyses be performed prior to unloading into the storage tanks. As required, a sample was collected from the diesel fuel oil truck and the appropriate analyses were performed with acceptable results before the truck was unloaded. Seven additional analyses are required to be completed within 30 days of sampling the truck, however the sample collected from the diesel fuel oil truck was mistakenly discarded before the remaining seven analyses were completed.

The cause was determined to be human error. The supervisor had told the individual that it was okay to discard all lube oil samples located on the sample cart. The individual discarded all the old samples on the cart including the diesel fuel oil sample.

The immediate corrective action was to perform all analyses on the contents of both storage tanks. The tanks were determined to be within specification. The emergency diesel generators were operable at all times.