RS-14-147, Additional Information Regarding Response to Bulletin 2012-01, Design Vulnerability in Electric Power System.

Additional Information Regarding Response to Bulletin 2012-01, Design Vulnerability in Electric Power System.

ML14120A271
Person / Time
Site:	Oyster Creek (DPR-016)
Issue date:	04/30/2014
From:	Jim Barstow Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
BL-12-001, RS-14-147, TAC ME9362
Download: ML14120A271 (7)

Text

1

)

RS-14-147 10 CFR SO.S4(f)

April 30, 2014 u.s. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20SSS-0001 Oyster Creek Nuclear Generating Station Renewed Facility Operating License No. DPR-16 NRC Docket No. SO-219

Subject:

Additional Information Regarding Response to Bulletin 2012-01, "Design Vulnerability in Electric Power System"

References:

1. NRC Bulletin 2012-01, "Design Vulnerability in Electric Power System," dated July 27,2012

2. Letter from K. R. Jury (EGC) to NRC, "Exelon Generation Company, LLC 90-Day Response to NRC Bulletin 2012-01, 'Design Vulnerability in Electric Power System,'"

dated October 2S, 2012

3. Letter from M. G. Evans (NRC) to M. J. Padlio (EGC), "Request for Additional Information Regarding Response to Bulletin 2012-01, 'Design Vulnerability in Electric Power System,'" dated December 20,2013

4. Letter from D. M. Gullott (EGC) to NRC, "Exelon Generation Company, LLC Additional Information Regarding Response to Bulletin 2012-01, 'Design Vulnerability in Electric Power System,'" dated February 3,2014 S. Letter from J. Lamb (NRC) to M. J. Padlio (EGC), "Oyster Creek Nuclear Generating Station - Request for Additional Information Regarding Bulletin 2012-01, 'Design Vulnerability in Electric Power System' (T AC No. ME9362)," dated March 31, 2014 On July 27, 2012, Reference 1, NRC Bulletin 2012-01, "Design Vulnerability in Electric Power System," was issued to all holders of operating licenses and combined licenses for nuclear power reactors, except those who have permanently ceased operation and have certified that fuel has been removed from the reactor vessel. The Bulletin requested that all addressees submit a written response within 90 days in accordance with 10 CFR SO.S4, "Conditions of licenses," paragraph (f). The Bulletin requested information to determine compliance with 10 CFR SO.SSa(h)(2), "Protection systems," 10 CFR SO.SSa(h)(3), "Safety systems,"

u.s. Nuclear Regulatory Commission April 30, 2014 Page 2 and Appendix A to 10 CFR Part 50, GDC 17, "Electric power systems," or equivalent principal design criteria as specified in licensee's Updated Final Safety Analysis Report. Exelon Generation Company, LLC (EGC) responded to the Bulletin in Reference 2. In Reference 3, the NRC requested additional information from licensees to verify that they have completed interim corrective actions and compensatory measures in response to the design vulnerability described in Reference 1, and to determine the status of each licensee's long-term corrective actions. That information was provided by EGC in Reference 4. In Reference 5, the NRC requested additional information for Oyster Creek Nuclear Generating Station (OCNGS) based on the Reference 4 response.

The attachment to this letter provides EGC's response to the Reference 5 request for OCNGS.

There are no regulatory commitments contained in this letter. Should you have any questions concerning this letter, please contact Mr. Mitchel Mathews at (630) 657-2819.

I declare under penalty of perjury that the foregoing is true and correct. Executed on the 30th day of April 2014.

Respectfully, d~~

James Barstow Director Licensing Exelon Generation Company, LLC

Attachment:

Oyster Creek Nuclear Generating Station Supplemental Information Related to Bulletin 2012-01, "Design Vulnerability in Electric Power System"

Oyster Creek Nuclear Generating Station Supplemental Information Related to Bulletin 2012-01, "Design Vulnerability in Electric Power System" NRC Request:

On July 27, 2012, the U.S. Nuclear Regulatory Commission (NRC) issued Bulletin 2012-01, "Design Vulnerability in Electric Power System" (Agencywide Documents Access and Management System (ADAMS) Accession No. ML12074A115), to all holders of operating licenses and combined licenses for nuclear power reactors.

By letter dated October 25,2012 (ADAMS Accession No. ML12300A106), Exelon Generation Company, LLC (Exelon), responded to Bulletin 2012-01. The NRC sent a request for additional information (RAI) regarding Bulletin 2012-01, by letter dated December 20, 2013 (ADAMS Accession No. ML13351A314). Exelon responded to the RAI in its letter dated February 3,2014 (ADAMS Accession No. ML14034A179).

The NRC staff has determined that additional information is required to complete its review.

The specific information requested is addressed below.

NRC Request No.1. In addition to the information provided in the RAI response, please describe specific electrical system parameters monitored / instructions provided to the control room operators to validate, on a daily basis, that there are no open phase issues at Oyster Creek Nuclear Generating Station (OCNGS).

Exelon Generation Company. LLC (EGC) Response to NRC Request No.1:

To ensure that there are no open phase conditions (OPCs) at OCNGS, conditions permitting, the operators conduct daily switchyard tours. During these tours the operators monitor the physical condition of the switchyard which includes monitoring of the insulators for all distribution and transmission lines. Each phase that is supplied to the startup transformers (SUTs) is also monitored. This monitoring includes checking the individual phase voltage regulators for proper operation and to verify the tap position of each phase of the voltage regulator is consistent with the corresponding phase on the other bank. Any abnormal indication including any difference of two or more tap positions on the voltage regulators would not meet the acceptance criteria contained in OCNGS Procedure 681.4.005, "Substation Tour Sheet." Failure to meet these acceptance criteria would prompt operators to contact OCNGS Engineering, who would investigate the abnormal conditions.

In the Main Control Room, operators have a low voltage alarm that annunciates when line-to-line voltage between any two phases drops below 3800 volts on the safety buses. The operators will monitor the individual phase currents on a low voltage condition and trip the supply breaker to the safety bus on an open phase. This guidance is provided in Alarm Response Procedures RAP-T3a, "Bus 1C Volts Lo," and RAP-T3e, "Bus 1D Volts Lo," for Banks 5 and 6, respectively.

Details on system monitoring are as follows:

• Daily inspections of the switchyard insulators.

• Once per shift inspections of the dilution pumps and dilution pump switchgear which are normally supplied from SUT SA. Operators monitor for any abnormal condition, which may be indicative of an OPC during these inspections. Abnormal conditions identified during Page 1 of 5

Oyster Creek Nuclear Generating Station Supplemental Information Related to Bulletin 2012*01, "Design Vulnerability in Electric Power System" these inspections would be promptly evaluated by shift management and entered into the Corrective Action Program for further evaluation.

• In accordance with OCNGS procedure 681.4.005, "Substation Tour Sheet," the tap positions for all three phases of the voltage regulators for the startup transformers are recorded. The tap positions of the voltage regulators for each transformer must be within two steps of the corresponding tap on the other divisional bank. If an OPC existed on either of the two SUT supply lines, the tap position for the affected phase would be different (Le., by more than two steps) than the corresponding phase on the other division. If the tap positions are out of position by more than two steps, the acceptance criteria of OCNGS procedure 681.4.005 is not met and an engineering assessment of the condition is performed. The SUbstation tour is normally performed on a daily basis (Le., conditions permitting). Therefore, in the unlikely event of an OPC on either of the offsite power sources, it should be detected within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> and actions will be taken to correct the situation.

• When an Engineered Safety Feature (ESF) system is out of service, the Substation Tour is performed every eight (8) hours in accordance with OCNGS Procedure 681.4.004 and OP-OC-1 000-1 003, "Redundant System Operability Verification Checksheet," Section 8, "Technical Specification Log Sheet." Therefore, in the unlikely event of an OPC on either of the offsite power sources, it would be detected within eight (8) hours and actions will be taken to correct the situation.

• OCNGS design does not provide voltage monitoring on the ESF buses. Therefore, the plant operating procedures, including operating procedures for off-normal plant conditions, do not call for verification of the voltages on all three phases of the ESF buses. However, if the plant electrical auxiliary power is being supplied by the SUTs and the line-to-line voltage between any of the three phases goes below the degraded-grid voltage relay setpoint, associated alarms will be received in the main control room. The alarm response procedures for these alarms require operators to check the currents on the primary side of the SUTs. If the current is zero (0) in one of the phases, the procedure directs operators to trip the corresponding breaker to separate the affected ESF bus from its offsite power source. This will start the emergency diesel generator associated with that ESF bus.

• Alarm response procedures for "BUS 1C VOLTS LO" and "BUS 10 VOLTS LO" have been revised to provide guidance to operators to monitor individual phase currents for all three phases of power for the respective SUT on control room panel 12F-8 for detection and action to separate the ESF buses from the offsite power source if a loss of phase event occurs.

These actions minimize the time that an undetected open phase condition could exist and minimize the impact on plant equipment.

Page 2 of 5

Oyster Creek Nuclear Generating Station Supplemental Information Related to Bulletin 2012*01, "Design Vulnerability in Electric Power System" NRC Request No.2. In response to long-term actions proposed for correcting open phase conditions (OPC), Exelon stated that:

The design of the OCNGS auxiliary power system and the General Design Criterion (GOC) 17, "Electric power systems,"

power sources are robust in that the sources are physically independent and completely redundant. As a result, even though the OPC may not be detected and could impact one division of [engineered safety system] ESF function, all ESF and important-to-safety functions are preserved in the other division. Based on the physical independence of the two GOC 17 sources, the compensatory actions, the small potential for inadvertent actuation of the OPC protection scheme, and the short time the OPC protection scheme would be in operation (i.e., OCNGS will be shut down in the year 2019), there are no plans to implement any OPC protection scheme at OCNGS.

Please provide an engineering analysis of the OCNGS vulnerability to single failures in the electrical systems. Please provide a basis why operating the plant until the end of its license without the OPC protection maintains adequate protection and maintains public health and safety.

EGC Response to NRC Request No.2:

The offsite power configuration at OCNGS is different than Byron Station. Byron Station has a single 345 kV power feed that normally powers both ESF divisions. The OCNGS configuration has two independent feeders from the offsite power sources. Each of these feeders only powers one of the ESF buses. During normal power operations, an OPC on one feeder would not result in a loss of any of the safety functions that are required to meet accident analyses.

The details of the OCNGS power configuration are discussed below.

Description of the Auxiliary Power System:

The normal source of auxiliary power for OCNGS is the main generator through the Station Auxiliary Transformer (SAT). The preferred power sources during startup, shutdown, and abnormal or accident conditions are the SUTs.

Normal startup, shutdown, and standby back-up power is provided by two physically independent off-site sources that are provided from the 34.5 kV buses. The power is supplied from 34.5 kV Buses A and B to SUT SB and SA through voltage regulators (Le., Transformer Bank 6 and Bank 5, respectively). The secondary side of SUTs SA and SB are connected to Non-Class 1E Switchgear Bus 1A and Non-Class 1E Switchgear Bus 1B, respectively.

Switchgear Bus 1A is connected to Class 1E Switchgear Bus 1C, and Switchgear Bus 1B is connected to Class 1E Switchgear Bus 1D.

Page 3 of 5

Oyster Creek Nuclear Generating Station Supplemental Information Related to Bulletin 2012-01, "Design Vulnerability in Electric Power System" Normal Operation:

During normal operation, excluding the maintenance periods, the main generator supplies the SAT, which supplies Switchgear Bus 1A and SWitchgear Bus 1B from two different 4160 V secondary windings.

In the event the main generator trips (e.g., from a reactor trip or generator protection),

Switchgear Buses 1A and 1B are automatically transferred to the SUTs. If the SUTs are not available or trip for any reason, Breakers 1C and/or 1D open and the EDGs start and load onto Switchgear Buses 1C and 1D within 20 seconds.

Each safety bus (Le., 1C and 1D) has three (3) single phase potential transformers (Le., 4200V /

120V). The loss of voltage relays (Le., 27-1 C for Bus 1C and 27-1 D for Bus 1D) monitor the A and B potential transformer secondary phases, while the degraded voltage relays monitor all three line-to-line voltages (Le., 27-11 C for Bus 1C and 27-11 D for Bus 1D monitor between A and B phases; 27 -12C for Bus 1C and 27-12D for Bus 1D monitor between Band C phases; and 27 -13C for Bus 1C and 27 -13D for Bus 1D monitor between A and C phases).

The 1E 4160 V buses trip the respective breakers (1 C or 1D) when the loss of voltage condition on the buses is detected. In addition, the degraded grid voltage relays that are arranged in a two-out-of-three configuration trip the breakers for the 1E buses with a 10-second time delay when at least two of three relays are actuated. If the loss of voltage relay does not actuate under OPC, the likelihood of two of the degraded grid relays tripping under OPC is high. This will trip the breakers for the 1E buses in 10 seconds.

Shutdown/Abnormal Operation:

During shutdown operation, to facilitate maintenance on any of the 4160 volt buses, the electrical power sources may be limited to one of the two offsite power sources for a short duration (e.g., 105 hours0.00122 days <br />0.0292 hours <br />1.736111e-4 weeks <br />3.99525e-5 months <br /> planned during the upcoming 1R25 refueling outage). During this evolution, the 480 volt unit substations are cross tied to provide power to plant loads in accordance with site procedures. The prerequisites for this evolution are that the reactor mode switch is in SHUTDOWN or REFUEL, Reactor Coolant System is less than 212 'F with reactor vented, and the voltages at the 4160 volt buses are within the acceptable limits. Plant risk is further limited by ensuring that the reactor cavity is flooded and the fuel pool gates removed in accordance with OU-OC-1 03-1 001, "Oyster Creek Shutdown Safety Management Program,"

during plant evolutions such as abnormal electrical lineups with minimum power sources available. This allows much more inventory of reactor coolant than what will be available without the fuel pool gates removed.

OCNGS has the capability to back feed power from the 230 kV offsite source through the main power transformer (MPT) in accordance with OCNGS Procedure 337, "4160 Volt Electrical System." In this configuration the MPT can act as step-down transformers to power the isophase bus and the SAT from offsite. In turn, the SAT can power 4160 volt Bus 1A and Class 1E Bus 1C independent of the SUTs.

In addition to EDGs, OCNGS can also restore power to 4160 V Bus 1B by starting the Station Blackout (SBO) combustion turbines (CTs) in accordance with Abnormal Operating Procedure, Page 4 of 5

Oyster Creek Nuclear Generating Station Supplemental Information Related to Bulletin 2012-01, "Design Vulnerability in Electric Power System" ABN-36, "Loss of Offsite Power and Station Blackout." The OCNGS SBO power source has the capability to carry all ESF loads independent of the SUTs.

Lastly, operators have the option to trip Breakers 1C and/or 1D to start the Emergency Diesel Generators (EDGs) should the OPC condition occur. Therefore, due to the multiple, diverse and independent power sources available during normal operation, and the additional risk-reducing measures taken during off-normal electric lineups, the existing OCNGS electric power system provides adequate protection to maintain public health and safety.

Risk Assessment:

A risk assessment for an OPC at OCNGS was completed with the following results:

The overall characterization of the risk impact from an OPC at OCNGS would be of very low safety significance (Le., core damage frequency on the order of 7.0 E-9 per year). This is primarily due to the switchyard design, the independence of the SUTs and associated buses, the low frequency of an initiating event due to an OPC, and the mitigating capabilities of the isolation condenser during SBO events. Sensitivity evaluations show a low quantitative impact on core damage frequency from the operator action to diagnose and execute steps to separate the ESF buses from the degraded SUT. Improving procedures and design to make the human action success more likely would only improve risk by a small amount (Le., change in core damage frequency on the order of 6.3 E-9 per year). Based on the results of this risk assessment, OCNGS can maintain adequate protection and maintain public health and safety while operating the plant until the end of its license without implementing any open phase isolation system (OPIS).

==

Conclusion:==

Based on the multiple, diverse, and independent power sources available during normal operation, the minimal time off-normal electrical lineups are used, additional plant risk-reducing measures taken during off-normal electric lineups, and the results of the risk assessment discussed above, the operation of the existing OCNGS electric power system continues to maintain adequate protection and maintain public health and safety without the implementation of an OPIS.

Page 5 of 5

Text

1

)

RS-14-147 10 CFR SO.S4(f)

April 30, 2014 u.s. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20SSS-0001 Oyster Creek Nuclear Generating Station Renewed Facility Operating License No. DPR-16 NRC Docket No. SO-219

Subject:

Additional Information Regarding Response to Bulletin 2012-01, "Design Vulnerability in Electric Power System"

References:

1. NRC Bulletin 2012-01, "Design Vulnerability in Electric Power System," dated July 27,2012

2. Letter from K. R. Jury (EGC) to NRC, "Exelon Generation Company, LLC 90-Day Response to NRC Bulletin 2012-01, 'Design Vulnerability in Electric Power System,'"

dated October 2S, 2012

3. Letter from M. G. Evans (NRC) to M. J. Padlio (EGC), "Request for Additional Information Regarding Response to Bulletin 2012-01, 'Design Vulnerability in Electric Power System,'" dated December 20,2013

4. Letter from D. M. Gullott (EGC) to NRC, "Exelon Generation Company, LLC Additional Information Regarding Response to Bulletin 2012-01, 'Design Vulnerability in Electric Power System,'" dated February 3,2014 S. Letter from J. Lamb (NRC) to M. J. Padlio (EGC), "Oyster Creek Nuclear Generating Station - Request for Additional Information Regarding Bulletin 2012-01, 'Design Vulnerability in Electric Power System' (T AC No. ME9362)," dated March 31, 2014 On July 27, 2012, Reference 1, NRC Bulletin 2012-01, "Design Vulnerability in Electric Power System," was issued to all holders of operating licenses and combined licenses for nuclear power reactors, except those who have permanently ceased operation and have certified that fuel has been removed from the reactor vessel. The Bulletin requested that all addressees submit a written response within 90 days in accordance with 10 CFR SO.S4, "Conditions of licenses," paragraph (f). The Bulletin requested information to determine compliance with 10 CFR SO.SSa(h)(2), "Protection systems," 10 CFR SO.SSa(h)(3), "Safety systems,"

u.s. Nuclear Regulatory Commission April 30, 2014 Page 2 and Appendix A to 10 CFR Part 50, GDC 17, "Electric power systems," or equivalent principal design criteria as specified in licensee's Updated Final Safety Analysis Report. Exelon Generation Company, LLC (EGC) responded to the Bulletin in Reference 2. In Reference 3, the NRC requested additional information from licensees to verify that they have completed interim corrective actions and compensatory measures in response to the design vulnerability described in Reference 1, and to determine the status of each licensee's long-term corrective actions. That information was provided by EGC in Reference 4. In Reference 5, the NRC requested additional information for Oyster Creek Nuclear Generating Station (OCNGS) based on the Reference 4 response.

The attachment to this letter provides EGC's response to the Reference 5 request for OCNGS.

There are no regulatory commitments contained in this letter. Should you have any questions concerning this letter, please contact Mr. Mitchel Mathews at (630) 657-2819.

I declare under penalty of perjury that the foregoing is true and correct. Executed on the 30th day of April 2014.

Respectfully, d~~

James Barstow Director Licensing Exelon Generation Company, LLC

Attachment:

Oyster Creek Nuclear Generating Station Supplemental Information Related to Bulletin 2012-01, "Design Vulnerability in Electric Power System"

Oyster Creek Nuclear Generating Station Supplemental Information Related to Bulletin 2012-01, "Design Vulnerability in Electric Power System" NRC Request:

On July 27, 2012, the U.S. Nuclear Regulatory Commission (NRC) issued Bulletin 2012-01, "Design Vulnerability in Electric Power System" (Agencywide Documents Access and Management System (ADAMS) Accession No. ML12074A115), to all holders of operating licenses and combined licenses for nuclear power reactors.

By letter dated October 25,2012 (ADAMS Accession No. ML12300A106), Exelon Generation Company, LLC (Exelon), responded to Bulletin 2012-01. The NRC sent a request for additional information (RAI) regarding Bulletin 2012-01, by letter dated December 20, 2013 (ADAMS Accession No. ML13351A314). Exelon responded to the RAI in its letter dated February 3,2014 (ADAMS Accession No. ML14034A179).

The NRC staff has determined that additional information is required to complete its review.

The specific information requested is addressed below.

NRC Request No.1. In addition to the information provided in the RAI response, please describe specific electrical system parameters monitored / instructions provided to the control room operators to validate, on a daily basis, that there are no open phase issues at Oyster Creek Nuclear Generating Station (OCNGS).

Exelon Generation Company. LLC (EGC) Response to NRC Request No.1:

To ensure that there are no open phase conditions (OPCs) at OCNGS, conditions permitting, the operators conduct daily switchyard tours. During these tours the operators monitor the physical condition of the switchyard which includes monitoring of the insulators for all distribution and transmission lines. Each phase that is supplied to the startup transformers (SUTs) is also monitored. This monitoring includes checking the individual phase voltage regulators for proper operation and to verify the tap position of each phase of the voltage regulator is consistent with the corresponding phase on the other bank. Any abnormal indication including any difference of two or more tap positions on the voltage regulators would not meet the acceptance criteria contained in OCNGS Procedure 681.4.005, "Substation Tour Sheet." Failure to meet these acceptance criteria would prompt operators to contact OCNGS Engineering, who would investigate the abnormal conditions.

In the Main Control Room, operators have a low voltage alarm that annunciates when line-to-line voltage between any two phases drops below 3800 volts on the safety buses. The operators will monitor the individual phase currents on a low voltage condition and trip the supply breaker to the safety bus on an open phase. This guidance is provided in Alarm Response Procedures RAP-T3a, "Bus 1C Volts Lo," and RAP-T3e, "Bus 1D Volts Lo," for Banks 5 and 6, respectively.

Details on system monitoring are as follows:

• Daily inspections of the switchyard insulators.

• Once per shift inspections of the dilution pumps and dilution pump switchgear which are normally supplied from SUT SA. Operators monitor for any abnormal condition, which may be indicative of an OPC during these inspections. Abnormal conditions identified during Page 1 of 5

Oyster Creek Nuclear Generating Station Supplemental Information Related to Bulletin 2012*01, "Design Vulnerability in Electric Power System" these inspections would be promptly evaluated by shift management and entered into the Corrective Action Program for further evaluation.

• In accordance with OCNGS procedure 681.4.005, "Substation Tour Sheet," the tap positions for all three phases of the voltage regulators for the startup transformers are recorded. The tap positions of the voltage regulators for each transformer must be within two steps of the corresponding tap on the other divisional bank. If an OPC existed on either of the two SUT supply lines, the tap position for the affected phase would be different (Le., by more than two steps) than the corresponding phase on the other division. If the tap positions are out of position by more than two steps, the acceptance criteria of OCNGS procedure 681.4.005 is not met and an engineering assessment of the condition is performed. The SUbstation tour is normally performed on a daily basis (Le., conditions permitting). Therefore, in the unlikely event of an OPC on either of the offsite power sources, it should be detected within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> and actions will be taken to correct the situation.

• When an Engineered Safety Feature (ESF) system is out of service, the Substation Tour is performed every eight (8) hours in accordance with OCNGS Procedure 681.4.004 and OP-OC-1 000-1 003, "Redundant System Operability Verification Checksheet," Section 8, "Technical Specification Log Sheet." Therefore, in the unlikely event of an OPC on either of the offsite power sources, it would be detected within eight (8) hours and actions will be taken to correct the situation.

• OCNGS design does not provide voltage monitoring on the ESF buses. Therefore, the plant operating procedures, including operating procedures for off-normal plant conditions, do not call for verification of the voltages on all three phases of the ESF buses. However, if the plant electrical auxiliary power is being supplied by the SUTs and the line-to-line voltage between any of the three phases goes below the degraded-grid voltage relay setpoint, associated alarms will be received in the main control room. The alarm response procedures for these alarms require operators to check the currents on the primary side of the SUTs. If the current is zero (0) in one of the phases, the procedure directs operators to trip the corresponding breaker to separate the affected ESF bus from its offsite power source. This will start the emergency diesel generator associated with that ESF bus.

• Alarm response procedures for "BUS 1C VOLTS LO" and "BUS 10 VOLTS LO" have been revised to provide guidance to operators to monitor individual phase currents for all three phases of power for the respective SUT on control room panel 12F-8 for detection and action to separate the ESF buses from the offsite power source if a loss of phase event occurs.

These actions minimize the time that an undetected open phase condition could exist and minimize the impact on plant equipment.

Page 2 of 5

Oyster Creek Nuclear Generating Station Supplemental Information Related to Bulletin 2012*01, "Design Vulnerability in Electric Power System" NRC Request No.2. In response to long-term actions proposed for correcting open phase conditions (OPC), Exelon stated that:

The design of the OCNGS auxiliary power system and the General Design Criterion (GOC) 17, "Electric power systems,"

power sources are robust in that the sources are physically independent and completely redundant. As a result, even though the OPC may not be detected and could impact one division of [engineered safety system] ESF function, all ESF and important-to-safety functions are preserved in the other division. Based on the physical independence of the two GOC 17 sources, the compensatory actions, the small potential for inadvertent actuation of the OPC protection scheme, and the short time the OPC protection scheme would be in operation (i.e., OCNGS will be shut down in the year 2019), there are no plans to implement any OPC protection scheme at OCNGS.

Please provide an engineering analysis of the OCNGS vulnerability to single failures in the electrical systems. Please provide a basis why operating the plant until the end of its license without the OPC protection maintains adequate protection and maintains public health and safety.

EGC Response to NRC Request No.2:

The offsite power configuration at OCNGS is different than Byron Station. Byron Station has a single 345 kV power feed that normally powers both ESF divisions. The OCNGS configuration has two independent feeders from the offsite power sources. Each of these feeders only powers one of the ESF buses. During normal power operations, an OPC on one feeder would not result in a loss of any of the safety functions that are required to meet accident analyses.

The details of the OCNGS power configuration are discussed below.

Description of the Auxiliary Power System:

The normal source of auxiliary power for OCNGS is the main generator through the Station Auxiliary Transformer (SAT). The preferred power sources during startup, shutdown, and abnormal or accident conditions are the SUTs.

Normal startup, shutdown, and standby back-up power is provided by two physically independent off-site sources that are provided from the 34.5 kV buses. The power is supplied from 34.5 kV Buses A and B to SUT SB and SA through voltage regulators (Le., Transformer Bank 6 and Bank 5, respectively). The secondary side of SUTs SA and SB are connected to Non-Class 1E Switchgear Bus 1A and Non-Class 1E Switchgear Bus 1B, respectively.

Switchgear Bus 1A is connected to Class 1E Switchgear Bus 1C, and Switchgear Bus 1B is connected to Class 1E Switchgear Bus 1D.

Page 3 of 5

Oyster Creek Nuclear Generating Station Supplemental Information Related to Bulletin 2012-01, "Design Vulnerability in Electric Power System" Normal Operation:

During normal operation, excluding the maintenance periods, the main generator supplies the SAT, which supplies Switchgear Bus 1A and SWitchgear Bus 1B from two different 4160 V secondary windings.

In the event the main generator trips (e.g., from a reactor trip or generator protection),

Switchgear Buses 1A and 1B are automatically transferred to the SUTs. If the SUTs are not available or trip for any reason, Breakers 1C and/or 1D open and the EDGs start and load onto Switchgear Buses 1C and 1D within 20 seconds.

Each safety bus (Le., 1C and 1D) has three (3) single phase potential transformers (Le., 4200V /

120V). The loss of voltage relays (Le., 27-1 C for Bus 1C and 27-1 D for Bus 1D) monitor the A and B potential transformer secondary phases, while the degraded voltage relays monitor all three line-to-line voltages (Le., 27-11 C for Bus 1C and 27-11 D for Bus 1D monitor between A and B phases; 27 -12C for Bus 1C and 27-12D for Bus 1D monitor between Band C phases; and 27 -13C for Bus 1C and 27 -13D for Bus 1D monitor between A and C phases).

The 1E 4160 V buses trip the respective breakers (1 C or 1D) when the loss of voltage condition on the buses is detected. In addition, the degraded grid voltage relays that are arranged in a two-out-of-three configuration trip the breakers for the 1E buses with a 10-second time delay when at least two of three relays are actuated. If the loss of voltage relay does not actuate under OPC, the likelihood of two of the degraded grid relays tripping under OPC is high. This will trip the breakers for the 1E buses in 10 seconds.

Shutdown/Abnormal Operation:

During shutdown operation, to facilitate maintenance on any of the 4160 volt buses, the electrical power sources may be limited to one of the two offsite power sources for a short duration (e.g., 105 hours0.00122 days <br />0.0292 hours <br />1.736111e-4 weeks <br />3.99525e-5 months <br /> planned during the upcoming 1R25 refueling outage). During this evolution, the 480 volt unit substations are cross tied to provide power to plant loads in accordance with site procedures. The prerequisites for this evolution are that the reactor mode switch is in SHUTDOWN or REFUEL, Reactor Coolant System is less than 212 'F with reactor vented, and the voltages at the 4160 volt buses are within the acceptable limits. Plant risk is further limited by ensuring that the reactor cavity is flooded and the fuel pool gates removed in accordance with OU-OC-1 03-1 001, "Oyster Creek Shutdown Safety Management Program,"

during plant evolutions such as abnormal electrical lineups with minimum power sources available. This allows much more inventory of reactor coolant than what will be available without the fuel pool gates removed.

OCNGS has the capability to back feed power from the 230 kV offsite source through the main power transformer (MPT) in accordance with OCNGS Procedure 337, "4160 Volt Electrical System." In this configuration the MPT can act as step-down transformers to power the isophase bus and the SAT from offsite. In turn, the SAT can power 4160 volt Bus 1A and Class 1E Bus 1C independent of the SUTs.

In addition to EDGs, OCNGS can also restore power to 4160 V Bus 1B by starting the Station Blackout (SBO) combustion turbines (CTs) in accordance with Abnormal Operating Procedure, Page 4 of 5

Oyster Creek Nuclear Generating Station Supplemental Information Related to Bulletin 2012-01, "Design Vulnerability in Electric Power System" ABN-36, "Loss of Offsite Power and Station Blackout." The OCNGS SBO power source has the capability to carry all ESF loads independent of the SUTs.

Lastly, operators have the option to trip Breakers 1C and/or 1D to start the Emergency Diesel Generators (EDGs) should the OPC condition occur. Therefore, due to the multiple, diverse and independent power sources available during normal operation, and the additional risk-reducing measures taken during off-normal electric lineups, the existing OCNGS electric power system provides adequate protection to maintain public health and safety.

Risk Assessment:

A risk assessment for an OPC at OCNGS was completed with the following results:

The overall characterization of the risk impact from an OPC at OCNGS would be of very low safety significance (Le., core damage frequency on the order of 7.0 E-9 per year). This is primarily due to the switchyard design, the independence of the SUTs and associated buses, the low frequency of an initiating event due to an OPC, and the mitigating capabilities of the isolation condenser during SBO events. Sensitivity evaluations show a low quantitative impact on core damage frequency from the operator action to diagnose and execute steps to separate the ESF buses from the degraded SUT. Improving procedures and design to make the human action success more likely would only improve risk by a small amount (Le., change in core damage frequency on the order of 6.3 E-9 per year). Based on the results of this risk assessment, OCNGS can maintain adequate protection and maintain public health and safety while operating the plant until the end of its license without implementing any open phase isolation system (OPIS).

==

Conclusion:==

Based on the multiple, diverse, and independent power sources available during normal operation, the minimal time off-normal electrical lineups are used, additional plant risk-reducing measures taken during off-normal electric lineups, and the results of the risk assessment discussed above, the operation of the existing OCNGS electric power system continues to maintain adequate protection and maintain public health and safety without the implementation of an OPIS.

Page 5 of 5