ML12307A018

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Response to NRC Bulletin 2012-01
ML12307A018
Person / Time
Site: McGuire, Mcguire  Duke Energy icon.png
Issue date: 10/25/2012
From: Capps S
Duke Energy Carolinas
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
BL-12-001
Download: ML12307A018 (16)
v • d • e


Text

STEVEN D. CAPPS WDUke Vice President

-Ener~fgy McGuire Nuclear Station Duke Energy MGO0 VP / 12700 Hagers Ferry Rd.

Huntersville, NC 28078 980-875-4805 980-875-4809 fax Steven. Capps@duke-energy. corn October 25, 2012 10 CFR 50.54(f)

U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555-0001

Subject:

Duke Energy Carolinas, LLC (Duke Energy)

McGuire Nuclear Station, Units 1 and 2 Docket Nos. 50-369 and 50-370 Response to NRC Bulletin 2012-01

Reference:

1. NRC Bulletin 2012-01: Design Vulnerability in Electric Power System, dated July 27, 2012 On July 27, 2012, the Nuclear Regulatory Commission issued NRC Bulletin 2012-01 "Design Vulnerability in Electric Power System" to all power reactor licensees and holders of combined licenses for nuclear power reactors. The purpose of this bulletin is to notify Licensees of a recent operating experience concerning the loss of one of the three phases of the offsite power circuit at Byron Station, Unit 2 in order to determine if further regulatory action is warranted.

NRC Bulletin 2012-01 requires that each licensee provide a response to the Requested Actions within 90 days of the date of this bulletin. Enclosure 1 provides the responses to the Requested Actions.

There are no regulatory commitments contained in this letter, Please address any comments or questions regarding this matter to Rick Abbott at 980-875-4685.

I declare under penalty of perjury that the foregoing is true and correct.

Respectfully, Steven D. Capps :

www. duke-energy com

U.S. Nuclear Regulatory Commission October 25, 2012 Page 2 xc: with enclosure V. M. McCree, Region II Administrator U.S. Nuclear Regulatory Commission Marquis One Tower 245 Peachtree Center Avenue NE, Suite 1200 Atlanta, Georgia 30303-1257 E. J. Leeds Director, Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission 11555 Rockville Pike Mail Stop 13-H16M Rockville, MD 20852-2738 J. H. Thompson, Project Manager U. S. Nuclear Regulatory Commission 11555 Rockville Pike Mail Stop 0-8 G9A Rockville, MD 20852-2738 John Zeiler NRC Senior Resident Inspector McGuire Nuclear Station Bulletin 2012-01 Response Overview:

System Description - Items 2., 1.d, 2.a, 2.c System Protection - 1., 1.a, 2.b, 2.d Consequences - 1.b, 1.c, 2.e

  • - Unit 1 Simplified One-Line Diagram
  • a - Unit 2 Simplified One-Line Diagram
  • - Tables o

Table 1 - ESF Buses Continuously Powered From Offsite Power Source(s) o Table 2 - ESF Buses Not Continuously Powered From Offsite Power Source(s) o Table 3 - ESF Buses Major Loads o

Table 4 - Offsite Power Transformers o

Table 5 - Protective Devices

System Description

Items 2, 1.d, 2.a, and 2.c request system information and will be addressed in this section:

2. Briefly describe the operating configuration of the ESF buses (Class 1E for current operating plants or non-Class 1E for passive plants) at power (normal operating condition).

Response

McGuire is connected to offsite power through parallel Main Step-Up (MSU) transformers and parallel Bus Lines (BL). Each of these transformers is connected to parallel Generator Circuit Breakers (GCB) which serves as the connection point of the main generator to offsite and onsite power system.

During normal loading conditions, the main generator supplies power to the ESF busses through the GCBs and various auxiliary transformers. McGuire Unit 1 connects to a 230 kV Switchyard, and McGuire Unit 2 connects to a 525 kV Switchyard.

Page 1 Bulletin 2012-01 Response Upon a reactor, turbine, or generator trip, the GCBs open which separates the main generator from the offsite and onsite power systems. By opening the GCBs, the parallel MSU transformers immediately provide offsite power to the ESF busses with no bus transfer schemes required.

See Attachment 2 and 2a, for simplified one-line diagrams of the Unit 1 and Unit 2 power system. Note Attachment 2 and 2a shows transformers 1ATE and 2ATE. These transformers supply power to the auxiliary electric boilers and do not supply power to any ESF loads.

1.d. Describe the offsite power transformer (e.g., start-up, reserve, station auxiliary) winding and grounding configurations.

Response

See Attachment 3, Table 4 for offsite power transformer winding and grounding configurations.

2. a. Are the ESF buses powered by offsite power sources? If so, explain what major loads are connected to the buses including their ratings.

Response

Yes, for normal operating condition configurations Emergency Safeguard Feature (ESF) buses and non-safety loads are aligned to offsite sources, but powered from the main generator.

See Attachment 3, Tables 1 and 2 for ESF bus power sources.

See Attachment 3, Table 3 for ESF bus major loads energized during normal power operations, including their ratings.

2.c. Confirm that the operating configuration of the ESF buses is consistent with the current licensing basis. Describe any changes in offsite power source alignment to the ESF buses from the original plant licensing.

Response

The current licensing basis for McGuire Nuclear Station credits two independent offsite power sources that are capable of supplying power to two independent ESF busses. This confirms the operating configuration of the ESF buses is consistent with the current licensing basis. No changes from the original licensing basis have been made for offsite power alignments to the ESF busses.

Each nuclear generating unit is provided with two independent immediate access circuits of offsite power from the Transmission System. For Unit 1, each circuit consists of a connection from the 230 kV switching station over an independent 230 kV overhead transmission line through one of the two half-sized step-up transformers (1A, 1 B) to one of the two unit auxiliary Page 2 Bulletin 2012-01 Response transformers. For Unit 2, each circuit consists of a connection from the 525 kV Switching Station over an independent 525 kV overhead transmission line through one of the two half-sized step-up transformers (2A, 2B) to one of the two unit auxiliary transformers.

Each unit is provided with two full size auxiliary power transformers (1ATA, 1ATB, 2ATA, 2ATB) which are rated 60/80/100 MVA and are sized to carry all of the auxiliaries of one operating nuclear unit plus the safety shutdown loads of the other nuclear unit. Each auxiliary transformer has two secondary windings, with each winding normally energizing one 6900 Volt unit normal auxiliary switchgear assembly.

The 6900 Volt Normal Auxiliary Power System of each nuclear unit consists of four assemblies of auxiliary switchgear with each assembly connected through two main breakers and buses to the two unit auxiliary power transformers (1TA, 1TB, 1TC, 1TD, 2TA, 2TB, 2TC, 2TD). With the two full-sized power transformers available, only two of the four switchgear assemblies normally receive power from each auxiliary power transformer. The 6900 Volt Normal Auxiliary Power System furnishes power to all of the large station auxiliary loads such as the reactor coolant pumps, condenser circulating water pumps, hotwell pumps, etc.; in addition, the system normally furnishes power to the two redundant and independent 4160 Volt Essential Auxiliary Power Systems of each unit through 6900/4160 Volt transformers (1ATC, 1ATD, 2ATC, 2ATD, SATA, SATB).

In the event of the loss of one of the unit auxiliary transformers, the two 6900V switchgear assemblies that are normally energized from that transformer will automatically transfer to the alternate unit auxiliary transformer, which will then furnish power to the four switchgear assemblies.

Each unit has two redundant and independent 4160 Volt Essential Auxiliary Power Systems (1ETA, 1ETB, 2ETA, and 2ETB). All ESF loads are supplied power during a blackout or accident condition from the 4160 Volt Essential Auxiliary Power System. Each of the 4160 Volt Essential Auxiliary Power Systems is provided with a diesel-engine generator (DG) connected to automatically start and supply power in the event that power from the 6900 Volt bus is not available.

The following at power (normal operating condition) configurations have been confirmed to be consistent with the current licensing basis as described in Technical Specifications Bases 3.8.1 (AC Sources, Operating) (refer to Attachment 2 and 2a for a one-line diagram for the offsite power design):

Unit 1 A Train

1. BL1A-1ATA-1TA-1ATC-1 ETA
2. BLI B-1ATB-1TA-1ATC-1 ETA
3. BL1A-1ATA-1TC-SATA-1 ETA
4. BLI B-1ATB-1TC-SATA-1 ETA
5. BL2A-2ATA-2TC-SATA-1 ETA
6. BL2B-2ATB-2TC-SATA-1 ETA Page 3 Bulletin 2012-01 Response Unit 1 B Train
1. BL B-1ATB-1TD-1ATD-1 ETB
2. BL1A-1ATA-1TD-1ATD-1 ETB
3.

BL B-1ATB-1TB-SATB-1 ETB

4. BL1A-1ATA-1TB-SATB-1 ETB
5. BL2B-2ATB-2TB-SATB-1 ETB
6. BL2A-2ATA-2TB-SATB-1 ETB Unit 2 A Train
1. BL2A-2ATA-2TA-2ATC-2ETA
2. BL2B-2ATB-2TA-2ATC-2ETA
3. BL2A-2ATA-2TC-SATA-2ETA
4. BL2B-2ATB-2TC-SATA-2ETA
5. BL1A-1ATA-1TC-SATA-2ETA
6. BL B-1ATB-1TC-SATA-2ETA Unit 2 B Train
1. BL2B-2ATB-2TD-2ATD-2ETB
2. BL2A-2ATA-2TD-2ATD-2ETB
3. BL2B-2ATB-2TB-SATB-2ETB
4. BL2A-2ATA-2TB-SATB-2 ETB
5. BL B-1ATB-1TB-SATB-2ETB
6. BL1A-1ATA-1TB-SATB-2ETB Page 4 Bulletin 2012-01 Response System Protection Items 1, 1.a, 2.b, and 2.d request information regarding electrical system protection and will be addressed in this section:
1. Given the requirements above, describe how the protection scheme for ESF buses (Class 1E for current operating plants or non-Class 1E for passive plants) is designed to detect and automatically respond to a single-phase open circuit condition or high impedance ground fault condition on a credited off-site power circuit or another power sources. Also, include the following information:

Response

Consistent with the current licensing basis and GDC 17, existing protective circuitry will separate the ESF buses from a connected failed offsite source due to a loss of voltage or a sustained, balanced degraded grid voltage concurrent with certain design basis accidents. The relay systems were not specifically designed to detect an open single phase of a three phase system.

Each of the redundant 4160 Volt Essential Auxiliary Power System buses is provided with two levels of undervoltage protection to monitor bus voltage. Each level is provided with a separate set of three undervoltage relays which are utilized in a two-out-of-three logic scheme.

The first level of undervoltage relays detects a loss of voltage condition on the 4160 Volt Essential Auxiliary Power System bus. The loss of voltage relays drop out at approximately 76%

voltage. The loss of voltage setpoint was selected such that relay operation will not be initiated during normal power transients. If two-out-of-three relays detect a loss of voltage condition, the 4160 Volt Essential Auxiliary Power System bus will be separated from offsite power.

The second level of undervoltage relays detects a degraded voltage condition on the 4160 Volt Essential Auxiliary Power System bus. The degraded voltage relays drop out at approximately 88% voltage for Unit I and approximately 89% voltage for Unit 2. This second level of protection employs two time delays. If two-out-of-three relays on a bus detect a degraded voltage condition, the two timing relays are started. One timing relay, set at less than or equal to 11 seconds, ensures that the degraded voltage condition is not a short-duration transient. If the degraded voltage persists until after this relay has completed its timing cycle, an annunciator alarm is activated in the control room. The second timing relay, set at less than or equal to 600 seconds, continues its timing cycle to allow a period in which the operators can implement actions to correct the degraded voltage condition. If the degraded voltage condition remains present until the completion of the second timing cycle, the 4160 Volt Essential Auxiliary Power System bus will be separated from offsite power. In addition, at any time after the first timing cycle and before the end of the second timing cycle, separation of the 4160 Volt Essential Auxiliary Power System bus from offsite power will occur automatically in the event of a safety injection (SI) actuation signal during degraded voltage conditions.

During normal plant operation, offsite power is aligned to the essential buses through Main Step-Up Transformers (MSUs), Unit Auxiliary Transformers (UATs), and Auxiliary Power Page 5 Bulletin 2012-01 Response Transformers. An open phase on the primary-side of the MSU Transformer would have no direct effect on essential bus voltage with the unit on-line since the main generator feeds three phase power to the secondary-side of the MSUs through the UATs and onto the essential buses (reference Attachment 2 and 2a). If the main generator trips from such a condition (possibly due to high negative sequencing currents), power may be lost to the essential bus(es) thus actuating the protective relaying at the essential bus(es).

Response of the essential buses, or the response of the main generator to such a condition, was not specifically analyzed for an open MSU phase. As stated above, even in the presence of such a condition, if voltage at the essential buses degrade to the point of detection by the protective relaying circuitry, the essential buses will be isolated from offsite power (reference Request 1.a, below, for further detail).

High impedance ground faults were not specifically analyzed on offsite power circuits.

However, similar to the response from the previous paragraph, if a high impedance fault on an offsite power circuit is such that it affects the essential buses, the protective relaying will respond by isolating the offsite power circuit. This level of coordination ensures equipment supplied by the essential buses are not impaired or operated outside of their designed ratings.

l.a. The sensitivity of protective devices to detect abnormal operating conditions and the basis for the protective device setpoint(s).

Response

Consistent with the current licensing basis and GDC 17, existing electrical protective devices are sufficiently sensitive to detect design basis conditions like a loss of voltage or a degraded voltage, but were not designed to detect a single phase open circuit condition. See Attachment 3, Table 5 for undervoltage protective devices and the basis for the device setpoint(s).

Existing electrical protective devices are also sufficiently sensitive to detect a ground fault., Table 5 lists ground protection/alarms on the ESF buses and the basis for the device setpoint(s).

2.b. If the ESF buses are not powered by offsite power sources, explain how the surveillance tests are performed to verify that a single-phase open circuit condition or high impedance ground fault condition on an off-site power circuit is detected.

Response

The essential buses at McGuire are aligned to offsite power sources during normal operation, but are powered by the main generator.

Technical Specification Section 3.8 requires two OPERABLE circuits between the offsite transmission system and the onsite essential power system during Modes 1 through 4 and one OPERABLE circuit during Modes 5, 6, and during irradiated fuel movement. Surveillance procedures verify proper breaker alignment from the Switchyard to the essential buses, and Page 6 Bulletin 2012-01 Response verification of voltage from the 24 kV, the 6.9 kV, and the 4.16 kV levels. This surveillance is performed weekly. An additional surveillance is performed when verifying remaining operable power sources if an offsite power source or DIG becomes inoperable.

2.d. Do the plant operating procedures, including off-normal operating procedures, specifically call for verification of the voltages on all three phases of the ESF buses

Response

At the 4.16 kV level, metering for the secondary-side of the 6.9/4.16 kV Transformers includes a voltmeter and a voltage transducer. Though this voltage is not monitored on all three phases, provisions are included at the 4.16 kV buses with a switched voltmeter to monitor all three phases. The current plant surveillance procedures record this voltage on a weekly basis and when verifying remaining operable power sources if an offsite power source or D/G becomes inoperable. However, voltage from only a single phase is verified and recorded.

Consequences Items 1.b, 1.c, and 2.e request information regarding the electrical consequences of an event and will be addressed in this section:

1.b. The differences (if any) of the consequences of a loaded (i.e., ESF bus normally aligned to offsite power transformer) or unloaded (e.g., ESF buses normally aligned to unit auxiliary transformer) power source.

Response

Installed relays were not designed to detect single phase open circuit conditions. Existing loss of voltage and degraded voltage relays may respond depending on load and possible grounds.

In general, there will be no plant response for an unloaded power source in the event of a single-phase open circuit on a credited off-site power circuit because there is insufficient current to detect a single-phase open circuit for this configuration.

The plant response for a loaded power source cannot be calculated without specifying the amount of loading and the specific loads involved.

Page 7 Bulletin 2012-01 Response 1.c. If the design does not detect and automatically respond to a single-phase open circuit condition or high impedance ground fault condition on a credited offsite power circuit or another power sources, describe the consequences of such an event and the plant response.

Response

A high impedance ground will have no immediate effect on plant operation. If the ground is sufficiently large to affect plant operation, protective relaying will isolate the ground automatically.

McGuire Nuclear Station did not describe in the Current Licensing Basis (CLB) that the Class 1 E protection scheme (for the emergency safeguard feature (ESF) buses) was designed to detect and automatically respond to a single-phase open circuit condition and therefore was not described in the UFSAR or Technical Specifications.

Since McGuire Nuclear Station did not credit the ESF bus protection scheme as being capable of detecting and automatically responding to a single phase open circuit condition, an open phase fault was not included in the design criteria for either the loss of voltage, the degraded voltage relay (DVR) scheme or secondary level undervoltage protection system (SLUPS) design criteria. Since open phase detection was not credited in the McGuire Nuclear Station design or licensing basis, no design basis calculations or design documents exist that previously considered this condition. Detailed plant specific models would need to be developed (e.g.,

transformer magnetic circuit models, electric distribution models, motor models; including positive, negative, and zero sequence impedances (voltage and currents), and the models would need to be compiled and analyzed for the McGuire Nuclear Station specific Class 1 E electric distribution system (EDS)).

2.e. If a common or single offsite circuit is used to supply redundant ESF buses, explain why a failure, such as a single-phase open circuit or high impedance ground fault condition, would not adversely affect redundant ESF buses.

Response

Not applicable since McGuire Nuclear Station does not use a common or single offsite circuit to supply redundant ESF busses.

Page 8

Enclosure Bulletin 2012-01 Unit 1 Simplified One-Line Diagram TO 230 KV To 230 KV Switchyard Switchyard I

T02=TB

  • To 2ETA 4----.---

Unit 2 A e a Equipment Area 1 ETA,-

1 ETB UnitI 1

4.16 KV Essential Auxiliary Power G

I System 13 Other 13 Other IA Circuits Circuits 1B

.... _ l Page 9

Enclosure Bulletin 2012-01 a Unit 2 Simplified One-Line Diagram To 525 KV To 525 KV Switchyard Switchyard 2B 2A 6.9KV2244KKVV Swgr.

'-Swgr.

S To iTB

  • [
  • TolTC 2AT2ATC SATA SATB

'Equipment Area I

13 Other 13 Other 2B Circuits Circuits 2A Page 10

Enclosure Bulletin 2012-01 - Tables McGuire Nuclear Station Table I - ESF Buses Continuously Powered From Offsite Power Source(s)

Description 6oESF Bus Po*wer ESF Bus Name (normal operatnig Original licensing Soure condition),

basis conf&iguration Busline 1A 1 ETA Y

Busline 1 B 1 ETB Y

Busline 2A 2ETA Y

Busline 2B 2ETB Y

Table 2 - ESF Buses Not Continuously Powered From Offsite Power Source(s)

Table 3 - ESF Buses Normally Energized Major Loads ESF Bus:

Load Voltage.LeVel (V)

' Rating (HP):

1 ETA Nuclear Service Water 4160 1000 Pump 1A 1 ETA Component Cooling 4160 200 Water Pump 1A1 1 ETA Component Cooling 4160 200 Water Pump 1A2 1 ETA Centrifugal Charging 4160 600 Pump 1A 1ETA Fuel Pool Cooling Pump 4160 200 1A 1 ETA Control Area Ventilation 4160 495 System A*

1ETB Nuclear Service Water 4160 1000 Pump lB 1ETB Component Cooling 4160 200 Water Pump 1B1 1ETB Component Cooling 4160 200 Water Pump 1B2 1ETB Centrifugal Charging 4160 600 Pump 1B 1ETB Fuel Pool Cooling Pump 4160 200 1B 1 ETB Control Area Ventilation 4160 495 System B*

  • Shared Between Units Page 11

Enclosure Bulletin 2012-01 Table 3 (continued)

ESFIBus '

t La Voltage Level (V)~;

Rating,(HP.)

2ETA Nuclear Service Water 4160 1000 Pump 2A 2ETA Component Cooling 4160 200 Water Pump 2A1 2ETA Component Cooling 4160 200 Water Pump 2A2 2ETA Centrifugal Charging 4160 600 Pump2A 2ETA Fuel Pool Cooling Pump 4160 200 2A 2ETA Control Area Ventilation 4160 495 System A*

2ETB Nuclear Service Water 4160 1000 Pump2B 2ETB Component Cooling 4160 200 Water Pump 2B1 2ETB Component Cooling 4160 200 Water Pump 2B2 2ETB Centrifugal Charging 4160 600 Pump2B 2ETB Fuel Pool Cooling Pump 4160 200 2B 2ETB Control Area Ventilation 4160 495 System B*

  • Shared Between Units Page 12

Enclosure Bulletin 2012-01 Table 4 - Offsite Power Transformers Transformer Winding MVA Size Voltage Rating Grounding Configuration (AOIFA/FOA)

(PrimarylSecondary)

Configuration MSU IA and lB Wye-Delta 760 MVA FOA 230/22.8 kV Neutral Solidly Grounded MSU 2A and 2B Wye-Delta 750 MVA FOA 525/22.8 kV Neutral Solidly Grounded IATA, IATB, Delta-Wye/Wye 60/80/100 MVA 22.8/6.9 kV Neutral 2A TA, 2A TB (Three Winding)

Resistively Grounded 1ATC, 1ATD, Delta-Wye 5.5 MVA (Air Cooled) 6.9/4.16 kV Neutral Solidly 2ATC, 2ATD, Grounded SA TA, SA TB Page 13

Enclosure Bulletin 2012-01 Table 5 - Protective Devices Protection~ Protective UV

etpon, Basis for Setpoint ~

j*Zone>

Device Logic: :(Nominal 4 KV ESF Loss of Voltage 2 of 3 Unit 1: 3174.5 To actuate upon complete loss of ESF Bus Relay (76.33%)

Bus voltage condition Unit 2: 3157 (75.9%)

4 KV ESF Degraded Grid 2 of 3 Unit 1: 3689 To actuate upon sustained degraded ESF Bus (88.77%)

Bus voltage condition Unit 2: 3713.5 (89.3%)

4 kV ESF Motor and NA 5 Amps Zero To provide sensitive monitoring for ground Bus Transformer Sequence faults on ESF loads. This protection trips Ground Current, 0.1 the load's breaker during all modes of Protection (50G) second delay operation including emergency scenarios.

4 kV ESF Normal and NA 120 Amps Zero To provide backup protection to sustained Bus Standby Feeder Sequence ground faults on the ESF bus or loads.

Breaker Ground Current, 0.4 The pickup and time delay settings were Protection (51G) second delay @

chosen to provide sufficient coordination 500% of tap with 50G relays. This protective device trips the ESF bus incoming breaker during all modes of operation including emergency scenarios.

4 kV Neutral Ground NA 5 Volts on The emergency diesel generator is Emergency Overvoltage secondary of resistively grounded through a neutral Diesel (59DGN) neutral grounding transformer. This protective Generator grounding device provides backup protection to transformer, 2 sustained ground faults on the ESF bus, second delay loads, or generator while it is supplying power to the ESF bus. The pickup and time delay settings were chosen to provide sufficient coordination with 5OG relays. This protective device trips the diesel breaker during non-emergency scenarios. The relay provides an alarm-only function during emergency scenarios and does not provide a trip signal to the diesel breaker.

Page 14

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