Text

Response to Request for Additional Information Re Chapter 8, Electrical Power - Supplemental Safety Evaluation Report (Sser 22, Open Item 30)
	ML15030A511
Person / Time
Site:	Watts Bar
Issue date:	01/30/2015
From:	James Shea; Tennessee Valley Authority
To:	; Document Control Desk, Office of Nuclear Reactor Regulation
References
	TAC ME2731
	Download: ML15030A511 (79)
	v • d • e

Tennessee Valley Authority, 1101 Market Street, Chattanooga, Tennessee 37402 CNL-15-030 January 30, 2015 10 CFR 50.4 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Watts Bar Nuclear Plant, Unit 2 Construction Permit No. CPPR-92 NRC Docket No. 50-391

Subject:

Response to Watts Bar Nuclear Plant Unit 2 Request for Additional Information Regarding Chapter 8, Electrical Power - Supplemental Safety Evaluation Report (SSER 22, OPEN ITEM 30) (TAC NO. ME2731)

Reference:

Email from Michael Miernicki to Gordon Arent, FW: Watts Bar 2 - RAIs -

Degraded Voltage Relay Issue dated November 7, 2014 [ML14365A048].

Attachment:

Watts Bar RAI Final 11-07-2014.docx In accordance with Title 10 of the Code of Federal Regulations (10 CFR) Section 50.4, Written Communications, Tennessee Valley Authority (TVA) hereby submits the response to the U. S. Nuclear Regulatory Commissions (NRC) request for additional information (RAI) regarding Chapter 8, Electrical Power of the Watts Bar Nuclear Plant Unit 2 (WBN-2)

Supplemental Safety Evaluation Report (SSER) 22, Open Item 30 (Reference).

To support the staffs review of the response, Enclosure 1 contains a regulatory analysis that provides a detailed explanation of how the degraded voltage protection scheme complies with the applicable design and licensing basis requirements for the capacity and capability of the offsite power sources for WBN-2. The regulatory analysis describes the onsite, Class 1E safety bus Level 1 and Level 2 undervoltage protection schemes and how the offsite power sources and Class 1E bus undervoltage protection schemes fully comply with applicable design and licensing basis requirements, including General Design Criterion 17, Electric Power Systems, (GDC 17) of 10 CFR 50, Appendix A. Enclosure 2 contains the specific responses to each of the staffs seven specific questions identified in the RAI.

TVA has prepared this response to address each of the questions contained in the staffs RAI as well as associated issues considered during the related audit, public meetings, teleconferences, and topical industry conferences. For reference, Enclosure 3 contains answers to additional staff questions that arose during the December 10, 2014 meeting and provides a chronology of staff interactions and correspondences related to the staffs review of TVAs degraded voltage protection scheme.

U.S. Nuclear Regulatory Commission CNL-15-030 Page 2 January 30, 2015 There are no new commitments contained in this submittal. Should you have questions regarding this response , please contact Gordon Arent, Director, Watts Bar Licensing at (423) 365-2004.

I declare under penalty of perjury that the foregoing is true and correct. Executed on the 30 1h day of January, 2015.

Enclosures:

1. Degraded Voltage Scheme Regulatory Analysis

2. Response to Watts Bar Nuclear Plant Unit 2, Request for Additional Information Regarding Chapter 8, "Electrical Power" - Supplemental Safety Evaluation Report, Open Item 30

3. Responses to Additional Questions from December 10, 2014 Meeting

4. Chronology of Staff Interactions and Related Correspondence cc (Enclosures):

NRC Regional Administrator - Region II NRC Senior Resident Inspector - Watts Bar Nuclear Plant, Unit 1 NRC Senior Resident Inspector - Watts Bar Nuclear Plant, Unit 2 NRC Project Manager - Watts Bar Nuclear Plant, Unit 1 NRC Project Manager - Watts Bar Nuclear Plant, Unit 2

ENCLOSURE 1 TENNESSEE VALLEY AUTHORITY DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS E1-1

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS EXECUTIVE

SUMMARY

The design, layout, and plant equipment for Tennessee Valley Authoritys (TVAs) Watts Bar Nuclear Unit 2 (WBN-2) is virtually identical to Watts Bar Nuclear Plant Unit 1 (WBN-1). The design and licensing bases for Unit 2 are essentially the same as what presently exists for Unit 1, which received its full power operating license (OL) in 1996.

This document provides relevant background information regarding NUREG-0847, Supplemental Safety Evaluation Report (SSER) 22, Open Item No. 30 as well as detailed descriptions of the bases for regulatory compliance of WBNs offsite power sources and Class 1E safety bus undervoltage protection schemes with the current licensing basis of the plant.

The Watts Bar Nuclear Plant (WBN) offsite power sources and Class 1E bus undervoltage protection schemes fully comply with the applicable design and licensing basis requirements, including General Design Criterion 17, Electric Power Systems, (GDC 17) of Title 10 of the Code of Federal Regulations (10 CFR), Appendix A. A comprehensive set of power system analyses demonstrates that the offsite power system can perform its intended safety functions assuming the onsite standby power supplies are not functioning. Specifically, the offsite power sources have sufficient capacity and capability to assure that (1) specified acceptable fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded as a result of anticipated operational occurrences and (2) the core is cooled and containment integrity and other vital functions are maintained in the event of postulated accidents. The analyses provide the technical basis for the key Analytical Limits which are used to determine the Operability of the offsite power sources and the appropriate setpoints for the Class 1E bus undervoltage protective relays, consistent with NRC Branch Technical Position (BTP) PSB-1, Adequacy of Station Electric Distribution System Voltages.

The WBN Class 1E, 6.9kV shutdown boards are provided with two levels of undervoltage protection. Loss of Voltage Relays (LVRs) provide the first level of protection and Degraded Voltage Relays (DVRs) provide a second level of protection for the Class 1E loads. Consistent with NRC Branch Technical Position PSB-1, the primary function of the LVRs is to detect loss of offsite power at the Class 1E buses and disconnect the safety-related buses in the event a loss of offsite power (LOOP). The primary function of the DVRs is provided to protect Class 1E equipment from sustained low voltage conditions on the safety-related buses. When actuated (dropped out or tripped) and timed out, the LVRs and DVRs initiate a transfer of the Class 1E safety buses from the normal offsite power supply to the onsite standby power supplies (i.e.,

diesel generators). The voltage and time delay setpoints established for the WBN Level 1 and E1-2

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS Level 2 undervoltage protective relays support the plant Technical Specifications and provide adequate protection to prevent damage to the permanently connected Class 1E loads.

In all cases, the capability to successfully actuate the ESF loads is maintained, from either the offsite source (block start) or the onsite source (sequenced start from the emergency diesel generator). At the DVR Dropout Voltage Analytical Limit there is adequate operating voltage for (a) powering the safety related accident loads required to be operating under the maximum loading conditions (running loads), and (b) starting individual motor loads and stroking individual NRC Generic Letter 89-10 motor operated valves. In all cases, the DVR provides adequate protection for the Class 1E loads and prevents inadvertent trip of safety loads.

Tennessee Valley Authority has applied the results of detailed power system analyses, incorporated relevant industry operating experience, and implemented the under-voltage protection schemes so as to minimize the probability of losing power from any of the offsite power supplies as a result of, or coincident with, the loss of power generated by the nuclear units, the loss of power from the transmission network, or the loss of power from the onsite WBN standby power supplies. Tennessee Valley Authority has established the appropriate setpoints and administrative processes and procedures to achieve a well balanced allocation between conservative design margin and safe plant operating margins.

The WBN design includes adequate indicators and alarms, and TVA has established an appropriate set of procedures and protocols to ensure that the necessary operational limits for the offsite power sources are maintained. Formal agreements, which define the necessary Nuclear Plant Interface Requirements (NPIRs), are in place with the transmission system operator. A real-time transmission system state estimator is used to ensure that the operational limits for the offsite power sources are maintained.

The WBN offsite power sources and Class 1E bus undervoltage protection schemes fully comply with the applicable plant design and licensing basis requirements, including General Design Criterion 17, Electric Power Systems, (GDC 17) of 10CFR50, Appendix A and NRC Branch Technical Position (BTP) PSB-1, Adequacy of Station Electric Distribution System Voltages. The approaches used for TVAs degraded voltage relaying analyses and offsite source / station AC power analyses are consistent with the applicable NRC guidance.

Therefore, there are adequate bases for closure of NUREG-0847, SSER 22, Open Item No.

30.

E1-3

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS TABLE OF CONTENTS PART ONE - INTRODUCTION

1. Background

2. Purpose

3. Summary of Degraded Voltage Events and Relevant Industry Operating Experience 3.1 Millstone Station Unit 2 - 1976 3.2 Safety Bus Undervoltage Protection 3.3 Arkansas Nuclear One - 1978 3.4 Callaway Plant - 1999 3.5 2013 Blackout and Electric Reliability Organization (ERO)

4. Relevant WBN Licensing Basis Documents and Submittals PART TWO - WBN OFFSITE SOURCES AND BUS UNDERVOLTAGE PROTECTION

5. Glossary of Defined Terms

6. Description of WBN Offsite Sources and Class 1E Bus Undervoltage Protection 6.1 Preferred Power Supply 6.2 Class 1E Safety Bus Undervoltage Protection

7. WBN Design and Licensing Bases for Offsite Sources 7.1 General Design Criterion 17 7.2 Analysis of WBN Offsite Sources 7.3 Nuclear Plant Interface Requirements (NPIRs) 7.4 Transmission System Operator and State Estimator 7.5 Applicable Plant Procedures 7.6 WBN Plant Technical Specifications

8. WBN Class 1E Bus Undervoltage Protection 8.1 NRC Branch Technical Position PSB-1 8.2 Analysis to Establish DVR and LVR Analytical Limits E1-4

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS 8.3 Summary of Analysis

9. Design Basis Compliance 9.1 GDC 17 and Offsite Power System Requirements 9.2 PSB-1 and Undervoltage Protection Scheme Requirements

10. Summary and Conclusions

11. References Attachments General Design Criterion 17 of 10CFR50 Appendix A U.S. NRC Branch Technical Position PSB-1 E1-5

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS PART ONE - INTRODUCTION 1 BACKGROUND The Watts Bar Nuclear Plant (WBN) is located in southeastern Tennessee approximately 50 miles northeast of Chattanooga. The plant is owned by the Tennessee Valley Authority (TVA). The WBN site consists of two Westinghouse pressurized water reactor (PWR) units.

WBN Unit 1 received a full power operating license (OL) from the NRC in February 1996.

Tennessee Valley Authority is in the final process of licensing WBN Unit 2, which is of the same vintage as WBN Unit 1. The design, layout, and plant equipment for Unit 2 are virtually identical to Unit 1. NRC staff paper SECY-07-0096 (Reference 1) describes to the NRC Commission the general approach for the NRCs licensing activities and the licensing bases for WBN Unit 2. The NRC staff is conducting its licensing review of Unit 2 under 10 CFR Part 50 and the design and licensing bases for Unit 2 is essentially the same as what presently exists for Unit 1. The NRC Commission supports a licensing review approach that employs the current licensing basis for Unit 1 and approved the NRC staffs recommendations for the licensing approach for Unit 2 as discussed in SRM-SECY-07-0096 (see Reference 2).

The NRC staff is in the process of completing its technical review of TVA's Final Safety Analysis Report for WBN Unit 2. An open item for licensing WBN Unit 2 is to resolve the degraded voltage issue that is documented as Open Item No. 30 in Section 8.3.1.2 of Supplement 22 to NUREG-0847 (SSER 22, Reference 3).

Open Item No. 30 states the following.

TVA should confirm that all safety-related equipment (in addition to the Class 1E motors) will have adequate starting and running voltage at the most limiting safety-related components (such as motor-operated valves (MOVs), contactors, solenoid valves or relays) at the DVR setpoint dropout setting. TVA should also confirm that (1) the motor starting transient studies are based on the dropout voltage value of DVR and time delay,(2) the steady-state voltage drop studies are carried out by maximizing running loads on the Class 1E distribution system (bounding combination of safety systems loads), with the voltage at 6.9-kV [kilovolt] Class 1E buses (monitored by the DVRs) at or just above the DVR dropout setting, and (3) the DVR settings do not credit any equipment operation (such as LTC [load tap changer] transformers) upstream of the 6.9-kV Class 1E buses. TVA should also confirm that the final technical specifications (TSs) are properly derived from these analytical values for the degraded voltage settings (Reference 3).

2 PURPOSE This document provides relevant background information and a detailed description of the WBN GDC 17 offsite power source capacity and capability and the Class 1E safety bus undervoltage protection schemes. This information is intended to support the NRC staff review of Supplemental Safety Evaluation Report SSER 22, Open Item No. 30 for WBN Unit 2.

General Design Criteria 17 (see Attachment 1) of 10CFR50, Appendix A requires two physically independent circuits (not necessarily on separate rights of way) designed and located so as to minimize to the extent practical the likelihood of their simultaneous failure E1-6

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS under operating and postulated accident and environmental conditions. GDC 17 requires the onsite and offsite power system to have sufficient capacity and capability to assure:

a. Specified fuel design limits and design conditions of reactor coolant pressure boundary are not exceeded as a result of anticipated operational occurrences, and the

b. Core is cooled and containment integrity and other vital functions are maintained in event of postulated accidents.

NRC Branch Technical Position PSB-1 (see Attachment 2) recommends that two levels of undervoltage protection be provided on the safety-related electrical distribution system. The first level of undervoltage protection is the loss of voltage relay (LVR), whose primary function is to detect loss of offsite power at the Class 1E buses and disconnect the safety-related buses in the event a loss of offsite power (LOOP). The second level of undervoltage protection is the degraded voltage relay (DVR), which is provided to protect Class 1E equipment from sustained low voltage conditions on the safety-related buses.

This document:

Summarizes relevant industry operating experience concerning offsite power reliability and degraded voltage conditions;

Describes the applicable design and licensing basis requirements for the WBN offsite power sources and onsite Class 1E safety bus undervoltage protection schemes;

Describes the analyses performed to demonstrate that the WBN offsite sources have sufficient capacity and capability to perform their intended functions and the resultant Analytical Limits established from those analyses;

Discusses the applicable processes and procedures established to ensure reliability and operability of the WBN offsite sources;

Describes the analyses performed to establish the appropriate Analytical Limits for the Class 1E safety bus undervoltage protection schemes and how these schemes (a) adequately protect safety related equipment during accident and non-accident conditions and (b) preserve safe operational margin for the offsite power system, i.e., the preferred power source for WBN; and

Summarizes the bases for WBNs compliance with the applicable design and licensing basis requirements for the WBN offsite power sources and onsite Class 1E safety bus undervoltage protection schemes.

This document provides the bases for closure of Open Item No. 30 in Section 8.3.1.2 of Supplement 22 to NUREG-0847 (SSER 22, Reference 3).

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ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS 3

SUMMARY

OF DEGRADED VOLTAGE EVENTS AND RELEVANT INDUSTRY OPERATING EXPERIENCE This section summarizes industry operating experience regarding degraded voltage events.

In addition, key observations for each event are provided.

3.1 MILLSTONE STATION UNIT 2 - 1976 Following a plant trip on July 5, 1976, several motors powered from 480 V motor control centers (MCCs) at Millstone Unit 2 failed to start as required. The motors failed to start due to open (blown) fuses on the 480/120 V control power transformers (CPTs) powering the control circuits associated with the motors. Subsequent investigation determined that the cause of the fuse failures was due to the low available 480 V bus voltage as a result of the plant trip.

When the plant tripped, the 345 kV transmission system voltage dropped from 352 kV (1.02 per unit) to 333 kV (0.965 per unit). With the onsite AC distribution system powered from the offsite transmission system, there was a corresponding drop in onsite bus voltages.

Control power fuses opened due to the low bus voltage when their associated 480 V motors received signals to start following the plant trip. This rendered the motors In-Operable.

Subsequent investigations concluded that under similar low voltage conditions, the Operability of Millstone Unit 2 480 V Engineered Safety Features (ESF) equipment could not be assured.

The initial corrective action was to raise the setpoint of the Engineered Safeguards Actuation System (ESAS) loss of power undervoltage relays 1 to assure that the plant would separate from the grid and onsite standby power systems would start before the control voltage fell below that required for contactor operation. Two weeks following this modification, the plants safety buses were inadvertently de-energized during the start of a circulating water pump. The inrush current associated with the large circulating water pump caused the safety bus voltage to drop below the new ESAS undervoltage relay setting.

In August 1976, the NRC provided licensees with a description of the Millstone Unit 2 degraded voltage event along with a request for information (see Reference 5 for a representative example). The NRC asked licensees to evaluate the Class 1E electrical distribution system to determine if the operability of safety related equipment, including associated control circuitry or instrumentation, can be adversely affected by short term or long term degradation in the grid system voltage within the range where the offsite power is counted on to supply important equipment. The NRC also asked licensees to Define the normal operating range of your grid system voltage and the corresponding voltage values at the safety related buses.

In its 1976 request for information, the NRC asked licensees to define the voltage range over which the safety related components, and non-safety components, can operate continuously in the performance of their design function. The NRC also stated that The 1

At Millstone Unit the loss of power undervoltage relays are an integral part of the Engineered Safeguards Actuation System (ESAS). This relay function is commonly referred to the first level on Class 1E safety bus undervoltage protection and the sensing devices that implement the function are commonly referred to as loss of voltage relays (LVRs). At the time of the Millstone Unit 2 1976 degraded voltage event there was only one level of safety bus undervoltage protection.

E1-8

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS functional safety requirement of the undervoltage trip 2 is to detect the loss of offsite (preferred) power system voltage and initiate the necessary actions required to transfer safety related buses to the onsite power system.

Observations Regarding the Millstone Unit 2 Experience Based on a current review of this event, TVA noted the following insights regarding the 1976 Millstone Unit 2 degraded voltage event.

a. At the time of the 1976 Millstone event, it appears the licensee may not have had a clear understanding and supporting analysis of the impact of a plant trip on grid voltage and the minimum voltages required to support operation of plant safety equipment.

b. Due to the level of technology, there was a lack of capability to perform detailed load flow and voltage drop analyses of the onsite distribution systems and components with the same level of detail and fidelity that exists today.

c. There were no formal protocols in place between the licensee and the transmission system operator such as those now mandated by NERC Standard NUC-001.2.1 (Reference 27).

d. The transmission system operator likely did not have a real-time system state estimator to predict the impact of a plant trip on the plants switchyard voltage given the actual transmission system conditions since real time state estimator technology was not widely used at the time to predict nuclear plant switchyard voltage.

e. Raising bus undervoltage protection setpoints can have an adverse impact on plant safety as demonstrated by the Millstone event two weeks after raising the undervoltage relay setpoint.

TVA has addressed each of these insights items at WBN.

3.2 SAFETY BUS UNDERVOLTAGE PROTECTION General Design Criteria 17 found in Appendix A of 10 CFR 50 requires that provisions shall be included to minimize 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, the loss of power from the transmission network, or the loss of power from the onsite electric power supplies (Reference 6). In response to the 1976 Millstone event, licensees analyzed their AC power system to determine that the voltage levels at the safety-related buses were optimized for the full load and minimum load conditions that are expected throughout the anticipated range of voltage variations for the offsite power sources (Reference 7).

The safety function of nuclear plant Class 1E loss of power monitors, commonly referred to as the first level (or Level 1) undervoltage protection, is to detect the loss of voltage from the offsite (preferred) power supply and to initiate the necessary actions required to transfer the 2

The undervoltage trip referred to is the first level of safety bus undervoltage protection. At the time of this letter second level undervoltage (or degraded voltage) protection schemes did not exist.

E1-9

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS safety related buses to the onsite standby power supplies. In response to the 1976 Millstone event, the NRC issued Staff Positions Relative to the Emergency Power Systems for Operating Reactors, see Reference 7.

In 1977, the NRC staff concluded that the operating procedures and guidelines utilized by electric utilities and their interconnected cooperative organizations minimized the probability for degraded voltage conditions to occur. However, the NRC Staff Positions required licensees to provide a second level on safety bus undervoltage protection because sustained degradation of the offsite power systems voltage could result in the loss of capability of the redundant safety loads, their control circuitry and the associated electrical components required for performing safety functions (Reference 7).

In response to the NRC Staff Positions, licensees such as TVA installed additional protective relaying to address sustained degradation safety bus voltage. At WBN, TVA implemented a second level of undervoltage protection referred to as Degraded Voltage Relays (DVRs). As discussed further in Section 6 of this Enclosure, the WBN 6.9 kV safety bus DVRs are arranged in two out of three coincident logic with a time delay to trip. If the 6.9 kV safety bus voltage drops below the DVR trip setpoint and remains below the relays reset setpoint for period longer than the time delay the following automatic actions occur:

An alarm annunciates in the Main Control Room,

Supply breakers to 6.9 kV shutdown board trip open,

Load shedding from the 6.9 kV and 480 V shutdown boards,

480 V shutdown-board current-limiting reactor-bypass breaker closes, and

Diesel generator starts.

Degraded Voltage Relays (DVRs) which provide the second level (Level 2) undervoltage protection for safety buses, have three setpoints which should be based on an Analytical Limit (AL) 3 derived from the design basis analysis of the plants AC power system and the plants overall safety analysis as documented in the Updated Final Safety Analysis Report (UFSAR). The three DVR setpoints are the:

Pickup (or reset) voltage,

Dropout (or trip) voltage, and

Relay trip time delay.

The DVR pickup (or reset) voltage should be based on the minimum post reactor and turbine trip grid voltage required to actuate the safety loads successfully and reset the DVR.

TVA refer to this value as the Minimum Grid Voltage for Operable Offsite Sources because it is associated with the minimum transmission system voltage for Operable offsite sources.

The resultant post actuation safety bus voltage value is referred to as the DVR Pickup Voltage Analytical Limit. The DVR trip (or dropout) voltage should be based on the 3

The difference between the Analytical Limit (AL) and the relay setpoint and allowable value is the instrument uncertainty.

E1-10

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS minimum Class 1E bus voltage required for the safety loads to operate. This value is referred to as the DVR Dropout Voltage Analytical Limit. The DVR time delay should be set short enough to provide adequate protection for the safety related equipment and long enough to avoid inadvertent trips during starting transients including the automatic actuation of the engineered safety features (ESF) loads. Any additional appreciable load added to the Class 1E safety bus with the voltage at the DVR dropout (or trip) voltage setpoint will cause the relay to trip and separation of the Class 1E distribution system from the offsite power system. A more detailed discussion of the DVR voltage and time delay Analytical Limits, trip setpoints, and allowable values for WBN is provided in Section 8 of this report.

In 1981, the NRCs criteria for the Level 2 undervoltage protection scheme were incorporated into NRC Branch Technical Position PSB-1, Adequacy of Station Electric Distribution System Voltages (Reference 8).

3.3 ARKANSAS NUCLEAR ONE - 1978 On September 16, 1978, an unusual sequence of events involving the offsite to onsite power system occurred at Arkansas Nuclear One (ANO) Units 1 and 2; see NRC Information Notice 79 04 and NRC Generic Letter 79 36 (References 9 and 10, respectively). At the time of the event, ANO Unit 1 was operating at 100 percent power and Unit 2 was in hot standby, performing (pre-license) hot functional testing in preparation for initial criticality and power operations. The Unit 1 electrical loads were being supplied from the units main generator via the unit auxiliary transformer (UAT). The Unit 2 electrical loads were being fed from the offsite transmission system through Startup Transformer No. 3.

The Unit 1 reactor protection system initiated a reactor trip and the turbine-generator tripped concurrently. As a result, the Unit 1 electrical loads were transferred automatically to Startup Transformer No. 1, which is powered from the offsite transmission system.

At ANO, a single Auto-Transformer supplies power from the offsite transmission system to Startup Transformer No. 1 and Startup Transformer No. 3. The Auto-Transformer was sized to carry power for both units, but at the time of the event the protective relays were still adjusted for operation of Unit 1 only. As a result, when both units concurrently drew power from the Auto-Transformer these protection relays tripped and cut off power to Startup Transformer No. 1 and Startup Transformer No. 3. Consequently, Startup Transformer No. 2 became the only source of offsite power for both Units 1 and 2.

The onsite switching equipment automatically transferred the full auxiliary loads for both units to Startup Transformer No. 2. However, Startup Transformer No. 2 was not designed to carry full auxiliary loads for both units. Therefore, Startup Transformer No. 2 became overloaded and the voltage dropped on the station distribution system for offsite power. At Unit 2, eight seconds after the switch to Startup Transformer No. 2, the relays which operate to protect Engineered Safety Feature (ESF) equipment from low (degraded) voltage disconnected and therefore de-energized both Unit 2 ESF buses as designed.

Subsequent analysis indicated that in the event of a loss of coolant accident (LOCA) at Unit 1, the resultant system voltages supplied by Startup Transformer No. 1 may have been inadequate to support the non-safety related loads running and the starting of the required ESF loads. Furthermore, the resultant voltage drops would not actuate the safety bus undervoltage protection and transfer the safety loads to the emergency diesel generators.

E1-11

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS Following the September 16, 1978 event at ANO, the NRC issued Information Notice 79-04 (Reference 9). On August 8, 1979, the NRC issued Generic Letter 79-36 (Reference 10) which required licensees:

to determine analytically if, assuming all onsite sources of AC power are not available, the offsite power system and the onsite distribution system is of sufficient capacity and capability to automatically start as well as operate all required safety loads within their required voltage ratings in the event of (1) an anticipated transient (such as unit trip) or (2) an accident (such as a LOCA) regardless of other actions the electric power system is designed to automatically initiate and without the need for manual shedding of any electric loads.

Generic Letter 79-36 also addressed undervoltage protection of safety loads. Generic Letter 79 36 states that Protection of safety loads from undervoltage conditions must be designed to provide the required protection without causing voltages in excess of maximum voltage ratings of safety loads and without causing spurious separations of safety buses from offsite power.

Guidelines for the required analyses were provided in Enclosure 2 of Generic Letter 79-36.

With respect to appropriate grid voltage to use in the required analyses, Generic Letter 79-36 stated that the minimum expected value should be selected based on the least of the following:

a. The minimum steady-state voltage experienced at the connection to the offsite circuit;

b. The minimum voltage expected at the connection to the offsite circuit due to contingency plans which may result in reduced voltage from this grid; and

c. The minimum predicted voltage from grid stability analysis. (e.g., load flow studies).

For multi-unit stations, separate analysis were required for each unit assuming (1) an accident in the unit being analyzed and simultaneous shutdown of all other units at the station; or (2) an anticipated transient in the unit being analyzed (e.g., unit trip) and simultaneous shutdown of all other units at that station, whichever presents the largest load situation.

Observations Regarding the ANO Experience Based on a current review of this event, TVA noted the following insights regarding the ANO experience:

a. At the time of the September 1978 event, it appears that the design of the ANO offsite to onsite electric power system did not fully meet the General Design Criterion 17 of 10CFR50, Appendix A, in that during certain circumstances loss of one of the two offsite power circuits could also result in a loss of the other offsite to onsite circuit. As indicated in the event report, this condition was due to improper E1-12

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS calibration of relays and not an inherent deficiency in the offsite to onsite circuit design or the capacity and capability of the transmission system.

b. Tennessee Valley Authoritys interpretation of the ANO event is that at the time of the event the offsite power supply for ANO Unit 1 engineered safety feature (ESF) loads was deficient in that degraded voltage could have resulted in the unavailability of ESF equipment. However, at the time of the event, the second level of undervoltage protection (commonly referred to as degraded voltage protection) was not installed on the Unit 1 Class 1E safety buses.

The WBN electrical system design includes two levels of Class 1E undervoltage protection and TVA has established a strong analytical basis for the voltage and time delay setpoints for these protective relays. TVA also has fully addressed this operating experience for the dual unit operation of WBN.

3.4 CALLAWAY PLANT - 1999 On August 12, 1999, with the Callaway plant in Mode 3 (Hot Standby), the switchyard voltage supplied from the transmission system decreased below the minimum Operability limit established in station procedures. The plants Technical Specification action statement for both offsite sources In-Operable was entered and the transmission control area operator was contacted to initiate actions to increase switchyard voltage. The transmission control area operator's actions, combined with a decreasing system demand, restored switchyard voltages above the minimum operability limitation, and the Technical Specification action statement was exited. Both offsite power sources were In-Operable for approximately 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months , see Reference 11.

Due to high ambient temperatures, service territory loading was near peak levels. Even at these peak levels, it was predicted that switchyard voltage would remain above the established operability limits based on previous load flow analyses. During investigations of the August 12, 1999 event at Callaway, it was determined that large amounts of power were being transported across the grid on the day of the event. This power was being transported from northern utilities to the southern portion of the United States due to a generation shortfall in that area and regions high ambient temperatures in the South and lower Midwest, and cool temperatures in the North. The magnitude of the power being wheeled across the grid had not been previously observed and was far in excess of typical levels.

The deregulated wholesale power market contributed to the higher grid power flows that occurred. Since load flow analyses had not analyzed this level of system loading, the minimum voltage previously established was not valid for verifying that the offsite source would have adequate capability to supply station loads during a design basis accident.

At the time of the August 12, 1999 event at Callaway, switchyard voltage indications were not adequate for determining Operability of the offsite source in the event of a plant trip offline. Similar grid loading conditions were present at Callaway on August 10, 1999.

However, low switchyard voltages were not observed at that time since Callaway generation was locally supporting grid voltage. Consequently, the capability of the offsite source could not be readily verified when the unit was in operation. Offsite source capability is normally confirmed by an analysis that considers the anticipated loading conditions on the grid. With grid loading above these previously analyzed values, the plant was placed in a condition E1-13

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS which was outside of its design basis analysis for verifying the offsite source would have adequate capacity to supply station loads during a design basis accident.

Due to bus loading conditions at the time of the Callaway event, adequate voltage was maintained on the Class 1E power system such that the safety related components were operated within design parameters. If additional loads would have been in operation at the time, the Class 1E power system voltages may have decreased below the setpoint of the second level undervoltage relays. Had this occurred, the DVRs would have initiated a bus transfer to the emergency diesel generators.

Observations Regarding the Callaway Experience Based on a current review of this event, TVA noted the following insights regarding the Callaway event:

a. Our interpretation of the event is that there was no real-time, system state estimator in use to predict the impact of the Callaway plant trip on the plants switchyard voltage given the actual transmission system conditions.

b. It appears that there was no process in place to periodically review the grid load flow analysis prior to each peak loading season to ensure that anticipated changes in transmission system operation are incorporated into the Callaways design basis AC system analysis.

c. It also appears that if the degraded voltage relay dropout (or trip) setpoint was raised to a higher value at the time, the Callaway plant would have been subjected to an inadvertent loss of offsite power.

Enforceable NERC transmission planning and reliability standards established following the 2003 Blackout (see Section3.5), help ensure that the impact of worst case grid power flows are understood on the TVA transmission system. TVA uses state estimators and formal written agreements between WBN and TVA transmission to ensure the necessary Nuclear Plant Interface Requirements (NPIRs) are met. As discussed in Section 8 of this Enclosure, TVA has established a strong analytical basis for the voltage and time delay setpoints for WBN Class 1E, undervoltage protective relays and has a process in place to periodically review the grid load flow.

3.5 2003 BLACKOUT AND ELECTRIC RELIABILITY ORGANIZATION (ERO)

The way that TVA transmission system is operated today is significantly more reliable than the manner in which transmission systems were operated at the time of the 1976 Millstone event, the 1978 ANO event, and 1999 event at Callaway. For example, a real time transmission system state estimator is used to predict the post-trip switchyard voltage at WBN and written protocol which define Nuclear Plant Interface Requirements (NPIRs) are in place between WBN and the transmission system operator. This section describes the role of the North American Electric Reliability Corporation (NERC) and the Federal Energy Regulatory Commission (FERC) play in maintaining a seamless environment for satisfying nuclear plant offsite sources requirements.

On August 14, 2003, large portions of the Midwest and Northeast United States and Ontario, Canada, experienced an electric power blackout. The outage affected an area with E1-14

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS an estimated 50 million people and 61,800 megawatts (MW) of electric load in the states of Ohio, Michigan, Pennsylvania, New York, Vermont, Massachusetts, Connecticut, New Jersey and the Canadian province of Ontario. During the August 2003 blackout, nine U.S.

nuclear power plants experienced rapid shutdowns (reactor trips) as a consequence of the power outage. Seven nuclear power plants in Canada operating at high power levels at the time of the event also experienced rapid shutdowns. Four other Canadian nuclear plants automatically disconnected from the grid due to the electrical transient but were able to continue operating at a reduced power level and were available to supply power to the grid as it was restored by the transmission system operators. Six nuclear plants in the United States and one in Canada experienced significant electrical disturbances but were able to continue generating electricity. Many non-nuclear generating plants in both countries also tripped during the event. Numerous other nuclear plants observed disturbances on the electrical grid but continued to generate electrical power without interruption. See Section 1 and 8 of Reference 12.

In light of the August 2003 Northeast Blackout, the U.S. Congress added Section 215 to the Federal Power Act as part of the Energy Policy Act of 2005 to create a regime for mandatory reliability standards, to be developed and enforced in the U.S. by an Electric Reliability Organization (ERO). The Policy Act mandates that the ERO be certified by, and operate under the oversight of, the Federal Energy Regulatory Commission (FERC). See Reference 13.

In July 2006, the North American Electric Reliability Corporation (NERC) was certified as the ERO. Thereafter, NERC established delegation agreements with eight Regional Entities. WBN and TVAs transmission service territory operates within and is governed by the SERC Reliability Corporation (SERC), which is a nonprofit corporation responsible for promoting and improving the reliability, adequacy, and critical infrastructure of the bulk power supply systems in all or portions of sixteen central and southeastern states.

With the establishment of NERC operating as the ERO, having the legal authority to issue and enforce mandatory reliability standards for the bulk electric system, there is a seamless environment for satisfying nuclear plant offsite sources requirements. This continuity was further re-enforced by the Interpretive Order issued by FERC on February 16, 2006 (Reference 13).

The Interpretive Order clarified that Transmission Providers may communicate with affiliated nuclear power plants regarding certain matters related to the safety and reliability of the transmission system on the nuclear power plants, in order to comply with requirements of the Nuclear Regulatory Commission. Further, the FERC regulations do not prohibit a Transmission Provider and its affiliated nuclear power plant from engaging in necessary communications related to the safety and reliability of the transmission system or the nuclear power plant, including information relating to the loss of or potential loss of transmission lines that provide off-site power to the nuclear power plant regardless of ownership of those lines.

4 RELEVANT WBN LICENSING BASIS DOCUMENTS AND SUBMITTALS provides a summary of relevant history of licensing basis documents and submittals associated with the GDC 17 offsite power source capacity and capability and the Class 1E safety bus undervoltage protection schemes for Watts Bar Nuclear Plant. The design and licensing bases for WBN Unit 2 are essentially the same as what presently E1-15

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS exists for Unit 1, which received its full power operating license (OL) in 1996. Design and licensing bases compliance of the WBN offsite sources and Class 1E safety bus undervoltage protection was reviewed by the NRC staff as part of the licensing process for Unit 1 and found to be acceptable.

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ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS PART TWO - WBN OFFSITE SOURCES AND BUS UNDERVOLTAGE PROTECTION 5 GLOSSARY OF DEFINED TERMS This section defines specific terms that are used in Part Two of this document. These terms are italicized throughout the document as an aid to the reader.

Adequate Offsite Power - The existence of power from the transmission system of sufficient voltage and capacity to power the safety-related loads under defined plant load conditions.

Sufficient voltage is related to the capability to reset the degraded voltage relay post actuation of the loading required for safe shutdown or to mitigate the consequences of a design basis event and thereby allow the safety buses to remain powered from the offsite source.

Adapted from NRC Generic Letter 2006-02, Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power, dated February 1, 2006.

(Reference 15)

Analytical Limit (AL) - A limit of a measured or calculated variable established by the analysis to ensure that a safety limit is not exceeded.

ANSI/ISA Standard 67.04.01-2000, Setpoints for Nuclear Safety-Related Instrumentation (Reference 16).

Degraded Safety Bus Voltage - Class 1E, 6.9 kV safety bus voltage that is below the minimum value required to ensure adequate voltage to operate the running loads associated with maximum loading conditions, and start individual motor loads and stroke individual NRC Generic Letter 89-10 motor operated valves.

Degraded Voltage Relays (DVRs) - Degraded Voltage Relays (DVRs) provide a second level (Level 2) undervoltage protection with time delay to protect the Class 1E equipment. The DVRs monitor the Class 1E, 6.9 kV safety bus voltages and separate the Class 1E AC distribution system from the offsite power system after the trip (or dropout) setpoint is reached for the established time delay.

NRC Branch Technical Position PSB-1, Adequacy of Station Electric Distribution System Voltages, Revision 0, dated July 1981.

DVR Dropout Voltage Analytical Limit - The Analytical Limit associated with the DVR dropout (or trip) value is based on the minimum analyzed Class 1E, 6.9 kV bus voltage that provides adequate operating voltage for (a) powering the safety related loads required to be operating under the maximum loading conditions (running loads), and (b) starting individual motor loads and stroking individual NRC Generic Letter 89-10 motor operated valves.

DVR Pickup Voltage Analytical Limit - The Analytical Limit associated with the DVR pickup (reset) value is the maximum Class 1E, 6.9 kV bus voltage permissible to prevent the premature separation from the offsite supply due to improper operation of the degraded voltage relays during temporary voltage dips (caused by bus transfers, motor starts, engineered safety features actuation).

E1-17

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS DVR Minimum Time Delay Analytical Limit - The Analytical Limit associated with the DVR minimum time delay value required to prevent the premature separation from the offsite supply due to improper (or undesired) operation of the Degraded Voltage Relays during temporary voltage dips (caused by bus transfers, motor starts, engineered safety features actuation).

DVR Maximum Time Delay Analytical Limit - The Analytical Limit associated with the DVR maximum time delay value permissible to (a) ensure the Class 1E power system can perform its intended safety function within the timeframe of the USFAR Chapter 15 safety analysis assumptions if the offsite source cannot perform its intended safety function and (b) adequately protect the Class 1E loads from damage and inadvertent trip in the event of sustained degraded bus voltage conditions.

Loss of Voltage Relays (LVRs) - Loss of Voltage Relays (LVRs) provide the first (or lower) level (Level 1) undervoltage protection for the Class 1E equipment. The safety function of the LVRs is to detect a loss of offsite power (LOOP), isolate the Class 1E buses from the offsite source, and start the onsite standby AC power sources. The LVRs monitor the Class 1E, 6.9 kV safety bus voltages.

NRC Branch Technical Position PSB-1, Adequacy of Station Electric Distribution System Voltages, Revision 0, dated July 1981.

LVR Dropout Voltage Analytical Limit - The Analytical Limit associated with the LVR dropout (or trip) value is based on the minimum permissible Class 1E, 6.9 kV safety bus voltage to prevent motor stall or and inadvertent trip of the Class 1E loads.

LVR Minimum Time Delay Analytical Limit - The Analytical Limit associated with the LVR minimum time delay value required to prevent premature separation from the offsite supply due to short duration, momentary bus voltage dips due to switching transients, electrical faults (which are cleared), and other transient events.

LVR Maximum Time Delay Analytical Limit - The Analytical Limit associated with the LVR maximum time delay value permissible to ensure the Class 1E loads are not damaged due to low bus voltage.

Minimum Grid Voltage for Operable Offsite Sources - The Minimum Grid Voltage for Operable Offsite Sources is the minimum 161 kV bus voltage at Watts Bar Hydro Station required for Adequate Offsite Power for Watts Bar Nuclear Plant such that the WBN offsite sources can perform their intended GDC 17 safety functions. Adequate Offsite Power requires the offsite source to have sufficient capacity and capability to perform its intended function (which includes the ability to block start the accident loads). For WBN, the acceptance criteria for the offsite source voltage is established by analysis in terms of (1) a maximum voltage drop from pre-event to post-event (e.g., design basis accident at WBN) and (2) a minimum post-event grid voltage at the Watts Bar Hydro Station. These criteria are based not only on current grid conditions but also the predicted grid conditions assuming an N-1 Contingency.

Minimum Safety Bus Voltage - The minimum Class 1E, 6.9 kV safety bus voltage that corresponds to the DVR Dropout Voltage Analytical Limit.

E1-18

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS N-1 Contingency - The unexpected failure or outage of a single system component, such as a generator, transmission line, circuit breaker, switch or other electrical element. An N-1 Contingency would include the trip of the nuclear unit, the trip of the largest generator on the system, trip of a transmission path or loss of a power transformer.

NRC Generic Letter 2006-02, Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power, dated February 1, 2006. (Reference 15)

Nuclear Plant Interface Requirements (NPIRs) - The requirements based on nuclear plant licensing requirement and Bulk Electric System requirements that have been mutually agreed to by the Nuclear Plant Generator Operator and the applicable Transmission Entities.

See NERC Glossary of Terms at http://www.nerc.com/pa/stand/glossary of terms/glossary_of_terms.pdf Operable - A system, subsystem, train, component, or device is Operable or has Operability when it is capable of performing its specified safety function(s) and when all necessary attendant instrumentation, controls, normal or emergency electrical power, cooling and seal water, lubrication and other auxiliary equipment that are required for the system, subsystem, train, component, or device to perform its function(s) are also capable of performing their related support function(s).

NRC Inspection Manual Part 9900, Operability Determinations & Functionality Assessments for Resolution of Degraded or Nonconforming Conditions Adverse to Quality or Safety; Attachment to NRC RIS 2005-20 dated September 26, 2005.

(Reference 17)

E1-19

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS 6 DESCRIPTION OF WBN OFFSITE SOURCES AND CLASS 1E BUS UNDERVOLTAGE PROTECTION 6.1 PREFERRED POWER SUPPLY The offsite power system is referred to as the preferred power supply - see Section 8.2 of the NRCs Standard Review Plan (Reference 18) and IEEE Standard 308 (Reference 19).

The Standard Format and Content of Safety Analysis Reports for Nuclear Power Plants issued by the Atomic Energy Commission in February 1972 also refers to the two circuits required by GDC 17 to supply power for safety loads from the transmission network system as the preferred power supply (Reference 20). However, the preferred power supply is not a Class 1E system.

The Watts Bar Nuclear Plant (WBN) is located approximately 50 miles northeast of Chattanooga, Tennessee, on the west bank of the Tennessee River. The plant is connected into a strong 500 kV transmission grid via a 500 kV switchyard with a double breaker -

double bus configuration. The main generator for each reactor unit and each of five 500 kV transmission lines can be connected to either or both buses through a single 500 kV breaker.

The preferred power supply for WBN is supplied from the TVAs Watts Bar Hydro Plant 161 kV Switchyard via two radial transmission lines. The two overhead transmission lines are approximately 1.5 miles long and are located entirely on TVA property. The Watts Bar Hydro 161 kV Switchyard is interconnected with the TVA power system through six 161 kV transmission lines and five hydro-electric generators. The two 161 kV offsite power circuits to WBN are supported on separate towers, and the separation between the two transmission lines is sufficient to ensure that the failure of any tower in one circuit will not endanger the other circuit.

Figure 6-1 is a simplified one line diagram of WBN offsite to onsite power circuits. In the current licensing basis (CLB) for WBN, each of the 161 kV offsite power circuits connects to two 161 kV-6.9 kV common station service transformers (CSST) at WBN. One transmission line feeds WBN CSST A and D and the other transmission line feeds WBN CSST B and C.

Offsite power to the onsite Class 1E power system is normally supplied from CSSTs C and D. Common station service transformers CSST A and B normally supply power to the 6.9 kV non-essential common, unit, and RCP buses. Either CSST A or B can be aligned to supply offsite power to two 6.9 kV shutdown boards in place of CSST C or D.

Common station service transformers CSST C and CSST D and combinations of CSSTs A, B, C, and D are capable of powering shutdown loads during anticipated operational occurrences and design basis events, including:

a. Dual-unit trip as a result of an abnormal operational occurrence;

b. Accident in one unit and concurrent shutdown of the second unit; and

c. Accident in one unit and spurious Engineered Safety Feature (ESF) actuation in the other Unit.

For completeness, it is noted that in a letter to the NRC dated August 1, 2013 (Reference 21), TVA submitted a license amendment request (LAR) for WBN Unit 1. The license amendment credits upgrades made to CSST A and B to provide two new sources of E1-20

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS preferred Class 1E power supply feeds in addition to the two normal Class 1E power supply feeds. This proposed request achieves licensing basis commonality for the current Operating Unit 1 license with respect to those approved elements of the Unit 2 application as docketed in NUREG-0847, Supplements 22 and 24 (Reference 21). The August 1, 2013 LAR is under review by the NRC staff.

6.2 Class 1E Safety Bus Undervoltage Protection The WBN Class 1E, 6.9kV shutdown boards are provided with two levels of undervoltage protection. Loss of Voltage Relays (LVRs) provide the first level of protection and Degraded Voltage Relays (DVRs) provide a second level of protection for the Class 1E loads.

Consistent with NRC Branch Technical Position PSB-1, the primary function of the LVRs is to detect loss of offsite power at the Class 1E buses and disconnect the safety-related buses in the event a loss of offsite power (LOOP). The loss of voltage protection consists of three sets of relays and is described in Section 8.2.2 of the WBN FSAR. The first set of these relays (27LVA, LVB, LVC) has a voltage setpoint of 87% of 6.9kV (nominal, decreasing).

These relays are arranged in a two-out-of-three coincidence logic to initiate a time delay that is set at 0.75 seconds. At the end of this time delay, if the voltage is still low, a trip of the 6.9 kV shutdown-board supply breaker will occur. Once the supply breakers have been opened, a second set of induction disk-type undervoltage relays, 27D, which has a voltage setpoint of 70% of 6.9kV (nominal, decreasing) and an internal time delay of 0.5 seconds (nominal) at zero volts, will start the diesel generator. A third set of induction disk-type undervoltage relays, 27S, which has a voltage setpoint of 70% of 6.9kV (nominal, decreasing) and an internal time delay of 3 seconds (nominal) at zero volts, will initiate load shedding of the loads on the 6.9 kV shutdown board, selected loads on the 480V shutdown board, and closure of the 480 V shutdown-board current-limiting reactor bypass breaker.

The primary function of the DVRs is to protect Class 1E equipment from sustained low voltage conditions on the safety-related buses. When actuated (dropped out or tripped) and timed out, the LVRs and DVRs initiate a transfer of the Class 1E safety buses from the normal offsite power supply to the onsite standby power supplies (i.e., diesel generators).

The DVRs have a nominal voltage setpoint of 96 percent of 6.9 kV (nominal, decreasing) and are arranged in a two-out-of-three coincidence logic, with a 10-second (nominal) time delay.

If the 6.9 kV bus voltage is still below the relays reset value at the end of 10 seconds, the following automatic actions occur:

An alarm annunciates in the Main Control Room,

Supply breakers to 6.9 kV shutdown board trip open,

Load shedding from the 6.9 kV and 480 V shutdown boards,

480 V shutdown-board current-limiting reactor-bypass breaker closes, and

Diesel generator starts.

The Analytical Limits established for the WBN DVRs and LVRs are summarized in Table .

From these Analytical Limits, the plants Technical Specification trip set points and allowable values are calculated by accounting for the tolerances and uncertainties associated with measurement errors, drift errors, and other errors. This is done in E1-21

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS accordance with TVAs setpoint methodology documented Technical Instruction TI-28 (Reference 25). TVA Calculation WBPE21192022001 (Reference 26) and TVA Calculation TDR SYS.211-LV1 (Reference 38) determines the applicable instrument loop uncertainties and establishes setpoints and allowable values for the WBN Class 1E, safety bus DVR and LVR undervoltage protection schemes.

Table 6-1. Class 1E, 6.9 kV Safety Bus DVR and LVR Analytical Limits Degraded Voltage Relays (DVRs)

DVR Pickup (Reset) Voltage Analytical Limit 6681 V (0.9683 pu)

DVR Dropout (Trip) Voltage Analytical Limit 6555 V (0.9500 pu)

DVR Minimum Time Delay Analytical Limit 8.5 sec DVR Maximum Time Delay Analytical Limit 11.5 sec Loss of Voltage Relays (LVRs)

LVR Pickup (Reset) Voltage Analytical Limit 6072 V (0.8800 pu)

LVR Dropout (Trip) Voltage Analytical Limit 5934 V (0.8600 pu)

LVR Minimum Time Delay Analytical Limit 0.4 sec LVR Maximum Time Delay Analytical Limit 1.14 sec E1-22

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS 161 kV WBN HYDRO PLANT BUS WBN 500 kV SWITCHYARD 500 kV BUS 161 kV BUS 500 kV BUS 161 kV BUS GENERATOR 2 CSST C CSST D GENERATOR 1 COM STA COM STA MAIN 2 SWGR C SWGR D MAIN 1 CSST B CSST A USST 2B USST 2A USST 1B USST 1A NON-ESSENTIAL 6.9 kV BUSES NON-ESSENTIAL 6.9 kV BUSES DG DG DG DG 6.9 kV SD BD 2B-B 6.9 kV SD BD 2A-A 6.9 kV SD BD 1B-B 6.9 kV SD BD 1A-A Figure 6-1: Simplified One Line Diagram of Watts Bar Offsite to Onsite Power Circuits E1-23

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS 7 WBN DESIGN AND LICENSING BASES FOR OFFSITE SOURCES The General Design Criteria (GDC) contained in Appendix A of 10 CFR 50 establish minimum requirements for the principal design criteria for water-cooled nuclear power plants. The following GDC, regulatory documents, and industry standards establish specific design requirements applicable to the capacity and capability of the offsite power sources for WBN:

GDC 17, Electric Power Systems;

GDC-18, Inspection and Testing of Electric Power Systems;

GDC-5, Sharing of Structures, Systems, and Components;

IEEE Standard 308-1971, Criteria for Class 1E Electric Systems for Nuclear Power Generating Stations; and

Regulatory Guide 1.32 (AEC Safety Guide 32), Revision 0.

Regarding the WBN offsite power sources, this document addresses GDC 17 offsite source capacity and capability compliance.

7.1 GENERAL DESIGN CRITERION 17 GDC 17 defines requirements for the onsite electric power system and the offsite electric power system, and is part of the design and licensing bases for WBN. The NRC staff reviewed the GDC 17 compliance of the WBN offsite and onsite power systems as part of the licensing review of WBN Unit 1, and is in the process of completing its technical review of GDC 17 compliance for Unit 2.

GDC 17 requires the onsite and offsite power system (assuming the other system is not functioning) to have sufficient capacity and capability to assure:

a. Specified fuel design limits and design conditions of reactor coolant pressure boundary are not exceeded as a result of anticipated operational occurrences, and the

b. Core is cooled and containment integrity and other vital functions are maintained in event of postulated accidents.

Anticipated Operational Occurrences (AOOs) are conditions of normal operation expected to occur one or more times during life of plant. Loss of Offsite Power (LOOP) events are AOOs per the definitions section of 10CFR50, Appendix A. Postulated accidents include those design bases events (moderate frequency, in-frequent, and low probability design basis limiting events) addressed in Chapter 15 of the FSAR. contains the entire text of GDC 17. Key elements of GDC 17 requirements, as well as a brief overview of WBN design features related to those requirements, are described below.

a. Electric power from the transmission system to onsite distribution system to be supplied by two physically independent offsite circuits.

The WBN preferred power supply consists of two independent 161 kV overhead lines that are located entirely on TVA property. The two circuits are supported on separate sets of E1-24

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS transmission towers. The separation between the two lines is sufficient to ensure that the failure of any tower in one circuit will not endanger the other offsite circuit.

b. Each offsite circuit shall be designed and located so as to minimize to the extent practical the likelihood of their simultaneous failure under operating and postulated accident and environmental conditions. A switchyard common to both circuits is acceptable.

The bus arrangement at Watts Bar Hydro Plant 161 kV switchyard is designed so that the loss of any one of the four main bus sections will not cause loss of power to either of the two preferred power source lines to WBN. The transmission line structures for 161 kV lines are designed to meet or exceed load requirements specified in the National Bureau of Standards Handbook No. 81 (National Electric Safety Code Part 2). Designing to these requirements ensures the adequacy of lines for wind and heavy icing conditions in excess of those that would be expected to occur in this area. The phase conductor and shield wire design tensions are selected to avoid vibration problems. The 161 kV transmission lines provide power to WBN common station service transformers CSSTs A and D and CSSTs B and C. The lines are routed to the east and north of the nuclear plant transformer yard so as to minimize the likelihood of their simultaneous failure.

c. GDC 17 does not require the independent circuits to be installed on separate rights of way (ROW).

Consistent with GDC-17, some of the transmission lines into the Watts Bar Hydro (WBH) 161 kV switchyard are installed on a common ROW. However, the two offsite power circuits from the WBH 161 kV switchyard to the WBN CSSTs are supported on separate sets of transmission structures and the separation between the two lines is sufficient to ensure that the failure of any structure in one circuit will not endanger the other offsite circuit.

d. Each offsite circuit shall be available in sufficient time following a loss of all onsite alternating current power supplies and the other offsite electric power circuit, to assure that specified acceptable fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded. GDC 17 requires one of the offsite circuits to be available within a few seconds following a loss-of-coolant accident to assure that core cooling, containment integrity, and other vital safety functions are maintained. This is typically referred to as the immediately available source.

Two immediately available offsite circuits are provided to the WBN Class 1E safety buses via common station service transformers CSSTs C and D. Consistent with the Bases document for the Westinghouse Standardized Technical Specifications, each WBN offsite to onsite transmission circuit is capable of maintaining rated frequency and voltage, and accepting required loads during an accident, while connected to the ESF buses. See Section B3.8.1 of NUREG 1431 (Reference 21).

e. The last paragraph of GDC 17 states that Provisions shall be included to minimize 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, the loss of power from the transmission network, or the loss of power from the onsite electric power supplies.

Physical separation of lines, primary and backup protection systems, and a strong transmission grid minimize the probability of simultaneous failures of offsite power sources. TVAs offsite power analysis for WBN is based on consideration of an N-1 Contingency which includes trip of the nuclear unit and postulated trips of the largest generator on the system, a transmission E1-25

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS path, or loss of a power transformer. TVAs offsite power analysis for WBN also considers the complete loss of the onsite power supplies.

GDC 17 defines design requirements. It does not specify operating requirements or stipulate operational restrictions on the loss of offsite power sources. These are derived from analysis of the plants AC power systems, which are described in Section 7.2 of this document. One of the underlying purposes of GDC 17 is that a failure of the offsite source (including degraded voltage) should not cause a failure of the onsite source to perform its safety function. TVAs design complies with this purpose.

With respect to GDC 17, the regulatory position of Regulatory Guide 1.32 (Reference 22) states Criterion 17 delineates the design requirements regarding availability of power from the transmission network. Accordingly, a preferred design would include two immediate access circuits from the transmission network. An acceptable design would substitute a delayed access circuit for one of the immediate access circuits provided that availability of the delayed access circuit conforms to Criterion 17. The WBN design provides two immediate access offsite to onsite circuits from the transmission network.

7.2 ANALYSIS OF WBN OFFSITE SOURCES As described in Section 8.2.2 of the WBN UFSAR, steady-state and transient stability studies of the transmission system demonstrate that the offsite power sources remain intact and are reliable sources to supply the onsite electric power system for a:

1. Safety Injection (SI) actuation in a WBN nuclear unit with an electrical fault in the generator step-up transformer, or

2. SI actuation in a WBN nuclear unit and either the loss of Sequoyah Nuclear Unit 2, the loss of the largest load on the grid (Bowater 161-kV substation), or the loss of the most critical transmission line.

A comprehensive set of analyses of the WBN AC power distribution system and associated equipment loads was performed to demonstrate that the offsite power system can perform its intended GDC 17 safety functions for assuming the onsite standby power supplies are not functioning. Specifically, the analysis demonstrates that the offsite power sources have sufficient capacity and capability to assure that (1) specified acceptable fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded as a result of anticipated operational occurrences and (2) the core is cooled and containment integrity and other vital functions are maintained in the event of postulated accidents. The analyses performed, which are described below, provide the technical basis for the Analytical Limits used to determine the Operability of offsite power sources.

Calculation EDQ00099920070002 (Reference 24) documents a comprehensive analyses of the WBN 6.9 kV and 480 V AC power distribution system and associated equipment loads for dual unit operation. The calculation evaluates the system performance and resultant equipment voltages when powered from the 161 kV offsite preferred power supply and the 500 kV system. The AC system analyses were performed for the various transformer, bus, and circuit breaker alignments and design basis events, including the 6.9 kV Shutdown Boards aligned to a single CSST (C or D), the 6.9 kV Shutdown Boards aligned to CSST A or B, LOCA in one unit with orderly shutdown of the other, and Safety Injection actuation Phase A or Safety Injection actuation Phase B. The analysis considers the operation of the safety-related electrical distribution system under accident conditions (accident initiated block starting) with the offsite power supply at the Minimum Grid Voltage for Operable Offsite Sources, which bounds to a minimum expected grid voltage and capacity for an N-1 Contingency.

E1-26

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS The scope of TVA Calculation EDQ00099920070002 (Reference 23) includes the Common Station Service Transformers (CSSTs) A, B, C and D, Unit Station Service Transformers (USSTs) 1 A, 1 B, 2A, and 2B, 6.9kV Reactor Coolant Pump (RCP) Boards, 6.9kV Start Buses, 6.9kV Common Station Switchgear C and D, 6.9kV Shutdown Boards, 6.9kV Unit Boards, 6.9kV Common Boards, and the downstream 6900-480V transformers, 480V distribution boards and all interconnections. The analysis assumes that the:

Equipment started by an automatic Safety Injection actuation is started at the same time (block start) unless the loads control circuitry has a sequential time delay,

Equipment that is tripped off by a Safety Injection actuation trips, and

Loads that could be operating immediately after the Safety Injection, whether safety-related or not, are running.

The Class 1E, 6.9 kV Shutdown Boards have the capability to be connected to either CSST C or CSST D or split between them. These transformers have fast-acting automatic load tap changers (LTCs) installed on the secondary windings to regulate the Shutdown Boards voltage for variations in the grid voltage, bus loading or both. Common station service transformers CSST A and CSST B were retrofitted with fast-acting automatic LTCs. Consequently, the adequacy of CSST A and CSST B for use as offsite power source for Class 1E, 6.9 kV safety buses is evaluated (see Section 6). Only one 6.9 kV ESF train (6.9 kV Shutdown Boards 1A-A and 2A-A or 6.9 kV Shutdown Boards 1B-B and 2B-B) may to be transferred to CSSTs A or B at any given time.

The analysis documented in TVA Calculation EDQ00099920070002 considers the maximum expected loading conditions associated with a design basis accident in one reactor unit and the other reactor unit in a simultaneous orderly shutdown. Normal operation loads are considered for the non accident unit under shutdown and these loads are considered to envelop the loads for orderly shutdown of the unit.

The residual heat removal (RHR) pump for the non-accident unit is not considered since if it is only required for reactor cool down about 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months later.

The offsite source analysis indicates that although the safety-related bus voltage drops below the DVR Dropout Voltage Analytical Limit (6555 V) during the accident (block-start) transient, it recovers above the DVR Pickup Voltage Analytical Limit (6681 V) before the DVR Minimum Time Delay Analytical Limit (8.5 seconds) is exceeded. The upper bound for the reset setting of the DVR was established using the tightest possible tolerance between the nominal dropout and reset set points. The Analytical Limits for DVR dropout and reset are equal to the nominal set points plus all tolerances from measurement errors, drift errors, and other errors in accordance with TVA setpoint methodology documented in Technical Instruction TI-28 (Reference 25).

Some difference (a dead band) between the DVR dropout and reset is necessary for reliable and predictable operation of the relay. With the relay set with a minimum deadband, raising the relay dropout to a higher voltage value requires a corresponding increase in the reset voltage, i.e., the two setpoints are coupled together. Setting the dead band as tight as possible provides a greater degree of operation margin and increases plant safety by reducing the potential for premature separation from the offsite supply during transient loading conditions which is consistent with regulatory guidance.

7.3 NUCLEAR PLANT INTERFACE REQUIREMENTS (NPIRS)

North American Electric Reliability Corporation (NERC) Standard NUC-001-2.1 (Reference 26) is a mandatory standard that requires coordination between Nuclear Plant Generator Operators and Transmission Entities for the purpose of ensuring safe nuclear plant operation. Each Nuclear Plant E1-27

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS Generator Operator and the applicable Transmission Entities are required under NERC/FERC regulation to have in effect one or more Agreements that include mutually agreed to Nuclear Plant Interface Requirements (NPIRs) and document how the Nuclear Plant Generator Operator and the applicable Transmission Entities shall address and implement these NPIRs.

7.4 TRANSMISSION SYSTEM OPERATOR AND STATE ESTIMATOR Transmission and Power Supply (TPS) is a TVA owned and operated Transmission System Operating entity, which provides the interconnection of WBN to the bulk electric system. TVA Intergroup Agreement TVA-SPP-10.010 (Reference 27) is the business agreement that establish the roles, responsibilities, accountability, authority, and the relationship between TVA Nuclear Power Group (NPG) and TPS. This TVA Intergroup Agreement defines how and when grid status, and changes to grid status, is communicated between TPS and WBN.

Historically, TVA issued transmission system operating guides that (1) identify key parameters required to assure offsite power adequacy, (2) identify when grid conditions are outside the bounds of the analysis, and (3) direct prompt notification to the affected TVA nuclear site when offsite power adequacy cannot be assured. In accordance with the transmission system operating guidelines, TPS performed bounding analyses using power system analysis software tools. The bounding analyses evaluated the ability of the grid to provide Adequate Offsite Power under a range of conditions and contingencies. The analyses identified a set of conditions and parameters (e.g. lines and transformers in service, system loading, and voltage levels etc.) that determine if and when offsite power is adequate. Today, in order to comply with NERC standards, such as NUC-001-2.1 (Reference 27) and FAC-011-02 (Reference 29),

operating guides and/or a real-time state estimator are used to determine the ability of the transmission system to provide adequate voltage support for the actual grid conditions in effect at any time.

Power system state estimators are real time data processing and analysis algorithms that utilize available power system measurements (e.g., voltage, current, real and reactive power) obtained from various parts of a large integrated power system to determine the actual state (i.e., the operating conditions) of the system as a whole. The system state is determined based on the laws of physics and a model of the system. The current system state can be further used to analyze the impact of certain what if scenarios such as the trip of the nuclear unit, or the loss of a transmission line, a large loss of load, or a power transformer.

As required by the TVA Intergroup Agreement (TVA-SPP-10.010), planning and operating Transmission System Studies (TSS) are performed by TPS for each nuclear site for the purpose of determining the conditions under which the transmission system can provide Adequate Offsite Power for the safe shutdown of the nuclear plant per the requirements of GDC 17. NPG and TPS periodically review the TSS assumptions to ensure they remain valid following modifications to the plant and to the grid.

Modifications/changes either to the plant or transmission system may require revisions to the planning and/or operating TSSs and/or FSAR.

A Supervisory Control and Data Acquisition (SCADA) system gathers the status of grid components and quantities every few seconds. The system status information feed into a state estimator program is used to run the RTNET Real Time Contingency Analysis software. The state estimator runs a calculation every two minutes and has four redundant servers. If real time functionality fails, offsite power is qualified using archived cases and offline software. The reliability monitor alarms if system conditions indicate offsite power is disqualified. The alarms trigger TPS recognition of problem conditions so the TSO operator can recommend actions to correct the cause of the problem. The state estimator at TVA has been in place since September 2013, and since that time, no valid alarms have occurred at WBN (Reference 30).

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ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS The state estimator model is updated at least quarterly. Additionally, the interface requirements within Transmission System Reliability Standard Operating Procedure TRO-EA-SOP-30.406 (Reference 31) state when the TPS is to notify WBN when the 161 kV system is disqualified as defined by a minimum grid voltage and maximum voltage drop. The state estimator model depicting the offsite source in the various offsite configurations to WBN during a design bases event and normal operating modes for the 161 kV system is also referenced in TRO-EA-SOP-30.406.

Tennessee Valley Authority Transmission System Reliability Standard Operating Procedure TRO-EA-SOP-30.405 (Reference 32) documents the procedures necessary to operate the electric system to meet the Nuclear Plant Interface Requirements (NPIRS) of the TVA-owned nuclear units, including WBN. If Adequate Offsite Power cannot be assured and the condition cannot be corrected in 15 minutes, then TPS promptly notifies the WBN site operations shift management. This condition may be due to either actual system conditions or grid parameter(s) being outside the bounds of the analysis, or loss of relevant system monitoring capability. Additionally, TPS issues System Alerts that are used to warn of unusual, impending emergency, and emergency conditions pertaining to power supply adequacy, transmission system integrity, grid reliability, and weather. However, these alerts do not necessarily affect the Operability of the nuclear power plants offsite power supply.

7.5 APPLICABLE PLANT PROCEDURES WBN Technical Instruction TI-12.15 (Reference 33) directs operator actions to be taken when notified the 161kV power grid is unable to provide qualified offsite power to the Watts Bar Nuclear Plant (i.e. off site power requirements cannot be met for either incoming Watts Bar Hydro line). In the event that WBN is notified by the Transmission System Operator that the off site power requirements cannot be met for either incoming 161 kV line, Technical Instruction TI-12.15 directs the plant operator to enter the appropriate WBN Technical Specification Limiting Condition of Operation (LCO) - see Section 7.6 below.

Adequate grid voltage is related to the capability to reset the degraded voltage relay post actuation of the loads required for safe shutdown or to mitigate the consequences of a design basis event and thereby enabling the safety buses to remain powered from the offsite source. The Minimum Grid Voltage for Operable Offsite Sources is the minimum 161 kV bus voltage at Watts Bar Hydro Station required for Adequate Offsite Power for Watts Bar Nuclear Plant such that the WBN offsite sources can perform their intended GDC 17 safety functions.

At present, the NORMAL offsite voltage acceptance criteria for the 161 kV source for WBN is as follows:

Maximum voltage drop from pre-event to post-event for a design bases event is a 11 kV drop with a minimum post-event grid voltage at WBN of 153 kV. The ALTERNATE offsite acceptance criteria for the 161 kV source for WBN is a maximum voltage drop from pre-event to post-event for a a design bases event is 6 kV drop with a minimum post-event grid voltage at WBN of 153 kV. (Reference 31)

With dual unit operation, the offsite voltage acceptance criteria for the 161 kV source will be revised as follows to account for the additional load associated with an accident in one unit and concurrent shutdown of the second unit:

A maximum pre-event to post-event voltage drop of 9 kV with a minimum grid of 153 kV in NORMAL and a 6 kV drop with a minimum grid voltage of 153 kV in ALTERNATE.

(Reference 34)

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ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS Communication is also required from WBN to TPS. The WBN site has procedures that direct the plant operators to contact the TPS dispatcher if indications of abnormal grid or switchyard conditions, such as low frequency or voltage, are identified. Additionally, TVA NPG site operating procedure NPG-SPP-07.1.6, On Line Work Control Power System Alerts / Offsite Power (Reference 44), provides guidance for the operators to notify TVA TPS anytime equipment that has the potential to affect off-site power operability (e.g. generator voltage regulators, capacitor banks, etc.) are out-of-service or their risk level changes due to maintenance. Based on this new information, TVA TPS will perform an evaluation and determination of offsite power qualification based on current system configuration.

TVAs TPS has developed an internal website that identifies each plant's anticipated offsite power status as green (no offsite power risks), yellow (offsite power vulnerable), or red (offsite power disqualified).

This website is used to assess grid condition as part of the 12 week scheduling process and at the beginning of each day. If TPS determines that a plant's risk color has changed, TPS will inform the plant so the plant can take actions to reduce risk or correct the condition.

In addition to the qualification determination, individual problems which may affect offsite power are also alarmed. Alarms on the Electrical Control Panel in the WBN main control room (MCR) are provided for indication of CSST Failure and CSST Abnormal along with alarms to warn of bus transfers, undervoltage, and control power failures. Additional indications, to monitor offsite power conditions, include voltage on the downstream side of each CSST and load tap changer settings, voltage available to each 6.9 kV Shutdown Board, and amperage supplied through each supply breaker to the board.

7.6 WBN PLANT TECHNICAL SPECIFICATIONS The WBN plant Technical Specifications (Reference 35) require Two qualified circuits between the offsite transmission network and the onsite Class 1E AC Electrical Power Distribution System be Operable in operating Modes 1, 2, 3, and 4 (TS 3.8.1). With one offsite circuit In-Operable, Surveillance Requirement 3.8.1.1 must be performed for the remaining offsite circuit within 1 hour1.157407e-5 days 2.777778e-4 hours 1.653439e-6 weeks 3.805e-7 months prior to or after entering the In-Operable condition, and at least once per 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months thereafter, until Operability is restored (TS 3.8.1.A). Surveillance Requirement 3.8.1.1 involves verifying correct breaker alignments and power available from the offsite circuit. The action statement associated with TS Requirement 3.8.1 for one offsite circuit In-Operable also requires restoration of the In-Operable offsite circuit to Operable status within 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months or the plant must be in Mode 3 (Hot Standby) within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and Mode 5 (Cold Shutdown) within 36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months (see TS 3.8.1.G) of not meeting the action statement.

With two offsite circuits In-Operable in Modes 1, 2, 3, or 4, the action statement associated with TS Requirement 3.8.1.D requires restoration of one In-Operable offsite circuit to Operable status within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months or the plant must be in Mode 3 (Hot Standby) within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and Mode 5 (Cold Shutdown) within 36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months (see TS 3.8.1.G) of not meeting the action statement.

For plant operating Modes 5 and 6, One qualified circuit between the offsite transmission network and the onsite Class 1E AC electrical power distribution subsystem(s) is required to be Operable to support each AC electrical distribution subsystem required by LCO 3.8.10 (TS 3.8.2).

The offsite sources are considered Operable if they can perform their intended safety function.

Accordingly, each offsite source must have sufficient capacity and capability of starting and running the safety loads, including the automatic block loading associated with a Safety Injection actuation. This requires the offsite source voltage to be sufficient to pickup the low voltage motor contactors, provide adequate terminal voltage to the individual loads (during motor starting / inrush conditions as well as running conditions), and reset the DVRs.

E1-30

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS The offsite sources are Operable when there is Adequate Offsite Power. Consistent with NRC Generic Letter 2006-02 (Reference 15), Adequate Offsite Power requires the existence of power from the transmission system with sufficient voltage and capacity to power the safety-related loads under defined plant load conditions. Sufficient grid voltage is related to the capability to reset the degraded voltage relay post actuation of the loading required for safe shutdown or to mitigate the consequences of a design basis event and thereby allow the safety buses to remain powered from the offsite source.

The Minimum Grid Voltage for Operable Offsite Sources is the minimum 161 kV bus voltage at Watts Bar Hydro Station required for Adequate Offsite Power for Watts Bar Nuclear Plant such that the WBN offsite sources can perform their intended GDC 17 safety functions.

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ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS

8. WBN CLASS 1E BUS UNDERVOLTAGE PROTECTION 8.1 NRC BRANCH TECHNICAL POSITION PSB-1 NRC Branch Technical Position PSB-1 (Reference 8) provides regulatory guidance for Class 1E bus undervoltage protection. Branch Technical Position PSB-1 is part of the design and licensing bases for WBN. Branch Technical Position PSB-1 recommends that two levels of undervoltage protection be provided on the safety-related electrical distribution system. The first level of undervoltage protection is the loss of voltage relay (LVR), whose primary function is to detect and disconnect the safety-related boards upon a loss of offsite power. The second level of undervoltage protection is the degraded voltage relay (DVR), which is provided to protect Class 1E equipment from sustained low voltage conditions on the offsite power grid (i.e., transmission system voltages below Minimum Grid Voltage for Operable Offsite Sources). The combination of the LVR system and the DVR system provides protection for the safety-related electrical distribution systems from degraded and loss of voltage conditions on the offsite power supply. to this document contains the complete text of Branch Technical Position PSB-1. The analyses performed by TVA to establish the appropriate Analytical Limits (voltage and time delays) for the Class 1E safety bus undervoltage protection schemes are described in Section 8.2 of this report.

These analyses demonstrate that the Class 1E safety bus undervoltage protection schemes are capable of performing their intended design functions and fully comply with the applicable design and licensing basis requirements, including GDC 17 and NRC Branch Technical Position PSB-1.

8.2 ANALYSIS TO ESTABLISH DVR AND LVR ANALYTICAL LIMITS Branch Technical Position PSB-1 requires the selection of undervoltage and time delay setpoints to be determined based on an analysis of the voltage requirements of the Class 1E loads at all onsite system distribution levels. Time delay setpoints should be sufficiently long to establish the existence of a sustained degraded voltage condition (i.e., something longer than a motor starting transient) and sufficiently short enough such that the permanently connected Class 1E loads will not be damaged.

The Analytical Limits established for the WBN DVRs and LVRs in accordance with NRC Branch Technical Position PSB-1 are summarized in Table 6-1. The TVA analyses that established the DVR and LVR voltage and time delay Analytical Limits were based on the operating (running) voltage for required safety-related loads. TVAs analyses demonstrate that the DVR and LVR set points protect the safety-related loads when offsite power is not capable to accept the required accident loads and allows the safety-related buses to remain connected to offsite power when offsite power is capable to accept the required accident loads. The TVA design, supporting analyses, and established Analytical Limits allow the safety-related buses to remain connected to the offsite sources when there is Adequate Offsite Power, i.e., power from the transmission system has sufficient voltage and capacity to power the safety-related loads under worst case accident loading conditions (including ESF block starting and running).

TVA Calculation WBN-EEB-MST1060029 (Reference 35) establishes the basis for the Class 1E, 6.9 kV safety bus DVR and LVR Analytical Limits. The analyses performed ensure that the DVR and LVR voltage and time delays settings:

Ensure that the available equipment terminal voltage is maintained at or above a level required for the safety related loads to operate, and

Ensure that the duration of the degraded voltage at a given voltage level does not result in thermal degradation or damage of any equipment.

E1-32

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS Degraded Voltage Relay (DVR) Analytical Limits The TVA analyses that established the Analytical Limits (voltages and time delays) for the dropout (or trip) of the Class 1E, 6.9 kV safety bus DVRs were initially based on the operating (running) voltage for the required safety-related loads under worst case loading conditions. This approach was consistent with the original licensing basis for WBN Unit 1. Subsequently, TVA performed an additional sensitivity analysis using the DVR Dropout Voltage Analytical Limit and DVR Minimum and Maximum Time Delay Analytical Limits to ensure adequate voltage to operate the running loads associated with maximum loading conditions, and start individual motor loads and stroke individual NRC Generic Letter 89-10 motor operated valves.

TVA established the DVR Dropout Voltage Analytical Limit based on a bottom up analysis of the required voltage limits and protective device settings of individual loads.

TVA Calculation WBN-EEB-MST1060029 (Reference 35) uses the lowest possible voltage at the 6.9kV Shutdown Boards without tripping (actuating) the DVR. The result value of 6555 V is the DVR Dropout (Trip) Voltage Analytical Limit. This value was determined using a fixed 6.9 kV bus voltage. Key elements of the analysis include the following.

The connected medium and low voltage safety-related loads required for accident and normal operating conditions are verified to have an acceptable operating (running) voltage.

Each safety-related motor connected to the Class 1E system under normal loading conditions as well as Safety Injection actuation Phase A and Safety Injection Phase B loading conditions is verified to have adequate starting voltage (individual motor starting cases).

Motor terminal voltage is verified to be sufficient to prevent motor overheating damage.

Motor control circuits and 120 V distribution panel loads are verified to have adequate voltages.

Resultant terminal voltages are verified to be adequate to stroke individual NRC Generic Letter 89-10 motor operated valves.

The DVR time delay setpoint is short enough to provide adequate protection for the safety related equipment and long enough to avoid trips during starting transients including the automatic actuation of the engineered safety features (ESF) loads. The upper boundary of the DVR time delay is selected to be less than the safety analysis time allowed for the emergency diesel generators (EDGs) to come up to rated speed and voltage and be ready to accept load. The lower boundary of the DVR time delay should be greater than the time it takes the safety bus voltage to recover above the upper boundary of the degraded voltage relay reset setting discussed in Section 7.2 of this report.

The nominal setpoint for the DVR voltage and time delay are selected considering all applicable errors and tolerances in accordance with TVA Set point Technical Instruction TI-28 (Reference 24).

Loss of Voltage Relay (LVR) Analytical Limits The Analytical Limits associated the Class 1E, 6.9 kV safety bus LVRs are shown in Table 6-1. During the worst case accident block loading with the Minimum Grid Voltage for Operable Offsite Sources, analysis demonstrated that the minimum Class 1E, 6.9 kV bus voltage will not drop below the LVR Dropout Voltage Analytical Limit.

E1-33

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS The LVR is an integral part of the safety bus undervoltage voltage protection scheme described in NRC Branch Technical Position PSB-1. The LVR Analytical Limits (voltage and time delay) are selected to detect loss of offsite power (LOOP) to the Class 1E safety buses and prevent premature separation from the offsite supply due to short duration, momentary bus voltage dips due to switching transients, electrical faults (which are cleared), and other transient events. The LVR Analytical Limits (voltage and time delay) also limit the magnitude and duration of degraded voltage the safety-related buses can be exposed to.

TVA Calculations WBPE2119202001 (Reference 36) and TDR SYS.211-LV1 (Reference 37) document the analysis of the specific instrument loop uncertainties associated with the Class 1E safety LVRs.

These calculations, along with TVA Calculation WBN EEB MST1060029 (Reference 35) demonstrate that Analytical Limits established for the LVRs provide adequate protection for the safety related equipment, support the plants Technical Specifications, and compliance with the NRC Branch Technical Position PSB-1.

The LVR Reset Voltage Analytical Limit reset was established to be less than, with some margin, the minimum safety-related bus voltage that occurs during the accident (block-start) transient. The LVR Dropout Voltage Analytical Limit is greater than the safety-related bus voltage that produces motor stall conditions. The LVR nominal setpoint was then selected considering tolerances from measurement errors, drift errors, and other errors in accordance with TVA Set point Technical Instruction TI-28 (Reference 24).

The LVR Minimum Time Delay Analytical Limit was selected to be long enough to ride through short-circuits and other short-time system transients (for example, lightning strikes or switching transients),

taking into account the total sensing and clearing times for these types of events. The LVR Maximum Time Delay Analytical Limit selected to be less than the time allowed in the plant safety analysis for loss of voltage detection. The nominal set point for the time delay was selected considering all applicable errors and tolerances in accordance with TVA Setpoint Technical Instruction TI-28 (Reference 24).

8.3

SUMMARY

OF ANALYSIS TVA has confirmed the plant Technical Specification trip setpoints and allowable values for the DVR and LVR voltage and time delay values are properly derived from established Analytical Limits. The nominal setpoints and allowable values associated with the DVR and LVR Analytical Limits voltage and time delays are selected considering all applicable errors and tolerances in accordance with TVA Setpoint Technical Instruction TI-28 (Reference 25). TVA Calculations WBPE2119202001 (Reference 37) and TDR SYS.211-LV1 (Reference 38) document the analysis of the specific instrument loop uncertainties associated with the Class 1E bus undervoltage protection schemes.

Figure 8-1 provides a graphical summary of the key operational limits and Analytical Limits associated with the Operability of the offsite sources and class 1E safety bus undervoltage protection schemes.

These limits are derived from the supporting analysis described in Sections 7 and 8 of this report. The graphical summary and representation of these analysis results indicates that in all scenarios the safety related Class 1E equipment is protected and can perform their intended safety functions.

Figure 8-1 highlights three distinct regions.

In Region 1, where the pre-ESF actuation safety bus voltage is at or above the lower limit of the normal 6.9 kV bus operating band, the offsite source is Operable and therefore capable of performing its intended safety function during worst case loading accident conditions and the limiting system alignments (single common station service transformer CSST C or D carrying all the safety loads). In E1-34

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS Region 1 the automatic load tap changer (LTC) on the CSSTs maintain the pre-event 6.9 kV bus voltage within the normal operating band prior to the design basis event. The grid voltage assumed for the associated transient loading analysis includes the impact of the limit N-1 Contingency on the grid. In Region 1, analysis show that safety related 6.9 kV and 480 V Class 1E system and associated loads have sufficient voltage to actuate (block start) the ESF loads and reset the DVR under worst case loading conditions.

In Region 1, the Class 1E loads remain connected to the Operable offsite source and the DVR provides adequate protection for the operating safety loads.

At the transition point to Region 3, i.e., at the DVR Dropout Voltage Analytical Limit, the medium and low voltage safety-related loads required for normal operation and accident mitigation are verified to have acceptable operating (running) voltage, and the individual safety loads have adequate voltage to start and the GL 89-10 MOVs have adequate voltage to stroke. When the safety bus voltages remains below the DVR Dropout Voltage Analytical Limit (Region 3) for a duration longer than the DVR time delay, the DVR automatically transfers the safety buses to the onsite standby sources in a timeframe consistent with the plants safety analysis assumptions. In Region 3, the safety buses are not connected to the offsite sources for any time longer than the DVR Maximum Time Delay Analytical Limit.

In Region 3, the DVR adequately protects the operating Class 1E loads by transferring 6.9 kV Class 1E buses to the onsite power sources and the onsite power sources perform their GDC 17 design function.

In Region 2, the offsite sources are declared In-Operable. However, in Region 2 the offsite sources may be capable of performing their intended safety function depending on the capacity of the grid (i.e., if there is Adequate Offsite Power), the actual plant transformer and bus alignments, and the actual safety bus loading. For example, under certain grid conditions with the Unit 1 and Unit 2 safety bus loads split between CSST A and CSST B and approximately 6700 V on the 6.9 kV safety buses (i.e., approximately 20 Volts above the DVR Pickup Voltage Analytical Limit) the offsite sources are capable of successfully actuating (block start) the ESF loads in one unit, resetting the DVRs, and servicing the safe shutdown loads in the other unit. Conversely, a slightly lower 6.9 kV bus voltage would result in a dropout and timeout of the DVR in the unit with the ESF actuation.

If the offsite source is incapable of performing its intended safety function, the DVR automatically transfers safety buses to the onsite standby sources in a timeframe consistent with the plants safety analysis assumptions. To confirm this, TVA performed the sensitivity analysis, which is documented in Calculation STUDY-EEB-WBN-12-001, Appendix B and H (Reference 39). This study evaluates various possible grid conditions to distinguish between those that allow successful performance of design basis requirements and those that do not (degraded grid conditions). This analysis considered variations of grid conditions (infinite combinations of grid voltage and capacity) and ensured that for any motor starting event where voltage drops below the DVR dropout setting and then recovers to the reset value, it will recover above the DVR Dropout Analytical Limit (6555 volts) within a maximum four (4) second time delay. This time delay is acceptable based on WBNs accident analysis assuming a five (5) second time delay for restoration of safety-related bus voltage, when relying on offsite power (Reference 40).

The analysis demonstrates that adequate voltage is provided for the Class 1E loads at all onsite distribution levels once the voltage has recovered to the DVR Dropout Analytical Limit. TVAs sensitivity analysis did not credit operation of the CSST LTC for voltage recovery.

In all cases in Region 2, the capability to successfully actuate the ESF loads is maintained and either the offsite or the onsite source has sufficient capacity and capability to assure that (1) specified acceptable fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded as a result of anticipated operational occurrences and (2) the core is cooled and containment integrity and other vital functions are maintained in the event of postulated accidents within the time limits assumed in E1-35

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS the plant safety analysis. In all cases in Region 2, the DVR provides adequate protection for the Class 1E loads and prevents inadvertent trip of safety loads.

In Region 2, the Class 1E loads remain connected to either the on-site or offsite source and the DVR provides adequate protection for the Class 1E loads and prevents inadvertent trip of safety loads.

E1-36

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS Worst Case Design Basis Normal Plant Accident 161 kV Bus Operation w/ Offsite Sources Operable Voltage Region 1

> The offsite power source is capable of 168 kV performing its intended safety (1.043 pu) function. Sufficient voltage is available to Normal Grid actuate ESF loads (block start and run) and Operating Voltage (1) reset the DVR under worst case loading 161kV conditions.

162 kV (1.006 pu)

WBN > Class 1E loads remain connected to the offsite Hydro Bus power source.

> DVR provides adequate protection of the Class 161 kV 1E loads.

(1.000 pu)

Region 2 LTC 153 kV Minimum Grid Voltage for > The Class 1E, 6.9 kV safety bus voltage is (0.950 pu) Operable Offsite Source below the minimum pre-ESF actuation bus voltage. Offsite source is declared In-Operable and Tech Spec LCO action statement is entered.

6.9 kV Bus Offsite Source Safety Bus Transfers Inoperable T/S LCO to Onsite Source > The offsite power source may be capable of Voltage performing its intended safety function depending on the capacity of the grid, 7135 kV transformer and bus alignments, and Class 1E (1.034 pu)

Region 1 bus loading.

Normal Bus Minimum Minimum Operating Band Pre Actuation Post Actuation > DVR provides adequate protection of the Class 7010 kV w/ LTC Bus Voltage Voltage w/ LTC 1E loads and prevents inadvertent trip of safety (1.016 pu) loads.

Class 1E Loads Operable

> DVR automatically transfers safety buses to the 6.9 kV 6900 kV onsite standby sources in a timeframe (1.000 pu) consistent with the plants safety analysis WBN Region 2 from Offsite Source assumptions, if offsite source is incapable of DVR / LVR Class 1E Bus performing its intended safety function.

Monitoring Point 6681 kV DVR Reset AL

> All cases Class 1E loads are capable of (0.968 pu) Required Minimum performing their intended safety function.

Post Actuation Voltage 6555 kV DVR Dropout AL (0.950 pu)

Region 3 Class 1E Operable from T1 (Note 2) > At the DVR dropout AL, the offsite power source 6072 kV LVR Reset AL is not capable of block starting the safety (0.880 pu) Region 3 loads. However, the offsite source is capable of Onsite Source providing adequate voltage for the safety-related 5934 kV LVR Dropout AL loads required for normal operation and accident (0.860 pu) mitigation (running voltage) and starting individual safety loads including 89-10 MOVs.

> DVR automatically transfers safety buses to the onsite standby sources in a timeframe consistent with the plants safety analysis assumptions.

Notes:

> In Region 3, the safety buses are not connected

1. Axis not drawn to scale to the offsite sources.

2. Voltage Recovery Duration T1 must be less than DVR time delay setting for a successful ESF actuation and voltage recovery above the DVR Reset AL.

Figure 8-1: Offsite Sources and Class 1E Safety Bus Undervoltage Operating and Protection Bands E1-37

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS 9 DESIGN BASIS COMPLIANCE WBN Unit 1 received a full power operating license (OL) from the NRC in early 1996. TVA is in the final process of licensing WBN Unit 2, which is of the same vintage as WBN Unit 1. The design, layout, and plant equipment for Unit 2 are virtually identical to Unit 1, and the Unit 1 design basis compliance required for offsite power systems and undervoltage protection scheme applies to Unit 2.

Regarding the capacity and capability of the offsite power sources and the Class 1E safety bus undervoltage protection schemes, GDC 17 and NRC Branch Technical Position PSB-1 define the principal design basis requirements for WBN. During the licensing process for Unit 1, the NRC staff concluded that TVA met the requirements of GDC 17 with respect to the offsite power systems having the (1) the capacity and capability to permit the functioning of structures, systems, and components

[SSCs] important to safety; (2) provisions to minimize 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 loss of power from the onsite electric power supplies; (3) the physical independence of the offsite power circuits; and (4) availability of circuits (Section 8.2.2. of Reference 41). During the licensing process for Unit 1, the NRC staff concluded that TVAs design and analysis for the Class 1E safety bus DVR and LVR protection schemes satisfied the regulatory positions of NRC Branch Technical Position PSB-1 and that the units preoperational testing program verified that the design conforms to PSB-1; see Section 8.3.1.2 of Reference 43.

The specific compliance requirements described in Section 7 and 8 above are summarized below:

9.1 GDC 17 AND OFFSITE POWER SYSTEM REQUIREMENTS With respect to the offsite power system, GDC 17 requires the following.

a. Electric power from the transmission system to onsite distribution system to be supplied by two physically independent offsite circuits.

The WBN preferred power supply consists of two independent 161 kV overhead lines that are located entirely on TVA property. The two circuits are supported on separate sets of transmission towers. The separation between the two lines is sufficient to ensure that the failure of any tower in one circuit will not endanger the other offsite circuit.

b. Each offsite circuit shall be designed and located so as to minimize to the extent practical the likelihood of their simultaneous failure under operating and postulated accident and environmental conditions. A switchyard common to both circuits is acceptable.

The bus arrangement at Watts Bar Hydro Plant 161 kV switchyard is designed so that the loss of any one of the four main bus sections will not cause loss of power to either of the two preferred power source lines to WBN. The transmission line structures for 161 kV lines are designed to meet or exceed load requirements specified in the National Bureau of Standards Handbook No. 81 (National Electric Safety Code Part 2). Designing to these requirements ensures the adequacy of lines for wind and heavy icing conditions in excess of those that would be expected to occur in this area. The phase conductor and shield wire design tensions are selected to avoid vibration problems. The 161 kV transmission lines provide power to WBN common station service transformers CSSTs A and D and CSSTs B and C. The lines are routed to the east and north of the nuclear plant transformer yard so as to minimize the likelihood of their simultaneous failure.

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ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS

c. GDC 17 does not require the independent circuits to be installed on separate rights of way (ROW).

Consistent with GDC 17 the WBN offsite transmission circuits are installed on a common ROW.

However, the two circuits are supported on separate sets of transmission towers and the separation between the two lines is sufficient to ensure that the failure of any tower in one circuit will not endanger the other offsite circuit.

d. Each offsite circuit shall be available in sufficient time following a loss of all onsite alternating current power supplies and the other offsite electric power circuit, to assure that specified acceptable fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded. GDC 17 requires one of the offsite circuits to be available within a few seconds following a loss-of-coolant accident to assure that core cooling, containment integrity, and other vital safety functions are maintained. This is typically referred to as the immediately available source.

Two immediately available offsite circuits to the WBN Class 1E safety buses via common station service transformers CSSTs C and D. Consistent with the Bases document for the Westinghouse Standardized Technical Specifications, each WBN offsite to onsite transmission circuit is capable of maintaining rated frequency and voltage, and accepting required loads during an accident, while connected to the ESF buses. See Section B3.8.1 of NUREG 1431 (Reference 21).

e. The last paragraph of GDC 17 states that Provisions shall be included to minimize 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, the loss of power from the transmission network, or the loss of power from the onsite electric power supplies.

The WBN offsite and onsite power system design minimizes the probability of simultaneous failures of offsite and onsite power sources.

Additionally, by verifying that the requirements of GDC 17 are satisfied and that the offsite source can perform its intended safety function, the following can be stated for WBN 2.

WBN has met the requirements of GDC 17, "Electric Power Systems," with respect to the offsite power system's (a) capacity and capability to permit functioning of structures, systems, and components important to safety; (b) provisions to minimize 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 loss of power from the onsite electric power supplies; c) physical independence of circuits; and (d) availability of circuits.

The preferred power system consists of at least two physically independent circuits routed from the electrical grid system by transmission lines to the onsite power distribution system. At least one circuit will be available within a few seconds following a loss of coolant accident and is considered an immediate access circuit. Each circuit is designed and located so as to minimize to extent practical the likelihood of their simultaneous failure under operating and postulated accident and environmental conditions.

Each circuit has been sized with sufficient capacity to supply all connected loads. Each circuit can be made available to the onsite power system, assuming loss of the onsite ac standby power supplies and loss of the other offsite circuit, to ensure that fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded. The switchyard is E1-39

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS arranged such that each offsite circuit can be isolated from other circuits to permit reestablishment of offsite power to the onsite distribution system. The switchyard is also arranged such that single events (e.g., a spurious relay trip or a breaker not operating during fault conditions) will not cause simultaneous failure of all offsite circuits to the switchyard. The results of WBNs stability analysis indicated that loss of the largest generating capacity being supplied to the grid, loss of largest load from the grid, loss of the most critical transmission line, or loss of the unit itself will not cause grid instability.

9.2 PSB-1 AND UNDERVOLTAGE PROTECTION SCHEME REQUIREMENTS NRC Branch Technical Position PSB-1 establishes a technical approach determined by the NRC staff to be an acceptable means for addressing Class 1E safety bus undervoltage protection schemes. Branch Technical Position PSB-1 includes the following regulatory positions.

Positions B.1(a)(b) recommends that two levels of undervoltage protection be provided on the safety-related electrical distribution system. The first level of undervoltage protection is the loss of voltage relay (LVR). The loss of voltage scheme is provided to detect loss of offsite power at the Class 1E buses and disconnect the safety-related buses in the event a loss of offsite power (LOOP). The second level of undervoltage protection is the degraded voltage relay (DVR), which is provided to protect Class 1E equipment from sustained low voltage conditions on the safety-related buses.

The WBN design includes two levels of undervoltage protection consistent with PSB-1. The WBN DVR and LVR design was reviewed by the NRC staff as part of the licensing process for Unit 1. Section 8.3.1.2 of NUREG-0847 (Reference 41) states that The staff concludes that the proposed design meets BTP positions and is acceptable.

As part of the completion of WBN Unit 2, TVA has performed detailed analyses of the WBN offsite and onsite power systems. TVA has also updated the existing plant electrical analyses to address the impacts of dual reactor unit operation.

The TVA analyses that established the Analytical Limits (voltages and time delays) for the dropout (or trip) of the Class 1E, 6.9 kV safety bus DVRs were initially based on the operating (running) voltage for the required safety-related loads under worst case loading conditions. This approach was consistent with the original licensing basis for WBN Unit 1. Subsequently, TVA performed an additional sensitivity analysis using the DVR Dropout Voltage Analytical Limit and DVR Minimum and Maximum Time Delay Analytical Limits to ensure adequate voltage to operate the running loads associated with maximum loading conditions, and start individual motor loads and stroke individual NRC Generic Letter 89-10 motor operated valves. See Section 8.2 of Enclosure 1.

The DVR time delay setpoint is short enough to provide adequate protection for the safety related equipment and long enough to avoid trips during starting transients including the automatic actuation of the engineered safety features (ESF) loads. The upper boundary of the DVR time delay is selected to be less than the safety analysis time allowed for the emergency diesel generators (EDGs) to come up to rated speed and voltage and be ready to accept load.

The lower boundary of the DVR time delay should be greater than the time it takes the safety bus voltage to recover above the upper boundary of the degraded voltage relay reset setting discussed in Section 7.2 of Enclosure 1.

Position B.1(c) states, in part, that The voltage sensors shall be designed to satisfy the following applicable requirements derived from IEEE Std. 279-1971, Criteria for Protection Systems for Nuclear Power Generating Stations.

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ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS The voltage sensors and relays for WBN Unit 1 and Unit 2 DVR and LVR undervoltage protection schemes fully complies with IEEE Standard 279-1971.

Position B.1(d) states, The Technical Specifications shall include limiting conditions for operations, surveillance requirements, trip setpoints with minimum and maximum limits, and allowable values for the second-level voltage protection sensors and associated time delay devices.

TVA Calculation WBNEEBMSTI060029 (Reference 36) describes the necessary analysis to support the determination of the setpoints and time delays outlined in PSB-1. WBN Technical Specifications supports the requirements listed above (Reference 35).

Position B.2 states The Class 1E bus loading scheme should automatically prevent shedding during sequencing of the emergency loads to the bus. The load shedding feature should, however, be reinstated upon completion of the load sequencing action. The technical specifications must include a test requirement to demonstrate the operability of the automatic bypass and reinstatement features at least once per 18 months during shutdown.

Position B.2 also states In the event an adequate basis can be provided for reinstating the load shed feature during the above transient conditions, the setpoint value in the Technical Specifications for the first level of undervoltage protection must specify a value having maximum and minimum limits. The bases for the setpoints and limits selected must be documented.

Enclosure 2 to TVAs October 9, 1981 letter to the NRC (Reference 42) states, in part, Degraded voltage relaying will not open the standby supply breaker and will not initiate load shedding and resequencing if a 6900-volt shutdown board is supplied by its diesel generator.

The loss of-voltage relays will initiate load shedding and resequencing. However, the voltage set point and time delay for these relays prevent their operation for any motor starting transients when adequate power is being supplied to the shutdown board. Maximum and minimum limits for the loss-of-voltage relay set points will be included in the Technical Specifications. Section 8.3.1.2 of NUREG-0847 (Reference 41) states, in part, that By letter dated October 9, 1981

[Reference 42] the staff concludes that the proposed design meets BTP positions and is acceptable. The DVR and LVR design for WBN Unit 1 and Unit 2 are the same.

Position B.3, states The voltage levels at the safety-related buses should be optimized for the maximum and minimum load conditions that are expected throughout the anticipated range of voltage variations of the offsite power sources by appropriate adjustment of the voltage tap settings of the intervening transformers. The tap settings selected should be based on an analysis of the voltage at the terminals of the Class 1E loads. The analyses performed to determine minimum operating voltages should typically consider maximum unit steady-state and transient loads for events, such as a unit trip, loss-of-coolant accident, startup or shutdown, with the offsite power supply (grid) at minimum anticipated voltage and only the offsite source being considered available. Maximum voltages should be analyzed with the offsite power supply (grid) at maximum expected voltage concurrent with minimum unit loads. A separate set of the above analyses should be performed for each available connection to the offsite power supply.

Tennessee Valley Authority has established the appropriate setpoints and administrative processes and procedures to achieve a well balanced allocation between conservative design margin and safe plant operating margins. TVA performed detailed analyses of the AC system performance during maximum and minimum load conditions and selected transformer tap positions to ensure adequate terminal voltage for the Class 1E loads for single unit reactor operation and dual unit reactor operation. The analyses performed and documented in TVA Calculation EDQ00099920070002 (Reference 24) consider the maximum unit steady-state and E1-41

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS transient loads for events, such as a unit trip, loss-of-coolant accident, startup or shutdown, with the offsite power supply (grid) at Minimum Grid Voltage for Operable Offsite Sources. The analyses of the offsite power sources credit only the offsite source being considered available.

The impact of maximum grid voltages concurrent with light loading conditions were considered in TVAs analysis.

Position B.4, states, in part, that The analytical techniques and assumptions used in the voltage analyses cited in item 3 above must be verified by actual measurement. The verification and test should be performed before initial full-power reactor operation on all sources of offsite power ...

Based on the similarities between WBN Unit 1 and Unit 2, the testing utilized for Unit 1 is applicable to the Unit 2. Regarding BTP PSB-1 Position 4, NUREG 0847, SSER 20 states, in part, that In SSER 13, the staff stated that Confirmatory Issue 28 was resolved on the basis of a preoperational test documented in Inspection Report 50-390/84-90, dated February 11, 1985.

However, the staff stated that the results obtained from that test were no longer valid since TVA was re-performing the preoperational tests. The preoperational test was conducted [re-performed] by TVA and reviewed by the staff in Inspection Reports 50-390/95-22 (September 8, 1995) and 50-390/95-77 (December 6, 1995). This update does not change the staff's conclusion regarding Confirmatory Issue 28 (Reference 43).

10

SUMMARY

AND CONCLUSIONS The design, layout, and plant equipment for TVAs Watts Bar Nuclear (WBN) Unit 2 are virtually identical to WBN Unit 1. The design and licensing bases for Unit 2 are essentially the same as what presently exists for Unit 1, which received its full power operating license (OL) in 1996. The compliance of the WBN offsite sources and Class 1E safety bus undervoltage protection was reviewed by the NRC staff as part of the licensing process for Unit 1 and found to be acceptance. As part of the licensing process for Unit 2, TVA has performed the detailed analysis of the Unit 2 AC power system and updated the appropriate analyses to account for dual reactor unit operation.

The NRC staff is in the process of completing its technical review of TVA Final Safety Analysis Report for Watts Bar Unit 2. An open item for licensing WBN Unit 2 is the resolution of the degraded voltage question documented as Open Item No. 30 in Section 8.3.1.2 of Supplement 22 to NUREG-0847 (SSER 22, Reference 3). This document provides relevant background information regarding Open Item No. 30 as well as detailed descriptions of the bases for regulatory compliance of WBNs offsite power sources and Class 1E safety bus undervoltage protection schemes with the current licensing basis of the plant.

The intended safety functions resulting from meeting the requirements for GDC 17 and PSB-1 requirements have different and distinct purposes. For GDC 17 requirements being met, the offsite source is considered Operable and capable to handle the worst case accident loading conditions while remaining on offsite power. The design feature and intended safety function of the undervoltage protection scheme as required by PSB-1 is to provide protection to the Class 1E equipment such that they may perform their required safety functions regardless of whether they are connected to the offsite power source or the onsite standby power source.

Electric power from the TVA 161 kV transmission system to onsite WBN AC distribution system is supplied by two physically independent offsite circuits. The WBN offsite power sources and Class 1E bus undervoltage protection schemes fully comply with the applicable plant design and licensing basis requirements, including General Design Criterion 17, Electric Power Systems, (GDC 17) of 10CFR50, Appendix A and NRC Branch Technical Position (BTP) PSB-1, Adequacy of Station Electric Distribution System Voltages.

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ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS Tennessee Valley Authority has performed a comprehensive set of power system analyses that demonstrate that the offsite power system can perform its intended GDC 17 safety functions for assuming the onsite standby power supplies are not functioning. Specifically, the analyses performed demonstrate that the offsite power sources have sufficient capacity and capability to assure that (1) specified acceptable fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded as a result of anticipated operational occurrences and (2) the core is cooled and containment integrity and other vital functions are maintained in the event of postulated accidents.

Tennessee Valley Authoritys analyses address the full load and minimum load conditions which are expected and consider the anticipated range of voltage variations for the offsite power sources.

Consistent with NRC staff guidance, TVAs analyses of the offsite power system demonstrate that the safety-related electrical distribution system will perform its intended design functions under accident conditions (accident initiated starting) with the offsite power supply at the minimum allowable (operable) voltage and capacity (minimum post-event voltage and maximum post-event voltage drop). Based on the analysis results, TVA has defined the expected range for the offsite transmission system. This includes consideration of N-1 Contingencies on the grid. TVA has thoroughly analyzed the adequacy of the voltage provided to the onsite safety equipment when powered from the offsite sources.

The way that TVA transmission system is operated today is significantly more reliable than the manner in which it was operated at the time of the 1976 Millstone event, the 1978 ANO event, and 1999 event at Callaway. With NERC operating as the Electric Reliability Organization (ERO), having the legal authority to issue and enforce mandatory reliability standards for the bulk electric system, there is a seamless environment for satisfying nuclear plant offsite sources requirements. To that end, TVA has established an appropriate set of procedures and protocols to ensure that the necessary operational limits for the offsite power sources are understood and maintained by the Transmission System Operator (TVA-TPS). Formal agreements, which define the necessary Nuclear Plant Interface Requirements (NPIRs), are in place with the TVA Transmission System Operator. A real-time transmission system state estimator is used to ensure that the operational limits for the offsite power sources are maintained.

In the event that the WBN is notified by the Transmission System Operator that the offsite power requirements cannot be met, plant procedures directs the operators to enter the appropriate WBN Technical Specification Limiting Condition of Operation (LCO).

The WBN Class 1E, 6.9kV shutdown boards are provided with two levels of undervoltage protection.

Loss of Voltage Relays (LVRs) provide the first level of protection and Degraded Voltage Relays (DVRs) provide a second level of protection for the Class 1E loads. Consistent with NRC Branch Technical Position PSB-1, the primary function of the LVRs is to detect loss of offsite power at the Class 1E buses and disconnect the safety-related buses in the event a loss of offsite power (LOOP). The primary function of the DVRs is provided to protect Class 1E equipment from sustained low voltage conditions on the safety-related buses. When actuated (dropped out or tripped) the LVRs and DVRs initiate a transfer of the Class 1E safety buses from the normal offsite power supply to the onsite standby power supplies (i.e., diesel generators).

TVAs analyses demonstrate that the DVR and LVR Analytical Limits protect the safety-related loads when offsite power is not capable to accept the required accident loads and allows the safety-related buses to remain connected to offsite power when offsite power is capable to accept the required accident loads. The DVR time delay setpoint is short enough to provide adequate protection for the safety related equipment and long enough to avoid trips during starting transients including the automatic actuation of the engineered safety features (ESF) loads. The DVR Dropout Voltage Analytical Limit assures there is adequate operating voltage for (a) powering the safety related accident loads E1-43

ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS required to be operating under the maximum loading conditions (running loads), and (b) starting individual motor loads and stroking individual NRC Generic Letter 89-10 motor operated valves.

Consequently, TVA has established adequate provisions to minimize the probability of losing electric power from either the offsite or onsite power supplies as a result of, or coincident with, the loss of power generated by the nuclear power unit, the loss of power from the transmission network, or the loss of power from the onsite power supplies and provided adequate protection of the Class 1E loads from degraded voltage conditions.

The WBN offsite power sources and Class 1E bus undervoltage protection schemes fully comply with the applicable plant design and licensing basis requirements, including General Design Criterion 17, Electric Power Systems, (GDC 17) of 10CFR50, Appendix A and NRC Branch Technical Position (BTP) PSB-1, Adequacy of Station Electric Distribution System Voltages. The approaches used for TVAs degraded voltage relaying analyses and offsite source / station AC power analyses are consistent with the applicable NRC guidance. TVA has established the appropriate setpoints and administrative processes and procedures to achieve a well balanced allocation between conservative design margin and safe plant operating margins. Therefore, there are adequate bases for closure of NUREG-0847, SSER 22, Open Item No. 30.

11 REFERENCES

1. U.S. NRC Staff SECY Paper to the NRC Commission SECY-07-0096, Possible Reactivation of Construction and Licensing Activities for the Watts Bar Nuclear Plant Unit 2, dated June 7, 2007.

2. U.S. NRC Commission Staff Requirements Memorandum SRM-SECY-07-0096, Possible Reactivation of Construction and Licensing Activities for the Watts Bar Nuclear Plant Unit 2, dated July 25, 2007.

3. NUREG-0847, Supplement No. 22, Safety Evaluation Report Related to the Operation of Watts Bar Nuclear Plant, Unit 2, Docket No. 50-391, ADAMS Accession No. ML110390197.

4. U.S. NRC Email (M. J. Miernicki) to TVA dated November 12, 2014, Watts Bar 2 - RAIs -

Degraded Voltage Relay Issue, Watts Bar Nuclear Plant Unit 2 (WBN-2) Request For Additional Information Regarding Chapter 8, "Electrical Power"- Supplemental Safety Evaluation Report (SSER 22, Open Item 30) (TAC No. ME2731), ADAMS Accession No. ML1465A048.

5. U.S. NRC Letter to Northern States Power Company dated August 13, 1976, Monticello Nuclear Generating Plant and Prairie Island Nuclear Generating Plant Unit Nos. 1 and 2, Enclosure 1 -

Description of Millstone Unit 2 Events, Enclosure 2 - Request for Information, ADAMS Accession No. ML112920787.

6. General Design Criterion 17, Electric Power Systems, of 10CFR50, Appendix A, General Design Criteria for Nuclear Power Plants.

7. U.S. NRC Letter to Iowa Electric & Power Company dated June 2, 1977, Re: Duane Arnold Energy Center, Enclosure 1 - Safety Evaluation and Statement of Staff Positions Relative to the Emergency Power Systems for Operating Reactors, Docket No. 50-331, ADAMS Accession No. ML090130692.

8. NRC Branch Technical Position PSB-1, Adequacy of Station Electric Distribution System Voltages, Revision 0, dated July 1981.

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ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS

9. U.S. NRC Information Notice No. 79-04, Degradation of Engineered Safety Features, dated February 16, 1979.

10. U.S. NRC Generic Letter 79-36, Adequacy of Station Electric Distribution Systems Voltages, dated August 8, 1979.

11. Callaway Plant Unit 1, Licensee Event Report 1999-005-02 (Docket No. 50-483), Operating Conditions Exceeding Previously Analyzed Values Results in Inoperability of Both Offsite Sources, dated April 14, 2000, ADAMS Accession No. ML003706314.

12. Final Report on the August 14, 2003 Blackout in the United States and Canada - Causes and Recommendations, prepared by the U.S. - Canada Power System Outage Task Force, April 2004.

13. North American Electric Reliability Corporation (NERC), Improving Coordinated Operations Across the Electric Reliability Organization (ERO) Enterprise, dated February 2014.

http://www.nerc.com/AboutNERC/keyplayers/Documents/ERO_Enterprise_

Operating_Model_Feb2014.pdf

14. U.S. Federal Energy Regulatory Commission Interpretive Order Relating to the Standards of Conduct, Docket No. RM01-10-005, issued February 16, 2006.

15. NRC Generic Letter 2006-02, Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power, dated February 1, 2006.

16. ANSI/ISA Standard 67.04.01-2000, Setpoints for Nuclear Safety-Related Instrumentation.

17. NRC Inspection Manual Part 9900, Operability Determinations & Functionality Assessments for Resolution of Degraded or Nonconforming Conditions Adverse to Quality or Safety; Attachment to NRC RIS 2005-20 dated September 26, 2005.

18. NUREG 0800, Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants (initially issued as NUREG-75/087), Section 8.2 Offsite Power System, Revision 5, May 2010.

19. IEEE Standard 308-1971, IEEE Standard Criteria for Class 1E Electric Systems for Nuclear Power Generating Stations.

20. U.S. Atomic Energy Commission, Standard Format and Content of Safety Analysis Reports for Nuclear Power Plants, February 1972, ADAMS Accession No. ML13350A353.

21. TVA Letter to NRC dated August 1, 2013, Application to Modify Watts Bar Nuclear Plant Unit 1 Technical Specifications Regarding AC Sources - Operating (TS-WBN-13-02), ADAMS Accession No. ML13220A103.

22. NUREG-1431, Volume 2, Standard Technical Specifications - Westinghouse Plants, Bases, Revision 3.

23. U.S. NRC Regulatory Guide 1.32 (AEC Safety Guide 32), Revision 0, Use of IEEE Std 308-1971, Criteria for Class 1E Electric Systems for Nuclear Power Generating Stations, dated August 11, 1972.

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ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS

24. TVA Calculation EDQ00099920070002, AC Auxiliary Power System Analysis (Dual Unit Operation), Revision 43.

25. TVA Branch Technical Instruction BTI-EEB-TI-28, Setpoint Calculations, Revision 10.

26. TVA Calculation WBPE2119202001, 6.9 kV Shutdown & Logic Boards Undervoltage Relays Requirement / Demonstrated Accuracy Calculation, Revision 11.

27. NERC Standard NUC-001.2.1, Nuclear Plant Interface Coordination, effective date April 1, 2010.

28. TVA Standard Program and Process TVA-SPP-10.010, NERC Standard Compliance Processes Shared by TVA's Nuclear Power Group and Transmission Organizations, Revision 4.

29. NERC Standard FAC-001-2, System Operating Limits Methodology for the Operations Horizon, effective date April 29, 2009.

30. TVA Transmission Email (J.D. Porter) to TVA Design Engineering (A. J. Taylor) dated January 20, 2015 , Confirmation of No State Estimator Alarms for WBN.

31. TVA Transmission System Reliability Standard Operating Procedure TRO-EA-SOP-30.406, Incorporation of Nuclear Offsite Power NPIRs and Accident Loading into Operating System Models, Revision 0.

32. TVA Transmission System Reliability Standard Operating Procedure TRO-EA-SOP-30.405, Nuclear Offsite Power Operating Requirements, Revision 0.

33. WBN Technical Instruction TI-12.15, 161 kV Offsite Power Requirements, Revision 26.

34. TVA Calculation EDX-000-999-2004-0002, Watts Bar Nuclear Plant (WBN) - Transmission System Study (TSS) - Grid Voltage Study of WBNs Off-site Power System, Revision 3.

35. Watts Bar Nuclear Plant, Unit 1, NRC Facility Operating License No. NPF-90, Appendix A - Technical Specifications, Amendment 96, dated August 22, 2014.

36. TVA Calculation WBN-EEB-MST1060029, Degraded Voltage Analysis, Revision 37.

37. TVA Calculation WBPE2119202001, 6.9 kV Shutdown & Logic Undervoltage Relays Requirement / Demonstrated Accuracy Calculation, Revision 11.

38. TVA Calculation TDR SYS.211-LV1, Demonstrated Accuracy Calculation TDR SYS.211-LV1, Revision 4.

39. TVA WBN Calculation STUDY-EEB-WBN-12-001, Sensitivity Study of Degraded Voltage Relay (DVR) Protection During Motor Starting, Revision 2.

40. Westinghouse Letter WAT-D-9235 (J.W. Irons) to TVA (W.L. Elliot) dated March 12, 1993, Tennessee Valley Authority Watts Bar Unit Numbers 1 and 2, Five Second Delay for Safety Injection and Feedwater Isolation (SECL-92-028, Revision 1).

41. NUREG 0847, NRC Safety Evaluation Report Related to the Operation of Watts Bar Nuclear Plant, Units 1 and 2, Docket Nos. 50-390 and 50-391, June 1982.

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ENCLOSURE 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS

42. TVA Letter (L. M. Mills) to NRC (E. Adensam) dated October 9, 1981, Enclosure 2: Watts Bar Nuclear Plant Units 1 and 2, Responses to NRC Power Systems (Electrical) Branch Concerns, ADAMS Accession No. ML073521461.

43. NUREG 0847, Supplement No. 20, NRC Safety Evaluation Report Related to the Operation of Watts Bar Nuclear Plant, Units 1 and 2, Docket Nos. 50-390 and 50-391, February 1996, ADAM Accession No. ML072060498.

44. TVA procedure NPG-SPP-07.1.6, On Line Work Control Power System Alerts / Offsite Power, Revision 004.

E1-47

ENCLOSURE 1 - ATTACHMENT 1 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS APPENDIX A TO PART 50 - GENERAL DESIGN CRITERIA FOR NUCLEAR POWER PLANTS Criterion 17Electric power systems. An onsite electric power system and an offsite electric power system shall be provided to permit functioning of structures, systems, and components important to safety. The safety function for each system (assuming the other system is not functioning) shall be to provide sufficient capacity and capability to assure that (1) specified acceptable fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded as a result of anticipated operational occurrences and (2) the core is cooled and containment integrity and other vital functions are maintained in the event of postulated accidents.

The onsite electric power supplies, including the batteries, and the onsite electric distribution system, shall have sufficient independence, redundancy, and testability to perform their safety functions assuming a single failure.

Electric power from the transmission network to the onsite electric distribution system shall be supplied by two physically independent circuits (not necessarily on separate rights of way) designed and located so as to minimize to the extent practical the likelihood of their simultaneous failure under operating and postulated accident and environmental conditions. A switchyard common to both circuits is acceptable. Each of these circuits shall be designed to be available in sufficient time following a loss of all onsite alternating current power supplies and the other offsite electric power circuit, to assure that specified acceptable fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded. One of these circuits shall be designed to be available within a few seconds following a loss-of-coolant accident to assure that core cooling, containment integrity, and other vital safety functions are maintained.

Provisions shall be included to minimize 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, the loss of power from the transmission network, or the loss of power from the onsite electric power supplies.

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ENCLOSURE 1 - ATTACHMENT 2 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants:

LWR Edition (NUREG-0800, Formerly issued as NUREG-75/087)

E1-49

ENCLOSURE 1 - ATTACHMENT 2 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS E1-50

ENCLOSURE 1 - ATTACHMENT 2 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS E1-51

ENCLOSURE 1 - ATTACHMENT 2 DEGRADED VOLTAGE SCHEME REGULATORY ANALYSIS E1-52

ENCLOSURE 2 TENNESSEE VALLEY AUTHORITY RESPONSE TO WATTS BAR NUCLEAR PLANT UNIT 2, REQUEST FOR ADDITIONAL INFORMATION REGARDING CHAPTER 8, ELECTRICAL POWER - SUPPLEMENTAL SAFETY EVALUATION REPORT, OPEN ITEM 30 E2-1

ENCLOSURE 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION QUESTION 1 Describe the analysis that was performed and the methodology used to demonstrate that for the worst-case design basis operating configuration, the existing DVR Analytical Limit (drop-out) of 6555 volts is adequate to automatically start as well as operate all required safety-related loads thus meeting the NRC staff positions established in SRP BTP PSB-1 and comply with the requirements of GDC 17 concerning offsite power source.

RESPONSE 1 NRC regulatory requirements stated in 10 CFR 50, Appendix A, General Design Criteria (GDC

17) (Reference 1) and NRC Branch Technical Position PSB-1 (Reference 2) govern the application of the degraded and loss of voltage relaying systems for TVA Nuclear Plants. PSB-1 recommends that two levels of undervoltage protection be provided on the safety-related electrical distribution system. The first level of undervoltage protection is the loss of voltage relay (LVR), whose primary function is to detect and disconnect the safety-related boards upon a loss of offsite power. The second level of undervoltage protection is the degraded voltage relay (DVR), which is provided to protect Class 1E equipment from sustained low voltage conditions on the offsite power grid. The combination of the LVR system and the DVR system provides protection for the safety-related electrical distribution systems from degraded and loss of voltage conditions on the offsite power supply.

The TVA analyses that established the DVR Analytical Limit (dropout) was originally based on the operating (running) voltage for required safety-related loads. This Analytical Limit has also been shown by analysis to be adequate to start all safety-related motors, one at a time, under worst-case loading conditions. The associated DVR time delay setting has been shown by analyses to be long enough to allow for motor starting and short enough to ensure safety-related equipment will perform its functions within accident analysis time limits during postulated worst-case voltage transients.

The TVA analyses that originally established the LVR Analytical Limit (dropout) was based on the voltage required to prevent safety-related motors from stalling. This LVR setting is integral to the degraded voltage protection scheme by limiting the magnitude and duration of degraded voltage on the safety-related buses and ensuring that safety-related motors can accelerate.

A confirmatory sensitivity analysis was also performed by TVA using these established DVR/LVR set points and time delays. The confirmatory analysis demonstrated that the DVR/LVR scheme (analytical basis for the set points and time delays) protects the safety-related loads and ensures proper transfer to the onsite power supply (within safety analysis time E2-2

ENCLOSURE 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION limits) when offsite power is not capable to maintain voltage and accept the required accident loads, but allows the safety-related buses to remain connected to offsite power when offsite power is capable to accept the required accident loads within safety analysis time limits. These results are true for any possible voltage outcome at the DVR monitored bus for any grid voltage and capability. All required design basis configurations and scenarios are considered (all 6.9kV Shutdown Boards aligned to a single CSST C or D, 6.9kV Shutdown Boards aligned to CSST A or B, LOCA in one unit with orderly shutdown of the other, SI Phase A or SI Phase B). The DVR/LVR scheme thereby meets the NRC staff positions established in PSB-1 and complies with the requirements of GDC 17 concerning offsite power sources.

A. Analysis for DVR and LVR Set Points and Time Delays This is a summary of the analysis for establishing the DVR and LVR set points and time delays. Five analyses were performed to establish the protection scheme and they are as follows.

1. Component Analysis Detailed voltage drop analyses were performed to identify the anticipated terminal voltages under postulated accident and operating conditions. The voltage drop analyses identified the enveloping components of the safety-related electrical distribution system loads for:

a) operation of the safety-related electrical distribution system under steady-state (running) conditions with the safety-related bus voltage as low as possible while still keeping all connected safety-related loads within their design operating voltage range (Reference 3);

b) operation of the safety-related electrical distribution system under steady-state (running) conditions with the safety-related bus voltage as low as possible while keeping all connected safety-related motor loads above their stall voltage (Reference 3); and c) operation of the safety-related electrical distribution system under accident conditions (accident initiated starting) with the offsite power supply at the minimum allowable (Operable) voltage and capacity (minimum post-event voltage and maximum post-event voltage drop) (Reference 4)

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ENCLOSURE 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION

2. Determination of DVR Dropout Setting An Analytical Limit was established for the set point (dropout setting) of the DVR, which is greater than the minimum safety-related bus voltage established in analysis A.1.a.

This analysis starts at the component level and is used to ensure safety-related loads are not damaged during a degraded voltage event. The nominal set point of the dropout setting is selected considering tolerances from measurement errors, drift errors, and other errors in accordance with TVA Set point Technical Instruction TI-28 (Reference 7).

In addition, a motor starting analysis (Reference 5, Appendix A) was performed to demonstrate the adequacy of the DVR dropout Analytical Limit for starting all safety-related motors, one at a time, assuming maximum system loading for all required operating and accident scenarios. As a practical matter, this analysis provides confidence for the starting of components from a design basis accident signal, and that individual components will start and run when called upon by Operators.

3. Determination of DVR Reset Setting An upper boundary was also established for the reset setting of the DVR. The tightest possible tolerance was employed between the nominal dropout and reset set points.

The Analytical Limit for DVR reset is equal to the nominal set point plus all tolerances from measurement errors, drift errors, and other errors in accordance with TVA Set point Technical Instruction TI-28. Since analysis A.1.c determines that the safety-related bus voltage drops below the dropout setting of the relay during the accident (block-start) transient, then it must recover above the Analytical Limit for DVR reset prior to the lower Analytical Limit of the DVR time delay selected in Part 4 (DVR Time Delay Setting).

4. Determination of DVR Time Delay Setting The DVR time delay allows the safety-related electrical distribution system to overcome expected motor starting transients and other transients (such as lightning strikes or switching transients) without DVR actuation and also ensures protective overcurrent devices for the connected equipment do not actuate (lock-out) prior to transfer to the onsite power supply. The upper Analytical Limit of the time delay is less than the total time allowed in the safety analysis for the emergency diesel generators to come up to rated speed and voltage and be ready to accept load (11.5 sec).

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ENCLOSURE 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION In addition, a protective device evaluation was performed for the safety-related motors that start immediately upon receipt of an accident signal (Reference 3). The evaluation considered that the safety-related bus voltage was at the calculated worst-case transient voltage drop during the accident loading sequence (from analysis A.1.c). This is conservative with respect to the current drawn by the motors because it assumes the system voltage does not recover after the initial voltage drop created by the starting accident loads (does not reset the DVR) and any lesser voltage would result in less current drawn by the starting motors. The protective devices were evaluated for a duration equal to the upper Analytical Limit of the DVR time delay plus the motor acceleration time when applied on the onsite power supply at rated motor voltage.

Control power transformer circuits were also evaluated for the safety-related, accident-initiated 480 volt motors to ensure that the control circuit fuses will not blow when carrying inrush current of the starter plus any other normally energized devices, for the duration of the safety analysis time limit. This is a conservative approach, because the actual current would be less during a degraded voltage scenario. Since analysis A.1.c determines that the safety-related bus voltage drops below the DVR dropout setting during the accident (block-start) transient with minimum allowable offsite power supply voltage, the DVR time delay lower Analytical Limit is more than the time required for the voltage to recover above the DVR reset Analytical Limit (see part A.3). The nominal setpoint was selected considering all applicable errors and tolerances in accordance with TVA Setpoint Technical Instruction TI-28.

5. Determination of the LVR Set Point and Time Delay Settings The LVR is an integral part of the degraded voltage protection scheme, and its setting and time delay is selected to limit the magnitude and duration of degraded voltage on the safety-related buses (Reference 3). The Analytical Limit for LVR reset was established to be less than, with some margin, the minimum safety-related bus voltage that occurs during the accident (block-start) transient from analysis A.1.c. In addition, the Analytical Limit for LVR dropout is greater than the safety-related bus voltage that produces motor stall conditions from analysis A.1.b. The LVR nominal set point was then selected considering tolerances from measurement errors, drift errors, and other errors in accordance with TVA Set point Technical Instruction TI-28.

The lower Analytical Limit of the LVR time delay was chosen long enough to ride through short-circuits and other short-time system transients (for example, lightning strikes or switching transients), taking into account the total sensing and clearing times for these types of events. The upper Analytical Limit of the LVR time delay is less than the time allowed in the safety analysis for loss of voltage detection (1.5 sec). The E2-5

ENCLOSURE 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION nominal set point was selected considering all applicable errors and tolerances in accordance with TVA Set point Technical Instruction TI-28.

B. Sensitivity Analysis This is a summary of a confirmatory sensitivity study that was performed to ensure the previously established DVR and LVR set points and time delays were also adequate to protect the starting function of required safety-related motors.

The method TVA chose demonstrates that the safety-related bus voltage during a motor starting event is acceptable to assure the starting function and is not dependent on any assumption of grid capability (voltage or capacity) prior to the event. The sensitivity analysis (Reference 5, Appendix B and H) was performed to distinguish between all possible grid conditions that provide sufficient capability for successful performance of design basis requirements and those that do not (degraded grid conditions). This analysis considered all variations of grid conditions (infinite combinations of grid voltage and capacity) and ensured that for any motor starting event where voltage drops below the DVR dropout setting and then recovers to the reset value, it will recover above the Analytical Limit for DVR dropout (6555 volts) within a maximum 4 second time delay. This time delay is acceptable based on WBNs safety analysis limit of 5 seconds for restoration of safety-related bus voltage, when relying on offsite power (Reference 6).

The detailed analytical approach used is based on the fact that for any motor starting event (single motor or group of motors), the voltage at the DVR monitored bus can respond in only one of three ways:

1. The voltage decreases to the DVR dropout, or above (DVR does not actuate)

2. The voltage decreases below the DVR dropout and recover (DVR actuates, but does not initiate transfer to the onsite supply since voltage remains above the LVR setpoint and recovers above the DVR reset value prior to timeout); or

3. The voltage decreases below the DVR setpoint and does not recover (DVR actuates and times out or the LVR actuates and times out. In either case, the safety-related buses are transferred to the onsite power supply after the associated time delay.)

Detailed analysis was performed to address each of the three scenarios as follows:

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ENCLOSURE 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION

For any design basis operating or accident scenario where the safety-related bus voltage responds per scenario 1, a motor starting analysis (Reference 5, Appendix A) was performed to demonstrate the adequacy of the DVR dropout Analytical Limit (see part A.2). This voltage level is conservative since it bounds all possible safety-related bus voltage outcomes for scenario 1. For additional information, refer to the Region 1 discussion in Section 8.3 of Enclosure 1.

For any design basis operating or accident scenario where the safety-related bus voltage responds per scenario 2, a sensitivity analysis (Reference 5, Appendix B and H) was performed using a detailed dynamic model of the WBN onsite electrical distribution system and offsite power grid. The purpose of this sensitivity analysis was to determine the maximum possible voltage recovery time for all grid conditions that allow reset of the DVR. In order to determine the worst-case grid condition that will (minimally) allow reset of the DVR, all variations of grid conditions were considered (infinite combinations of grid voltage and capacity, including degraded conditions) at the onset of the motor starting event (t=0+). Automatic load tap changers were not credited. All required operating and accident scenarios and alignments were considered (LOCA in one unit with orderly shutdown of the other, SI Phase A or SI Phase B, all 6.9kV Shutdown Boards aligned to a single CSST C or D, 6.9kV Shutdown Boards aligned to CSST A or B). The analysis was performed specifically with 6.9kV Shutdown Board 1A-A as a representative case. Any voltage variation using the other shutdown boards would be minor and would not change the outcome of the analysis. This analysis demonstrates that for any possible safety-related bus voltage transient where voltage drops below the DVR dropout setting and then recovers to the reset value, including a drop to the LVR setpoint, it will recover above the DVR dropout Analytical Limit (6555 volts) within a maximum 4 second time delay. Since the DVR dropout Analytical Limit was previously demonstrated as adequate to automatically start all required safety-related motors (scenario 1), and since WBNs accident analysis assumes an acceptable maximum 5 second time delay for restoration of safety-related bus voltage when relying on offsite power (Reference 6), this analysis confirms the DVR dropout Analytical Limit is also adequate to automatically start all required safety-related motors for scenario 2. The following graphic Figure 1-1, shows an example of the sensitivity analysis outcome.

For additional information, refer to the Region 1 discussion in Section 8.3 of Enclosure 1.

For any design basis operating or accident scenario where the safety-related bus voltage responds per scenario 3, the protective device analysis previously performed (part A.4) demonstrates that the safety-related buses will be successfully transferred E2-7

ENCLOSURE 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION to the onsite power supply and all required loads automatically started prior to any protective device lock out. For additional information, refer to the Region 3 discussion in Section 8.3 of Enclosure 1.

These results are true for any possible voltage outcome at the DVR monitored bus and during all required design basis scenarios and operating configurations. Collectively, the analyses for these three scenarios demonstrate that for all grid conditions, the DVR/LVR scheme, including established set points and time delays, 1) properly protects the safety-related loads and ensures adequate transfer to the onsite power supply when offsite power voltage is not capable of maintaining voltage and accepting required accident loads, and 2) properly allows the safety-related buses to remain connected to offsite power when grid voltage is capable of maintaining voltage and accepting required safety-related loads.

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ENCLOSURE 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION References

1. Appendix A to Part 50 - General Design Criteria for Nuclear Power Plants, Criterion 17 -

Electric Power Systems (GDC 17)

2. Branch Technical Position PSB-1, Adequacy of Station Electric Distribution System Voltages

3. WBNEEBMSTI060029, Revision 37 Degraded Voltage Analysis

4. EDQ00099920070002, Revision 41 AC Auxiliary Power System Analysis (dual unit operation)

5. WBN Calculation STUDY-EEB-WBN-12-001 R2, Sensitivity Study of Degraded Voltage Relay (DVR) Protection During Motor Starting

6. WAT-D-9235, Tennessee Valley Authority Watts Bar Unit Numbers 1 and 2, Five Second Delay for Safety Injection and Feedwater Isolation

7. TVA Branch Technical Instruction BTI-EEB-TI-28 R10, Setpoint Calculations E2-9

ENCLOSURE 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION E2-10

ENCLOSURE 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION QUESTION 2 If the analysis concludes that all safety-related loads cannot perform their intended safety functions, while relying on offsite power, at the DVR Analytical Limit of 6555 volts for the worst-case design basis operating configuration, identify the loads that are affected, the required safety function and explain the consequences of their failure to perform the intended safety functions. In addition, provide all design basis operating configurations that TVA can demonstrate that all safety-related loads can perform their intended safety functions at the DVR Analytical Limit of 6555 volts.

RESPONSE 2 The analysis (in RAI Question 1) concluded that safety related loads can perform their intended safety functions.

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ENCLOSURE 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION QUESTION 3 WBN design requires a minimum of 432V on the 480V busses to satisfactorily mitigate the consequences of a design basis event coupled with degraded voltage conditions. The analytical studies indicate that the 480V board voltage may result in a voltage lower than the required 432V during medium voltage motor starts. The analyses assumes that either :

a. The grid is sufficiently viable to allow recovery to the Degraded Voltage Drop out value (6555), within 5 seconds the 480V system voltage will recover to greater than 432 Volts, OR

b. If the voltage does not reach the DVR drop out, then the voltage is inconsequential since it will result in a transfer to the DGs.

Given that the 5 second limitation is related to plant operation on offsite power, explain the rationale for statement (b) above, whereby the plant may be on a degraded offsite power source for more than 5 seconds until transfer to the onsite power system.

RESPONSE 3 The scenarios described in this question occur in Region 2 of Figure 8-1 and are addressed by Section 8.3 of .

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ENCLOSURE 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION QUESTION 4 Please provide the minimum grid voltage range(s) for the offsite power source(s) to be considered available and Operable in accordance with the Plant Technical Specifications requirements and GL 2006-002 guidance.

Also, provide the corresponding required minimum offsite power source voltage at the 6.9 kV safety-related buses for it to be considered available and Operable. If the DVR analytical voltage limit is not the basis for the operability, provide technical and regulatory basis for not changing the existing Analytical Limit specified for the DVR.

RESPONSE 4 For WBN, the specific grid voltage criteria for offsite power to be considered available and Operable in accordance with the Plant Technical Specifications requirements and GL 2006-002 guidance is:

U1 Operation (existing):

>153kV (minimum post-event grid voltage) and <11kV drop (from pre-event voltage)

Dual Unit Operation:

>153kV (minimum post-event grid voltage) and <9kV drop (from pre-event voltage)

The DVR Analytical Limits are not the basis for operability. The technical basis is described in Section 8.2 and the regulatory basis is described in Section 9 of Enclosure 1.

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ENCLOSURE 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION QUESTION 5 Confirm that Watts Bar Units 1 and 2 will maintain the compliance with guidance provided in GL 2006- 02 concerning the use of protocols between the nuclear power plant and the transmission system operator (TSO) to determine the operability of offsite power systems under plant technical specifications requirements. In addition, describe the design basis requirements that will be established for dual unit operation at Watts Bar and the agreement that will be established between the Watts Bar Units and the grid operator to ensure that offsite power system has adequate capacity and capability to meet the GDC 17 requirements.

RESPONSE 5 Watts Bar Nuclear Plant (WBN) Units 1 and 2 will maintain existing protocols between the nuclear power plant and the transmission system operator (TSO) to determine the operability of offsite power. These protocols are consistent with the guidance provided in GL 2006-02 and described in section 7.4 of Enclosure 1. The design basis requirements established for dual unit operation are discussed in Section 7.5 of Enclosure 1.

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ENCLOSURE 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION QUESTION 6 TVA Nuclear (TVAN) provided additional information in letter dated January 31, 2007 (ADAMS ML070330051) to supplement the responses to questions 1 and 5 in in GL 2006-02. The response to question 1 states The DVR dropout point is based on the minimum safety buss voltage that provides adequate voltage to required safety loads during worst-case loading conditions. Please provide an explanation for this statement based on the definition for operability of offsite power sources provided in GL 2006-02.

RESPONSE 6 An analysis of WBN offsite sources is presented in section 7.2 of Enclosure 1.

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ENCLOSURE 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION QUESTION 7 Please describe the plant operating and alarm response procedures, control room alarms and indications available to the operator to monitor the offsite power system availability and operability and take the required actions in accordance with plant operating and alarm response procedures.

RESPONSE 7 Sections 7.4, 7.5, and 7.6 of Enclosure 1 provide descriptions of the transmission and nuclear plant operator interface procedures, alarms, and responses.

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ENCLOSURE 3 RESPONSES TO ADDITIONAL QUESTIONS FROM DECEMBER 10, 2014 MEETING

1. Clearly describe how and why TVAs degraded voltage protection scheme meets GDC 17, PSB-1, and RIS requirements. Be specific about how and why TVA complies with the specific portions of GDC 17, PSB-1, and the RIS. Clearly document if TVAs position is different so the NRC can review and explain why TVAs interpretation is safe.

Specifically explain how we address the last paragraph of the RIS on page 6 through the 3rd paragraph on page 7.

Section 9 of Enclosure 1 describes how TVA meets the regulatory requirements. Per January 27, 2015 teleconference between TVA (E. Cobey) and NRC (J. Lubinski), NRC confirmed that TVAs response should address specific regulatory requirements and guidance and that a discussion relative to RIS 2011-12 was not required.

2. Define capable grid and contrast to Operable grid.

Definitions for Operable and Adequate Offsite Power (capable grid) are in section 5 of Enclosure 1. Additionally, Figure 8-1 and the associated summary in section 8.3 illustrate the differences between the two.

3. Provide a worst case analysis where the starting point is 6555V at T=0- for the 6.9KV and 480V loads. The analysis will assume a 95/95 reliability normal grid capacity.

Based on discussion during the December 17, 2014 meeting, NRC confirmed this question no longer required a response.

4. Clarify where starting voltages were used in TVAs DRAFT response to RAI 1.

Based on discussion during the December 17, 2014 meeting, NRC confirmed this question no longer required a response.

5. Revise the response to RAI 2 to include the consequences of a transfer to onsite power starting at 6555V for T=0+ and T=0-.

Please see the revised response to RAI 2.

6. State what minimum voltage is necessary to remain on offsite power.

Figure 8-1 and the associated summary in section 8.3 describe what is needed to remain on offsite power.

7. Address how TVA has used IEEE-741.

Based on discussion during the December 17, 2014 meeting, NRC confirmed this question no longer required a response.

8. Define T=0, T=0-, and T=0+.

T=0 is the time at initiation of an ESF actuation T=0- is pre-ESF actuation T=0+ is post-ESF actuation E3-1

ENCLOSURE 3 RESPONSES TO ADDITIONAL QUESTIONS FROM DECEMBER 10, 2014 MEETING Figure 8-1 and the associated summary in section 8.3 describe these times.

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ENCLOSURE 4 TENNESSEE VALLEY AUTHORITY CHRONOLOGY OF STAFF INTERACTIONS AND RELATED CORRESPONDENCE E4-1

ENCLOSURE 4 CHRONOLOGY OF STAFF INTERACTIONS AND RELATED CORRESPONDENCE This attachment provides a summary of the relevant history of licensing basis documents and submittals associated with the GDC 17 offsite power source capacity and capability and the Class 1E safety bus undervoltage protection schemes for Watts Bar Nuclear Plant Unit 2 (WBN-2).

Watts Bar Nuclear Plant Unit 1 Relevant Timeline to Licensing Watts Bar Unit Nuclear Plant Unit 1 (WBN-1) was licensed Feb 7, 1996 under docket 50-390.

The aspect of the WBN-1 licensing basis, as related to the degraded voltage relay (DVR) setting, is documented in two Safety Evaluation Reports (SERs) and the corresponding Tennessee Valley Authority (TVA) submittals: NUREG-0847 Safety Evaluation Report related to operation of Watts Bar Nuclear Plant, Units 1 and 2, Docket Nos. 50-390 and 50-391, June 1982 (Reference 1); and Watts Bar Nuclear Plant, Unit 1 - Issuance of Amendment Regarding Increase of the Time Delay Setting Of 6.9 kV Emergency Bus Degraded Voltage Function from 6 to 10 Seconds (TAC No. MB1954); dated January 23, 2002 (Reference 2).

As illustrated below, the Watts Bar Nuclear Plant (WBN) 1982 SER and its Supplements document compliance of WBN, Units 1 and 2 designs with respect to PSB-1.

WBN 1982 SER- Section 8.3.1.2 (Reference 1):

in IEEE Standard 308-1974, which states that preferred offsite and the standby onsite emergency power supplies shall not have a common mode failure between them. The positions that the staff have developed are being used in the evaluation of electrical power designs for operating plants, and CP [Construction Permit] and OL [Operating License] applications. The applicant was made aware of these positions, which have been incorporated into SRP

[Standard Review Plan] Appendix 8A as BTP [Branch Technical Position] PSB-1[Power Systems Branch]. The applicant documented that the Watts Bar design will be modified as shown on FSAR [Final Safety Analysis Report] Figure 040.62-1 to meet BTP PSB-1. By letter dated October 9, 1981, the applicant provided additional descriptive information to support the conclusion that the Watts Bar design, once modified, will be in conformance with positions B-1 and B-2 of BTP PSB-1. The staff concludes that the proposed design meets BTP positions and is acceptable. In addition, design implementation will be verified as part of the site visit/drawing review. In regard to positions 3 and 4, the applicant has documented that the auxiliary power system meets these positions and that the analyses will be verified in the preoperational testing program. This meets the staff positions and is acceptable, pending verification of the analyses.

The staff will verify the test results.

WBN 1982 Supplemental Safety Evaluation Report (SSER), Supplement 7, Section 8.3.1.2 dated September 1991 (Reference 3):

In the SER, the staff stated that it would verify the adequacy of the applicant's analysis regarding compliance with Branch Technical Position (BTP) PSB-1 once the preoperational test was completed. The staff noted that the preoperational test has shown that the Watts Bar design conforms with BTP PSB-1 (see Inspection Report 50-390/84-90, dated February 11, 1985). The staff is still evaluating the status of this issue and will update the status in a future SSER.

WBN 1982 SER, SSER Supplement 13, Section 8.3.1.2 dated April 1994 (Reference 4):

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ENCLOSURE 4 CHRONOLOGY OF STAFF INTERACTIONS AND RELATED CORRESPONDENCE In the SER, the staff stated that it would verify the adequacy of the applicant's analysis regarding compliance with BTP PSB-1 once the preoperational test was completed. The staff had confirmed that a preoperational test had shown that the Watts Bar design conforms with BTP PSB-1 (see Inspection Report 50-390/84-90, dated February 11, 1985). Hence, Confirmatory Issue 28 was resolved. However, due to design changes, the results obtained from the previous test are no longer valid and the applicant is performing preoperational tests again. The staff will review this issue when it inspects the applicant's preoperational test program.

WBN 1982 SER, SSER Supplement 14, Section 8.3.1.2 dated December 1994 (Reference 5):

The material that follows revises the discussion in SSER 13.

(1) Allowable Technical Specification Limits for the Inverse Time Delay Relay. In SSER 13, the staff stated that Technical Specifications should require, for example, that the capability of the relays not to trip when subjected to a voltage of 75 percent for 30 seconds be demonstrated.

The staff implied that this had been included in the draft Technical Specifications. This statement was wrong. Instead, the staff required that the setpoints and allowable values for the load-shed and diesel start relays be included in the plant's Technical Specifications to resolve the concerns.

WBN 1982 SER, SSER Supplement 20, Section 8.3.1.2 dated February 1996 (Reference 6):

In SSER 13, the staff stated that Confirmatory Issue 28 was resolved on the basis of a preoperational test documented in Inspection Report 50-390/84-90, dated February 11, 1985.

However, the staff stated that the results obtained from that test were no longer valid since TVA was re-performing the preoperational tests. The preoperational test was conducted by TVA and reviewed by the staff in Inspection Reports 50-390/95-22 (September 8, 1995) and 50-390/95-77 (December 6, 1995). This update does not change the staff's conclusion regarding Confirmatory Issue 28.

As illustrated below WBN Unit1 2002 SER explicitly credits Load Tap Changers (LTCs) for both the dynamic block-start motor loading and the static individual start motors at steady-state conditions. (Reference 2):

The Degraded Voltage Relays have a voltage setpoint of 96% of 6900 V (nominal) and are arranged in a two out of three coincidence logic to initiate a nominal time delay. This arrangement provides time for the load tap changers (LTCs) on CSSTs [Common Station Service Transformers] C and D to recover the degraded voltage condition at the 6.9 kV shutdown boards back to an acceptable level resetting the Degraded Voltage Relays. The CSST LTCs maintain the nominal steady state 6.9 kV shutdown board voltage between 7010 and 7132 V for variation in board loading and grid voltage The analysis was performed using the Electrical Transient Analyzer Program The calculations demonstrated the ability of the offsite power system to start and operate all required loads for a worst case DBE without transferring to the EDGs The lower boundary dropout and the upper reset setpoint of the Degraded Voltage Relays remains unchanged. Analyses have shown that operating equipment, such as motors, would not be damaged and would accelerate back to rated speed, thus ensuring their continued availability to perform their intended safety function.

Specifically, the analysis demonstrated that the required safety-related equipment in operation at the time a degraded voltage condition occurred would continue to operate throughout the 10-E4-2

ENCLOSURE 4 CHRONOLOGY OF STAFF INTERACTIONS AND RELATED CORRESPONDENCE second delay. If the degraded voltage condition cleared during this time period, the voltage would return to nominal levels and be available for equipment required to perform safety functions. Calculations demonstrated that the automatic LTCs remain capable of regulating the 6.9 kV shutdown board voltage within the present voltage relay setpoints. The LTCs will restore 6.9 kV shutdown board voltage for a safety injection signal with a simultaneous worst case grid drop before the Degraded Voltage Relays actuate to transfer power supply to the EDGs.

Engineered safeguard motors will have sufficient voltage available at the terminals to ensure proper starting and operation, when supplied by offsite power. Maximum loading on transformers, distribution system cables, and 6900 V and 480 V boards is bounded by current analyses and remains below component ratings The 1982 SER and Supplement 1 were based on estimates that preceded formal calculations.

The 1982 SER Supplement 2 and beyond were based on the TVA submittal that was based on calculation WNB-EEB-MS-TI06-0029, Degraded Voltage Analysis. The 2002 SER was based on the TVA submittal that was based on calculation WNB-EEB-MS-TI06-0029, Degraded Voltage Analysis (Reference 7). Review of calculation WNB-EEB-MS-TI06-0029, Degraded Voltage Analysis, identified that this calculation:

Used IEEE Standard 741-1997, IEEE Standard Criteria for the Protection of Class 1E Power Systems and Equipment in Nuclear Power Generating Stations a design input.

Addressed both dynamic and static motor loading conditions.

Credited LTCs in modeling of the system response to the dynamic block-start motor loading and the static individual start motors at steady-state conditions.

Did not credit LTCs in development of DVR setpoints.

Established the DVR set point and reset values such that bus voltage would recover above the reset value prior to the lower boundary of the time delay.

The WBN 1982 SER and its Supplements document compliance of WBN Unit 1 design with PSB-1. WBN Unit 1 2002 SER explicitly credits LTCs, for both the Design Basis Event block-start motor loading and also for the individual starting of motors at steady-state loading conditions.

WBN Unit 2 Relevant Timeline to Licensing In SSER 21 issued Feb 2009 (Reference 8), the staff addressed TVA's application for a license to operate WBN Unit 2, and provided information regarding the status of the items remaining to be resolved, which were outstanding at the time that TVA deferred construction of WBN Unit 2, and were not evaluated and resolved as part of the licensing of WBN Unit 1.

The SSER also stated that in its SER and Supplemental SER (SSER) Nos. 1 through 20 issued by the Office of Nuclear Reactor Regulation (NRR) of the U.S. Nuclear Regulatory Commission (NRC or the staff), the staff documented its safety evaluation and determination that WBN Unit 1 met all applicable regulations and regulatory guidance. Based on satisfactory findings from all applicable inspections, on February 7, 1996, the NRC issued a full-power operating license (OL) to WBN Unit 1, authorizing operation up to 100-percent power. In SSER 21 and future SSERs, the staff will document its evaluation and closure of open items in support of TVA's application for a license to operate WBN Unit 2.

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ENCLOSURE 4 CHRONOLOGY OF STAFF INTERACTIONS AND RELATED CORRESPONDENCE NUREG-0847 Supplement 22, dated January 2011, under section 8.3.1.2 (Reference 9),

establishes the DVR setpoint as an open item 30 under Appendix HH. This establishment is in conjunction with the NRC letter to TVA, "Watts Bar Nuclear Plant Unit 2 Construction - NRC Integrated Inspection Report 05000391/2010603 and Notice of Violation," dated August 5, 2010 (Reference 10).

08/12/2011 - TVA formally responds to Open Action Item 30 established by the SSER.

(Reference 11) 06/18/2012 - TVA formally again responds by letter dated June 7, 2012 (Reference 12) to Open Action Item 30 by submitting additional calculation STUDY-EEB-WBN-12-001 (Reference 13).

08/02/2013 - NRC Announcement of 08/19/13 Public Meeting with TVA. The meeting covered the open items remaining for WBN Unit 2 and specifically discussed the degraded voltage relay design as it related to WBN2 (ADAMS Accession No. ML13213A410 ).

01/28/2014 - NRC Summary of Public Meeting with TVA held on January 28, 2014 (ADAMS Accession No. ML14091A272). The NRC and TVA discussed the status of each of the electrical open items documented in supplements to NUREG-0847 "Safety Evaluation Report Related to the Operation of Watts Bar Nuclear Plant, Unit 2." The main focus was on open item number 30 related to degraded voltage issues. TVA stated that they believe they addressed this issue already and that if the NRC staff doesn't agree with TVA's approach that the NRC needs to formally document this in an RAI so that they could find a path to resolution. In the meeting, the NRC staff agreed to review TVA's last submittal attempting to resolve this issue and would issue RAIs if required. Subsequent to the meeting the staff issued an RAI on February 27, 2014 (Reference 14).

02/27/2014 - NRC Submits RAI to TVA as a response to TVA letter dated June 7, 2012 (Reference 14). Five questions were submitted for TVA response.

04/24/2014 -TVA slides for Public Meeting with NRC and - NRC Summary of Public Meeting with TVA. This meeting was held to provide additional information to the NRC regarding the upcoming responses to the RAIs issued on 2/27/2014 (ADAMS Accession No. ML14118A194 and ML14118A201).

06/13/2014 - TVA responds to NRC RAIs from 2/27/14 (Reference 15) plus submits a revised STUDY-EEB-WBN-12-001 Rev 2 (Reference 16) for NRC review.

11/04/2014 - NRC issues its audit plan for the inspection scheduled to start on 11/4/14 (Reference 17). The NRC stated this is in support of Open Item 30, the current RAI responses from those dated on 2/27/14, and the revised calculation STUDY-EEB-WBN-12-001 Rev 2.

11/12/2014 - NRC withdraws previous RAIs on 11/7/14 and submits the current set of RAI as a result of the audit held on 11/04/2014 (Reference 18).

12/4/2014 - TVA submits its draft responses to the NRCs RAI dated 11/12/14 (Reference 19).

12/10/2014 - NRC holds a public meeting to discuss with TVA its draft RAI responses submitted on 12/04/2014 and asks for eight additional items of clarification.

12/17/2014 - NRC holds a second public meeting to discuss with TVA its draft RAI responses submitted on 12/04/2014.

E4-4

ENCLOSURE 4 CHRONOLOGY OF STAFF INTERACTIONS AND RELATED CORRESPONDENCE References

1. .U.S. NRC NUREG-0847, Safety Evaluation Report related to the operation of Watts Bar Unit 1 and 2 Dockets Nos. 50-390 and 50-391, June 1982. ADAM Accession No. ML073450539

2. U.S. NRC Letter to Mr. J. A. Scalice, Chief Nuclear Officer and Executive Vice President of the Tennessee Valley authority, dated January 23, 2002 Watts Bar Nuclear Plant ,

Unit 1 - Issuance of Amendment regarding increase of the time delay setting of 6.9 kV emergency bus degraded voltage function from 6 to 10 seconds (TAC No. MB 1954).

ADAMS Accession No. ML020230334.

3. U.S. NRC NUREG-0847, Supplement Safety Evaluation Report 7 related to the operation of Watts Bar Unit 1 and 2 Dockets Nos. 50-390 and 50-391, September 1991.

ADAMS Accession No. ML073520438.

4. U.S. NRC NUREG-0847, Supplement Safety Evaluation Report 13 related to the operation of Watts Bar Unit 1 and 2 Dockets Nos. 50-390 and 50-391, April 1994.

ADAMS Accession No. ML072060484.

5. U.S. NRC NUREG-0847, Supplement Safety Evaluation Report 14 related to the operation of Watts Bar Unit 1 and 2 Dockets Nos. 50-390 and 50-391, December 1994.

ADAMS Accession No. ML072060486.

6. U.S. NRC NUREG-0847, Supplement Safety Evaluation Report 20 related to the operation of Watts Bar Unit 1 and 2 Dockets Nos. 50-390 and 50-391, February 1996.

ADAM Accession No. ML072060498.

7. WBN Calculation WBNEEBMST1060029, Revision 37, Degraded Voltage Analysis.

8. U.S. NRC NUREG-0847, Supplement Safety Evaluation Report 21 related to the operation of Watts Bar Unit 1 and 2 Dockets Nos. 50-390 and 50-391, February 2009, ADAM Accession No. ML090340719.

9. U.S. NRC NUREG-0847, Supplement Safety Evaluation Report 22 related to the operation of Watts Bar Unit 1 and 2 Dockets No 50-391, January 2011, ADAM Accession No. ML110390197.

10. U.S. NRC Letter to NRC letter to TVA, "Watts Bar Nuclear Plant Unit 2 Construction -

NRC Integrated Inspection Report 05000391/2010603 and Notice of Violation," dated August 5, 2010, ADAM Accession No. ML102170465.

11. TVA Letter to the NRC, Watts Bar Nuclear Plant (WBN) Unit 2 - Safety Evaluation Report Supplement 22 (SSER 22) - Response to NRC Required Action Item, dated Aug 12, 2011, ADAM Accession No. ML11229A020.

12. TVA Letter to the NRC, Watts Bar Nuclear Plant (WBN) Unit 2 - NUREG-0847 Supplemental Safety Evaluation Report (SSER) Related to the Operation of Watts Bar Nuclear Plant, Unit 2, Appendix HH Open Item 30 - Power System Degraded Voltage, dated June 7, 2012, ADAM Accession No. ML12160A350.

E4-5

ENCLOSURE 4 CHRONOLOGY OF STAFF INTERACTIONS AND RELATED CORRESPONDENCE

13. WBN Calculation STUDY-EEB-WBN-12-001 Revision1, Sensitivity Study of Degraded Voltage Relay (DVR) Protection During Motor Starting.

14. NRC Email (S. Lingam) to TVA (G. Arent) dated Feb 27, 2014, RAIs - Watts Bar Nuclear Plant Unit- 2 (WBN-2) Electrical Open Items Request for Additional Information for FSAE Section 8.3, NRR/DE/EEEB (TAC NO. ME2731), ADAMS Accession No. ML14059A158.

15. TVA Letter to the NRC, Responses to Degraded Voltage Issue Requests for Additional Information, dated June 13, 2014, ADAM Accession No. ML14198A597.

16. WBN Calculation STUDY-EEB-WBN-12-001 Revision 2, Sensitivity Study of Degraded Voltage Relay (DVR) Protection During Motor Starting.

17. U.S NRC Letter to NRC letter to TVA (Shea), "Watts Bar Nuclear Plant, Unit-2, Audit Plan for Review of Degraded Voltage Protection Scheme" dated October 31, 2014, ADAM Accession No. ML14300A104.

18. NRC Email (M. Miernicki) to TVA (G. Arent), dated November 12, 2014,

Subject:

Watts Bar 2 - RAIs - Degraded Voltage Relay Issue.

Attachment:

Watts Bar DVR RAIs Final 11-12-2014.docx

19. TVA Email (R. Stroud) to NRC (M. Miernicki), dated Dec 4, 2014, Draft RAIs - DRAFT Response to Degraded Voltage RAIs.

E4-6

v t e Letter Sequence Supplement
TAC:ME2731, (Withdrawn, Closed)
Initiation Request, Request, Request, Request, Request, Request Acceptance... Supplement, Supplement Administration Withholding Request Acceptance Questions 28 January 2014 11 March 2010 23 June 2010 12 July 2010 2 July 2010 24 June 2010 8 July 2010 29 June 2010 15 July 2010 27 August 2010 8 September 2010 Answers 9 April 2010 6 August 2010 5 August 2010 4 October 2010 10 February 2011 13 June 2012 18 March 2013 Results Withdrawal Other: CNL-14-077, Responses to Degraded Voltage Issue Requests for Additional Information, ML102290358, ML102290360, ML102290361, ML102290362, ML102290363, ML102290364, ML102290365, ML102290366, ML102290367, ML102290368, ML102290383, ML102290384, ML102290385, ML102290386, ML102290387, ML102290388, ML102290389, ML102290390, ML102290391, ML102290392, ML102290403, ML102290404, ML102290405, ML102290406, ML102290407, ML102290408, ML102290409, ML102290410, ML102290411, ML102290414, ML102290439, ML102290440, ML102290441, ML102290442, ML102290443, ML102290444, ML102290445, ML102290446, ML102290447, ML102290448, ML102290458, ML102290459, ML102290460, ML102290461, ML102290462, ML102290463, ML102290464, ML102290465, ML102290466... further results
MONTHYEAR2010-06-28 [Table View]

ML15030A511

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