Text

Revised Response to NRC Bulletin 2012-01, Design Vulnerability in Electrical Power System
	ML19360A259
Person / Time
Site:	Browns Ferry, Watts Bar, Sequoyah
Issue date:	12/26/2019
From:	Polickoski J; Tennessee Valley Authority
To:	; Document Control Desk, Office of Nuclear Reactor Regulation
References
	BL-12-001, CNL-19-121
	Download: ML19360A259 (40)
	v • d • e

Tennessee Valley Authority, 1101 Market Street, Chattanooga, Tennessee 37402 CNL-19-121 December 26, 2019 10 CFR 50.4 10 CFR 50.54(f)

ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Browns Ferry Nuclear Plant, Units 1, 2, and 3 Renewed Facility Operating License Nos. DPR-33, DPR-52, and DPR-68 NRC Docket Nos. 50-259, 50-260, and 50-296 Sequoyah Nuclear Plant, Units 1 and 2 Renewed Facility Operating License Nos. DPR-77 and DPR-79 NRC Docket Nos. 50-327 and 50-328 Watts Bar Nuclear Plant, Units 1 and 2 Facility Operating License Nos. NPF-90 and NPF-96 NRC Docket Nos. 50-390 and 50-391

Subject:

Revised Response to NRC Bulletin 2012-01, Design Vulnerability in Electrical Power System

References:

1. NRC Bulletin 2012-01, Design Vulnerability in Electric Power System, dated July 27, 2012 (ML12074A115)

2. TVA Letter to NRC, 90-Day Response to NRC Bulletin 2012-01, Design Vulnerability in Electric Power System, dated October 25, 2012 (ML12312A167)

3. NRC Letter to TVA, Browns Ferry Nuclear Plant, Units 1, 2, and 3; Sequoyah Nuclear Plant, Units 1 and 2; Watts Bar Nuclear Plant, Units 1 and 2 -

Issuance of Amendment Nos. 309, 332, 292, 345, 339, 128, and 31 Regarding Unbalanced Voltage Protection (EPID L-2017-LLA-0391), dated August 27, 2019 (ML18277A110)

4. NRC Letter to TVA, Watts Bar Nuclear Plant, Unit 2 - Issuance of Amendment No. 23 Regarding the Actions to Resolve Issues Identified in NRC Bulletin 2012-01, Design Vulnerability in Electric Power System (EPID L-2018-LLA-0239), dated December 21, 2018 (ML18334A333)

U.S. Nuclear Regulatory Commission CNL-19-121 Page 2 December 26, 2019 In Reference 1, the Nuclear Regulatory Commission (NRC) issued Bulletin 2012-01, Design Vulnerability in Electric Power System, which requested information regarding how the design of the protection scheme for Engineered Safety Feature (ESF) buses detects and automatically responds to single-phase open circuit conditions or high impedance ground fault conditions, including a description of the operating configuration of the ESF buses.

In Reference 2, Tennessee Valley Authority (TVA) provided the information requested in Reference 1 for the Browns Ferry (BFN), Sequoyah (SQN), and Watts Bar Nuclear (WBN)

Plants.

In Reference 3, the NRC approved a license amendment request (LAR) for TVA to add a new level of protection, Unbalanced Voltage to the BFN, SQN, and WBN Technical Specifications for the loss of power instrumentation. TVAs undervoltage protection scheme, located on the medium voltage Class 1E buses, includes loss of voltage protection, degraded voltage protection, and unbalanced voltage protection. The undervoltage protection scheme was designed to detect and provide automatic protection for the connected Class 1E system from offsite power that is not capable of performing its safety function due to undervoltage conditions.

This undervoltage protection scheme minimizes the probability of losing electric power from any of the remaining supplies as a result of, or coincident with, the loss of power generated by the nuclear power unit or the loss of power from the transmission network.

At the time of initial Bulletin response (Reference 2), unbalanced voltage protection was not approved by NRC for the TVA plants. With this level of undervoltage protection approved and enabled at BFN, SQN, and WBN, the station-specific system protection responses and consequences that were included in the original responses to Bulletin items 1, 1a, 1b, and 1c were no longer valid and have been revised. The responses to Bulletin items 2b, 2d, and 2e have been updated, where necessary, for clarity.

The Reference 2 responses to Bulletin items 2, 1d, and 2a included system descriptions specific to the electric power stations and require no modification or updates for BFN and WBN. As stated in the original response, a SQN LAR to restore the original unit station service transformer (USST) operating configuration was under review by the NRC at the time of the response. That LAR was approved on October 31, 2012 (ML12286A078), and responses to items 2, 1d, and 2a for SQN have been updated. The response to item 2c includes confirmation of consistency with the NRC-approved licensing basis and requires no modification or update for BFN and WBN. The updated response to item 2c for SQN reflects approval of the USST LAR.

Lastly, in Reference 4, the NRC approved an amendment for WBN Unit 2, which revised Operating License Condition 2.C(5) to state, By December 31, 2019, the licensee shall report to the NRC that the actions to resolve the issues identified in Bulletin 2012-01, Design Vulnerability in Electrical Power System, have been implemented. Therefore this submittal reports that the actions to resolve the issues identified in Bulletin 2012-01 have been implemented for WBN Unit 2.

U.S. Nuclear Regulatory Commission CNL-19-121 Page 3 December 26, 2019 TVA has determined that there is no design vulnerability associated with open phase events at TVA stations. All changes described above to the original Reference 2 response are noted by revision bars in the right margin for Enclosures 1, 2, and 3.

There are no new regulatory commitments associated with this submittal. Please address any questions regarding this request to Kimberly Hulvey at (423) 751-3275.

I declare under penalty of perjury that the foregoing is true and correct. Executed on this 26th day of December 2019.

Respectfully, James T. Polickoski Director, Nuclear Regulatory Affairs

Enclosures:

1. Browns Ferry Nuclear Plant, Units 1, 2, and 3 Revised Response to NRC Bulletin 2012-01, Design Vulnerability in Electric Power System

2. Sequoyah Nuclear Plant, Units 1 and 2 Revised Response to NRC Bulletin 2012-01, Design Vulnerability in Electric Power System

3. Watts Bar Nuclear Plant, Units 1 and 2 Revised Response to NRC Bulletin 2012-01, Design Vulnerability in Electric Power System cc (Enclosures):

NRC Regional Administrator - Region II NRC Senior Resident Inspector - Browns Ferry Nuclear Plant NRC Senior Resident Inspector - Sequoyah Nuclear Plant NRC Senior Resident Inspector - Watts Bar Nuclear Plant NRC Project Manager - Browns Ferry Nuclear Plant NRC Project Manager - Sequoyah Nuclear Plant NRC Project Manager - Watts Bar Nuclear Plant State Health Officer, Alabama Department of Public Health Director, Division of Radiological Health - Tennessee State Department of Environment and Conservation

ENCLOSURE 1 Browns Ferry Nuclear Plant, Units 1, 2, and 3 Revised Response to NRC Bulletin 2012-01, "Design Vulnerability in Electric Power System"

ENCLOSURE 1 Browns Ferry Nuclear Plant, Units 1, 2, and 3 90-Day Response to NRC Bulletin 2012-01, Design Vulnerability in Electric Power System Overview:

Statement of No Open Phase Design Vulnerability for Class 1E Equipment System Description - Items 2., 1.d, 2.a, 2.c System Protection - Items 1., 1.a, 2.b, 2.d Consequences - Items 1.b, 1.c, 2.e References Attachment 1 - Simplified One-Line Diagram Attachment 2 - 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 Normally Energized Major Loads o Table 4 - Offsite Power Transformers o Table 5 - Protective Devices E1-1

Statement of No Open Phase Design Vulnerability for Class 1E Equipment The U. S. Nuclear Regulatory Commission (NRC) issued Bulletin 2012-01, Design Vulnerability in Electrical Power System, which identified a vulnerability to an open phase condition (OPC) in electrical power systems at nuclear power plants. The bulletin required that licensees comprehensively verify their compliance with General Design Criterion (GDC) 17, Electric Power Systems, and to respond to the bulletin. GDC 17 corresponds in general to Atomic Energy Commission Criterion 39 to which the Browns Ferry Nuclear Plant (BFN) units are licensed. The bulletin also requested information regarding how the design of the protection scheme for engineered safety feature (ESF) buses detects and automatically responds to single-phase open circuit conditions or high impedance ground fault conditions, including a description of the operating configuration of the ESF buses.

As stated in the bulletin, At Byron, a failure to design the electric power systems protection scheme to sense the loss of a single phase between the transmission network and the onsite power distribution system resulted in unbalanced voltage at both ESF buses (degraded offsite power system), trip of several safety-related pieces of equipment such as Essential Service Water pumps, Centrifugal Charging Pumps, and Component Cooling Water Pumps and the unavailability of the onsite electric power system. This situation resulted in neither the onsite nor the offsite electric power system being able to perform its intended safety functions (i.e., to provide electric power to the ESF buses with sufficient capacity and capability to permit functioning of structures, systems, and components important to safety).

TVA has designed and implemented an unbalanced voltage protection scheme to protect structures, systems, and components (SSCs) important to safety, as discussed below. As stated in the NRCs safety evaluation (SE) (Reference 1) for TVAs unbalanced voltage relay (UVR) license amendment request (LAR), An open circuit in one phase of a three-phase power system results in an unbalanced voltage condition on plant buses. The unbalanced voltage condition results in unbalanced current flow that produces negative and zero sequence current.

Therefore, because TVA employs a UVR protection scheme, TVAs SSCs important to safety are protected from an OPC event. Therefore, BFN has no vulnerability to an OPC.

TVAs unbalanced voltage protection scheme is designed to detect and provide automatic protection for the connected Class 1E system from offsite power that is not capable of performing its safety function due to unbalanced voltage conditions. This unbalanced protection scheme is part of the undervoltage protection scheme that minimizes the probability of losing electric power from any of the remaining supplies as a result of, or coincident with, the loss of power generated by the nuclear power unit or the loss of power from the transmission network.

TVAs undervoltage protection scheme, located on the medium voltage Class 1E buses, includes loss of voltage protection, degraded voltage protection, and unbalanced voltage protection.

Because aspects of the implementation of the unbalanced voltage protection scheme design described above required a change to the Technical Specifications, TVA submitted an associated LAR. In the NRCs SE (Reference 1) for that LAR, the NRC staff determined that TVAs UVR protection schemes will provide reasonable assurance that the safety-related equipment will be adequately protected from the consequences of negative sequence currents resulting from unbalanced voltage conditions. The UVR protection will be part of the primary success path that functions or actuates to mitigate a design-basis accident or transient that either assumes the failure of or presents a challenge to the integrity of a fission product barrier E1-2

and, therefore, will meet Criterion 3 of 10 CFR 50.36(c)(2)(ii) for inclusion into the TSs. The NRC staff concludes that the implementation of the UVR protection schemes will provide reasonable assurance that GDC 17 (AEC Criterion 39 for the Browns Ferry units) required onsite electric power system will be available to permit functioning of SSCs important to safety following a detectable degradation related to unbalanced voltages in the offsite power system.

Therefore, as stated above, implementation of the undervoltage protection scheme will provide reasonable assurance that GDC 17 (AEC Criterion 39 for the Browns Ferry units) required onsite electric power system will be available to permit functioning of SSCs important to safety following a detectable degradation of voltages in the offsite power system, including those caused by an OPC.

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).

See Attachment 1 for a simplified one-line diagram.

The Engineered Safety Feature (ESF) buses are normally powered from the main generator through the unit station service transformers (USSTs). During normal operating conditions the USSTs are connected to the respective unit generator isolated-phase bus and receive power from Generators 1, 2, and 3. The ESF buses are supplied through the B USSTs and plant auxiliaries buses are supplied from the A USSTs. The USSTs 1B, 2B, and 3B supply normal power to the ESF buses via 4.16 kilovolt (kV) Unit Boards 1A, 1B, 2A, 2B, 3A, and 3B. For Units 1 and 2, USST 1B and USST 2B each supply a 4.16 kV Unit Board with each 4.16 kV Unit Board feeding a safety-related 4.16 kV Shutdown Bus 1 or 2, with each 4.16 kV Shutdown Bus then feeding two of the Unit 1 and 2 safety-related Division I (A and B) or Division II (C and D) 4.16 kV Shutdown Boards. For Unit 3, USST 3B supplies two 4.16 kV Unit Boards with each 4.16 kV Unit Board feeding two of the Unit 3 safety-related 4.16 kV Shutdown Boards (Division I 3EA & 3EB and Division II 3EC & 3ED).

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

The 4.16 kV secondary winding of each unit and common station service transformer (CSST) is wye-connected, with the neutral grounded through a resistor which will limit ground fault current to 1600 amperes maximum. The neutral resistor serves to prevent overvoltage on the winding which could occur in the event of a ground fault if the 4.16 kV system were not intentionally grounded.

See Attachment 2, 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.

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No, for the normal (at power) operating configuration, the ESF buses are powered from the main generator through the unit station service transformers.

See Attachment 2, Tables 1 and 2 for ESF bus power sources, and Table 3 for ESF bus major loads energized during normal power operations.

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.

The operating configuration, as described under item 2, has been confirmed to be consistent with the current licensing basis as discussed in the BFN Updated Final Safety Analysis Report (UFSAR), License Amendments 75 and 72, respectively, issued on September 3, 1981, and Unit 3 License Amendment 52, issued on March 29, 1982, and their associated safety evaluations. The design changes associated with these amendments included adding one additional USST per unit, adding on-load tap changer on the primary windings of USSTs 1B, 2B, and 3B, and the addition of generator low-side breakers to all three units. The installation of the generator circuit breaker changed the normal and alternate supply to the Class 1E distribution system. Operation of the generator circuit breaker allows backfeeding from the 500 kV switchyard as the immediate access source of offsite power to the onsite distribution system.

The breaker is designed to open automatically on a unit trip or maximum fault current.

See Attachment 2, Tables 1 and 2, for any changes in the offsite power source alignment to the ESF buses from the original plant licensing.

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.

Consistent with the current licensing basis documents (including those specified in item 2.c),

existing protective circuitry will separate the ESF buses from a connected failed offsite source as a result of a loss of voltage, a sustained balanced degraded voltage, or a sustained unbalanced voltage. While the relay systems were not specifically designed to detect and automatically respond only to an open phase on a three phase system, the undervoltage protection scheme will automatically detect and provide automatic protection from the unbalanced voltage and/or degraded voltage created by the open phase or high impedance ground fault that prevents the connected safety equipment from operating due to insufficient voltage (Reference 1).

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Normal Plant Operation Considerations:

During normal plant operation, the ESF buses sequence of events were described and evaluated as acceptable as part of the NRCs SE for TVAs UVRs (Reference 1, Section 3.2.2) for an open phase.

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

Consistent with the current licensing basis documents specified in item 2.c, existing electrical protective devices are sufficiently sensitive to detect design basis conditions like a loss of voltage or a sustained balanced degraded voltage, or a sustained unbalanced voltage. See , Table 5, for undervoltage protective devices and the basis for the device setpoint(s). Degraded voltage relay logic is 2 out of 3 and degraded voltage relays monitor A-B, B-C and C-A. The UVR logic is a permissive 1 out of 2 and each of the relays monitor A-B-C phases.

Existing electrical protective devices are also sufficiently sensitive to detect a ground fault. , Table 5 lists ground protection 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.

The current BFN, Units 1, 2, and 3, Technical Specifications do not contain surveillance requirements for offsite power circuits. In addition, the ESF buses at BFN are powered by unit generators. Phase voltage is monitored for generator output.

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?

No, the current plant operating procedures, including operating procedures for off-normal alignments, do not specifically call for verification of the voltages on all three phases of the ESF buses.

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.

There are no differences of consequences to any safety-related (Class 1E) equipment for an open phase occurring on a normally loaded power source (i.e., Class 1E bus normally aligned to E1-5

offsite power transformer) or an unloaded power source (i.e., standby offsite power source). No adverse consequences or loss of function occur in either case based on the Unbalanced Voltage setpoint methodology. (Reference 1, Section 3.4.4).

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.

The UVR scheme provides detection and automatic protection for the Class 1E equipment for any adverse single-phase open circuit condition or high impedance ground fault condition on the offsite power circuit.

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.

Not applicable since BFN does not use an offsite circuit to supply redundant ESF buses during normal operation.

REFERENCES

1. NRC Letter to TVA, Browns Ferry Nuclear Plant, Units 1, 2, and 3; Sequoyah Nuclear Plant, Units 1 and 2; Watts Bar Nuclear Plant, Units 1 and 2 - Issuance of Amendment Nos. 309, 332, 292, 345, 339, 128, and 31 Regarding Unbalanced Voltage Protection (EPID L-2017-LLA-0391), dated August 27, 2019 (ML18277A110)

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Attachment 1 Simplified One-Line Diagram (See next page)

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NC 500 KV BUS 2 - SECTION 1 500 KV BUS 2 - SECTION 2 5207 5217 5227 5237 5247 5257 5267 5277 5287 5297 ATHENS TRINITY NC NC NC NO NC NC NC NC TRINITY 2 NC NC 161 KV 161 KV WEST POINT MADISON 1 TRINITY 1 5240 MAURY UNION LIMESTONE TRICO 5208 NC 5218 NC 5228 NC 5248 NC 5258 5268 NC 5278 NC 5288 NC 5298 NC 5209 5219 5229 5249 5259 5269 5279 5289 5299 NC NC NC NC NC NC NC NC NC 91G2 91G1 93G1 93G2 5201G 5221G 5231G 5251G 5271G 5281G 5291G CAP BANK NO.2 892 NO NO NO NO NO NO NO 5205 5215 5235 5245 5255 5265 5275 5285 46 MVAR 8989 NC NC 5225 NC 6271R NC NC NC NC NC NC NC NO NC SHUNT NO NO 5204 NC 5214 NC 5224 NC 5234 NC 5244 NC 5254 5264 NC 5274 NC 5284 NC NC NO/AUTO 919 935 REACTOR NC NC 5203 5213 5223 5233 5243 5253 5263 33 MVAR 5273 5283 5293 NC NC NO 89G NC NC NC NC NC NC NC NC NC NO NO 161KV NC NC NC NC 161KV 500 KV BUS 1 - SECTION 1 500 KV BUS 1 - SECTION 2 BUS 2 BUS 1 MAIN XFMR 5250 MAIN XFMR MAIN XFMR 928 924 UNIT 1 UNIT 2 UNIT 3 90G NO 927 929 925 923 NC USST 1B USST 2B USST 3B 902 USST 1A USST 2A USST 3A NO 9089 1017 NO NO X Y X Y X Y X Y X Y X Y 57-2 57-2 57-2 NC NC NC NC 89 89 89 CAP BANK NO.1 NC NO/AUTO NC 52 NC 52 NC 52 38 MVAR 1027 NO NO NO 57-1 214 57-1 224 57-1 234 CSST A CSST B X Y X Y 4KV 4KV 4KV RECIRC RECIRC RECIRC BD 1 BD 2 BD 3 GEN GEN GEN 1 2 3 4KV 1412 1414 1516 1518 4KV 1416 1418 1512 1514 START NC NO NC NO START NC NO NC NO BD 1 BD 2 4KV START BUS 1A 4KV START BUS 1A 4KV START BUS 1A 4KV START BUS 1B 4KV START BUS 1B 4KV START BUS 1B 4KV START BUS 2A 4KV START BUS 2B USST 1A USST 2A USST 3A 4KV 4KV 4KV RECIRC RECIRC RECIRC BD 1 BD 2 BD 3 1112 101 1424 1114 102 1524 1116 103 1532 1118 1422 1212 201 1428 1214 202 1526 1216 203 1426 1218 1522 1312 301 1432 1314 302 1528 1316 303 1434 TC1A TC1B 1122 1436 1124 1534 1222 1438 1224 1536 1322 1442 1324 1538 NC 4KV NO NC NO NC NO NC 10 NO NC NO NC NO NC NO NC 20 NO NC 4KV NO NC NO NC NO 16 NC NO NC NO NC NO NC NO NC NO NC NO 4KV 4KV 4KV 4KV 4KV 4KV 4KV NC NC UB 1A UB 1B UB 1C 4KV COM BD A UB 2A UB 2B UB 2C 4KV COM BD B UB 3A UB 3B UB 3C 480V COM BD 1 1126 BUS A NO BUS B 1132 1232 1326 1332 NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC TU2 RECIRC RECIRC RECIRC RECIRC RECIRC TU1A TU1B TG1A TG2A TG3A TEA TU2A TEB TC3B TC2B TC1B TU3A TU3B RECIRC B

VFD 1A VFD 1B VFD 2A VFD 2B VFD 3A VFD 3B 480V CONT BAY VENT BD A ALT TC2A TC2B 17 NC NC 480V COM BD 2 BUS A NO BUS B NC RPT BD NC NC RPT BD NC NC RPT BD NC 1-I 2-I 3-I 1440 1550 1442 1542 1444 1544 RPT BD RPT BD RPT BD NC NC NC NC NC NC 1-II 2-II 3-II 1450 1540 1452 1552 1454 1554 NC NO NC NO NC NO NC NO NC NO NC NO TC3A TC3B 14 NC NC 480V UB 1A 480V UB 1B 480V UB 2A 480V UB 2B 480V UB 3A 480V UB 3B 480V COM BD 3 121 NC 132 221 NC 232 321 NC 332 BUS A NO BUS B RECIRC RECIRC RECIRC RECIRC RECIRC RECIRC PUMP 1A PUMP 1B PUMP 2A PUMP 2B PUMP 3A PUMP 3B NC COND COND COND 480V CONT BAY VENT BD B DEMIN DEMIN DEMIN ALT BD 1 BD 2 BD 3 1612 1622 NC (ON 4KV SD BD A) NO (ON 4KV SD BD D) 4KV SD BUS 1 4KV SD BUS 1 1712 1722 (ON 4KV SD BD B) NO (ON 4KV SD BD C) NC 4KV SD BUS 2 4KV SD BUS 2 340 350 360 370 140 150 260 270 4KV SD 4KV SD 4KV SD 4KV SD 4KV SD 4KV SD 4KV SD 4KV SD BD 3EA BD 3EB BD 3EC BD 3ED DG BD A DG BD B DG BD C DG BD D DG DG DG DG A B C D 3A 3B 3C 3D 133 1614 1716 1818 1824 1616 1714 1822 1828 1718 1624 1812 1814 1724 1618 1816 1826 1334 1726 1838 1844 1728 1842 1848 1338 1626 1832 1834 1342 1628 1836 1846 1632 1732 1742 1642 6

NC NO NO NO NC NO NO NO NC NO NO NO NC NO NO NO NC NO NO NO NC NO NO NO NC NO NO NO NC NO NO NO NO NO NO NO 4KV SD BD A 4KV SD BD B 4KV SD BD C 4KV SD BD D 4KV SD BD 3EA 4KV SD BD 3EB 4KV SD BD 3EC 4KV SD 4KV BUS TIE BD 1934 NC 140 NC 150 NC NC 260 NC NC 270 NC NC 340 NC 350 NC 360 NC BD 3ED NC NC 379 NO 370 RHRSW A1, A2 RHRSW C1, C2 RHRSW B2, B3 RHRSW D2, D3 RHRSW A3 RHRSW C3 RHRSW B1 RHRSW D1 CABLES LIFTED TDA TS1A TS1E TDE TS1B TS2A TS2E TS2B TDB TS3A TS3E TS3B THB TSG1A NC 180 NO NO 190 NC NC 280 NO 290 NC 380 NC NO NO 390 NC 480V SD BD 1A 480V SD BD 1B 480V SD BD 2A 480V SD BD 2B 480V SD BD 3A 480V SD BD 3B ABANDONED NC NC NO NO NC NO NC NC NC NC NC NC NO NC NO NO NC NO NC NC NO NC NC NC NC NC NC NO NC NO NO NO NO NC NC NO NC NC NC NC NO NC 181 NO NC 281 NO NO 381 NO 480V RMOV BD 1A 480V RMOV BD 2A 480V RMOV BD 3A NC NC NC NC NC 371 4KV CT SWGR F 4KV CT SWGR E I&C RPS 1A UP INV CREV A I&C CON UP MMG I&C RPS 2A I&C CON UP MMG I&C RPS 3A I&C COND 480V CABLES BUS 1A MG BUS 1B DEMIN AC FEED BUS 2A MG BUS 2B DEMIN AC FEED BUS 3A MG BUS 3B DEMIN NO HVAC NC 372 NC LIFTED NO 191 NC BD1 NO 291 NC BD2 NO 391 NC BD 3 BD B 480V SGT BD ALT ALT ALT 480V RMOV BD 1B 480V RMOV BD 2B 480V RMOV BD 3B ON 1920 1930 COOLING TOWER 4KV CT NC NC NO SWGR A NO XFMR 2 NC NC NC NC NC NC NC NC Y SPARE TRINITY RPS 1B RPS RPS 2B RPS UP RPS 3B RPS UP CREV SBGT C MG ALT MG ALT ALT MG ALT ALT B X NO 192 NC NO 292 NC NO 392 NC 480V RMOV BD 1C 480V RMOV BD 2C 480V RMOV BD 3C COOLING TOWER XFMR 1 NO NO NC NO Y

480V ATHENS COM BD 1 BUS B 183 NC 480V CONT BAY NO NC NO NO NC NO NO X VENT BD A 480V RMOV BD 2D 480V RMOV BD 3D 282 382 1922 1916 1918 1924 1912 1926 1928 1914 NC NO NC NO NC NO NC NO NO 293 NO NC NO 393 NO NC 4KV CT SWGR A 4KV CT SWGR B 4KV CT SWGR C 4KV CT SWGR D D7 B13 B14 C7 480V RMOV BD 2E 480V RMOV BD 3E PUMPS 1A, 1B, 2A NC PUMPS 2B, 3A, 3B NC PUMPS 5A, 6A, 6B NC PUMPS 4A, 4B, 5B NC XS-S-1 XS-N-1 NC NC POLE 1B POLE 1A 384 1 2 3 MAINTENANCE BLDG 142 271 SOUTH NORTH NC 480V DSL NO NO 480V DSL NC REFERENCE DRAWINGS NC 480V DSL NO OUTSIDE NO XS-N-0 OUTSIDE AUX BD A AUX BD B Unit Drawing Sheet Description AUX BD 3EA LOOP NC NC NC POLE 1 LOOP TPS DSL-6055 1, 2 Dispatching Single Line, Browns Ferry XS-N-2 NC NC 0, 3 15E500 Series Normal/Standby Auxiliary Power POLE 6 NC 0 35E713 1, 3, 4, 5 4kV Cooling Tower Switchgear A, B, C, D 0 35E713 2 4kV Outside and Construction Loops 395 NODE 2 BLDG MPC 0 45E506 - 161kV Main Single Line NO 480V DSL NC ADHR HWC SBGT A SBGT B 0 45E715 - 4kV Common Board A, B; 4kV Start Board 1 AUX BD 3EB SOUTH NORTH SOUTH NORTH 1, 2, 3 45E718 - 4kV Reactor Recirc Board A, B; 4kV Start Board 2 ADMIN BUILDINGS CONSTRUCTION LINE XS-S-3 NC XS-N-4 NO NC 1, 2, 3 45E719 Series 4kV Reactor Recirc RPT Board 1A to 3B 480V POLE 13 POLE 11 1, 2, 3 45E721 - 4kV Unit Board 1A to 3C COM BD 3 TELCOM (HYPOCHLORITE) BLDG ECOLOCHEM 0 45E724 1, 2, 3, 4 4kV Shutdown Board A to D BUS A 394 480V CONT BAY LEGEND 3 45E724 5 4kV Bus Tie Board NO VENT BD B NC NO BFTVC 3 45E724 6, 7, 8, 9 4kV Shutdown Board 3EA to 3ED XS-N-3 ALTERNATE FEED 0 45E729 1, 2, 3 480V Common Board 1, 2, 3 POLE 16 NORMAL FEED 0 45E732 1, 2, 3, 4 480V Diesel Generator Auxiliary Board A, B 3 45E732 5, 6 480V Diesel Generator Auxiliary Board 3EA, 3EB 500 KV SYSTEM 0 45E733 1 480V Standby Gas Treatment Board 161 KV SYSTEM 0 45E736 1, 2 480V Control Bay Vent Board A, B 3 45E736 6 480V HVAC Board B 22 KV MAIN GENERATOR TRANSFORMER LOAD TAP CHANGER POWER 1, 2, 3 45E747 1, 2 480V Unit Board 1A to 3B 4KV NON-DIVISIONAL 1, 2, 3 45E749 Series 480V Shutdown Board 1A to 3B Normal Auto Transfer To 4KV START BUS 1A 1, 2, 3 45E751 Series 480V RMOV Board 1A to 3E USST 1B 480V Unit Bd 1B 480V Common Bd 2, Bus B AC ELECTRICAL DISTRIBUTION SYSTEM 4KV START BUS 1B OR 4KV & 480V DIV I 0 45E1504 - Main Generator 1 and 500kV Main Single Line USST 2B 480V Unit Bd 2B 480V Common Bd 2, Bus A BROWNS FERRY NUCLEAR PLANT INFORMATION ONLY 0 45E1505 - Main Generator 2 and 500kV Main Single Line USST 3B 480V Unit Bd 3B 480V Common Bd 2, Bus A 4KV START BUS 2A OR 4KV & 480V DIV II 0 45E1506 - Main Generator 3 and 500kV Main Single Line CSST A 480V Common Bd 2, Bus B 480V Unit Bd 1B 4KV START BUS 2B OR 480V NON-DIVISIONAL CSST B 480V Common Bd 2, Bus B 480V Unit Bd 1B PIP-02-03 REVISION DRAFT 0 75E701 - 480V Transformer Yard Distribution Cabinet 1, 2, 3 NOTE: See reference drawings for all loads on a board.

XXX ETAP NODE NUMBERS

Attachment 2 - Tables Table 1 - ESF Buses Continuously Powered From Offsite Power Source(s)

Description of ESF Bus Power Source ESF Bus Name (normal operating condition). Original licensing basis configuration (Y/N)

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

Description of ESF Bus Power Source ESF Bus Name (normal operating condition). Original licensing basis configuration (Y/N)

Unit Station Service Transformer 1B or 2B 4.16 kV Shutdown Board A N Unit Station Service Transformer 1B or 2B 4.16 kV Shutdown Board B N Unit Station Service Transformer 1B or 2B 4.16 kV Shutdown Board C N Unit Station Service Transformer 1B or 2B 4.16 kV Shutdown Board D N Unit Station Service Transformer 3B 4.16 kV Shutdown Board 3EA N Unit Station Service Transformer 3B 4.16 kV Shutdown Board 3EB N Unit Station Service Transformer 3B 4.16 kV Shutdown Board 3EC N Unit Station Service Transformer 3B 4.16 kV Shutdown Board 3ED N E1-9

Table 3 - ESF Buses Normally Energized Major Loads Class 1E Bus Load Voltage Level Rating (Normally (kV) (HP)

Running) 4.16 kV Shutdown Board RHRSW (EECW) 4.16 400 A A1

4.16 kV Shutdown Board RHRSW (EECW) 4.16 400 B C1

4.16 kV Shutdown Board RHRSW (EECW) 4.16 400 C B3

4.16 kV Shutdown Board RHRSW (EECW) 4.16 400 D D3

4.16 kV Shutdown Board RHRSW (EECW) 4.16 400 3EA A3

4.16 kV Shutdown Board RHRSW (EECW) 4.16 400 3EB C3

4.16 kV Shutdown Board RHRSW (EECW) 4.16 400 3EC B1

4.16 kV Shutdown Board RHRSW (EECW) 4.16 400 3ED D1 *

Normally, only 2 of the 8 RHRSW (EECW) Pump Motors will be energized at anytime during Normal Operation.

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Table 4 - Offsite Power Transformers Transformer Winding MVA Size Voltage Rating Grounding Configuration Configuration (AO/FA/FA) (Primary/Secondary)

Main Transformer 1 Wye-Delta (-) 1500MVA (3-1Ø), OF AF 500 - 22 kV High Side Grounded Wye Main Transformer 2 Wye-Delta (-) 1500MVA (3-1Ø), OF AF 500 - 22 kV High Side Grounded Wye Main Transformer 3 Wye-Delta (-) 1500MVA (3-1Ø), OF AF 500 - 22 kV High Side Grounded Wye Unit Station Service Delta-Wye (-) 24/32 MVA, 12/16 MVA (X 20.7- 4.16 - 4.16kV Low Side Grounded Wye Transformer 1B Winding) 12/16 MVA (Y Resistance Grounded Neutral Winding), OA/FA Unit Station Service Delta-Wye (-) 24/32 MVA, 12/16 MVA (X 20.7- 4.16 - 4.16kV Low Side Grounded Wye Transformer 2B Winding) 12/16 MVA (Y Resistance Grounded Neutral Winding), OA/FA Unit Station Service Delta-Wye (-) 24/32 MVA, 12/16 MVA (X 20.7- 4.16 - 4.16kV Low Side Grounded Wye Transformer 3B Winding) 12/16 MVA (Y Resistance Grounded Neutral Winding), OA/FA Common Station Service Wye-Delta-Wye 21.9/29.2/36.5/ MVA, 161- 4.16 - 4.16kV High and Low Side Grounded Wye Transformer A (--) 12/16/20 MVA (X Winding) Resistance Grounded Neutral 9.9/13.2/16.5 MVA (Y Winding), OA/FA/FOA Common Station Service Wye-Delta-Wye 21.9/29.2/36.5/ MVA, 161- 4.16 - 4.16kV High and Low Side Grounded Wye Transformer B (--) 12/16/20 MVA (X Winding) Resistance Grounded Neutral 9.9/13.2/16.5 MVA (Y Winding), OA/FA/FOA E1-11

Table 5 - Protective Devices Protection Protective Logic Setpoint Basis for Setpoint Zone Device (Nominal)

A loss of voltage condition is a reduction in voltage to a Each 4.16 kV level which results in the immediate loss of equipment Shutdown capability to perform its intended function. For a loss of Board (A, B, C, Loss of Voltage 2870V shutdown board voltage of greater than 1.5 seconds, relays 2 of 2 D, 3EA, Relay (68.99% of 4160V) will drop out and start the diesel generator. Transfer from 3EB,3EC, or offsite power to diesel generator power will occur if voltage 3ED) does not recover to the reset setpoint (2870V) within 1.5 seconds.

Each 4.16 kV Operation of the auxiliary power system under steady-state Shutdown (running) conditions, with the safety-related bus voltage as Degraded 3920V Board (A, B, C, 2 of 3 low as possible while still keeping all connected safety-Voltage (94.23% of 4160V)

D, 3EA, 3EB, related loads within their rated operating voltage range 3EC, or 3ED) (within ANSI C84.1 utilization voltages, range B)

Each 4.16 kV Operation of the auxiliary power system, with the safety-Shutdown 1.34V @ 2.60 Sec related bus voltage unbalanced while still keeping all Unbalanced Permissive Board (A, B, C, 3.23V @ 7.86 Sec connected safety-related loads within their rated operating Voltage 1 out of 2 D, 3EA, 3EB, 19.5V @ 3.18 Sec voltage range (Reference 1) 3EC, or 3ED)

USST 1B, 2B, Transformer To provide protection for the neutral resistor for the 10 or 3B Neutral Ground N/A 180A second rating.

Relay E1-12

ENCLOSURE 2 Sequoyah Nuclear Plant, Units 1 and 2 Revised Response to NRC Bulletin 2012-01, "Design Vulnerability in Electric Power System"

ENCLOSURE 2 Sequoyah Nuclear Plant, Units 1 and 2 90-Day Response to NRC Bulletin 2012-01, Design Vulnerability in Electric Power System Overview:

Statement of No Open Phase Design Vulnerability for Class 1E Equipment System Description - Items 2., 1.d, 2.a, 2.c System Protection - Items 1., 1.a, 2.b, 2.d Consequences - Items 1.b, 1.c, 2.e References Attachment 1 - Simplified One-Line Diagram Attachment 2 - 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 Normally Energized Major Loads o Table 4 - Offsite Power Transformers o Table 5 - Protective Devices E2-1

Statement of No Open Phase Design Vulnerability for Class 1E Equipment The U. S. Nuclear Regulatory Commission (NRC) issued Bulletin 2012-01, Design Vulnerability in Electrical Power System, which identified a vulnerability to an open phase condition (OPC) in electrical power systems at nuclear power plants. The bulletin required that licensees comprehensively verify their compliance with General Design Criterion (GDC) 17, Electric Power Systems, and to respond to the bulletin. The Sequoyah Nuclear Plant (SQN) units are licensed to the Proposed General Design Criteria of 1967 and SQNs compliance with the intent of GDC 17 is discussed in the SQN Updated Final Safety Analysis Report (UFSAR). The bulletin also requested information regarding how the design of the protection scheme for engineered safety feature (ESF) buses detects and automatically responds to single-phase open circuit conditions or high impedance ground fault conditions, including a description of the operating configuration of the ESF buses.

As stated in the bulletin, At Byron, a failure to design the electric power systems protection scheme to sense the loss of a single phase between the transmission network and the onsite power distribution system resulted in unbalanced voltage at both ESF buses (degraded offsite power system), trip of several safety-related pieces of equipment such as Essential Service Water pumps, Centrifugal Charging Pumps, and Component Cooling Water Pumps and the unavailability of the onsite electric power system. This situation resulted in neither the onsite nor the offsite electric power system being able to perform its intended safety functions (i.e., to provide electric power to the ESF buses with sufficient capacity and capability to permit functioning of structures, systems, and components important to safety).

TVA has designed and implemented an unbalanced voltage protection scheme to protect structures, systems, and components (SSCs) important to safety, as discussed below. As stated in the NRCs safety evaluation (SE) (Reference 1) for TVAs unbalanced voltage relay (UVR) license amendment request (LAR), An open circuit in one phase of a three-phase power system results in an unbalanced voltage condition on plant buses. The unbalanced voltage condition results in unbalanced current flow that produces negative and zero sequence current.

Therefore, because TVA employs a UVR protection scheme, TVAs SSCs important to safety are protected from an OPC event. Therefore, SQN has no vulnerability to an OPC.

TVAs unbalanced voltage protection scheme is designed to detect and provide automatic protection for the connected Class 1E system from offsite power that is not capable of performing its safety function due to unbalanced voltage conditions. This unbalanced protection scheme is part of the undervoltage protection scheme that minimizes the probability of losing electric power from any of the remaining supplies as a result of, or coincident with, the loss of power generated by the nuclear power unit or the loss of power from the transmission network.

TVAs undervoltage protection scheme, located on the medium voltage Class 1E buses, includes loss of voltage protection, degraded voltage protection, and unbalanced voltage protection.

Because aspects of the implementation of the unbalanced voltage protection scheme design described above required a change to the Technical Specifications, TVA submitted an associated LAR. In the NRCs SE (Reference 1) for that LAR, the NRC staff determined that TVAs UVR protection schemes will provide reasonable assurance that the safety-related equipment will be adequately protected from the consequences of negative sequence currents resulting from unbalanced voltage conditions. The UVR protection will be part of the primary success path that functions or actuates to mitigate a design-basis accident or transient that either assumes the failure of or presents a challenge to the integrity of a fission product barrier E2-2

and, therefore, will meet Criterion 3 of 10 CFR 50.36(c)(2)(ii) for inclusion into the TSs. The NRC staff concludes that the implementation of the UVR protection schemes will provide reasonable assurance that GDC 17 (AEC Criterion 39 for the Browns Ferry units) required onsite electric power system will be available to permit functioning of SSCs important to safety following a detectable degradation related to unbalanced voltages in the offsite power system.

Therefore, as stated above, implementation of the undervoltage protection scheme will provide reasonable assurance that GDC 17 required onsite electric power system will be available to permit functioning of SSCs important to safety following a detectable degradation of voltages in the offsite power system, including those caused by an OPC.

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

[engineered safety feature] buses (Class 1E for current operating plants or non-Class 1E for passive plants) at power (normal operating condition).

See Attachment 1 for a simplified one-line diagram.

Preferred electric power (normal power supply) to the emergency buses at SQN is normally supplied by unit power through the unit station service transformers (USSTs). For Unit 1, the normal power supply to start up and shut down the generator is typically supplied by the 500-kV system through the main bank transformers and USSTs. For Unit 2, the normal power supply to start up and shut down the generator is typically supplied by the 161-kV system through the main bank transformers and unit station service transformers.

SQN UFSAR Section 8.2.1.1 describes the two GDC 17 required sources for the Class 1E system. The intent of GDC 17 has been implemented in the design of the preferred power system by providing two main bank transformers; six 24-kV isolated phase buses; four 24-6.9-kV unit station service transformers; four 6.9-kV unit station service transformer buses; three 161-6.9-kV CSSTs (A and C, energized spare B); a 6.9-kV start board; four 6.9-kV start buses; eight 6.9-kV unit boards; four 6.9-kV shutdown boards; and all overhead conductors, buses, cable, and distribution equipment that interconnect the off-site power circuits with the 6.9-kV shutdown boards. The Preferred Power System is supplied power by way of either the plant 161-kV or 500-kV switchyard. The combination of Unit 1 and 2 main bank transformers, USSTs, 24-kV isolated phase buses, and 6.9-kV unit station service transformer buses comprise one qualified independent off-site power circuit.

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

The 6.9 kV secondary winding of each USST is wye-connected, with the neutral grounded through a resistor which will limit ground fault current to 1600 amperes maximum. The 6.9 kV secondary winding of each CSST is wye-connected, with the neutral grounded through a resistor, which will limit ground fault current to 1600 amperes maximum on CSST A and 800 amperes on CSST C. The neutral resistor serves to prevent overvoltage on the winding which could occur in the event of a ground fault if the 6.9 kV system were not intentionally grounded.

Note: Installed spare (CSST B) is equivalent to CSST A.

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See Attachment 2, 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.

No, at normal (at power) operating condition configuration, the ESF buses are powered from the main generator through the unit station service transformers. For SQN, the ESF buses are the 6.9 kV shutdown boards and the associated downstream safety-related distribution.

See Attachment 2, Tables 1 and 2 for ESF bus power sources, and Table 3 for ESF bus major loads energized during normal power operations.

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.

The operating configuration, as described under item 2, has been confirmed to be consistent with the current licensing basis as discussed in the SQN UFSAR and Unit 1 and Unit 2 License Amendments 332 and 325, respectively, issued on October 31, 2012, and their associated safety evaluations.

In the original SQN design and alignment, the Unit Station Service Transformers (USST) supplied unit power to the 6.9 kV unit boards during plant operation, with the CSSTs available to provide offsite power via the Start Buses. In the event of a loss of a USST or a unit trip, offsite power to the Class 1E 6.9 kV Shutdown Boards was transferred at the 6.9 kV unit boards to the CSSTs. However, due to an issue identified in 1986 regarding USST impedance challenging the 6.9 kV Unit Board circuit breaker short circuit ratings, the CSSTs were replaced with transformers with auto tap changers to accommodate voltage variations in the 161 kV system.

This change in the offsite lineup resulted in the CSSTs providing power to the 6.9 kV unit boards via the Start Buses and removed the necessity of power transfer during unit startups and shutdowns or the reliance on automatic power transfer in the event of a fault or plant trip in order to maintain power to the 6.9 kV unit boards.

A license amendment request, dated May 23, 2012 (Reference 2), was submitted to restore the original USST operating configuration, with modifications. This request was accepted under the safety evaluation dated October 31, 2012 (Reference 3) and is currently in place under Unit 1 and Unit 2 License Amendments 332 and 335.

See Attachment 2, Tables 1 and 2, for any changes in the offsite power source alignment to the ESF buses from the original plant licensing.

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 E2-4

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.

Consistent with the current licensing basis documents specified in item 2.c, existing protective circuitry will separate the ESF buses from a connected failed offsite source as a result of a loss of voltage or a sustained, balanced degraded grid voltage, or a sustained unbalanced voltage.

While the relay systems were not specifically designed to detect and automatically respond only to an open phase on a three phase system, the undervoltage protection scheme will automatically detect and provide automatic protection from the unbalanced voltage and/or degraded voltage created by the open phase or high impedance ground fault that prevents the connected safety equipment from operating due to insufficient voltage. (See the NRCs SE for TVAs UVRs (Reference 1).)

Normal Plant Operation Considerations:

During normal plant operation the ESF buses sequence of events were described and evaluated as acceptable as part of the NRCs SE for TVAs UVRs (Reference 1, Section 3.2.2) for an open phase.

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

Consistent with the current licensing basis documents specified in item 2.c, existing electrical protective devices are sufficiently sensitive to detect design basis conditions like a loss of voltage or a sustained balanced degraded voltage, or a sustained unbalanced voltage. See , Table 5 for undervoltage protective devices and the basis for the device setpoint(s). Degraded voltage relay logic is 2 out of 3 and degraded voltage relays monitor phases A-B, B-C and C-A. The UVR logic is a permissive 1 out of 2 and each of the relays monitor A-B-C phases.

Existing electrical protective devices are also sufficiently sensitive to detect a ground fault. , Table 5 lists protection devices 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.

The current SQN, Units 1 and 2, Technical Specifications do not contain surveillance requirements for offsite power circuits. In addition, the ESF buses at SQN are powered by unit generators. Phase voltage is monitored for generator output.

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?

Yes, the current plant operating procedures, including operating procedures for off-normal alignments, do specifically call for verification of the voltages on all three phases of the ESF buses.

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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.

There are no differences of consequences to any safety-related (Class 1E) equipment for an open phase occurring on a normally loaded power source (i.e., Class 1E bus normally aligned to offsite power transformer) or an unloaded power source (i.e., standby offsite power source). No adverse consequences or loss of function occur in either case based on the Unbalanced Voltage setpoint methodology. (Reference 1, Section 3.4.4).

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.

The UVR scheme provides detection and automatic protection for the Class 1E equipment for any adverse single-phase open circuit condition or high impedance ground fault condition on the offsite power circuit.

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.

Not applicable since SQN does not use an offsite circuit to supply redundant ESF buses during normal operation.

REFERENCES

1. NRC Letter to TVA, Browns Ferry Nuclear Plant, Units 1, 2, and 3; Sequoyah Nuclear Plant, Units 1 and 2; Watts Bar Nuclear Plant, Units 1 and 2 - Issuance of Amendment Nos. 309, 332, 292, 345, 339, 128, and 31 Regarding Unbalanced Voltage Protection (EPID L-2017-LLA-0391), dated August 27, 2019 (ML18277A110)

2. TVA Letter to NRC, Application to Modify Technical Specifications in Support of Unit Station Service Transformer Modification (TS-SQN-12-01), dated May 23, 2012 (ML12146A385)

3. NRC Letter to TVA, Sequoyah Nuclear Plant, Units 1 and 2 - Issuance of Amendments to Revise the Technical Specifications 3/4.8.1 in Support of Unit Station Service Transformer Modification (TS-SQN-12-01) (TAC Nos. ME8772 and ME8773), dated October 31, 2012 (ML12286A078)

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Attachment 1 Simplified One-Line Diagram (See next page)

E2-7

1 2 3 4 5 6 7 8 9 10 402 403 A 161 KV SWITCHYARD 500 KV SWITCHYARD VW CHATTANOOGA WATTS BAR HIWASSEE EAST CLEVELAND FRANKLIN CHICKAMAUGA 1 BRADLEY WIDOWS NUCLEAR 1 HIWASSEE 1 HIWASSEE 2 WATTS BAR CREEK CONCORD HYDRO MOCCASIN 5040 161 KV BUS 2 500KV BUS 2 877 887 897 907 917 927 920 997 1007 5057 5037 5027 5017 940 937 947 5047 878 888 898 908 918 928 938 948 998 1008 5058 5048 5038 5028 5018 5079 5069 5059 5049 5039 5029 B 879 889 899 909 919 929 939 949 959 969 979 989 999 1009 5019 1032 5041G 5021G 5011G CAP BANK 3 84 MVR 5071G 5061G 875 885 895 905 915 925 935 945 955 965 975 985 995 5055 5045 5035 5025 1005 5075 5065 5015 1012 874 924 934 944 954 964 974 984 994 1004 5074 5054 5034 CAP BANK 1 84 MVR 5064 873 923 933 943 953 963 973 983 993 5073 5053 5033 5063 1003 910 161 KV BUS 1 500KV BUS 1 15 7 6 5 4 3 2 1 7 8 9 10 11 12 13 14 16 17 18 19 20 990 5050 INTERTIE BANK 5 C

GENERATOR 2 GENERATOR 1 MAIN 2 MAIN 1 D NC NC 324 TO CSST D TO CTT B TO CTT A GCB314 USST 2B USST 2A CSST C CSST B CSST A USST 1B USST 1A X Y X Y X Y 1412 1612 NO NO NC NC NO NO NC NC 1414 1418 1614 1416 1514 1512 START BUS 1A E START BUS 1B START BUS 2A START BUS 2B 1634 1224 1222 1534 1632 1214 1212 1532 1216 1626 1526 1126 1624 1124 1122 1524 1622 1114 1112 1522 NO 19 NC NC 18 NO NO 17 NC NC 16 NO NO 6 NC NC 5 NO NO 15 NC NC NC 14 NO NO 13 NC NC 12 NO 6.9KV UNIT BD 2D 6.9KV UNIT BD 2C 6.9KV UNIT BD 2B 6.9KV UNIT BD 2A 6.9KV COMMON BD B 6.9KV COMMON BD A 6.9KV UNIT BD 1D 6.9KV UNIT BD 1C 6.9KV UNIT BD 1B 6.9KV UNIT BD 1A 1824 1822 1814 1812 NC NC NC NC NO NC 1724 1722 1714 1712 F NC NC NC NC NC NC NC NC COM EMERG 2B 2A 1B 1A 480V UNIT BD UTILITY BUS NC 139,140 NO NC 137,138 NO NO 135,136 NC NO 133,134 NC 480V UNIT BD 2B 480V UNIT BD 2A 480V UNIT BD 1B 480V UNIT BD 1A SHUTDOWN UTILITY BUS G

DG DG DG DG LEGEND 2B-B 2A-A 1B-B 1A-A HIGH VOLTAGE 1826 1828 1924 1938 1816 1922 1936 1934 1914 1728 1726 1718 1932 11 1818 10 1912 1716 8 NC NO NO NO NC NO NO NO NO NO NO 9 NC NO NO NC NO OFFSITE POWER CKT 1 6.9KV SD BD 2B-B 6.9KV SD BD 2A-A 6.9KV SD BD 1B-B 6.9KV SD BD 1A-A OFFSITE POWER CKT 2 NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NON-SAFETY B.O.P.

2B2-B 2B-B 2B1-B ERCW 2B-B 2A2-A 2A-A 2A1-A ERCW 2A-A 1B2-B 1B-B 1B1-B ERCW 1B-B 1A2-A 1A-A 1A1-A ERCW 1A-A SAFETY RELATED - TRAIN A H NC NO NO NC NC NC NO NO NC NC NC NO NO NC NC NC 120 NO NO NC NC SAFETY RELATED - TRAIN B 126 125 124 123 122 121 119 XXX ETAP NODE NUMBERS 480V SD BD 2B2-B 480V SD BD 2B1-B 480V SD BD 2A2-A 480V SD BD 2A1-A 480V SD BD 1B2-B 480V SD BD 1B1-B 480V SD BD 1A2-A 480V SD BD 1A1-A NC NC NC NC NC NO NC NC NC NC NC NC NC NC NO NC NC NC NC NC NC NC NC NC NC NO NC NC NC NC NC NC NC NC NO NC NC NC NC NC Reference Drawings 1-15E500-1 45N502 230 229 VPS I 232 231 1-15E500-1-29 45N504 VPS IV VPS III VPS IV VPS III VPS II VPS II VPS I ERCW ERCW ERCW ERCW 1-15E500-2 45N505 MCC 1B-B MCC 1A-A MCC 2B-B MCC 2A-A 1-15E500-2-35 NO NO NC NO NO NC NO NO 253 NC 251 NO NC NO 247 1-75W500 249 NC RMOV BD 2B1-B NO NC RMOV BD 2A1-A NO NC NC RMOV BD 1B1-B NO NC NC RMOV BD 1A1-A NO NC 1-75W500-16 NPlant NC I O NC RMOV BD 2B2-B 254 NO ERCW MCC 2B-B NC RMOV BD 2A2-A 252 NO ERCW MCC 2A-A NC RMOV BD 1B2-B 250 NO ERCW MCC 1B-B NC RMOV BD 1A2-A 248 NO ERCW MCC 1A-A Sequoyah NuclearT MA System C&A VENT BD 2B2-B 210 232 C&A VENT BD 2A2-A 208 231 C&A VENT BD 1B2-B 206 230 204 229 NO NC NO NC NO NC NO C&A VENT BD 1A2-A NC I

R C&A VENT BD 2A1-A 207 ElectricalODistribution C&A VENT BD 2B1-B 209 NC C&A VENT BD 1B1-B 205 NC C&A VENT BD 1A1-A 203 INF480V-500KV NO NO NC NO NO NC Y

R VENT BD 2A-A ONL R VENT BD 2B-B 258 257 R VENT BD 1B-B 256 R VENT BD 1A-A 255 DSL AUX BD 2B1-B 224 DSL AUX BD 2A1-A 222 DSL AUX BD 1B1-B 220 DSL AUX BD 1A1-A 218 NO NC NO NC NO NC NO NC NC NO NC NO NC NO NC NO DSL AUX BD 2B2-B 225 DSL AUX BD 2A2-A 223 DSL AUX BD 1B2-B 221 DSL AUX BD 1A2-A 219 Engineer: Mark D. Bowman CAD MAINTAINED DRAWING DATE FILE NAME REVISION LEVEL 8-7-2015 SQN Electrical Distribution System 4 1 2 3 4 5 6 7 8 9 10

Attachment 2 - Tables Table 1 - ESF Buses Continuously Powered From Offsite Power Source(s)

Description of ESF Bus Power Source ESF Bus Name (normal operating condition). Original licensing basis configuration (Y/N)

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

Description of ESF Bus Power Source ESF Bus Name (normal operating Original licensing basis condition) configuration (Y/N)

Unit Station Service Transformer 1A (USST 1A) Shutdown Board 1A-A N via 6.9 kV Unit Board 1A Unit Station Service Transformer 1B (USST 1B) Shutdown Board 1B-B N via 6.9 kV Unit Board 1D Unit Station Service Transformer 2A (USST 2A) Shutdown Board 2A-A N via 6.9 kV Unit Board 2A Unit Station Service Transformer 2B (USST 2B) Shutdown Board 2B-B N via 6.9 kV Unit Board 2D E2-9

Table 3 - ESF Buses Normally Energized Major Loads Class 1E Bus Load Voltage Level Rating (Normally Running) (kV) (HP)

Shutdown Board 1A-A ERCW 6.9 700 Shutdown Board 1B-B ERCW 6.9 700 Shutdown Board 1B-B CCP 6.9 600 Shutdown Board 2A-A ERCW 6.9 700 Shutdown Board 2A-A CCP 6.9 600 Shutdown Board 2B-B ERCW 6.9 700 Table 4 - Offsite Power Transformers Transformer Winding MVA Size Voltage Rating Grounding Configuration Configuration (AO/FA/FA) (Primary/Secondary)

USSTs 1A, 1B, 2A, Delta-Wye 24 / 32 / 40 22.0kV / 6.9kV Low Side Grounded Wye and 2B (3Ø ) and Resistance Neutral Grounded CSST A(3Ø ) Wye-Wye (3 Leg) 24 / 32 / 40 161kV / 6.9kV / 6.9kV High and Low Side Grounded Wye Buried Delta and Resistance Neutral Grounded CSST C(3Ø ) Wye-Wye (3 Leg) 24 / 32 / 40 161kV / 6.9kV / 6.9kV High and Low Side Grounded Wye Buried Delta and Resistance Neutral Grounded E2-10

Table 5 - Protective Devices Protection Protective Logic Setpoint (Nominal) Basis for Setpoint Zone Device Operation of the auxiliary power system under steady-Each 6.9 kV state (running) conditions, with the safety-related bus Shutdown Loss of Voltage 5520V voltage as low as possible while keeping all 2 of 3 Board (1A, 1B, Relay (80% of 6900V) connected safety-related motor loads above their stall 2A, or 2B) voltage. (greater than 70.7 percent of rated motor voltage for NEMA Design "B" motors)

Operation of the auxiliary power system under steady-Each 6.9 kV state (running) conditions, with the safety-related bus Shutdown Degraded 6456V voltage as low as possible while still keeping all 2 of 3 Board (1A, 1B, Voltage (93.5% of 6900V) connected safety-related loads within their rated 2A, or 2B) operating voltage range (within ANSI C84.1 utilization voltages, range B)

Each 6.9 kV Operation of the auxiliary power system, with the 1.30V @ 2.95 Sec Shutdown Unbalanced Permissive safety-related bus voltage unbalanced while still 2.96V @ 9.95 Sec Board (1A, 1B, Voltage 1 out of 2 keeping all connected safety-related loads within their 18.13V @ 3.95 Sec 2A, or 2B) rated operating voltage range (Reference 1)

CSST A Transformer To provide protection for the neutral resistor for the 10 Neutral Ground N/A 360A second rating.

Relay CSST B Transformer To provide protection for the neutral resistor for the 10 Neutral Ground N/A 360A second rating.

Relay CSST C To provide protection for the neutral resistor for the 10 Transformer second rating.

Neutral Ground N/A 72A Relay E2-11

ENCLOSURE 3 Watts Bar Nuclear Plant, Units 1 and 2 Revised Response to NRC Bulletin 2012-01, "Design Vulnerability in Electric Power System"

ENCLOSURE 3 Watts Bar Nuclear Plant, Units 1 and 2 90-Day Response to NRC Bulletin 2012-01, Design Vulnerability in Electric Power System Overview:

Statement of No Open Phase Design Vulnerability for Class 1E Equipment System Description - Items 2., 1.d, 2.a, 2.c System Protection - Items 1., 1.a, 2.b, 2.d Consequences - Items 1.b, 1.c, 2.e References Attachment 1 - Simplified One-Line Diagram Attachment 2 - 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 Normally Energized Major Loads o Table 4 - Offsite Power Transformers o Table 5 - Protective Devices E3-1

Statement of No Open Phase Design Vulnerability for Class 1E Equipment The U. S. Nuclear Regulatory Commission (NRC) issued Bulletin 2012-01, Design Vulnerability in Electrical Power System, which identified a vulnerability to an open phase condition (OPC) in electrical power systems at nuclear power plants. The bulletin required that licensees comprehensively verify their compliance with General Design Criterion (GDC) 17, Electric Power Systems, and to respond to the bulletin. The Watts Bar Nuclear Plant (WBN) units are licensed to the Proposed General Design Criteria of 1967 and WBNs compliance with the intent of GDC 17 is discussed in the WBN Updated Final Safety Analysis Report (UFSAR). The bulletin also requested information regarding how the design of the protection scheme for engineered safety feature (ESF) buses detects and automatically responds to single-phase open circuit conditions or high impedance ground fault conditions, including a description of the operating configuration of the ESF buses.

As stated in the bulletin, At Byron, a failure to design the electric power systems protection scheme to sense the loss of a single phase between the transmission network and the onsite power distribution system resulted in unbalanced voltage at both ESF buses (degraded offsite power system), trip of several safety-related pieces of equipment such as Essential Service Water pumps, Centrifugal Charging Pumps, and Component Cooling Water Pumps and the unavailability of the onsite electric power system. This situation resulted in neither the onsite nor the offsite electric power system being able to perform its intended safety functions (i.e., to provide electric power to the ESF buses with sufficient capacity and capability to permit functioning of structures, systems, and components important to safety).

TVA has designed and implemented an unbalanced voltage protection scheme to protect structures, systems, and components (SSCs) important to safety, as discussed below. As stated in the NRCs safety evaluation (SE) (Reference 1) for TVAs unbalanced voltage relay (UVR) license amendment request (LAR), An open circuit in one phase of a three-phase power system results in an unbalanced voltage condition on plant buses. The unbalanced voltage condition results in unbalanced current flow that produces negative and zero sequence current.

Therefore, because TVA employs a UVR protection scheme, TVAs SSCs important to safety are protected from an OPC event. Therefore, WBN has no vulnerability to an OPC.

TVAs unbalanced voltage protection scheme is designed to detect and provide automatic protection for the connected Class 1E system from offsite power that is not capable of performing its safety function due to unbalanced voltage conditions. This unbalanced protection scheme is part of the undervoltage protection scheme that minimizes the probability of losing electric power from any of the remaining supplies as a result of, or coincident with, the loss of power generated by the nuclear power unit or the loss of power from the transmission network.

TVAs undervoltage protection scheme, located on the medium voltage Class 1E buses, includes loss of voltage protection, degraded voltage protection, and unbalanced voltage protection.

Because aspects of the implementation of the unbalanced voltage protection scheme design described above required a change to the Technical Specifications, TVA submitted an associated LAR. In the NRCs SE (Reference 1) for that LAR, the NRC staff determined that TVAs UVR protection schemes will provide reasonable assurance that the safety-related equipment will be adequately protected from the consequences of negative sequence currents resulting from unbalanced voltage conditions. The UVR protection will be part of the primary success path that functions or actuates to mitigate a design-basis accident or transient that either assumes the failure of or presents a challenge to the integrity of a fission product barrier E3-2

and, therefore, will meet Criterion 3 of 10 CFR 50.36(c)(2)(ii) for inclusion into the TSs. The NRC staff concludes that the implementation of the UVR protection schemes will provide reasonable assurance that GDC 17 (AEC Criterion 39 for the Browns Ferry units) required onsite electric power system will be available to permit functioning of SSCs important to safety following a detectable degradation related to unbalanced voltages in the offsite power system.

Therefore, as stated above, implementation of the undervoltage protection scheme will provide reasonable assurance that GDC 17 required onsite electric power system will be available to permit functioning of SSCs important to safety following a detectable degradation of voltages in the offsite power system, including those caused by an OPC.

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).

See Attachment 1 for a simplified one-line diagram.

The engineered safety features (ESF) buses are normally powered directly from offsite power via common station service transformer connections to the 161 kilovolt (kV) switchyard.

Preferred offsite power is supplied from TVAs 161 kV transmission grid at Watts Bar Hydro Plant switchyard over two separate transmission lines, each connecting to two 161-6.9 kV common station service transformers (CSSTs) at WBN. The Class 1E power system is normally supplied from offsite power through CSSTs C and D. CSSTs C and D are connected to 6.9 kV common switchgear C and D via a bus similar to 6.9 kV start buses A and B. The 6.9 kV common switchgear C and D are then connected to the 6.9 kV shutdown boards.

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

The 6.9 kV secondary winding of each unit and common station service transformer is wye-connected, with the neutral grounded through a resistor which will limit ground fault current to 1600 amperes maximum. The neutral resistor serves to prevent overvoltage on the winding which could occur in the event of a ground fault if the 6900V system were not intentionally grounded.

See Attachment 2, 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.

At normal (at power) operating condition configuration the ESF buses are powered by offsite power sources.

See Attachment 2, Tables 1 and 2 for ESF bus power sources, and Table 3 for ESF bus major loads energized during normal power operations.

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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.

The operating configuration, as described under item 2, has been confirmed to be consistent with the configuration described in WBNs UFSAR, Chapter 8.2. There have not been any changes to the offsite power source alignment to the ESF buses from the WBN original plant licensing.

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.

Consistent with the current licensing basis document specified in item 2.c, existing protective circuitry will separate the ESF buses from a connected failed offsite source as a result of a loss of voltage, a sustained balanced degraded grid voltage, or a sustained unbalanced voltage.

While the relay systems were not specifically designed to detect and automatically respond only to an open phase on a three phase system, the undervoltage protection scheme will automatically detect and provide automatic protection from the unbalanced voltage and/or degraded voltage created by the open phase or high impedance ground fault that prevents the connected safety equipment from operating due to insufficient voltage (Reference 1).

Normal Plant Operation Considerations:

During normal plant operation, the ESF buses sequence of events were described and evaluated as acceptable as part of NRCs SE for UVRs (Reference 1) for an open phase.

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

Consistent with the current licensing basis document specified in item 2.c, existing electrical protective devices are sufficiently sensitive to detect design basis conditions like a loss of voltage or sustained balanced degraded voltage, or a sustained unbalanced voltage. See , Table 5 for undervoltage protective devices and the basis for the device setpoint(s). Degraded voltage relay logic is 2 out of 3 and degraded voltage relays monitor phases A-B, B-C and C-A. The UVR logic is a permissive 1 out of 2 and each of the relays monitor A-B-C phases.

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

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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.

Not Applicable - the ESF buses at WBN are powered by offsite power sources.