Text

Application to Modify Technical Specifications in Support of Unit Station Service Transformer Modification (TS-SQN-12-01)
	ML12146A385
Person / Time
Site:	Sequoyah
Issue date:	05/23/2012
From:	James Shea; Tennessee Valley Authority
To:	; Document Control Desk, Office of Nuclear Reactor Regulation
References
	TS-SQN-12-01
	Download: ML12146A385 (87)
	v • d • e

Tennessee Valley Authority, 1101 Market Street, Chattanooga, Tennessee 37402 May 23, 2012 10 CFR 50.90 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Sequoyah Nuclear Plant, Units 1 and 2 Facility Operating License Nos. DPR-77 and DPR-79 NRC Docket Nos. 50-327 and 50-328

Subject:

Application to Modify Technical Specifications in Support of Unit Station Service Transformer Modification (TS-SQN-12-01)

In accordance with the provisions of 10 CFR 50.90, "Application for amendment of license, construction permit, or early site permit," the Tennessee Valley Authority (TVA) is submitting a request for an amendment to Facility Operating License Nos.

DPR-77 and DPR-79 for the Sequoyah Nuclear Plant (SQN), Units 1 and 2.

The proposed changes would revise the SQN Technical Specifications (TS) 3/4.8.1 to include a surveillance requirement (SR) to demonstrate the required offsite circuits OPERABLE at least once per 18 months by manually and automatically transferring the power supply to a 6.9 kV Unit Board from the normal supply to the alternate supply.

This change is necessary as a result of the planned modifications to the plant design and operating configuration that will allow use of the unit station service transformers (USSTs) as a power supply to an offsite circuit. In addition, typographical errors on SQN Unit 1 TS page 3/4 8-2 are corrected.

The enclosure provides a description of the proposed changes, technical evaluation of the proposed changes, regulatory evaluation, and a discussion of environmental considerations. Attachments 1 and 2 to the enclosure provide the existing TS and Bases pages marked-up to show the proposed changes. Attachments 3 and 4 to the enclosure provide the existing TS and Bases pages retyped to incorporate the proposed changes. Attachment 5 provides a diagram of the current SQN 6.9 kV AC Electrical Distribution System. Attachment 6 provides a probabilistic risk assessment of the proposed changes.

Printed on recycled paper

U.S. Nuclear Regulatory Commission Page 2 May 23, 2012 TVA requests approval of this amendment by December 1, 2012 to support SQN Unit 2 plant startup following its USST modification during the fall 2012 refueling outage. The SQN Unit 1 USST modification is planned during the fall 2013 refueling outage.

TVA has determined that there are no significant hazards considerations associated with the proposed change and that the change qualifies for a categorical exclusion from environmental review pursuant to the provisions of 10 CFR 51.22(c)(9).

The SQN Plant Operations Review Committee and the SQN Nuclear Safety Review Board have reviewed this proposed change and determined that operation of SQN in accordance with the proposed change will not endanger the health and safety of the public.

Additionally, in accordance with 10 CFR 50.91 (b)(1), TVA is sending a copy of this letter and the enclosure to the Tennessee Department of Environment and Conservation.

There are no regulatory commitments associated with this submittal. Please address any questions regarding this request to Clyde Mackaman at 423-751-2834.

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

Executed on this 23r.. day of May 2012.

Resp fully, J. aa/'S fM~nar, Corporate Nuclear Licensing

Enclosure:

Evaluation of Proposed Change cc (Enclosure):

NRC Regional Administrator - Region II NRC Resident Inspector- Sequoyah Nuclear Plant Director, Division of Radiological Health - Tennessee State Department of Environment and Conservation

ENCLOSURE TENNESSEE VALLEY AUTHORITY SEQUOYAH NUCLEAR PLANT UNITS I AND 2 EVALUATION OF PROPOSED CHANGE

Subject:

Application to Modify Technical Specifications in Support of Unit Station Service Transformer Modification (TS-SQN-12-01)

1.

SUMMARY

DESCRIPTION

2. DETAILED DESCRIPTION

3. TECHNICAL EVALUATION

4. REGULATORY EVALUATION 4.1 Applicable Regulatory Requirements/Criteria 4.2 Precedent 4.3 Significant Hazards Consideration 4.4 Conclusions

5. ENVIRONMENTAL CONSIDERATION

6. REFERENCES ATTACHMENTS

1. Proposed TS Changes (Mark-Ups) for SQN, Units 1 and 2

2. Proposed TS Bases Changes (Mark-Ups) for SQN, Units 1 and 2

3. Proposed TS Changes (Final Typed) for SQN, Units 1 and 2

4. Proposed TS Bases Changes (Final Typed) for SQN, Units 1 and 2

5. SQN 6.9 kV AC Electrical Distribution System Diagram

6. Probabilistic Risk Assessment of Proposed Changes

1.0

SUMMARY

DESCRIPTION This evaluation supports a request to amend Appendix A of Facility Operating License Nos. DPR-77 and DPR-79 for the Sequoyah Nuclear Plant (SQN), Units 1 and 2, by revising the Technical Specifications (TS) for SQN, Units 1 and 2. The proposed changes would revise TS 3/4.8.1 to include a surveillance requirement (SR) to demonstrate the required offsite circuits OPERABLE at least once per 18 months by manually and automatically transferring the power supply to a 6.9 kV Unit Board from the normal supply to the alternate supply. This change is necessary as a result of the planned modifications to the plant design and operating configuration that will facilitate use of the unit station service transformers (USSTs) as a power supply an offsite power circuit. Refer to Attachment 5 for a diagram of the current 6.9 kV AC electrical distribution system.

Implementation of the Unit 2 USST modifications will enable the output of USST 2A to be aligned as the normal power supply to 6.9 kilovolt (kV) Shutdown Board 2A-A via Unit Board 2B, and the output of USST 2B to be aligned as the normal power supply to 6.9 kV Shutdown Board 2B-B via Unit Board 2C, beginning with plant startup following the SQN Unit 2 refueling outage in the fall of 2012. Similarly, implementation of the Unit 1 USST modifications will enable the output of USST 1A to be aligned as the normal power supply to 6.9 kV Shutdown Board 1A-A via Unit Board 1B, and the output of USST 1B to be aligned as the normal power supply to 6.9 kV Shutdown Board 1 B-B via Unit Board 1C, beginning with plant startup following the SQN Unit 1 refueling outage in the fall of 2013.

2.0 DETAILED DESCRIPTION

Background

General Design Criterion 17 (GDC 17), 10 CFR Part 50, Appendix A, requires two independent offsite power circuits designed and located to minimize to the extent practical the likelihood of their simultaneous failure under operating and postulated accident and environmental conditions.

The offsite power distribution system at the SQN switchyard consists of two 161 kV buses supplied by eight 161 kV feeders and two 500 kV buses supplied by five 500 kV feeders. One of the 161 kV transmission lines connect to the Chickamauga Hydroelectric Plant and one connects to the Watts Bar Hydroelectric Plant. The electrical output of SQN Unit 1 is fed to the 500 kV buses and the electrical output of SQN Unit 2 is fed to the 161 kV buses.

The original SQN design and alignment was for the USSTs to supply unit power to the 6.9 kV Unit Boards during plant operation, with the CSSTs available to provide offsite power via the Start Buses. In the event of a loss of a USST or a unit trip, offsite power to the Class 1E 6.9 kV Shutdown-Boards was transferred at the 6.9, kV Unit Boards to the CSSTs. TS SR 4.8.1.1.1.b required demonstration of manual and automatic transfer capability from the normal power supply to the alternate power supply at least once every 18 months.

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Due to an issue identified in 1986 regarding USST impedance challenging the 6.9 kV Unit Board circuit breaker short circuit ratings, TVA implemented a design change to replace the CSST with transformers with auto tap changers to accommodate voltage variations in the 161 kV system. With installation of the new CSSTs, the offsite lineup was changed to the CSSTs providing power to the 6.9 kV Unit Boards via the Start Buses. Therefore, it was no longer necessary to transfer power during unit startups and shutdowns, or to rely on automatic power transfer in the event of a fault or plant trip in order to maintain power to the 6.9 kV Unit Boards.

SR 4.8.1.1.1.b was not deleted at the time of the design change, because it was believed that there could be instances where use of the USST as a power supply might be needed as a contingency for maintenance activities. On July 11, 1994, the NRC approved an amendment to change the wording of SR 4.8.1.1.1.b to reflect the use of the CSSTs with auto load tap changers as the normal power supply for the 6.9 kV Unit Boards (ADAMS Accession No. ML013320192).

In 1996, it was determined that placing the units on the unit generator supported supply would only be necessary for maintenance on the offsite circuits. In that situation, the TS requirements would require compliance with the actions for an inoperable offsite circuit, even if the transfer scheme was operable. Once it was determined there were no practical opportunities to utilize the unit generator support circuit, a license amendment request was submitted for elimination of SR 4.8.1.1.1 .b. On September 23, 1997, the NRC approved an amendment to eliminate SR 4.8.1.1.1.b from the SQN, Unit 1 and 2, Technical Specifications (ADAMS Accession No. ML013320312).

The CSSTs are currently arranged with CSST A and CSST C providing power to the plant through the Start Buses. CSST A supplies Start Buses 1A and 2A, and CSST C supplies Start Buses 1 B and 2B. (CSST B is a spare transformer with two sets of secondary windings that can be used to power two of the Start Buses, with each Start Bus on a separate secondary winding.) Normal unit power from the Start Buses to the 6.9 kV Unit Boards and Class 1E 6.9 kV Shutdown Boards is provided as follows (See ):

Start Bus 1A supplies 6.9 kV Shutdown Board 1B-B via Unit Board 1C;

Start Bus 1B supplies 6.9 kV Shutdown Board 1A-A via Unit Board 1B;

Start Bus 2A supplies 6.9 kV Shutdown Board 2B-B via Unit Board 2C; and

Start Bus 2B supplies 6.9 kV Shutdown Board 2A-A via Unit Board 2B.

This lineup provides two immediate access offsite circuits to each unit (one to each load group).

Proposed Plant Modification In March 2009, both SQN Units 1 and 2 received an automatic reactor trip on reactor coolant pump (RCP) undervoltage. A loss of CSST C caused a loss of power to two 6.9 kV Unit Boards on each unit that feed the RCPs. CSST B continued to supply offsite power to the Train B safety-related 6.9 kV Shutdown Boards for both units. The emergency diesel generators started and powered the Train A safety-related 6.9 kV Shutdown Boards for both units until offsite power was restored. Implementation of the Page 3 of 23

proposed USST modifications will eliminate this single point vulnerability for a dual unit trip.

The proposed USST modifications will install new USSTs and install generator circuit breakers (GCBs) in the isolated phase bus (IPB) between the generator and main bank transformer (See Attachment 5 for the location of the proposed new GCBs). To support this arrangement, the IPB will be modified to accept the connection to the GCBs. The new USSTs will be sized to meet offsite power loading requirements and will include automatic load tap changers to provide the required 6.9 kV voltage regulation.

The design function of each USST will be to provide unit power to its respective 6.9 kV Unit Boards and Class 1 E 6.9 kV Shutdown Board. The new USSTs will have a higher impedance to limit fault current to below the 6.9 kV Unit Board circuit breaker rating.

The USST ratings will ensure adequate capacity for normal and accident loads. To support the offsite power requirements and improve normal operations, the USSTs will have automatic load tap changers. The USST load tap changers will be similar to those installed on the CSSTs and will have equivalent operating times. The USST load tap changers will sense voltage at the 6.9 kV Unit Boards.

Power circuit breakers (PCBs) are located between the high side of the main bank transformers and the switchyard; one PCB for each of the two connections to the 161 kV switchyard and one PCB for each of the two connections to the 500 kV switchyard.

Currently, the PCBs serve to connect the onsite generation system with the offsite grid and provide fault protection for the offsite circuit and main generator. Following plant modifications, the main generator will be capable of supplying electrical power to the offsite grid, as well as supplying power to the offsite circuit (via the USSTs through the IPB). The GCBs will provide an isolation point between the main generator and the transformers and will trip open on a unit trip. The PCBs will trip open on a fault between the GCBs and the PCBs, or a failure of the GCBs to open on a valid command signal.

Isolating the main generator from the transformers enables the transformers to be energized from the electrical power grid without energizing the main generator. When the generator is not operating, the main bank transformers function as step down transformers and supply electrical power from the grid to the USSTs and the connected electrical loads.

Offsite power will normally be supplied from the USSTs to the 6.9 kV Unit Boards, and will automatically transfer to the alternate power supply to at least one circuit from the CSSTs on a trip of the PCBs. The transfer occurs upon receipt of a PCB trip signal, whereby the Unit Board normal supply breaker opens and the alternate supply breaker closes. The alternate supply breaker will not close until the normal supply breaker opens. This transfer typically occurs within 6 cycles and, therefore, will not result in downstream breaker action or power interruption to motors and other electrical loads.

If a 6.9 kV Unit Board normal supply breaker fails to open (or fails to receive an open signal), the undervoltage relay will initiate a trip of the normal supply breaker and will send a close signal to the alternate supply breaker. The alternate supply breaker is restrained from closing by the normal supply breaker position contact and by a residual voltage restraint relay. This residual voltage relay prevents the new alternate supply breaker from closing until the voltage on the bus has decayed to below 30 percent of Page 4 of 23

nominal. This will preclude individual motor protection trips and ensures the motors will restart when power is reapplied.

Implementation of the Unit 2 USST modifications will enable the output of USST 2A to be aligned as the normal power supply to 6.9 kV Shutdown Board 2A-A via Unit Board 2B, and the output of USST 2B to be aligned as the normal power supply to 6.9 kV Shutdown Board 2B-B via Unit Board 2C, beginning with plant startup following the SQN Unit 2 refueling outage in the fall of 2012. Similarly, implementation of the Unit 1 USST modifications will enable the output of USST 1A to be aligned as the normal power supply to 6.9 kV Shutdown Board 1A-A via Unit Board 1B, and the output of USST 1 B to be aligned as the normal power supply to 6.9 kV Shutdown Board 1 B-B via Unit Board 1C, beginning with plant startup following the SQN Unit 1 refueling outage in the fall of 2013.

Implementation of these design changes requires reinstatement of the surveillance requirement to demonstrate the capability to manually and automatically transfer the power supply to a 6.9 kV Unit Board from the normal supply to the alternate supply.

Proposed Technical Specification Changes Currently, TS 3.8.1. 1.a requires two independent circuits between the offsite transmission network and the onsite Class 1 E distribution system to be OPERABLE in MODES 1, 2, 3 and 4. TS SR4.8.1.1.1.a requires each of the above required independent circuits between the offsite transmission network and the onsite Class 1 E distribution system to be determined OPERABLE at least once per 7 days by verifying correct breaker alignments and indicated power availability.

The proposed change revises the TS with the addition of TS SR 4.8.1.1.1.b to require demonstration that the required offsite circuits are OPERABLE at least once per 18 months by manually and automatically transferring the power supply to a 6.9 kV Unit Board from the normal supply to the alternate supply. The SR is modified by two notes.

The first Note precludes performance of the SR on the associated unit's 6.9 kV Unit Boards in MODES 1 and 2. The second Note specifies that verification of transfer capability is only required to be met for 6.9 kV Unit Boards that require normal and alternate power supplies.

3.0 TECHNICAL EVALUATION

TVA performed an in-depth technical evaluation of the proposed changes as discussed below. Section 3.1 contains a description of the affected systems. Section 3.2 contains evaluations of the proposed TS change. Section 3.3 contains a discussion of the risk evaluation performed for the proposed TS change.

3.1 System Description

A description of the relevant portions of the SQN electrical distribution system is presented below as background for evaluation of the proposed changes. The offsite power system is described in Section 8.2 of the SQN Updated Final Safety Report (UFSAR) and the onsite power system is described in Section 8.3 of the UFSAR. A Page 5 of 23

diagram of the SQN 6.9 kV AC Electrical Distribution System is provided in Attachment 5 to this enclosure depicting the current offsite circuit configuration and the proposed location of the new GCBs to facilitate an offsite configuration with backfeed through the main transformers following the USST modification.

3.1.1 AC Sources The SQN AC Electrical Power Distribution System sources consist of the offsite power sources and the onsite standby power sources (diesel generator sets (DGs)). As required by 10 CFR 50, Appendix A, GDC 17, the design of the AC electrical power system provides independence and redundancy to ensure an available source of power to the Engineered Safety Feature (ESF) systems.

The onsite Class 1 E AC Distribution System supplies electrical power to two load groups (four Shutdown Boards), shared between the two units, with each load group powered by a pair of independent Class 1E 6.9 kV Shutdown Boards (i.e., 1A-A and 2A-A, or 1B-B and 2B-B). Each pair of 6.9 kV Shutdown Boards is normally powered by a separate and independent offsite power source. Each 6.9 kV Shutdown Board can also be powered by a dedicated DG. Two DGs associated with one load group (two 'A' or two 'B' 6.9 kV Shutdown Boards) can provide all safety related functions to mitigate a loss-of-coolant accident (LOCA) in one unit and safely shutdown the other unit. The Train A and Train B ESF systems each provide for the minimum safety functions necessary to shut down the plant and maintain it in a safe shutdown condition.

Offsite power is supplied to the Sequoyah 161 kV transformer yard by eight dedicated lines. From the 161 kV transformer yard, two electrically and physically separated circuits provide AC power, through step-down CSSTs, to the 6.9 kV Unit Boards via the 6.9 kV Start Buses. Power is then routed from the 6.9 kV Unit Boards to the 6.9 kV Shutdown Boards. The two physically independent circuits between the offsite transmission network and the onsite Class 1 E distribution system are the preferred power supply. Each offsite circuit consists of a common station service transformer (CSST), Start Bus, Unit Board, and the necessary conductors, buses, and cables to complete a circuit to the Class 1 E distribution system.

Each 6.9 kV Shutdown Board has a dedicated Class 1 E DG to provide emergency AC power to essential safety systems in the event of a loss of offsite power (LOOP) or degraded voltage on its respective 6.9 kV Shutdown Board. The DG associated with power train 1A is referred to as the 1A-A DG, the DG associated with power train 2A is referred to as the 2A-A DG. Similarly, the DG associated with power train 1 B is referred to as the 1 B-B DG, the DG associated with power train 2B is referred to as the 2B-B DG.

Each DG is complete with its own air starting system, fuel supply system, battery and automatic control circuitry. The DGs are designed, installed and tested to requirements that ensure their availability. The DGs are also designed to operate in parallel with the offsite electrical power sources for test and exercise purposes.

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3.1.2 Electrical Distribution Systems The onsite Class 1 E AC electrical power distribution system is divided into two redundant and independent AC electrical power distribution subsystems with two 6.9 kV Shutdown Boards in each subsystem.

The Class 1 E electrical power distribution system has connections to two separate and independent offsite sources of power, as well as connections to the onsite emergency power system supplies (DGs). One of the two offsite sources is the normal power source for the 6.9 kV Shutdown Boards associated with load group A, and the other offsite source is the normal power source for the 6.9 kV Shutdown Boards associated with load group B. Therefore, transfers from one offsite power source to the other offsite power source are not required. For a loss of offsite power (LOOP) to the 6.9 kV Shutdown Boards, the onsite emergency power system supplies power to the 6.9 kV Shutdown Boards. Control power for the 6.9 kV circuit breakers is supplied from the Class 1 E batteries.

The AC Distribution System includes the 480 VAC Shutdown Boards and associated supply transformers, load centers, and protective devices. Each of the two 480 VAC Shutdown Boards serving a load group has a normal and an alternate supply from its associated 6.9 kV Shutdown Board. Each normal supply is through a separate 6900 to 480 VAC stepdown transformer. The alternate supply is common to the two 480 VAC Shutdown Boards in a load group and has a separate 6900 to 480 VAC stepdown transformer.

The 120 VAC vital buses are arranged in four load groups and are normally powered from the unit inverters or spare inverters and DC Boards 1,11, 111, and IV. The normal source of power for each inverter is from a separate 480 VAC Shutdown Board through 480 VAC to 125 VDC rectifier power supplies. The backup power source for each inverter is from one of the 125 VDC vital battery buses through an auctioneering type circuit. The auctioneering type circuit, consisting of isolating diodes in the normal and backup power source, permits a switchless transfer to the backup source with minimum variation of the inverter output voltage and frequency.

There are four independent 125 VDC electrical power distribution buses. Each bus receives normal power from an independent 480 VAC Shutdown Board via its associated battery charger. Upon loss of 480 VAC Shutdown Board power, the DC buses are energized by their connected battery banks.

3.2 Technical Specification Change Evaluations The requirements for offsite AC power sources are specified in TS 3/4.8.1, "AC Sources, Operating." Limiting Condition for Operation (LCO) 3.8.1.1.a requires two physically independent circuits between the offsite transmission network and the onsite Class 1E distribution system to be OPERABLE in MODES 1, 2, 3, and 4. TS SR 4.8.1.1.1.a requires each of the above required independent circuits between the offsite transmission network and the onsite Class 1E distribution system to be determined OPERABLE at least once per 7 days by verifying correct breaker alignments and indicated power availability.

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Currently, two independent offsite power sources are provided by the CSSTs to the 6.9 kV Shutdown Boards via the 6.9 kV Start Buses and 6.9 kV Unit Boards. The output of CSST C is the normal power source for 6.9 kV Shutdown Boards 1A-A and 2A-A, and the output of CSST A is the normal power source for the 6.9 kV Shutdown Boards 1B-B and 2B-B. This lineup provides two immediate access offsite circuits to each unit (one to each load group). Therefore, transfers from one offsite power source to the other offsite power source are not required.

Following plant modifications, the main generator will be capable of supplying electrical power to the offsite grid, as well as supplying power to one offsite circuit (via the USSTs through the IPB) with GCBs providing isolation between the main generator and the main bank transformers. When the generator is not operating, the main bank transformers function as step down transformers and supply electrical power from the grid to the USSTs.

Offsite power will normally be supplied from the USSTs to the 6.9 kV Unit Boards, and will automatically transfer to the alternate power supply to at least one circuit from the CSSTs on a trip of the PCBs. The transfer occurs when the Unit Board normal supply breaker opens and the alternate supply breaker closes. The alternate supply breaker will not close until the normal supply breaker opens. This transfer typically occurs within 6 cycles and, therefore, will not result in a downstream breaker action or power interruption to motors or other electrical loads.

During the time period between the SQN Unit 2 USST modification implementation and the SQN Unit 1 USST modification implementation, the output of USST 2A will be used as the normal power supply to 6.9 kV Shutdown Board 2A-A via Unit Board 2B, and the output of USST 2B will be used as the normal power supply to 6.9 kV Shutdown Board 2B-B via Unit Board 2C. CSST C will provide an alternate source of power for 6.9 kV Shutdown Board 2A-A via Unit Board 2B. CSST A will provide an alternate source of power for 6.9 kV Shutdown-Board 2B-B via Unit Board 2C. Power to 6.9 kV Shutdown Board 1A-A will continue to be supplied by CSST C via Start Bus 1B and Unit Board 1B.

Power to 6.9 kV Shutdown Board 1 B-B will continue to be supplied by CSST A via Start Bus 1A and Unit Board 1C. This lineup essentially requires three power sources (USST 2A/2B, CSST A, and CSST C) to supply the normal and alternate offsite power circuits. Alternatively, the CSSTs can provide the two immediate access offsite circuits with no required transfer, consistent with the current licensing basis.

After the SQN Unit 1 USST modification has been implemented, the normal offsite power alignment will be power supplied by the USSTs to the 6.9 kV Shutdown Boards via the 6.9 kV Unit Boards. Alternatively, power can be provided by one or more CSSTs via automatic transfer at the 6.9 kV Unit Boards on a trip of the PCBs. Other alignment configurations using other combinations of available USSTs and CSSTs will also be possible, as long as the alignments are consistent with the analyzed configurations, and comply with the requirements of GDC 17.

For example, to support breaker testing, offsite power to the 6.9 kV Shutdown Boards can be realigned from normal feed to alternate feed. This would result in Shutdown Boards 1A-A and 2A-A being fed from Unit Boards 1A and 2A, respectively, and Page 8 of 23

Shutdown Boards 1 B-B and 2B-B being fed from Unit Boards 1 D and 2D, respectively.

The CSST being utilized as an alternate power source to one group of Shutdown Boards would also be realigned (normally CSST A available to Shutdown Boards 1 B-B and 2B-B or CSST C available to Shutdown Boards 1A-A and 2A-A, would be realigned to CSST A available to Shutdown Boards 1A-A and 2A-A or CSST C available to Shutdown Boards 1B-B and 2B-B).

Implementation of these design changes requires reinstatement of the surveillance requirement to demonstrate the capability to transfer the power supply to a 6.9 kV Unit Board from the normal supply to the alternate supply. The proposed change revises the TS with the addition of TS SR 4.8.1.1.1.b to require demonstration that the required offsite circuits are OPERABLE at least once per 18 months by manually and automatically transferring the power supply to a 6.9 kV Unit Board from the normal supply to the alternate supply. The SR is modified by two notes. The first Note precludes performance of the SR on the associated unit's 6.9 kV Unit Boards in MODES 1 and 2, to preclude perturbations to the operating unit's electrical distribution system that could challenge continued operation and, as a result, unit safety systems.

The second Note specifies that automatic transfer from the normal supply to the alternate supply is only required to be met for 6.9 kV Unit Boards that require normal and alternate power supplies. Although manual transfers between the normal supply and the alternate supply are not relied upon to meet the accident analysis, this verification is included in the SR to ensure the availability of a backup to the automatic transfer feature.

Relaxation of the restriction from performing this SR during MODES 1 and 2 is addressed in NUREG-1431, "Standard Technical Specifications - Westinghouse Plants,"

as introduced in NRC approved Technical Specification Task Force (TSTF) change TSTF-283, Revision 3. Although this LAR proposes to revise, not delete, the NUREG-1431 SR Note, it would result in the removal of the MODE restriction from performance of the SR on the opposite unit's 6.9 kV Unit Boards when the opposite unit is shutdown. Therefore, consistent with the NUREG, the following criteria are given consideration prior to the removal of the MODE restrictions:

The MODE restrictions may be deleted, if it can be demonstrated to the staff, on a plant specific basis, that performing the SR with the reactor in any of the restricted MODES can satisfy the following criteria, as applicable:

a. Performance of the SR will not render any safety system or component inoperable,

b. Performance of the SR will not cause perturbations to any of the electrical distribution systems that could result in a challenge to steady state operation or to plant safety systems, and

c. Performance of the SR, or failure of the SR, will not cause, or result in, an AOO with attendant challenge to plant safety systems.

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An evaluation for the proposed SR follows.

TS SR 4.8.1 1.1 .b will verify the automatic and manual transfer capability of power to a 6.9 kV Unit Board from the normal supply to the alternate supply. The SR is modified by two Notes. The first Note precludes performance of the SR on the associated unit's 6.9 kV Unit Boards in MODES 1 and 2. The intent of the first Note is to ensure no surveillance related manipulation or perturbation of the electrical distribution system is allowed that could challenge continued operation of a unit in MODE 1 or 2.

The surveillance is only performed on 6.9 kV Unit Boards associated with the unit not in MODE 1 or 2. Restricting performance of the SR to MODE 3, 4, 5 or 6 will minimize the potential for inducing an electrical distribution subsystem perturbation that could result in a unit trip or actuation of an ESF system. Perturbations to the electrical distribution subsystems further minimized during performance of the SR on a 6.9 kV Unit Board by transferring the associated 6.9 kV Shutdown Board to the alternate feed (another 6.9 kV Unit Board that is not being tested) prior to performing the SR. Since the 6.9 kV Unit Board surveillance is performed on an unloaded board, it will not initiate a perturbation in an electrical distribution system that could challenge a plant safety related system, or challenge steady state conditions to the unit in operation. If the alignment of the associated 6.9 kV Shutdown Board to the alternate feed results in an inoperable offsite circuit, the Actions of LCO 3.8.1.1 will be entered and the provisions of LCO 3.0.5 will be invoked. By invoking LCO 3.0.5, the affected safety related equipment may be considered operable for the purpose of satisfying their applicable LCOs. Prior to performance of the SR, safety related equipment redundant to the equipment powered from the affected 6.9 kV Shutdown Board will be verified operable, as required by LCO 3.0.5.

Failure of this surveillance will not challenge or cause an operational transient or challenge to plant safety systems for the unit in operation. The ability of the 6.9 kV Shutdown Board to transfer to the DG will not be inhibited should the ability of the associated 6.9 kV Unit Board to automatically or manually transfer to the alternate supply breaker not be functional. The LOOP circuits will still automatically strip the board and allow the diesel starting and loading sequence to occur providing power to the ESF systems as designed.

3.3 Risk Evaluation As discussed in Section 3.2 above, TVA is proposing a change to TS 3/4.8.1 by adding a new surveillance requirement (TS SR 4.8.1.1.1.b) to verify the manual and automatic transfer of power to a 6.9 kV Unit Board from the normal supply to the alternate supply.

TVA is not proposing this LAR on a plant specific, risk informed basis such as described in Regulatory Guide 1.177, "An Approach for Plant-Specific, Risk-Informed Decision making: Technical Specifications," Revision 1. However, to ensure an appropriate consideration of risk for this change, TVA conducted a risk evaluation of the proposed change. The probabilistic risk assessment (PRA) report is contained in Attachment 6 to this enclosure. The results and conclusions of the evaluation are as follows:

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3.3.1 Intermediate Alignment Results During the intermediate alignment stage, only Unit 2 will have the new USST alignment applied to the 6.9 kV Unit Boards.

Unit 1 Table 1 - Change in Risk with New USST Alignment on Unit 2 Only Risk Metric Model of Record Alignment Change Change in Value Value Value CDF 3.0253E-05 3.0253E-05 None LERF 4.3899E-06 4.3899E-06 None The change in CDF and LERF are presented in Table 1. There is no change in CDF or LERF, which shows there is no risk impact on Unit 1 from Unit 2 having the new USST alignment.

Unit 2 Table 2 - Change in Risk with New USST Alignment on Unit 2 Only Risk Metric Model of Record Alignment Change Change in Value Value Value CDF 3.5901E-05 3.5901E-05 None LERF 4.6332E-06 4.6332E-06 None The change in CDF and LERF are presented in Table 2. The change to the model adds more reliability to the offsite power supplies. The Loss of Offsite Power (LOOP) initiator is not a dominant contributor to the overall CDF and LERF. Therefore, there is no change in CDF or LERF.

In this case, CDF and LERF showed no changes, so the alignment changes to the Start Buses with the addition of the USSTs are considered risk neutral and acceptable.

3.3.2 Final Alignment Results During the final alignment stage, both Unit 1 and Unit 2 will have the new USST alignment implemented.

Unit 1 Table 3 - Change in Risk with New USST Alignment on Both Units Risk Metric Model of Record Alignment Change Change in Value Value Value CDF 3.0253E-05 3.0252E-05 -1.OOE-09 LERF 4.3899E-06 4.3898E-06 -1.O0E-10 Page 11 of 23

The change in CDF and LERF are presented in Table 3. The change to the model adds more reliability to the offsite power supplies. The LOOP initiator is not a dominant contributor to the overall CDF and LERF. Therefore, there is a minor risk reduction with the addition of the USSTs.

Regulatory Guide 1.174 states that if an application shows a decrease in CDF or LERF, the change will be considered to have satisfied the relevant principle of risk-informed regulation with respect to CDF or LERF. In this case, CDF and LERF were reduced, so the changes to the Start Bus alignments with the addition of the USSTs are acceptable.

Unit 2 Table 4 - Change in Risk with New USST Alignment on Both Units Risk Metric Model of Record Alignment Change Change in Value Value Value CDF 3.5901 E-05 3.5900E-05 -1.OOE-09 LERF 4.6332E-06 4.6331 E-06 -1.OOE-10 The change in CDF and LERF are presented in Table 4. The change to the model adds more reliability to the offsite power supplies. The LOOP initiator is not a dominant contributor to the overall CDF and LERF. Therefore, there is a minor risk reduction with the addition of the USSTs.

Regulatory Guide 1.174 states that if an application shows a decrease in CDF or LERF, the change will be considered to have satisfied the relevant principle of risk-informed regulation with respect to CDF or LERF. In this case, CDF and LERF were reduced, so the changes to the Start Bus alignments with the addition of the USSTs are acceptable.

3.3.3 Surveillance Testing Surveillance testing will require the manual and automatic transfer function of the 6.9 kV Unit Boards to be tested. If the surveillance is being performed on the 6.9 kV Unit Board, its associated Shutdown Board loads will be transferred to alternate (another Unit Board that is not being tested) prior to performing the surveillance. Since the Unit Board surveillance is performed on an unloaded board, it has no impact on risk.

3.3.4 Conclusion The proposed plant modification provides both units with additional offsite power sources. In the intermediate alignment phase, the USSTs will be implemented on Unit 2 only. The USSTs 2A and 2B will be realigned as the normal power source to the 6.9 kV Unit Boards 2A, 2B, 2C and 2D. With the addition of appropriate logic to the SQN PRA model to alter the power alignments to the 6.9 kV Unit Boards on Unit 2, there was no risk impact to Unit 1 or Unit 2 during this alignment. The LOOP initiator is not a dominant contributor to the overall CDF and LERF. Therefore, the intermediate alignment is risk neutral.

For the final alignment phase, the USSTs will be implemented on Unit 1 as well. Unit 1 will be realigned so the USSTs 1A and 1 B are the normal power source to the 6.9 kV Page 12 of 23

Unit Boards 1A, 1 B, 1C and 1 D. Changes to the SQN PRA model of record were incorporated to alter the power alignments for the 6.9 kV Unit Boards for Unit 1. The new alignments slightly reduced risk by adding redundancy to the Unit Boards on both units, which is considered risk neutral and is acceptable by Regulatory Guide 1.174.

The proposed plant modifications are being implemented as a response to the dual unit trip in March of 2009. A loss of CSST C caused a loss of power to two 6.9 kV Unit Boards on each unit that feed the RCPs. CSST B continued to supply offsite power to the Train B safety-related 6.9 kV Shutdown Boards for both units. The emergency diesel generators started and powered the Train A safety-related 6.9 kV Shutdown Boards for both units until offsite power was restored. Implementation of the proposed USST modifications will eliminate this single point vulnerability for a dual unit trip. The PRA model does not model dual unit trip initiating events. The new power alignments for the 6.9 kV Unit Boards with the USSTs installed will provide added redundancy (back-up power from the CSSTs), and allow a different offsite power supply to be utilized for each unit. Reducing the potential for a dual unit trip will reduce the overall plant risk to CDF and LERF by reducing reactor trip frequency, and reducing challenges to safety equipment and Operator actions during mitigation. The ability to power equipment from the either the 161 kV or the 500 kV switchyards will also reduce the risks to the plant from external events.

4.0 REGULATORY EVALUATION

4.1 Applicable Regulatory Requirements and Criteria The onsite standby AC power systems at SQN are designed to comply with the following applicable regulations and requirements:

10CFR50, Appendix A, GDC 17 states the following:

Criterion 17 - Electric 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 reactorcoolant pressure boundary are not exceeded as a result of anticipatedoperational 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 sources, 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 DistributionSystem shall be supplied by two physically independent circuits (not necessarilyon separate rights of way) designed and located so as to minimize to the extent practicalthe Page 13 of 23

likelihood of their simultaneous failure under operating and postulatedaccident 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 alternatingcurrent 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 LOCA to assure that core cooling, containmentintegrity, and other vital safety functions are maintained.

Provisionsshall be included to minimize the probabilityof losing electric power from any of the remainingsources as a result of, or coincident with, the loss of power generated by the nuclearpower unit, the loss of power from the transmissionnetwork, or the loss of power from the onsite electric power sources.

The following demonstrates TVA conformance to GDC 17.

Criterion 17- Electricpower systems. An onsite electricpower system and an offsite electric power system shall be provided to permit functioning of structures, systems, and components importantto safety.

Currently, the Class 1E buses (6.9 kV Shutdown Boards), and their associated equipment important to safety, can be powered from either offsite power through circuits connected to the Sequoyah 161 kV switchyard or through the DGs associated with each 6.9 kV Shutdown Board.

The USST modifications will install new USSTs and install GCBs in the IPB between the generator and main bank transformer. Following the plant modifications, the main generator will be capable of supplying electrical power to the offsite grid, as well as supplying power to an offsite circuit (via the USSTs through the IPB). The new USSTs will have sufficient capacity and capability to meet offsite power loading requirements and will include automatic load tap changers to provide the required 6.9 kV voltage regulation. In this configuration, the USSTs will be the normally aligned power supply, with an automatic transfer to the alternate supply powered from CSST A or CSST C on a trip of the PCBs. Operation of the GCBs will allow backfeeding from the 161 kV switchyard (Unit 1) or 500 kV switchyard (Unit 2) as the immediate access source of offsite power to the onsite distribution system. The breakers are designed to open automatically on a unit trip.

NUREG-0800, Section 8.2, "Offsite Power System," provides guidance for the evaluation of designs that utilize a backfeed path through the main generator step-up transformer.

If the circuit utilizes a GCB, evaluation of the GCB acceptability per Appendix A should be performed to assess compliance with GDC 17. TVA has performed an evaluation of the new GCBs and has determined that the GCB ratings and required capabilities are consistent with the conditions as defined in Institute of Electrical and Electronics Engineers (IEEE) Std C37.013 and meet the performance tests and capabilities from IEEE Std C37.013 as stated in NUREG-0800, Section 8.2, Appendix A.

During the time period between the SQN Unit 2 USST modification implementation and the SQN Unit 1 USST modification implementation, the output of the Unit 2 USSTs will Page 14 of 23

be used as the normal power supply to 6.9 kV Shutdown Boards 2A-A and 2B-B. CSST C will provide an alternate source of power for 6.9 kV Shutdown Board 2A-A, and CSST A will provide an alternate source of power for 6.9 kV Shutdown Board 2B-B. Power to 6.9 kV Shutdown Board 1A-A will continue to be supplied by CSST C, and power to 6.9 kV Shutdown Board 1B-B will continue to be supplied by CSST A. This lineup requires three power sources (USST 2A/2B, CSST A, and CSST C) to supply the two existing offsite power circuits. Alternatively, the CSSTs can provide the two immediate access offsite circuits with no required transfer, consistent with the current licensing basis.

Each 6.9 kV Shutdown Board has a dedicated Class 1 E DG to provide emergency AC power to essential safety systems in the event of a LOOP or degraded voltage on its respective 6.9 kV Shutdown Board.

The safety function for each system (assuming the other system is not functioning) shall be to provide sufficient capacity and capabilityto assure that (1) specified acceptable fuel design limits and design conditions of the reactor coolantpressureboundary are not exceeded as a result of anticipated operationaloccurrencesand (2) the core is cooled and containmentintegrity and other vital functions are maintainedin the event of postulatedaccidents.

Calculations have been performed to assess the capability of the offsite power sources (USSTs or CSSTs) and the onsite power sources (emergency DGs) to supply the required loading under normal and accident conditions. The calculations were performed based on the maximum loading of one ESF train without regard for the availability of the other train. For the offsite power sources, the analysis assumes that both trains are functioning, as this increases the demand on the system.

CSST A and CSST C each have two secondary windings. Each secondary winding normally supports a different 6.9 kV Shutdown Board (i.e., CSST A winding X supports Shutdown Board 1 B-B and CSST A winding Y supports Shutdown Board 2B-B.

Likewise, CSST C winding X supports Shutdown Board 2A-A and CSST C winding Y supports Shutdown Board 1A-A). Therefore, either CSST can provide the minimum required loads for both units.

The USSTs (two per nuclear unit) will be the normally aligned power source to the 6.9 kV Shutdown Boards. The USSTs are arranged so that each 6.9 kV Shutdown Board will be supplied by a separate USST. The USSTs are sized with adequate capacity to support normal operation with power from the unit main generator. The USSTs are also sized with adequate capacity to support accident loading with power to one unit supplied by backfeed from the transmission grid, or with power to both units supplied by backfeed from the transmission grid after an orderly shutdown of the non-accident unit.

The emergency DGs are sized to accommodate required loading with sequenced starts of the major loads. Either pair of DGs associated with a load group will provide adequate capacity to support accident loads on one unit and an orderly shutdown of the non-accident unit.

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Calculations demonstrate that the transmission grid can support the normal and accident condition loads that will be imposed on the grid.

Calculation SQNETAPAC evaluated the capability of offsite power sources and the capability of onsite power sources to supply the required loading. It demonstrates that offsite power provided by a CSST or by a USST with backfeed from the transmission grid, or two DGs associated with one load group can provide sufficient capacity to operate the plant components required to mitigate a design basis accident in one unit and safely shutdown the other unit.

The onsite electric power sources, including the batteries,and the onsite electric distributionsystem, shall have sufficient independence, redundancy,and testability to perform theirsafety functions assuming a single failure.

The onsite electrical power system consists of two redundant Class 1 E electric power distribution subsystems, with two independent DGs per subsystem, and four redundant vital instrument and control power channels, each provided with a battery, battery charger, and inverter for each unit. Each redundant onsite power supply, subsystem, and channel has the capability and capacity to supply the required safety loads assuming the failure of its redundant counterpart. All required surveillance testing can be performed, thereby satisfying the testability criterion.

Electric power from the transmissionnetwork to the onsite Electric Distribution System shall be supplied by two physically independentcircuits (not necessarily on separaterights of way) designed and located so as to minimize to the extent practicalthe likelihood of their simultaneous failure under operatingand postulatedaccidentand environmental conditions. A switchyard common to both circuits is acceptable.

Following plant modifications, the main generator will be capable of supplying electrical power to the offsite grid, as well as supplying power to an offsite circuit (via the USSTs through the IPB) with GCBs providing isolation between the main generator and the main bank transformers. In this configuration, the main bank transformers function as step down transformers and supply electrical power from the 500 kV grid to the USSTs (Unit 1) or from the 161 kV grid to the USSTs (Unit 2). Power can also be supplied from the Sequoyah 161 kV transformer yard via the CSSTs on a trip of the PCBs.

Each of these circuits shall be designed to be availablein sufficient time following a loss of all onsite alternatingcurrent power supplies and the other offsite electric power circuit,to assure that specified acceptable fuel design limits and design conditions of the reactorcoolantpressure boundary are not exceeded.

Implementation of the USST modifications will enable use of a USST as the normal power supply to a 6.9 kV Unit Board, with an automatic transfer to the alternate power supply from a CSST on a trip of the PCBs.

Either the USSTs or a CSST will be available in a few seconds to effect safe shutdown of both units in the event of a loss of all onsite power and the loss of the other offsite Page 16 of 23

circuit. The available offsite circuit will 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 LOCA to assure that core cooling, containmentintegrity, and other vital safety functions are maintained.

Following implementation of the USST modifications, the USSTs will be the continuously connected normal power supply to the Class 1E distribution subsystems. In addition, an alternate power supply will be available within a few seconds via automatic transfer to one or more CSSTs, in the event of on a trip of the PCBs. Each offsite circuit will be capable of supplying power to one or more ESF trains in the event of a LOCA. Each ESF train is sufficient to assure that core cooling, containment integrity, and other vital safety functions are maintained.

Provisionsshall be included to minimize the probabilityof losing electric power from any of the remaining sources as a result of, or coincident with, the loss of power generatedby the nuclearpower unit, the loss of power from the transmissionnetwork, or the loss of power from the onsite electric power sources.

A planning transmission system study was performed by TVA's Energy Delivery group, to determine if the SQN offsite power systems (161 kV and 500 kV) are adequate to meet GDC 17 requirements. These analyses were performed using loading for SQN USSTs and CSSTs. In determining adequate offsite power for SQN, the TVA Transmission System provides two offsite sources (161 kV and 500 kV). The SQN 161 kV bus and 500 kV bus voltages must be maintained above specified values during a simulated loss of coolant accident (LOCA). These studies considered all pre-event outages in the SQN area followed by a LOCA event.

Based on the above discussion, the analysis shows that no exceptions have been taken to GDC 17. Therefore, SQN fully complies with GDC 17.

10CFR50, Appendix A, GDC 18 states the following:

Criterion 18- Inspection and Testing of Electric Power Systems. Electric Power Systems importantto safety shall be designed to permit appropriateperiodic inspection and testing of importantareas and features, such as wiring, insulation, connections, and switchboards,to assess the continuity of the systems and the condition of their components. The systems shall be designed with a capability to test periodically(1) the operability and functional performance of the components of the systems, such as onsite power sources, relays, switches, and buses, and (2) the operability of the systems as a whole and, under conditions as close to design as practical,the full operation sequence that brings the systems into operation, including operation of applicableportions of the protection system, and the transfer of power among the nuclearpower unit, the offsite power system and the onsite power system.

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TVA will continue to conform to GDC 18 as follows:

The proposed design changes, including the USST and GCB modifications, will not alter the ability to perform inspections and checks of wiring, insulation, connections, and switchboards, the ability to perform periodic testing with the unit on line, including automatic starting and loading of onsite DGs, and starting and loading of individual or groups of ESF equipment to their respective buses, or the initiation of protection system, starting and loading of DGs, or the transfer of power sources.

The applicable portion of Regulatory Guide (RG) 1.6, Regulatory Position 2, states the following:

Each a-c load group should have a connection to the preferred (offsite) power source and to a standby (onsite) power source (usually a single diesel generator). The standby power source should have no automatic connection to any other redundantload group.

At multiple nuclear unit sites, the standby power source for one load group may have an automatic connection to a load group of a different unit. A preferredpower source bus, however, may serve redundantload groups.

The following demonstrates TVA conformance to RG 1.6, Regulatory Position 2.

Implementation of the USST modifications will enable use of the USSTs as the normal power supply to the 6.9 kV Unit Boards, with an automatic transfer to the alternate power supply from CSST A or CSST C on a trip of the PCBs. Each 6.9 kV Shutdown Board will continue to have access to the offsite power sources and a dedicated onsite DG source.

The onsite power sources will have no automatic connection between load groups.

The applicable portion of RG 1.32, Regulatory Position 1 .a, states the following:

a. Availability of Offsite Power. Consistent with the requirements of Criterion17, the phrase "within an acceptable time" in Section 5.2.3(4), first paragraph,of IEEE Std 308-1974 should be construed to mean "within a few seconds" A preferred design would include two immediate access circuits from the transmission network.

Detailed guidance for operating proceduresand restrictions acceptable to the staff, applicable where two immediate access circuits are available, is contained in Regulatory Guide 1.93, 'Availability of Electric Power Sources. "An acceptable design would substitute a delayed access circuit for one of the immediate access circuits provided the availabilityof the delayed access circuit conforms to Criterion 17.

The following demonstrates TVA conformance to RG 1.32, Regulatory Position 1.a.

In addition to the current alignment of two immediate access circuits provided by the CSSTs, implementation of the USST modifications will enable other offsite power configurations that provide two immediate access circuits. Following implementation of the modifications, normal offsite power will be provided by the continuously connected USSTs, with an alternate power supply available within a few seconds via automatic transfer to one or more CSSTs on a trip of the PCBs. Both the alignment of two immediate circuits provided by the CSSTs, and the alignment of power from the USSTs Page 18 of 23

with automatic transfer to CSST A or C providing an alternate power supply, meet the requirements of GDC 17, as stated in RG 1.32, Regulatory Position 1.a.

RG 1.155, Regulatory Position 2, states the following:

Proceduresshould include the actions necessary to restore offsite power and use nearby power sources when offsite power is unavailable. As a minimum, the following potential causes for loss of offsite power should be considered:

Grid undervoltage and collapse

Weather-inducedpower loss

Preferredpower distribution system faults that could result in the loss of normal power to essentialswitchgearbuses The following demonstrates TVA conformance to RG 1.155.

The modifications include changes to the 161 kV and 500 kV switchyards. Currently, the PCBs serve to connect the onsite generation with the offsite grid and provide fault protection for the offsite circuit and main generator. Following implementation of the USST modifications, the GCBs will provide an isolation point between the main generator and the transformers and will trip open on a unit trip. The PCBs will trip open on a fault between the GCBs and the PCBs, or the failure of the GCBs to open on a valid command signal. The change to the PCBs will not impact the station blackout coping time.

The modifications do not change the offsite transmission system or the onsite safety related power system, and therefore, do not change the site's compliance with RG 1.155. As the offsite connection remains unchanged, the station blackout coping time is not impacted.

With the implementation of the proposed change, SQN continues to meet the applicable regulations and requirements.

4.2 Precedent Original SQN, Unit 1 and 2, design included the capability for the USSTs to supply unit power to the 6.9 kV Unit Boards with the CSSTs providing the offsite power supply via the Start Buses. TS SR4.8.1.1.1.b required demonstration of manual and automatic transfer capability from the normal power supply to the alternate power supply at least once every 18 months.

Due to an issue identified in 1986 regarding USST impedance challenging the 6.9 kV Unit Board circuit breaker short circuit ratings, TVA implemented a design change to replace the CSSTs with transformers with auto tap changers to accommodate voltage variations in the 161 kV system. With installation of the new CSSTs, the normal lineup for powering the 6.9 kV Unit Boards was changed so that normal bus power is supplied from the CSSTs. Therefore, it was no longer necessary to transfer power during unit startups and shutdowns, or to rely on automatic power transfer in the event of a fault or plant trip in order to maintain power to the 6.9 kV Shutdown Boards.

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SR 4.8.1.1.1 .b was not deleted at the time of the design change, because it was believed that there could be instances where use of the USST as a power supply might be needed as a contingency for maintenance activities. On July 11, 1994, the NRC approved an amendment to change the wording of SR 4.8.1.1.1.b to reflect the use of the CSSTs with auto load tap changers as the normal power supply for the 6.9 kV Unit Boards (ADAMS Accession No. ML013320192).

In 1996, it was determined that placing the units on the unit generator supported supply would only be necessary for maintenance on the offsite circuits. In that situation, the TS requirements would require compliance with the actions for an inoperable offsite circuit, even if the transfer scheme was operable. Once it was determined there were no practical opportunities to utilize the unit generator support circuit, a license amendment request was submitted for elimination of SR 4.8.1.1. 1.b. On September 23, 1997, the NRC approved an amendment to eliminate SR 4.8.1.1.1 .b from the SQN, Unit 1 and 2, TS (ADAMS Accession No. ML013320312).

The USST modifications will install new USSTs. The new transformers will be sized to meet offsite power loading requirements and will include automatic load tap changers to provide the required 6.9 kV voltage regulation. Each new USST will provide power to its respective 6.9 kV Unit Board and Class 1 E 6.9 kV Shutdown Board. The new USSTs will have a higher impedance to limit fault current to below the 6.9 kV Unit Board circuit breaker rating. Offsite power from a USST to a 6.9 kV Unit Board will automatically transfer to power supplied from a CSST on a trip of the PCBs.

These modifications will allow an offsite power configuration where the USSTs are capable of supplying normal power, and alternate power is supplied by the CSSTs.

Implementation of these design changes requires reinstatement of the surveillance requirement to demonstrate the capability to manually and automatically transfer the power supply to a 6.9 kV Unit Board from the normal power supply to the alternate power supply. Verification of automatic transfer from the normal supply to the alternate power supply will only be required to be met for 6.9 kV Unit Boards requiring normal and alternate power supplies.

On November 22, 2011, the NRC approved an amendment to similarly modify the Note associated with SR 3.8.1.8 (automatic and manual transfer capability of offsite circuits test) for Watts Bar Nuclear Plant, Unit 1, thereby allowing performance of the SR on the Unit 2 Shutdown Boards during Unit 1 operation with Unit 2 shutdown (ADAMS Accession No. ML11234A258).

4.3 Significant Hazard Consideration The proposed change adds generator circuit breakers and installs new unit station service transformers (USSTs) to allow offsite power to the Class 1 E electrical distribution system to be supplied from the USSTs. Each 6.9 kV Unit Board will be equipped with an automatic method of transferring from the normal supply (via a USST) to the alternate supply (via a common station service transformer (CSST)).

Page 20 of 23

Technical Specifications (TS) 3/4.8.1 is revised to include a surveillance requirement (SR) to demonstrate that the required offsite circuits are operable at least once per 18 months by manually and automatically transferring the power supply to a 6.9 kV Unit Board from the normal supply to the alternate supply. The SR is modified by two notes.

The first Note precludes performance of the SR on the associated unit's 6.9 kV Unit Boards in MODES 1 and 2. The second Note specifies that verification of transfer capability is only required to be met for 6.9 kV Unit Boards requiring normal and alternate power supplies.

The Tennessee Valley Authority (TVA) has concluded that the changes to Sequoyah Nuclear Plant (SQN) Units 1 and 2 TS 3/4.8.1 do not involve a significant hazards consideration. TVA's conclusion is based on its evaluation in accordance with 10 CFR 50.91(a)(1) of the three standards set forth in 10 CFR 50.92, "Issuance of Amendment," as discussed below:

1. Does the proposed amendment involve a significant increase in the probabilityor consequence of an accidentpreviously evaluated?

Response: No.

The offsite circuits and their associated emergency loads are accident mitigating features. As such, testing of the transfer capability between the normal and alternate power supplies is not associated with a potential accident-initiating mechanism. Therefore, the changes do not affect accident or transient initiation or consequences. The probability or consequences of previously evaluated accidents will not be significantly affected by the addition of the proposed offsite power source or surveillance requirement. Verification of the capability to transfer power from the USSTs to the CSSTs demonstrates the availability of the offsite circuit to perform its accident mitigation functions as assumed in the accident analyses. Therefore, the proposed changes do not involve a significant increase in the probability or consequences of an accident previously evaluated.

2. Does the proposed amendment create the possibility of a new or different kind of accident from any accidentpreviously evaluated?

Response: No.

The proposed changes do not require any new or different accidents to be postulated, since no changes are being made to the plant that would introduce any new accident causal mechanisms. This license amendment request does not impact any plant systems in a manner that would create a new or different kind of accident; nor does it have any impact on any accident mitigating systems that would significantly degrade the plant's response to an accident previously evaluated.

Therefore, the proposed changes do not create the possibility of a new or different kind of accident from any accident previously evaluated.

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3. Does the proposedamendment involve a significant reduction in a margin of safety?

Response: No.

The proposed plant modifications will allow an offsite circuit configuration where the USSTs are capable of supplying normal power, and alternate power is supplied by CSST A or CSST C. These design changes require reinstatement of the TS SR to demonstrate the capability to automatically transfer the power supply to each 6.9 kV Unit Board from the normal supply to the alternate supply.

The proposed changes to the unit power operating configuration do not alter the assumptions contained in the safety analyses regarding the availability of the offsite circuits. The proposed changes do not adversely impact the redundancy or availability requirements of offsite power supplies or change the ability of the plant to cope with station blackout events. Therefore, the proposed changes do not involve a significant reduction in a margin of safety.

4.4 Conclusions In conclusion, based on the considerations discussed above, (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.

5.0 ENVIRONMENTAL CONSIDERATION

A review has determined that the proposed amendment would change a requirement with respect to installation or use of a facility component located within the restricted area, as defined in 10 CFR 20. However, the proposed amendment does not involve (i) a significant hazards consideration, (ii) a significant change in the types or significant increase in the amounts of any effluents that may be released offsite, or (iii) a significant increase in individual or cumulative occupational radiation exposure. Accordingly, the proposed amendment meets the eligibility criterion for categorical exclusion set forth in 10 CFR 51.22(c)(9). Therefore, pursuant to 10 CFR 51.22(b), no environmental impact statement or environmental assessment need be prepared in connection with the proposed amendment.

6.0 REFERENCES

1. NUREG-1431, "Standard Technical Specifications - Westinghouse Plants,"

Revision 4.0.

2. NRC Safety Evaluation, "Issuance of Amendments (TAC Nos. M89482 and M89483)

(TS 93-18)," dated July 11, 1994.

3. NRC Safety Evaluation, "Issuance of License Amendments for the Sequoyah Nuclear Plants, Units 1 and 2 (TAC Nos. M96596 and M96597) (TS 96-06)," dated September 23, 1997.

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4. Sequoyah Nuclear Plant, Units 1 and 2, Updated Final Safety Analysis Report

5. 10 CFR 50 Appendix A, General Design Criterion (GDC) 17, "Electric Power Systems"

6. NRC Safety Guide 6, "Independence Between Redundant Standby (Onsite) Power Sources and Between Their Distribution Systems," dated March 10, 1971

7. NRC Regulatory Guide 1.32, "Criteria for Safety Related Electric Power Systems for Nuclear Power Plants," Revision 2

8. NRC Regulatory Guide 1.155, "Station Blackout," Revision 0

9. SQNETAPAC, "AC Auxiliary Power System Analysis," Revisions 49 and 51

10. E32 080425 602, "Sequoyah Nuclear Plant (SQN) - Planning Transmission System Study (TSS) - Grid Offsite Power, Transient Stability, and Fault Current Calculation for 2008 and 2013," Revision 2 Page 23 of 23

ATTACHMENT I Proposed TS Changes (Mark-Ups) for SQN, Units I and 2

Proposed TS Changes (Mark-Ups) for SQN, Unit I ELECTRICAL POWER SYSTEMS ACTION (Continued)

c. With one offsite circuit and one diesel generator set of the above required A.C. electrical power sources inoperable, demonstrate the OPERABILITY of the remaining A.C. sources by performing Surveillance Requirements 4.8.1.1.1.a within one hour and at least once per 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months thereafter, and Surveillance Requirement 4.8.1.1.2.a.4 within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months ; restore at least one of the inoperable sources to OPERABLE status within 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and in COLD SHUTDOWN within the following 30 hours3.472222e-4 days 0.00833 hours 4.960317e-5 weeks 1.1415e-5 months .

d. With two of the above required offsite A.C. circuits inoperable, demonstrate the OPERABILITY of 4 diesel generator sets by performing Surveillance Requirement 4.8.1.1.2.a.4 within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months , unless the diesel generator sets are already operating; restore at least one of the inoperable offsite sources to OPERABLE status within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months .

e. With either diesel generator sets 1A-A and/or 2A-A inoperable simultaneous with 1B-B and/or 2B-B, demonstrate the OPERABILITY of two offsite A.C. circuits by performing Surveillance Requirement 4.8.1.1.1 .a within one hour and at least once per 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months thereafter; restore at least 1) 1A-A and 2A-A or 2) 1B-B and 2B-B to P-E-P-ABI-E-OPERABLE status within 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and in COLD SHUTOOWI4 SHUTDOWN within the following 30 hours3.472222e-4 days 0.00833 hours 4.960317e-5 weeks 1.1415e-5 months .

f. LCO 3.0.4.b is not applicable to diesel generators.

SURVEILLANCE REOQUIREMENTS 4.8.1.1.1 Each of the above required independent circuits between the offsite transmission network and the onsite Class lE distribution system shall be:

a. Determined OPERABLE at least once per 7 days by verifying correct breaker alignments and indicated power availability.

b. Demonstrated OPERABLE at least once per 18 months# by manually and automatically transferring the power supply to each 6.9 kV Unit Board## from the normal supply to the alternate supply.

For the 1A, 1B, 1C, and 1D 6.9 kV Unit Boards, this Surveillance shall not be performed in MODES 1 and 2.

1# Transfer capability is only required to be met for 6.9 kV Unit Boards that require normal and alternate power supplies.

April 11, 2005 SEQUOYAH - UNIT 1 3/4 8-2 Amendment No. 100, 137, 184, 205, 228, 255, 301

Proposed TS Changes (Mark-Ups) for SQN, Unit 2 ELECTRICAL POWER SYSTEMS ACTION (Continued)

c. With one offsite circuit and one diesel generator set of the above required A.C. electrical power sources inoperable, demonstrate the OPERABILITY of the remaining A.C. sources by performing Surveillance Requirements 4.8.1.1. 1.a within one hour and at least once per 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months thereafter, and Surveillance Requirement 4.8.1.1.2.a.4 within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months ; restore at least one of the inoperable sources to OPERABLE status within 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and in COLD SHUTDOWN within the following 30 hours3.472222e-4 days 0.00833 hours 4.960317e-5 weeks 1.1415e-5 months .

d. With two of the above required offsite A.C. circuits inoperable, demonstrate the OPERABILITY of 4 diesel generator sets by performing Surveillance Requirement 4.8.1.1.2.a.4 within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months , unless the diesel generator sets are already operating; restore at least one of the inoperable offsite sources to OPERABLE status within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months .

e. With either diesel generator sets 1A-A and/or 2 A-A inoperable simultaneous with 1B-B and/or 2B-B, demonstrate the OPERABILITY of two offsite A.C. circuits by performing Surveillance Requirement 4.8.1.1.1 .a within one hour and at least once per 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months thereafter; restore at least 1) 1A-A and 2A-A or 2) 1B-B and 2B-B to OPERABLE status within 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and in COLD SHUTDOWN within the following 30 hours3.472222e-4 days 0.00833 hours 4.960317e-5 weeks 1.1415e-5 months .

f. LCO 3.0.4.b is not applicable to diesel generators.

SURVEILLANCE REQUIREMENTS 4.8.1.1.1 Each of the above required independent circuits between the offsite transmission network and the onsite Class E distribution system shall be:

a. Determined OPERABLE at least once per 7 days by verifying correct breaker alignments and indicated power availability.

b. Demonstrated OPERABLE at least once per 18 months# by manually and automatically transferring the power supply to each 6.9 kV Unit Board## from the normal supply to the alternate supply.

For the 2A, 2B, 2C, and 2D 6.9 kV Unit Boards, this Surveillance shall not be performed in MODES 1 and 2.

1. Transfer capability is only required to be met for 6.9 kV Unit Boards that require normal and alternate power supplies.

April 11, 2005 SEQUOYAH - UNIT 2 3/4 8-2 Amendment No. 89, 123, 176, 195, 219, 246, 290

ATTACHMENT 2 Proposed TS Bases Changes (Mark-Ups) for SQN, Units I and 2

Proposed TS Bases Changes (Mark-Ups) for SQN, Unit I 3/4.8 ELECTRICAL POWER SYSTEMS BASES 3/4.8.1 and 3/4.8.2 A.C. SOURCES AND ONSITE POWER DISTRIBUTION SYSTEMS The OPERABILITY of the A.C. and D.C power sources and associated distribution systems during operation ensures that sufficient power will be available to supply the safety related equipment required for 1) the safe shutdown of the facility and 2) the mitigation and control of accident conditions within the facility. The minimum specified independent and redundant A.C. and D.C. power sources and distribution systems satisfy the requirements of General Design Criteria 17 of Appendix "A" to 10 CFR 50.

The electrically powered AC safety loads are separated into redundant load groups such that loss of any one load group will not prevent the minimum safety functions from being performed. Specification 3.8.1.1 requires two physically independent circuits between the offsite transmission network and the onsite Class 1 E Distribution System and four separate and independent diesel generator sets to be OPERABLE in MODES 1, 2, 3, and 4. These requirements ensure availability of the required power to shut down the reactor and maintain it in a safe shutdown condition after an abnormal operational transient or a postulated desiqn basis accident.

Each offsite circuit must be capable of maintaining rated frequency and voltage, and accepting required loads during an accident. Minimum required switchyard voltages are determined by evaluation of plant accident loading and the associated voltage drops between the transmission network and these loads. These minimum voltage values are provided to TVA's Transmission Operations for use in system studies to support operation of the transmission network in a manner that will maintain the necessary voltages. Transmission Operations is required to notify SQN Operations if it is determined that the transmission network may not be able to support accident loading or shutdown operations as required by 10 CFR 50, Appendix A, GDC-17. Any offsite power circuits supplied by that transmission network that are not able to support accident loading or shutdown operations are inoperable.

The unit station service transformers (USSTs) utilize auto load tap changers to provide the required voltage response for accident loading. The load tap changer associated with a USST is required to be functional and in "automatic" for the USST to supply power to a 6.9 kV Unit Board.

The inability to supply offsite power to a 6.9 kV Shutdown Board constitutes the failure of only one offsite circuit, as long as offsite power is available to the other load group's Shutdown Boards. Thus, if one or both 6.9 kV Shutdown Boards in a load group do not have an offsite circuit available, then only one offsite circuit would be inoperable. If one or more Shutdown Boards in each load group, or all four Shutdown Boards, do not have an offsite circuit available, then both offsite circuits would be inoperable.

An "available" offsite circuit meets the requirements of GDC-1 7, and is either connected to the 6.9 kV Shutdown Boards or can be connected to the 6.9 kV Shutdown Boards within a few seconds.

An offsite circuit consists of all breakers, transformers, switches, interrupting devices, cabling, and controls required to transmit power from the offsite transmission network (beginning at the switchyard) to one load group of Class 1E 6.9 kV Shutdown Boards (ending at the supply side of the normal or alternate supply circuit breaker). Each required offsite circuit is that combination of power sources described below that are normally connected to the Class 1E distribution system, or can be connected to the Class 1E distribution system through automatic transfer at the 6.9 kV Unit Boards.

The following offsite power configurations meet the requirements of LCO 3.8.1.1 .a:

(Note that common station service transformer (CSST) B is a spare transformer with two sets of secondary windings that can be used to supply a total of two Start Buses for CSST A and/or CSST C, with each supplied Start Bus on a separate CSST B secondary winding.)

February 11, 2003 SEQUOYAH - UNIT 1 B 3/4 8-1 Amendment No. 12, 137, 173, 205, 241, 281

3/4.8 ELECTRICAL POWER SYSTEMS BASES 3/4.8.1 and 3/4.8.2 A.C. SOURCES AND ONSITE POWER DISTRIBUTION SYSTEMS

1. Two offsite circuits consisting of a AND b (no board transfers required: a loss of either circuit will not prevent the minimum safety functions from being performed):

a. From the 161 kV transmission network, through CSST A (winding X) to Start Bus 1A to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board lC), and CSST A (winding Y) to Start Bus 2A to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C): AND

b. From the 161 kV transmission network, through CSST C (winding X) to Start Bus 2B to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B), and CSST C (winding Y) to Start Bus 1B to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B).

2. Two offsite circuits consisting of a AND b (relies on automatic transfer from alignment a. 1) to b.2)(b).

or a.2) to b.1)(a) on a loss of USSTs 1A and 1B, OR relies on automatic transfer from alignment a.3) to b.2)(a), or a.4) to b. 1)(b) on a loss of USSTs 2A and 2B):

a. Normal power source alignments

1) From the 500 kV switchyard through USST 1A to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B);

2) From the 500 kV switchyard through USST 1B to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board IC):

3) From the 161 kV switchyard through USST 2A to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B): AND

4) From the 161 kV switchyard througqh USST 2B to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C).

b. Alternate Power source alignments

1) From the 161 kV transmission network, through:

(a) CSST A (winding X) to Start Bus 1A to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board lC): AND (b) CSST A (winding Y) to Start Bus 2A to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C): OR

2) From the 161 kV transmission network, through:

(a) CSST C (winding X) to Start Bus 2B to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B), AND (b) CSST C (winding Y) to Start Bus 1B to 6.9 kV Shutdown Board lA-A (through 6.9 kV Unit Board 1B).

April 11, 2005 SEQUOYAH - UNIT 1 B 3/4 8-1a Amendment No. 12, 137, 173, 205, 234, 241,261,285, 301

3/4.8 ELECTRICAL POWER SYSTEMS BASES 3/4.8.1 and 3/4.8.2 A.C. SOURCES AND ONSITE POWER DISTRIBUTION SYSTEMS 3, Two offsite circuits consisting of a AND b (relies on automatic transfer from alignment a.1) to b.1) and b.2) on a loss of the Unit 2 USSTs: a loss of alignment a.2) or a.3) will not prevent the minimum safety functions from being performed):

a. Normal power source alignments

1) From the 161 kV switchyard through USST 2A to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B), and USST 2B to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C):

2) From the 161 kV transmission network, through CSST A (winding X) to Start Bus 1A to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board 1C): AND

3) From the 161 kV transmission network, through CSST C (winding Y) to Start Bus 1B to 6.9 kV Shutdown Board lA-A (through 6.9 kV Unit Board 1B).

b. Alternate Power source alignments

1) From the 161 kV transmission network, through CSST A (winding Y) to Start Bus 2A to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C): AND

2) From the 161 kV transmission network, through CSST C (winding X) to Start Bus 2B to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B).

4. Two offsite circuits consisting of a AND b (relies on automatic transfer from alignment a.1) to b.1) and b.2) on a loss of the Unit 1 USSTs: a loss of alignment a.2) or a.3) will not prevent the minimum safety functions from being performed):

a. Normal power source alignments

1) From the 500 kV switchyard through USST 1A to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B), and USST 1B to 6.9 kV Shutdown Board 1B-B (througqh 6.9 kV Unit Board 1C):

2) From the 161 kV transmission network, through CSST A (winding Y) to Start Bus 2A to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C): AND

3) From the 161 kV transmission network, through CSST C (winding X) to Start Bus 2B to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B).

b. Alternate power source alignments

1) From the 161 kV transmission network, through CSST A (winding X) to Start Bus 1A to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board 1C): AND

2) From the 161 kV transmission network, through CSST C (winding Y) to Start Bus 1B to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B).

Other offsite power configurations are possible using different combinations of available USSTs and CSSTs, as long as the alignments are consistent with the analyzed configurations, and the aliqnments otherwise comply with the requirements of GDC 17.

December 22, 2003 SEQUOYAH - UNIT 1 B 3/4 8-1b Amendment No. 12, 137, 173, 205, 234, 261 285

ELECTRICAL POWER SYSTEMS BASES A.C. SOURCES AND ONSITE POWER DISTRIBUTION SYSTEMS (Continued)

For example, to support breaker testing, offsite power to the 6.9 kV Shutdown Boards can be realigned from normal feed to alternate feed. This would result in Shutdown Boards 1A-A and 2A-A being fed from Unit Boards 1A and 2A, respectively, and Shutdown Boards 1B-B and 2B-B being fed from Unit Boards 1D and 2D, respectively. The CSST being utilized as the alternate power source to one load group of Shutdown Boards would also be realigned (normally CSST A available to Shutdown Boards 1B-B and 2B-B or CSST C available to Shutdown Boards 1A-A and 2A-A, would be realigned to CSST A available to Shutdown Boards 1A-A and 2A-A or CSST C available to Shutdown Boards 1B-B and 2B-B).

The ACTION requirements specified for the levels of degradation of the power sources provide restriction upon continued facility operation commensurate with the level of degradation. The OPERABILITY of the power sources are consistent with the initial condition assumptions of the accident analyses and are based upon maintaining at least one redundant set of onsite A.C. and D.C. power sources and associated distribution systems OPERABLE during accident conditions coincident with an assumed loss of offsite power and single failure of the other onsite A.C. source.

The footnote for Action b of LCO 3.8.1.1 requires completion of a determination that the OPERABLE diesel generators are not inoperable due to common cause failure or performance of Surveillance 4.8.1.1.2.a.4 if Action b is entered. The intent is that all diesel generator inoperabilities must be investigated for common cause failures regardless of how long the diesel generator inoperability persists.

Action b of LCO 3.8.1.1 is further modified by a second note which precludes making more than one diesel generator inoperable on a pre-planned basis for maintenance, modifications, or surveillance testing. The intent of this footnote is to explicitly exclude the flexibility of removing a diesel generator set from service as a part of a pre-planned activity. While the removal of a diesel generator set (A or B train) is consistent with the initial condition assumptions of the accident analysis, this configuration is judged as imprudent. The term pre-planned is to be taken in the context of those activities which are routinely scheduled and is not relative to conditions which arise as a result of emergent or unforeseen events. As an example, this footnote is not intended to preclude the actions necessary to perform the common mode testing requirements required by Action b. As another example, this footnote is not intended to prevent the required surveillance testing of the diesel generators should one diesel generator maintenance be unexpectedly extended and a second diesel generator fall within its required testing frequency. Thus, application of the note is intended for pre-planned activities.

In addition, this footnote is intended to apply only to those actions taken directly on the diesel generator. For those actions taken relative to common support systems (e.g. ERCW), the support function must be evaluated for impact on the diesel generator.

The action to determine that the OPERABLE diesel generators are not inoperable due to common cause failure provides an allowance to avoid unnecessary testing of OPERABLE diesel generators. If it can be determined that the cause of the inoperable diesel generator does not exist on the OPERABLE diesel generators, Surveillance Requirement 4.8.1.1.2.a.4 does not have to be performed. If the cause of inoperability exists on other diesel generator(s), the other diesel generator(s) would be declared inoperable upon discovery and Action e of LCO 3.8.1.1 would be entered as applicable. Once the common failure is repaired, the common cause no longer exists, and the action to determine inoperability due to common cause failure is satisfied. If the cause of the initial inoperable diesel generator cannot be confirmed not to exist on the remaining diesel generators, performance of Surveillance 4.8.1.1.2.a.4 suffices to provide assurance to continued OPERABILITY of the other diesel generators.

SEQUOYAH - UNIT 1 B 3/4 8-1 c Amendment No. XXX

3/4.8 ELECTRICAL POWER SYSTEMS BASES 3/4.8.1 and 3/4.8.2 A.C. SOURCES AND ONSITE POWER DISTRIBUTION SYSTEMS According to Generic Letter 84-15, 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months is reasonable to confirm that the OPERABLE diesel generators are not affected by the same problem as the inoperable diesel generator.

Action f prohibits the application of LCO 3.0.4.b to an inoperable diesel generator. There is an increased risk associated with entering a MODE or other specified condition in the Applicability with an inoperable diesel generator and the provisions of LCO 3.0.4.b, which allow entry into a MODE or other specified condition in the Applicability with the LCO not met after performance of a risk assessment addressing inoperable systems and components, should not be applied in this circumstance.

The OPERABILITY of the minimum specified A.C. and D.C. power sources and associated distribution systems during shutdown and refueling ensures that 1) the facility can be maintained in the shutdown or refueling condition for extended time periods and 2) sufficient instrumentation and control capability is available for monitoring and maintaining the unit status.

With the minimum required AC power sources not available, it is required to suspend CORE ALTERATIONS and operations involving positive reactivity additions that could result in loss of required SDM (Mode 5) or boron concentration (Mode 6). Suspending positive reactivity additions that could result in failure to meet minimum SDM or boron concentration limit is required to assure continued safe operation. Introduction of coolant inventory must be from sources that have a boron concentration greater than or equal to that required in the RCS for minimum SDM or refueling boron concentration.

This may result in an overall reduction in RCS boron concentration but provides acceptable margin to maintaining subcritical operation. Introduction of temperature changes including temperature increases when operating with a positive MTC must also be evaluated to ensure they do not result in a loss of required SDM.

The requirements of Specification 3.8.2.1 provide those actions to be taken for the inoperability of A.C. Distribution Systems. Action a of this specification provides an 8-hour action for the inoperability of one or more A.C. boards. Action b of this specification provides a relaxation of the 8-hour action to 24-hours provided the Vital Instrument Power Board is inoperable solely as a result of one inoperable inverter and the board has been energized within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months . In this condition the requirements of Action a do not have to be applied. Action b is not intended to provide actions for inoperable inverters, which is addressed by the operability requirements for the boards, and is included only for relief from the 8-hour action of Action a when only one inverter is affected. More than one inverter inoperable will result in the inoperability of the associated 120 Volt A.C. Vital Instrument Power Board(s) in accordance with Action a.

With more than one inverter inoperable entry into the actions of TS 3.0.3 is not applicable because Action a includes provisions for multiple inoperable inverters as attendant equipment to the boards.

The Surveillance Requirements for demonstrating the OPERABILITY of the diesel generators are in accordance with the recommendations of Regulatory Guides 1.9 "Selection of Diesel Generator Set Capacity for Standby Power Supplies," March 10, 1971, and 1.108 "Periodic Testing of Diesel Generator Units Used as Onsite Electric Power Systems at Nuclear Power Plants," Revision 1, August 1977, and 1.137 "Fuel-Oil Systems for Standby Diesel Generators," Revision 1, October 1979. The Surveillance Requirements for the diesel generator load-run test and the 24-hour endurance and margin test are in accordance with Regulatory Guide 1.9, Revision 3, July 1993, "Selection, Design, Qualification, and Testing of Emergency Diesel Generator Units Used as Class 1E Onsite Electric Power Systems at Nuclear Power Plants." During the diesel generator endurance and margin surveillance test, momentary transients outside the kw and kvar load ranges do not invalidate the test results. Similarly, during the diesel generator load-run test, momentary transients outside the kw load range do not invalidate the test results.

SEQUOYAH - UNIT 1 B 3/4 8-1 d Amendment No. XXX

3/4.8 ELECTRICAL POWER SYSTEMS BASES Where the SRs discussed herein specify voltage and frequency tolerances, the following is applicable. 6800 volts is the minimum steady state output voltage and the 10 second transient value.

6800 volts is 98.6% of nominal bus voltage of 6900 volts and is based on the minimum voltage required for the diesel generator supply breaker to close on the 6.9 kV Shutdown Board. The specified maximum steady state output voltage of 7260 volts is based on the degraded over voltage relay setpoint and is equivalent to 110% of the nameplate rating of the 6600 volt motors. The specified minimum and maximum frequencies of the diesel generator are 58.8 Hz and 61.2 Hz, respectively. These values are equal to +/- 2% of the 60 Hz nominal frequency and are derived from the recommendations given in regulatory Guide 1.9.

Where the SRs discuss maximum transient voltages during load rejection testing, the following is applicable. The maximum transient voltage of 8880 volts represents a conservative limit to ensure the resulting voltage will not exceed a level that will cause component damage. It is based on the manufacturer's recommended high potential test voltage of 60% of the original factory high potential test voltage (14.8 kV). The diesel generator manufacturer has determined that the engine and/or generator controls would not experience detrimental effects for transient voltages < 9000 volts. The maximum transient voltage of 8276 volts is retained from the original technical specifications to ensure that the voltage transient following rejection of the single largest load is within the limits originally considered acceptable. It was based on 114% of 7260 volts, which is the Range B service voltage per ANSI-C84.1.

The Surveillance Requirement (SR) to transfer the power supply to each 6.9 kV Unit Board from the normal supply to the alternate supply demonstrates the OPERABILITY of the alternate supply to power the shutdown loads. The 18 month Frequency of the Surveillance is based on engineering judament, taking into consideration the unit conditions required to perform the Surveillance, and is intended to be consistent with expected fuel cycle lengths. Operating experience has shown that these components usually pass the SR when performed at the 18 month Frequency. Therefore, the Frequency was concluded to be acceptable from a reliability standpoint. This SR is modified by two Notes. The reason for Note # is that, during operation with the reactor critical, performance of this SR for the Unit 1 Unit Boards could cause perturbations to the electrical distribution systems that could challenge continued steady state operation and, as a result, unit safety systems. Note ## specifies that transfer capability is only required to be met for 6.9 kV Unit Boards that require normal and alternate power supplies. When both load groups are being supplied power by the USSTs, only the 6.9 kV Unit Boards associated with one load group are required to have normal and alternate power supplies. Therefore, only one CSST is required to be OPERABLE and available as an alternate power supply. Additionally, manual transfers between the normal supply and the alternate supply are not relied upon to meet the accident analysis. Manual transfer capability is verified to ensure the availability of a backup to the automatic transfer feature.

The Surveillance Requirement for demonstrating the OPERABILITY of the Station batteries are based on the recommendations of Regulatory Guide 1.129 "Maintenance Testing and Replacement of Large Lead Storage Batteries for Nuclear Power Plants," February 1978, and IEEE Std 450-1980, "IEEE Recommended Practice for Maintenance, Testing, and Replacement of Large Lead Storage batteries for Generating Stations and Substations."

SEQUOYAH - UNIT 1 B 3/4 8-1 e Amendment No. XXX

Proposed TS Bases Changes (Mark-Ups) for SQN, Unit 2 3/4.8 ELECTRICAL POWER SYSTEMS BASES 3/4.8.1 AND 3/4.8.2 A.C. SOURCES AND ONSITE POWER DISTRIBUTION SYSTEMS The OPERABILITY of the A.C. and D.C power sources and associated distribution systems during operation ensures that sufficient power will be available to supply the safety related equipment required for 1) the safe shutdown of the facility and 2) the mitigation and control of accident conditions within the facility. The minimum specified independent and redundant A.C. and D.C. power sources and distribution systems satisfy the requirements of General Design Criterion 17 of Appendix."A" to 10 CFR 50.

The electrically powered AC safety loads are separated into redundant load groups such that loss of any one load group will not prevent the minimum safety functions from being performed. Specification 3.8.1.1 requires two physically independent circuits between the offsite transmission network and the onsite Class 1E Distribution System and four separate and independent diesel generator sets to be OPERABLE in MODES 1, 2, 3, and 4. These requirements ensure availability of the required power to shut down the reactor and maintain it in a safe shutdown condition after an abnormal operational transient or a postulated design basis accident.

Each offsite circuit must be capable of maintaining rated frequency and voltage, and accepting required loads during an accident. Minimum required switchyard voltages are determined by evaluation of plant accident loading and the associated voltage drops between the transmission network and these loads. These minimum voltage values are provided to TVA's Transmission Operations for use in system studies to support operation of the transmission network in a manner that will maintain the necessary voltages. Transmission Operations is required to notify SQN Operations if it is determined that the transmission network may not be able to support accident loading or shutdown operations as required by 10 CFR 50, Appendix A, GDC-17. Any offsite power circuits supplied by that transmission network that are not able to support accident loading or shutdown operations are inoperable.

The unit station service transformers (USSTs) utilize auto load tap changers to provide the required voltage response for accident loading. The load tap changer associated with a USST is required to be functional and in "automatic" for the USST to supply power to a 6.9 kV Unit Board.

The inability to supply offsite power to a 6.9 kV Shutdown Board constitutes the failure of only one offsite circuit, as longq as offsite power is available to the other load group's Shutdown Boards. Thus, if one or both 6.9 kV Shutdown Boards in a load group do not have an offsite circuit available, then only one offsite circuit would be inoperable. If one or more Shutdown Boards in each load group, or all four Shutdown Boards, do not have an offsite circuit available, then both offsite circuits would be inoperable.

An "available" offsite circuit meets the requirements of GDC-1 7, and is either connected to the 6.9 kV Shutdown Boards or can be connected to the 6.9 kV Shutdown Boards within a few seconds.

An offsite circuit consists of all breakers, transformers, switches, interrupting devices, cabling.,

and controls required to transmit power from the offsite transmission network (beginning at the switchyard) to one load group of Class 1E 6.9 kV Shutdown Boards (ending at the supply side of the normal or alternate supply circuit breaker). Each required offsite circuit is that combination of power sources described below that are normally connected to the Class 1 E distribution system, or can be connected to the Class 1 E distribution system through automatic transfer at the 6.9 kV Unit Boards.

The following offsite power configurations meet the requirements of LCO 3.8.1.1 .a:

(Note that common station service transformer (CSST) B is a spare transformer with two sets of secondary windings that can be used to supply a total of two Start Buses for CSST A and/or CSST C, with each supplied Start Bus on a separate CSST B secondary winding.)

February 11, 2003 SEQUOYAH - UNIT 2 B 3/4 8-1 Amendment No. 123, 164, 195, 231, 272

3/4.8 ELECTRICAL POWER SYSTEMS BASES 3/4.8.1 and 3/4.8.2 A.C. SOURCES AND ONSITE POWER DISTRIBUTION SYSTEMS

1. Two offsite circuits consisting of a AND b (no board transfers required: a loss of either circuit will not prevent the minimum safety functions from being performed):

a. From the 161 kV transmission network, through CSST A (winding X) to Start Bus 1A to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board 1C), and CSST A (winding Y) to Start Bus 2A to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C): AND

b. From the 161 kV transmission network, through CSST C (winding X) to Start Bus 2B to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B), and CSST C (winding Y) to Start Bus 1B to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B).

2. Two offsite circuits consisting of a AND b (relies on automatic transfer from alignment a.1) to b.2)(b).

or a.2) to b. 1)(a) on a loss of USSTs 1A and 1B, OR relies on automatic transfer from alignment a.3) to b.2)(a), or a.4) to b.1)(b) on a loss of USSTs 2A and 2B):

a. Normal power source alignments

1) From the 500 kV switchyard through USST 1A to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B):

2) From the 500 kV switchyard through USST 1B to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board 1C):

3) From the 161 kV switchyard through USST 2A to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B): AND

4) From the 161 kV switchyard through USST 2B to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C).

b. Alternate power source alignments

1) From the 161 kV transmission network, through:

(c) CSST A (winding X) to Start Bus 1A to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board 1C): AND (d) CSST A (winding Y) to Start Bus 2A to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C): OR

2) From the 161 kV transmission network, through:

(c) CSST C (winding X) to Start Bus 2B to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B), AND (d) CSST C (winding Y) to Start Bus 1 B to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B).

April 11, 2005 SEQUOYAH - UNIT 2 B 3/4 8-1a Amendment No. 123, 164, 195, 224, 231,274, 290

3/4.8 ELECTRICAL POWER SYSTEMS BASES

3. Two offsite circuits consisting of a AND b (relies on automatic transfer from alignment a.1) to b.1) and b.2) on a loss of the Unit 2 USSTs: a loss of alignment a.2) or a.3) will not prevent the minimum safety functions from being performed):

a. Normal power source alignments

1) From the 161 kV switchyard through USST 2A to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B), and USST 2B to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C):

2) From the 161 kV transmission network, through CSST A (winding X) to Start Bus 1A to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board 1C): AND

3) From the 161 kV transmission network, through CSST C (winding Y) to Start Bus 1B to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B).

b. Alternate power source alignments

1) From the 161 kV transmission network, through CSST A (winding Y) to Start Bus 2A to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C): AND

2) From the 161 kV transmission network, through CSST C (winding X) to Start Bus 2B to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B).

4. Two offsite circuits consisting of a AND b (relies on automatic transfer from alignment a.1) to b.1) and b.2) on a loss of the Unit 1 USSTs: a loss of alignment a.2) or a.3) will not prevent the minimum safety functions from being performed):

a. Normal power source alignments

1) From the 500 kV switchyard through USST 1A to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B), and USST 1B to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board 1C):

2) From the 161 kV transmission network, through CSST A (winding Y) to Start Bus 2A to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C): AND

3) From the 161 kV transmission network, through CSST C (winding X) to Start Bus 2B to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B).

b. Alternate power source alignments

1) From the 161 kV transmission network, through CSST A (winding X) to Start Bus 1A to 6.9 kV Shutdown Board 1 B-B (through 6.9 kV Unit Board IC): AND

2) From the 161 kV transmission network, through CSST C (winding Y) to Start Bus 1B to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B).

Other offsite power configurations are possible using different combinations of available USSTs and CSSTs, as long as the alignments are consistent with the analyzed configurations, and the alignments otherwise comply with the requirements of GDC 17.

December 22, 2003 SEQUOYAH - UNIT 2 B 3/4 8-1b Amendment No. 123, 164, 195, 224, 274 234, 261 285

3/4.8 ELECTRICAL POWER SYSTEMS BASES For example, to support breaker testing, offsite power to the 6.9 kV Shutdown Boards can be realigned from normal feed to alternate feed. This would result in Shutdown Boards 1A-A and 2A-A being fed from Unit Boards 1A and 2A, respectively, and Shutdown Boards 1 B-B and 2B-B being fed from Unit Boards 1D and 2D, respectively. The CSST being utilized as the alternate power source to one load group of Shutdown Boards would also be realigned (normally CSST A available to Shutdown Boards 1B-B and 2B-B or CSST C available to Shutdown Boards 1A-A and 2A-A, would be realigned to CSST A available to Shutdown Boards 1A-A and 2A-A or CSST C available to Shutdown Boards 1 B-B and 2B-B).

The ACTION requirements specified for the levels of degradation of the power sources provide restriction upon continued facility operation commensurate with the level of degradation. The OPERABILITY of the power sources are consistent with the initial condition assumptions of the safety analyses and are based upon maintaining at least one redundant set of onsite A.C. and D.C. power sources and associated distribution systems OPERABLE during accident conditions coincident with an assumed loss of offsite power and single failure of the other onsite A.C. source.

The footnote for Action b of LCO 3.8.1.1 requires completion of a determination that the OPERABLE diesel generators are not inoperable due to common cause failure or performance of Surveillance 4.8.1.1.2.a.4 if Action b is entered. The intent is that all diesel generator inoperabilities must be investigated for common cause failures regardless of how long the diesel generator inoperability persists.

Action b of LCO 3.8.1.1 is further modified by a second note which precludes making more than one diesel generator inoperable on a pre-planned basis for maintenance, modifications, or surveillance testing. The intent of this footnote is to explicitly exclude the flexibility of removing a diesel generator set from service as a part of a pre-planned activity. While the removal of a diesel generator set (A or B train) is consistent with the initial condition assumptions of the accident analysis, this configuration is judged as imprudent. The term pre-planned is to be taken in the context of those activities which are routinely scheduled and is not relative to conditions which arise as a result of emergent or unforeseen events. As an example, this footnote is not intended to preclude the actions necessary to perform the common mode testing requirements required by Action b. As another example, this footnote is not intended to prevent the required surveillance testing of the diesel generators should one diesel generator maintenance be unexpectedly extended and a second diesel generator fall within its required testing frequency. Thus, application of the note is intended for pre-planned activities.

In addition, this footnote is intended to apply only to those actions taken directly on the diesel generator. For those actions taken relative to common support systems (e.g. ERCW), the support function must be evaluated for impact on the diesel generator.

The action to determine that the OPERABLE diesel generators are not inoperable due to common cause failures provides an allowance to avoid unnecessary testing of OPERABLE diesel generators. If it can be determined that the cause of the inoperable diesel generator does not exist on the OPERABLE diesel generators, Surveillance Requirement 4.8.1.1.2.a.4 does not have to be performed. If the cause of inoperability exists on other diesel generator(s), the other diesel generator(s) would be declared inoperable upon discovery and Action e of LCO 3.8.1.1 would be entered as applicable. Once the common failure is repaired, the common cause no longer exists, and the action to determine inoperability due to common cause failure is satisfied. If the cause of the initial inoperable diesel generator cannot be confirmed not to exist on the remaining diesel generators, performance of Surveillance 4.8.1.1.2.a.4 suffices to provide assurance of continued OPERABILITY of the other diesel generators.

SEQUOYAH - UNIT 2 B 3/4 8-1 c Amendment No. XXX

3/4.8 ELECTRICAL POWER SYSTEMS BASES According to Ger~eric Letter 84-15, 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months is reasonable to confirm that the OPERABLE diesel generators are not affected by the same problem as the inoperable diesel generator.

Action f prohibits the application of LCO 3.0.4.b to an inoperable diesel generator. There is an increased risk associated with entering a MODE or other specified condition in the Applicability with an inoperable diesel generator and the provisions of LCO 3.0.4.b, which allow entry into a MODE or other specified condition in the Applicability with the LCO not met after performance of a risk assessment addressing inoperable systems and components, should not be applied in this circumstance.

The OPERABILITY of the minimum specified A.C. and D.C. power sources and associated distribution systems during shutdown and refueling ensures that 1) the facility can be maintained in the shutdown or refueling condition for extended time periods and 2) sufficient instrumentation and control capability is available for monitoring and maintaining the unit status.

With the minimum required AC power sources not available, it is required to suspend CORE ALTERATIONS and operations involving positive reactivity additions that could result in loss of required SDM (Mode 5) or boron concentration (Mode 6). Suspending positive reactivity additions that could result in failure to meet minimum SDM or boron concentration limit is required to assure continued safe operation. Introduction of coolant inventory must be from sources that have a boron concentration greater than or equal to that required in the RCS for minimum SDM or refueling boron concentration.

This may result in an overall reduction in RCS boron concentration but provides acceptable margin to maintaining subcritical operation. Introduction of temperature changes including temperature increases when operating with a positive MTC must also be evaluated to ensure they do not result in a loss of required SDM.

The requirements of Specification 3.8.2.1 provide those actions to be taken for the inoperability of A.C. Distribution Systems. Action a of this specification provides an 8-hour action for the inoperability of one or more A.C. boards. Action b of this specification provides a relaxation of the 8-hour action to 24-hours provided the Vital Instrument Power Board is inoperable solely as a result of one inoperable inverter and the board has been energized within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months . In this condition the requirements of Action a do not have to be applied. Action b is not intended to provide actions for inoperable inverters, which is addressed by the operability requirements for the boards, and is included only for relief from the 8-hour action of Action a when only one inverter is affected. More than one inverter inoperable will result in the inoperability of the associated 120 Volt A.C. Vital Instrument Power Board(s) in accordance with Action a.

With more than one inverter inoperable entry into the actions of TS 3.0.3 is not applicable because Action a includes provisions for multiple inoperable inverters as attendant equipment to the boards.

The Surveillance Requirements for demonstrating the OPERABILITY of the diesel generators are in accordance with the recommendations of Regulatory Guides 1.9 "Selection of Diesel Generator Set Capacity for Standby Power Supplies", March 10, 1971, 1.108 "Periodic Testing of Diesel Generator Units Used as Onsite Electric Power Systems at Nuclear Power Plants," Revision 1, August 1977, and 1.137 "Fuel-Oil Systems for Standby Diesel Generators," Revision 1, October 1979. The surveillance requirements for the diesel generator load-run test and the 24-hour endurance and margin test are in accordance with Regulatory Guide 1.9, Revision 3, July 1993, "Selection, Design, Qualification, and Testing of Emergency Diesel Generator Units Used as Class 1E Onsite Electric Power Systems at Nuclear Power Plant." During the diesel generator endurance and margin surveillance test, momentary transients outside the kw and kvar load ranges do not invalidate the test results. Similarly, during the diesel generator load-run test, momentary transients outside the kw load range do not invalidate the test results.

SEQUOYAH - UNIT 2 B 3/4 8-1 d Amendment No. XXX

3/4.8 ELECTRICAL POWER SYSTEMS BASES Where the SRs discussed herein specify voltage and frequency tolerances, the following is applicable. 6800 volts is the minimum steady state output voltage and the 10 second transient value.

6800 volts is 98.6% of nominal bus voltage of 6900 volts and is based on the minimum voltage required for the diesel generator supply breaker to close on the 6.9 kV Shutdown Board. The specified maximum steady state output voltage of 7260 volts is based on the degraded over voltage relay setpoint and is equivalent to 110% of the nameplate rating of the 6600 volt motors. The specified minimum and maximum frequencies of the diesel generator are 58.8 Hz and 61.2 Hz, respectively. These values are equal to +/- 2% of the 60 Hz nominal frequency and are derived from the recommendations given in regulatory Guide 1.9.

Where the SRs discuss maximum transient voltages during load rejection testing, the following is applicable. The maximum transient voltage of 8880 volts represents a conservative limit to ensure the resulting voltage will not exceed a level that will cause component damage. It is based on the manufacturer's recommended high potential test voltage of 60% of the original factory high potential test voltage (14.8 kV). The diesel generator manufacturer has determined that the engine and/or generator controls would not experience detrimental effects for transient voltages < 9000 volts. The maximum transient voltage of 8276 volts is retained from the original technical specifications to ensure that the voltage transient following rejection of the single largest load is within the limits originally considered acceptable. It was based on 114% of 7260 volts, which is the Range B service voltage per ANSI-C84.1.

The Surveillance Requirement (SR) to transfer the power supply to each 6.9 kV Unit Board from the normal supply to the alternate supply demonstrates the OPERABILITY of the alternate supply to power the shutdown loads. The 18 month Frequency of the Surveillance is based on engineering judgment, taking into consideration the unit conditions required to perform the Surveillance, and is intended to be consistent with expected fuel cycle lengths. Operating experience has shown that these components usually pass the SR when performed at the 18 month Frequency. Therefore, the Frequency was concluded to be acceptable from a reliability standpoint. This SR is modified by two Notes. The reason for Note # is that, during operation with the reactor critical, performance of this SR for the Unit 1 Unit Boards could cause perturbations to the electrical distribution systems that could challenqe continued steady state operation and, as a result, unit safety systems. Note ## specifies that transfer capability is only required to be met for 6.9 kV Unit Boards that require normal and alternate power supplies. When both load groups are being supplied power by the USSTs, only the 6.9 kV Unit Boards associated with one load group are required to have normal and alternate power supplies. Therefore, only one CSST is required to be OPERABLE and available as an alternate power supply. Additionally, manual transfers between the normal supply and the alternate supply are not relied upon to meet the accident analysis. Manual transfer capability is verified to ensure the availability of a backup to the automatic transfer feature.

The Surveillance Requirement for demonstrating the OPERABILITY of the Station batteries are based on the recommendations of Regulatory Guide 1.129 "Maintenance Testing and Replacement of Large Lead Storage Batteries for Nuclear Power Plants," February 1978, and IEEE Std 450-1980, "IEEE Recommended Practice for Maintenance, Testing and Replacement of Large Lead Storage Batteries for Generating Stations and Substations."

SEQUOYAH - UNIT 2 B 3/4 8-1 e Amendment No. XXX

3 ATTACHMENT SQN, Units I and 2 (Final Typed) for Proposed TS Changes

Proposed TS Changes (Final Typed) for SQN, Unit I ELECTRICAL POWER SYSTEMS ACTION (Continued)

c. With one offsite circuit and one diesel generator set of the above required A.C. electrical power sources inoperable, demonstrate the OPERABILITY of the remaining A.C. sources by performing Surveillance Requirements 4.8.1.1.1.a within one hour and at least once per 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months thereafter, and Surveillance Requirement 4.8.1.1.2.a.4 within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months ; restore at least one of the inoperable sources to OPERABLE status within 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and in COLD SHUTDOWN within the following 30 hours3.472222e-4 days 0.00833 hours 4.960317e-5 weeks 1.1415e-5 months .

d. With two of the above required offsite A.C. circuits inoperable, demonstrate the OPERABILITY of 4 diesel generator sets by performing Surveillance Requirement 4.8.1.1.2.a.4 within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months , unless the diesel generator sets are already operating; restore at least one of the inoperable offsite sources to OPERABLE status within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months .

e. With either diesel generator sets 1A-A and/or 2A-A inoperable simultaneous with 1B-B and/or 2B-B, demonstrate the OPERABILITY of two offsite A.C. circuits by performing Surveillance Requirement 4.8.1.1.1 .a within one hour and at least once per 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months thereafter; restore at least 1) 1A-A and 2A-A or 2) 1B-B and 2B-B to OPERABLE status within 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and in COLD SHUTDOWN within the following 30 hours3.472222e-4 days 0.00833 hours 4.960317e-5 weeks 1.1415e-5 months .

f. LCO 3.0.4.b is not applicable to diesel generators.

SURVEILLANCE REQUIREMENTS 4.8.1.1.1 Each of the above required independent circuits between the offsite transmission network and the onsite Class 1E distribution system shall be:

a. Determined OPERABLE at least once per 7 days by verifying correct breaker alignments and indicated power availability.

b. Demonstrated OPERABLE at least once per 18 months# by manually and automatically transferring the power supply to each 6.9 kV Unit Board## from the normal supply to the alternate supply.

For the 1A, 1B, IC, and 1D 6.9 kV Unit Boards, this Surveillance shall not be performed in MODES 1 and 2.

1. Automatic transfer capability is only required to be met for 6.9 kV Unit Boards that require normal and alternate power supplies.

April 11, 2005 SEQUOYAH - UNIT 1 3/4 8-2 Amendment No. 100, 137, 184, 205, 228, 255, 301

Proposed TS Changes (Final Typed) for SQN, Unit 2 ELECTRICAL POWER SYSTEMS ACTION (Continued)

c. With one offsite circuit and one diesel generator set of the above required A.C. electrical power sources inoperable, demonstrate the OPERABILITY of the remaining A.C. sources by performing Surveillance Requirements 4.8.1.1.1.a within one hour and at least once per 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months thereafter, and Surveillance Requirement 4.8.1.1.2.a.4 within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months ; restore at least one of the inoperable sources to OPERABLE status within 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and in COLD SHUTDOWN within the following 30 hours3.472222e-4 days 0.00833 hours 4.960317e-5 weeks 1.1415e-5 months .

d. With two of the above required offsite A.C. circuits inoperable, demonstrate the OPERABILITY of 4 diesel generator sets by performing Surveillance Requirement 4.8.1.1.2.a.4 within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months , unless the diesel generator sets are already operating; restore at least one of the inoperable offsite sources to OPERABLE status within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months .

e. With either diesel generator sets 1A-A and/or 2 A-A inoperable simultaneous with 1 B-B and/or 21B-B, demonstrate the OPERABILITY of two offsite A.C. circuits by performing Surveillance Requirement 4.8.1.1.1 .a within one hour and at least once per 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months thereafter; restore at least 1) 1A-A and 2A-A or 2) 1 B-B and 2B-B to OPERABLE status within 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and in COLD SHUTDOWN within the following 30 hours3.472222e-4 days 0.00833 hours 4.960317e-5 weeks 1.1415e-5 months .

f. LCO 3.0.4.b is not applicable to diesel generators.

SURVEILLANCE REQUIREMENTS 4.8.1.1.1 Each of the above required independent circuits between the offsite transmission network and the onsite Class E distribution system shall be:

a. Determined OPERABLE at least once per 7 days by verifying correct breaker alignments and indicated power availability.

b. Demonstrated OPERABLE at least once per 18 months# by manually and automatically transferring power supply to each 6.9 kV Unit Board## from the normal supply to the alternate supply.

For the 2A, 2B, 2C, and 2D 6.9 kV Unit Boards, this Surveillance shall not be performed in MODES 1 and 2.

1. Automatic transfer capability is only required to be met for 6.9 kV Unit Boards that require normal and alternate power supplies April 11, 2005 SEQUOYAH - UNIT 2 3/4 8-2 Amendment No. 89, 123, 176, 195, 219, 246, 290

ATTACHMENT 4 Proposed TS Bases Changes (Final Typed) for SQN, Units I and 2

Proposed TS Bases Changes (Final Typed) for SQN, Unit 1 3/4.8 ELECTRICAL POWER SYSTEMS BASES 3/4.8.1 and 3/4.8.2 A.C. SOURCES AND ONSITE POWER DISTRIBUTION SYSTEMS The OPERABILITY of the A.C. and D.C power sources and associated distribution systems during operation ensures that sufficient power will be available to supply the safety related equipment required for 1) the safe shutdown of the facility and 2) the mitigation and control of accident conditions within the facility. The minimum specified independent and redundant A.C. and D.C. power sources and distribution systems satisfy the requirements of General Design Criteria 17 of Appendix "A" to 10 CFR 50.

The electrically powered AC safety loads are separated into redundant load groups such that loss of any one load group will not prevent the minimum safety functions from being performed. Specification 3.8.1.1 requires two physically independent circuits between the offsite transmission network and the onsite Class 1E Distribution System and four separate and independent diesel generator sets to be OPERABLE in MODES 1, 2, 3, and 4. These requirements ensure availability of the required power to shut down the reactor and maintain it in a safe shutdown condition after an abnormal operational transient or a postulated design basis accident.

Each offsite circuit must be capable of maintaining rated frequency and voltage, and accepting required loads during an accident. Minimum required switchyard voltages are determined by evaluation of plant accident loading and the associated voltage drops between the transmission network and these loads. These minimum voltage values are provided to TVA's Transmission Operations for use in system studies to support operation of the transmission network in a manner that will maintain the necessary voltages. Transmission Operations is required to notify SQN Operations if it is determined that the transmission network may not be able to support accident loading or shutdown operations as required by 10 CFR 50, Appendix A, GDC-17. Any offsite power circuits supplied by that transmission network that are not able to support accident loading or shutdown operations are inoperable.

The unit station service transformers (USSTs) utilize auto load tap changers to provide the required voltage response for accident loading. The load tap changer associated with a USST is required to be functional and in "automatic" for the USST to supply power to a 6.9 kV Unit Board.

The inability to supply offsite power to a 6.9 kV Shutdown Board constitutes the failure of only one offsite circuit, as long as offsite power is available to the other load group's Shutdown Boards. Thus, if one or both 6.9 kV Shutdown Boards in a load group do not have an offsite circuit available, then only one offsite circuit would be inoperable. If one or more Shutdown Boards in each load group, or all four Shutdown Boards, do not have an offsite circuit available, then both offsite circuits would be inoperable.

An "available" offsite circuit meets the requirements of GDC-17, and is either connected to the 6.9 kV Shutdown Boards or can be connected to the 6.9 kV Shutdown Boards within a few seconds.

An offsite circuit consists of all breakers, transformers, switches, interrupting devices, cabling, and controls required to transmit power from the offsite transmission network (beginning at the switchyard) to one load group of Class 1E 6.9 kV Shutdown Boards (ending at the supply side of the normal or alternate supply circuit breaker). Each required offsite circuit is that combination of power sources described below that are normally connected to the Class 1 E distribution system, or can be connected to the Class 1E distribution system through automatic transfer at the 6.9 kV Unit Boards.

The following offsite power configurations meet the requirements of LCO 3.8.1.1 .a:

(Note that common station service transformer (CSST) B is a spare transformer with two sets of secondary windings that can be used to supply a total of two Start Buses for CSST A and/or CSST C, with each supplied Start Bus on a separate CSST B secondary winding.)

February 11, 2003 SEQUOYAH - UNIT 1 B 3/4 8-1 Amendment No. 12, 137, 173, 205, 241, 281

3/4.8 ELECTRICAL POWER SYSTEMS BASES 3/4.8.1 and 3/4.8.2 A.C. SOURCES AND ONSITE POWER DISTRIBUTION SYSTEMS

1. Two offsite circuits consisting of a AND b (no board transfers required; a loss of either circuit will not prevent the minimum safety functions from being performed):

a. From the 161 kV transmission network, through CSST A (winding X) to Start Bus 1A to 6.9 kV Shutdown Board 1 B-B (through 6.9 kV Unit Board 1C), and CSST A (winding Y) to Start Bus 2A to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C); AND

b. From the 161 kV transmission network, through CSST C (winding X) to Start Bus 2B to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B), and CSST C (winding Y) to Start Bus 1B to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1 B).

2. Two offsite circuits consisting of a AND b (relies on automatic transfer from alignment a.1) to b.2)(b),

or a.2) to b.1)(a) on a loss of USSTs 1A and 1 B, OR relies on automatic transfer from alignment a.3) to b.2)(a), or a.4) to b.1)(b) on a loss of USSTs 2A and 2B):

a. Normal power source alignments

1) From the 500 kV switchyard through USST 1A to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B);

2) From the 500 kV switchyard through USST 1B to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board 1C);

3) From the 161 kV switchyard through USST 2A to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B); AND

4) From the 161 kV switchyard through USST 2B to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C).

b. Alternate power source alignments

1) From the 161 kV transmission network, through:

(a) CSST A (winding X) to Start Bus 1A to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board 1C); AND (b) CSST A (winding Y) to Start Bus 2A to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C); OR

2) From the 161 kV transmission network, through:

(a) CSST C (winding X) to Start Bus 2B to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B), AND (b) CSST C (winding Y) to Start Bus 1B to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B).

April 11,2005 SEQUOYAH - UNIT 1 B 3/4 8-1a Amendment No. 12, 137, 173, 205, 234, 241,261,285, 301

3/4.8 ELECTRICAL POWER SYSTEMS BASES 3/4.8.1 and 3/4.8.2 A.C. SOURCES AND ONSITE POWER DISTRIBUTION SYSTEMS

3. Two offsite circuits consisting of a AND b (relies on automatic transfer from alignment a. 1) to b. 1) and b.2) on a loss of the Unit 2 USSTs; a loss of alignment a.2) or a.3) will not prevent the minimum safety functions from being performed):

a. Normal power source alignments

1) From the 161 kV switchyard through USST 2A to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B), and USST 2B to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C);

2) From the 161 kV transmission network, through CSST A (winding X) to Start Bus 1A to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board 1C); AND

3) From the 161 kV transmission network, through CSST C (winding Y) to Start Bus lB to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B).

b. Alternate power source alignments

1) From the 161 kV transmission network, through CSST A (winding Y) to Start Bus 2A to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C); AND

2) From the 161 kV transmission network, through CSST C (winding X) to Start Bus 2B to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B).

4. Two offsite circuits consisting of a AND b (relies on automatic transfer from alignment a.1) to b. 1) and b.2) on a loss of the Unit 1 USSTs; a loss of alignment a.2) or a.3) will not prevent the minimum safety functions from being performed):

a. Normal power source alignments

1) From the 500 kV switchyard through USST 1A to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B), and USST 1B to 6.9 kV Shutdown Board I B-B (through 6.9 kV Unit Board 1C);

2) From the 161 kV transmission network, through CSST A (winding Y) to Start Bus 2A to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C); AND

3) From the 161 kV transmission network, through CSST C (winding X) to Start Bus 2B to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B).

b. Alternate power source alignments

1) From the 161 kV transmission network, through CSST A (winding X) to Start Bus 1A to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board 1C); AND

2) From the 161 kV transmission network, through CSST C (winding Y) to Start Bus 1B to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B).

Other offsite power configurations are possible using different combinations of available USSTs and CSSTs, as long as the alignments are consistent with the analyzed configurations, and the alignments otherwise comply with the requirements of GDC 17.

December 22, 2003 SEQUOYAH - UNIT 1 B 3/4 8-1b Amendment No. 12, 137, 173, 205, 234, 261 285

ELECTRICAL POWER SYSTEMS BASES A.C. SOURCES AND ONSITE POWER DISTRIBUTION SYSTEMS (Continued)

For example, to support breaker testing, offsite power to the 6.9 kV Shutdown Boards can be realigned from normal feed to alternate feed. This would result in Shutdown Boards 1A-A and 2A-A being fed from Unit Boards 1A and 2A, respectively, and Shutdown Boards 1B-B and 2B-B being fed from Unit Boards 1D and 2D, respectively. The CSST being utilized as the alternate power source to one load group of Shutdown Boards would also be realigned (normally CSST A available to Shutdown Boards 1B-B and 2B-B or CSST C available to Shutdown Boards 1A-A and 2A-A, would be realigned to CSST A available to Shutdown Boards 1A-A and 2A-A or CSST C available to Shutdown Boards 1 B-B and 2B-B).

The ACTION requirements specified for the levels of degradation of the power sources provide restriction upon continued facility operation commensurate with the level of degradation. The OPERABILITY of the power sources are consistent with the initial condition assumptions of the accident analyses and are based upon maintaining at least one redundant set of onsite A.C. and D.C. power sources and associated distribution systems OPERABLE during accident conditions coincident with an assumed loss of offsite power and single failure of the other onsite A.C. source.

The footnote for Action b of LCO 3.8.1.1 requires completion of a determination that the OPERABLE diesel generators are not inoperable due to common cause failure or performance of Surveillance 4.8.1.1.2.a.4 if Action b is entered. The intent is that all diesel generator inoperabilities must be investigated for common cause failures regardless of how long the diesel generator inoperability persists.

Action b of LCO 3.8.1.1 is further modified by a second note which precludes making more than one diesel generator inoperable on a pre-planned basis for maintenance, modifications, or surveillance testing. The intent of this footnote is to explicitly exclude the flexibility of removing a diesel generator set from service as a part of a pre-planned activity. While the removal of a diesel generator set (A or B train) is consistent with the initial condition assumptions of the accident analysis, this configuration is judged as imprudent. The term pre-planned is to be taken in the context of those activities which are routinely scheduled and is not relative to conditions which arise as a result of emergent or unforeseen events. As an example, this footnote is not intended to preclude the actions necessary to perform the common mode testing requirements required by Action b. As another example, this footnote is not intended to prevent the required surveillance testing of the diesel generators should one diesel generator maintenance be unexpectedly extended and a second diesel generator fall within its required testing frequency. Thus, application of the note is intended for pre-planned activities.

In addition, this footnote is intended to apply only to those actions taken directly on the diesel generator. For those actions taken relative to common support systems (e.g. ERCW), the support function must be evaluated for impact on the diesel generator.

The action to determine that the OPERABLE diesel generators are not inoperable due to common cause failure provides an allowance to avoid unnecessary testing of OPERABLE diesel generators. If it can be determined that the cause of the inoperable diesel generator does not exist on the OPERABLE diesel generators, Surveillance Requirement 4.8.1.1.2.a.4 does not have to be performed. If the cause of inoperability exists on other diesel generator(s), the other diesel generator(s) would be declared inoperable upon discovery and Action e of LCO 3.8.1.1 would be entered as applicable. Once the common failure is repaired, the common cause no longer exists, and the action to determine inoperability due to common cause failure is satisfied. If the cause of the initial inoperable diesel generator cannot be confirmed not to exist on the remaining diesel generators, performance of Surveillance 4.8.1.1.2.a.4 suffices to provide assurance to continued OPERABILITY of the other diesel generators.

SEQUOYAH - UNIT 1 B 3/4 8-1 c Amendment No. XXX

3/4.8 ELECTRICAL POWER SYSTEMS BASES 3/4.8.1 and 3/4.8.2 A.C. SOURCES AND ONSITE POWER DISTRIBUTION SYSTEMS According to Generic Letter 84-15, 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months is reasonable to confirm that the OPERABLE diesel generators are not affected by the same problem as the inoperable diesel generator.

Action f prohibits the application of LCO 3.0.4.b to an inoperable diesel generator. There is an increased risk associated with entering a MODE or other specified condition in the Applicability with an inoperable diesel generator and the provisions of LCO 3.0.4.b, which allow entry into a MODE or other specified condition in the Applicability with the LCO not met after performance of a risk assessment addressing inoperable systems and components, should not be applied in this circumstance.

The OPERABILITY of the minimum specified A.C. and D.C. power sources and associated distribution systems during shutdown and refueling ensures that 1) the facility can be maintained in the shutdown or refueling condition for extended time periods and 2) sufficient instrumentation and control capability is available for monitoring and maintaining the unit status.

With the minimum required AC power sources not available, it is required to suspend CORE ALTERATIONS and operations involving positive reactivity additions that could result in loss of required SDM (Mode 5) or boron concentration (Mode 6). Suspending positive reactivity additions that could result in failure to meet minimum SDM or boron concentration limit is required to assure continued safe operation. Introduction of coolant inventory must be from sources that have a boron concentration greater than or equal to that required in the RCS for minimum SDM or refueling boron concentration.

This may result in an overall reduction in RCS boron concentration but provides acceptable margin to maintaining subcritical operation. Introduction of temperature changes including temperature increases when operating with a positive MTC must also be evaluated to ensure they do not result in a loss of required SDM.

The requirements of Specification 3.8.2.1 provide those actions to be taken for the inoperability of A.C. Distribution Systems. Action a of this specification provides an 8-hour action for the inoperability of one or more A.C. boards. Action b of this specification provides a relaxation of the 8-hour action to 24-hours provided the Vital Instrument Power Board is inoperable solely as a result of one inoperable inverter and the board has been energized within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months . In this condition the requirements of Action a do not have to be applied. Action b is not intended to provide actions for inoperable inverters, which is addressed by the operability requirements for the boards, and is included only for relief from the 8-hour action of Action a when only one inverter is affected. More than one inverter inoperable will result in the inoperability of the associated 120 Volt A.C. Vital Instrument Power Board(s) in accordance with Action a.

With more than one inverter inoperable entry into the actions of TS 3.0.3 is not applicable because Action a includes provisions for multiple inoperable inverters as attendant equipment to the boards.

The Surveillance Requirements for demonstrating the OPERABILITY of the diesel generators are in accordance with the recommendations of Regulatory Guides 1.9 "Selection of Diesel Generator Set Capacity for Standby Power Supplies," March 10, 1971, and 1.108 "Periodic Testing of Diesel Generator Units Used as Onsite Electric Power Systems at Nuclear Power Plants," Revision 1, August 1977, and 1.137 "Fuel-Oil Systems for Standby Diesel Generators," Revision 1, October 1979. The Surveillance Requirements for the diesel generator load-run test and the 24-hour endurance and margin test are in accordance with Regulatory Guide 1.9, Revision 3, July 1993, "Selection, Design, Qualification, and Testing of Emergency Diesel Generator Units Used as Class 1 E Onsite Electric Power Systems at Nuclear Power Plants." During the diesel generator endurance and margin surveillance test, momentary transients outside the kw and kvar load ranges do not invalidate the test results. Similarly, during the diesel generator load-run test, momentary transients outside the kw load range do not invalidate the test results.

SEQUOYAH - UNIT 1 B 3/4 8-1 d Amendment No. XXX

3/4.8 ELECTRICAL POWER SYSTEMS BASES Where the SRs discussed herein specify voltage and frequency tolerances, the following is applicable. 6800 volts is the minimum steady state output voltage and the 10 second transient value.

6800 volts is 98.6% of nominal bus voltage of 6900 volts and is based on the minimum voltage required for the diesel generator supply breaker to close on the 6.9 kV Shutdown Board. The specified maximum steady state output voltage of 7260 volts is based on the degraded over voltage relay setpoint and is equivalent to 110% of the nameplate rating of the 6600 volt motors. The specified minimum and maximum frequencies of the diesel generator are 58.8 Hz and 61.2 Hz, respectively. These values are equal to +/- 2% of the 60 Hz nominal frequency and are derived from the recommendations given in regulatory Guide 1.9.

Where the SRs discuss maximum transient voltages during load rejection testing, the following is applicable. The maximum transient voltage of 8880 volts represents a conservative limit to ensure the resulting voltage will not exceed a level that will cause component damage. It is based on the manufacturer's recommended high potential test voltage of 60% of the original factory high potential test voltage (14.8 kV). The diesel generator manufacturer has determined that the engine and/or generator controls would not experience detrimental effects for transient voltages < 9000 volts. The maximum transient voltage of 8276 volts is retained from the original technical specifications to ensure that the voltage transient following rejection of the single largest load is within the limits originally considered acceptable. It was based on 114% of 7260 volts, which is the Range B service voltage per ANSI-C84.1.

The Surveillance Requirement (SR) to transfer the power supply to each 6.9 kV Unit Board from the normal supply to the alternate supply demonstrates the OPERABILITY of the alternate supply to power the shutdown loads. The 18 month Frequency of the Surveillance is based on engineering judgment, taking into consideration the unit conditions required to perform the Surveillance, and is intended to be consistent with expected fuel cycle lengths. Operating experience has shown that these components usually pass the SR when performed at the 18 month Frequency. Therefore, the Frequency was concluded to be acceptable from a reliability standpoint. This SR is modified by two Notes. The reason for Note # is that, during operation with the reactor critical, performance of this SR for the Unit 1 Unit Boards could cause perturbations to the electrical distribution systems that could challenge continued steady state operation and, as a result, unit safety systems. Note ## specifies that transfer capability is only required to be met for 6.9 kV Unit Boards that require normal and alternate power supplies. When both load groups are being supplied power by the USSTs, only the 6.9 kV Unit Boards associated with one load group are required to have normal and alternate power supplies. Therefore, only one CSST is required to be OPERABLE and available as an alternate power supply. Additionally, manual transfers between the normal supply and the alternate supply are not relied upon to meet the accident analysis. Manual transfer capability is verified to ensure the availability of a backup to the automatic transfer feature.

The Surveillance Requirement for demonstrating the OPERABILITY of the Station batteries are based on the recommendations of Regulatory Guide 1.129 "Maintenance Testing and Replacement of Large Lead Storage Batteries for Nuclear Power Plants," February 1978, and IEEE Std 450-1980, "IEEE Recommended Practice for Maintenance, Testing, and Replacement of Large Lead Storage batteries for Generating Stations and Substations."

SEQUOYAH - UNIT 1 B 3/4 8-1 e Amendment No. XXX

Proposed TS Bases Changes (Final Typed) for SQN, Unit 2 3/4.8 ELECTRICAL POWER SYSTEMS BASES 3/4.8.1 AND 3/4.8.2 A.C. SOURCES AND ONSITE POWER DISTRIBUTION SYSTEMS The OPERABILITY of the A.C. and D.C power sources and associated distribution systems during operation ensures that sufficient power will be available to supply the safety related equipment required for 1) the safe shutdown of the facility and 2) the mitigation and control of accident conditions within the facility. The minimum specified independent and redundant A.C. and D.C. power sources and distribution systems satisfy the requirements of General Design Criterion 17 of Appendix "A" to 10 CFR 50.

The electrically powered AC safety loads are separated into redundant load groups such that loss of any one load group will not prevent the minimum safety functions from being performed. Specification 3.8.1.1 requires two physically independent circuits between the offsite transmission network and the onsite Class 1E Distribution System and four separate and independent diesel generator sets to be OPERABLE in MODES 1, 2, 3, and 4. These requirements ensure availability of the required power to shut down the reactor and maintain it in a safe shutdown condition after an abnormal operational transient or a postulated design basis accident.

Each offsite circuit must be capable of maintaining rated frequency and voltage, and accepting required loads during an accident. Minimum required switchyard voltages are determined by evaluation of plant accident loading and the associated voltage drops between the transmission network and these loads. These minimum voltage values are provided to TVA's Transmission Operations for use in system studies to support operation of the transmission network in a manner that will maintain the necessary voltages. Transmission Operations is required to notify SQN Operations if it is determined that the transmission network may not be able to support accident loading or shutdown operations as required by 10 CFR 50, Appendix A, GDC-17. Any offsite power circuits supplied by that transmission network that are not able to support accident loading or shutdown operations are inoperable.

The unit station service transformers (USSTs) utilize auto load tap changers to provide the required voltage response for accident loading. The load tap changer associated with a USST is required to be functional and in "automatic" for the USST to supply power to a 6.9 kV Unit Board.

The inability to supply offsite power to a 6.9 kV Shutdown Board constitutes the failure of only one offsite circuit, as long as offsite power is available to the other load group's Shutdown Boards. Thus, if one or both 6.9 kV Shutdown Boards in a load group do not have an offsite circuit available, then only one offsite circuit would be inoperable. If one or more Shutdown Boards in each load group, or all four Shutdown Boards, do not have an offsite circuit available, then both offsite circuits would be inoperable.

An "available" offsite circuit meets the requirements of GDC-17, and is either connected to the 6.9 kV Shutdown Boards or can be connected to the 6.9 kV Shutdown Boards within a few seconds.

An offsite circuit consists of all breakers, transformers, switches, interrupting devices, cabling, and controls required to transmit power from the offsite transmission network (beginning at the switchyard) to one load group of Class 1 E 6.9 kV Shutdown Boards (ending at the supply side of the normal or alternate supply circuit breaker). Each required offsite circuit is that combination of power sources described below that are normally connected to the Class 1E distribution system, or can be connected to the Class 1E distribution system through automatic transfer at the 6.9 kV Unit Boards.

The following offsite power configurations meet the requirements of LCO 3.8. 1.1 .a:

(Note that common station service transformer (CSST) B is a spare transformer with two sets of secondary windings that can be used to supply a total of two Start Buses for CSST A and/or CSST C, with each supplied Start Bus on a separate CSST B secondary winding.)

February 11, 2003 SEQUOYAH - UNIT 2 B 3/4 8-1 Amendment No. 123, 164, 195, 231, 272

3/4.8 ELECTRICAL POWER SYSTEMS BASES 3/4.8.1 and 3/4.8.2 A.C. SOURCES AND ONSITE POWER DISTRIBUTION SYSTEMS

1. Two offsite circuits consisting of a AND b (no board transfers required; a loss of either circuit will not prevent the minimum safety functions from being performed):

a. From the 161 kV transmission network, through CSST A (winding X) to Start Bus 1A to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board 1C), and CSST A (winding Y) to Start Bus 2A to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C); AND

b. From the 161 kV transmission network, through CSST C (winding X) to Start Bus 2B to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B), and CSST C (winding Y) to Start Bus 1B to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B).

2. Two offsite circuits consisting of a AND b (relies on automatic transfer from alignment a.1) to b.2)(b),

or a.2) to b.1)(a) on a loss of unit station service transformer (USSTs) 1A and 1B, OR relies on automatic transfer from alignment a.3) to b.2)(a), or a.4) to b.1)(b) on a loss of USSTs 2A and 2B):

a. Normal power source alignments

1) From the 500 kV switchyard through USST 1A to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1 B);

2) From the 500 kV switchyard through USST 1B to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board 1C);

3) From the 161 kV switchyard through USST 2A to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B); AND

4) From the 161 kV switchyard through USST 2B to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C).

b. Alternate power source alignments

1) From the 161 kV transmission network, through:

(a) CSST A (winding X) to Start Bus 1A to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board 1C); AND (b) CSST A (winding Y) to Start Bus 2A to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C); OR

2) From the 161 kV transmission network, through:

(a) CSST C (winding X) to Start Bus 2B to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B), AND (b) CSST C (winding Y) to Start Bus 1B to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B).

April 11, 2005 SEQUOYAH - UNIT 2 B 3/4 8-1a Amendment No. 123, 164, 195, 224, 231,274,290

3/4.8 ELECTRICAL POWER SYSTEMS BASES

3. Two offsite circuits consisting of a AND b (relies on automatic transfer from alignment a.1) to b.1) and b.2) on a loss of the Unit 2 USSTs; a loss of alignment a.2) or a.3) will not prevent the minimum safety functions from being performed):

a. Normal power source alignments

1) From the 161 kV switchyard through USST 2A to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B), and USST 2B to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C);

2) From the 161 kV transmission network, through CSST A (winding X) to Start Bus 1A to 6.9 kV Shutdown Board 1 B-B (through 6.9 kV Unit Board 1C); AND

3) From the 161 kV transmission network, through CSST C (winding Y) to Start Bus 1 B to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B).

b. Alternate power source alignments

1) From the 161 kV transmission network, through CSST A (winding Y) to Start Bus 2A to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C); AND

2) From the 161 kV transmission network, through CSST C (winding X) to Start Bus 2B to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B).

4. Two offsite circuits consisting of a AND b (relies on automatic transfer from alignment a.1) to b.1) and b.2) on a loss of the Unit 1 USSTs; a loss of alignment a.2) or a.3) will not prevent the minimum safety functions from being performed):

a. Normal power source alignments

1) From the 500 kV switchyard through USST 1A to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B), and USST 1B to 6.9 kV Shutdown Board 1B-B (through 6.9 kV Unit Board 1C);

2) From the 161 kV transmission network, through CSST A (winding Y) to Start Bus 2A to 6.9 kV Shutdown Board 2B-B (through 6.9 kV Unit Board 2C); AND

3) From the 161 kV transmission network, through CSST C (winding X) to Start Bus 2B to 6.9 kV Shutdown Board 2A-A (through 6.9 kV Unit Board 2B).

b. Alternate power source alignments

1) From the 161 kV transmission network, through CSST A (winding X) to Start Bus 1A to 6.9 kV Shutdown Board 1 B-B (through 6.9 kV Unit Board 1C); AND

2) From the 161 kV transmission network, through CSST C (winding Y) to Start Bus 1B to 6.9 kV Shutdown Board 1A-A (through 6.9 kV Unit Board 1B).

Other offsite configurations are possible using different combinations of available USSTs and CSSTs, as long as the alignments are consistent with the analyzed configurations, and the alignments otherwise comply with the requirements of GDC 17.

December 22, 2003 SEQUOYAH - UNIT 2 B 3/4 8-1b Amendment No. 123, 164, 195, 224, 274 234, 261 285

3/4.8 ELECTRICAL POWER SYSTEMS BASES For example, to support breaker testing, offsite power to the 6.9 kV Shutdown Boards can be realigned from normal feed to alternate feed. This would result in Shutdown Boards 1A-A and 2A-A being fed from Unit Boards 1A and 2A, respectively, and Shutdown Boards 1 B-B and 2B-B being fed from Unit Boards 1D and 2D, respectively. The CSST being utilized as the alternate power source to one load group of Shutdown Boards would also be realigned (normally CSST A available to Shutdown Boards 1B-B and 2B-B or CSST C available to Shutdown Boards lA-A and 2A-A, would be realigned to CSST A available to Shutdown Boards 1A-A and 2A-A or CSST C available to Shutdown Boards 1 B-B and 2B-B).

The ACTION requirements specified for the levels of degradation of the power sources provide restriction upon continued facility operation commensurate with the level of degradation. The OPERABILITY of the power sources are consistent with the initial condition assumptions of the safety analyses and are based upon maintaining at least one redundant set of onsite A.C. and D.C. power sources and associated distribution systems OPERABLE during accident conditions coincident with an assumed loss of offsite power and single failure of the other onsite A.C. source.

The footnote for Action b of LCO 3.8.1.1 requires completion of a determination that the OPERABLE diesel generators are not inoperable due to common cause failure or performance of Surveillance 4.8.1.1.2.a.4 if Action b is entered. The intent is that all diesel generator inoperabilities must be investigated for common cause failures regardless of how long the diesel generator inoperability persists.

Action b of LCO 3.8.1.1 is further modified by a second note which precludes making more than one diesel generator inoperable on a pre-planned basis for maintenance, modifications, or surveillance testing. The intent of this footnote is to explicitly exclude the flexibility of removing a diesel generator set from service as a part of a pre-planned activity. While the removal of a diesel generator set (A or B train) is consistent with the initial condition assumptions of the accident analysis, this configuration is judged as imprudent. The term pre-planned is to be taken in the context of those activities which are routinely scheduled and is not relative to conditions which arise as a result of emergent or unforeseen events. As an example, this footnote is not intended to preclude the actions necessary to perform the common mode testing requirements required by Action b. As another example, this footnote is not intended to prevent the required surveillance testing of the diesel generators should one diesel generator maintenance be unexpectedly extended and a second diesel generator fall within its required testing frequency. Thus, application of the note is intended for pre-planned activities.

In addition, this footnote is intended to apply only to those actions taken directly on the diesel generator. For those actions taken relative to common support systems (e.g. ERCW), the support function must be evaluated for impact on the diesel generator.

The action to determine that the OPERABLE diesel generators are not inoperable due to common cause failures provides an allowance to avoid unnecessary testing of OPERABLE diesel generators. If it can be determined that the cause of the inoperable diesel generator does not exist on the OPERABLE diesel generators, Surveillance Requirement 4.8.1.1.2.a.4 does not have to'be performed. If the cause of inoperability exists on other diesel generator(s), the other diesel generator(s) would be declared inoperable upon discovery and Action e of LCO 3.8.1.1 would be entered as applicable. Once the common failure is repaired, the common cause no longer exists, and the action to determine inoperability due to common cause failure is satisfied. If the cause of the initial inoperable diesel generator cannot be confirmed not to exist on the remaining diesel generators, performance of Surveillance 4.8.1.1.2.a.4 suffices to provide assurance of continued OPERABILITY of the other diesel generators.

SEQUOYAH - UNIT 2 B 3/4 8-1 c Amendment No. XXX

3/4.8 ELECTRICAL POWER SYSTEMS BASES According to Generic Letter 84-15, 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months is reasonable to confirm that the OPERABLE diesel generators are not affected by the same problem as the inoperable diesel generator.

Action f prohibits the application of LCO 3.0.4.b to an inoperable diesel generator. There is an increased risk associated with entering a MODE or other specified condition in the Applicability with an inoperable diesel generator and the provisions of LCO 3.0.4.b, which allow entry into a MODE or other specified condition in the Applicability with the LCO not met after performance of a risk assessment addressing inoperable systems and components, should not be applied in this circumstance.

The OPERABILITY of the minimum specified A.C. and D.C. power sources and associated distribution systems during shutdown and refueling ensures that 1) the facility can be maintained in the shutdown or refueling condition for extended time periods and 2) sufficient instrumentation and control capability is available for monitoring and maintaining the unit status.

With the minimum required AC power sources not available, it is required to suspend CORE ALTERATIONS and operations involving positive reactivity additions that could result in loss of required SDM (Mode 5) or boron concentration (Mode 6). Suspending positive reactivity additions that could result in failure to meet minimum SDM or boron concentration limit is required to assure continued safe operation. Introduction of coolant inventory must be from sources that have a boron concentration greater than or equal to that required in the RCS for minimum SDM or refueling boron concentration.

This may result in an overall reduction in RCS boron concentration but provides acceptable margin to maintaining subcritical operation. Introduction of temperature changes including temperature increases when operating with a positive MTC must also be evaluated to ensure they do not result in a loss of required SDM.

The requirements of Specification 3.8.2.1 provide those actions to be taken for the inoperability of A.C. Distribution Systems. Action a of this specification provides an 8-hour action for the inoperability of one or more A.C. boards. Action b of this specification provides a relaxation of the 8-hour action to 24-hours provided the Vital Instrument Power Board is inoperable solely as a result of one inoperable inverter and the board has been energized within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months . In this condition the requirements of Action a do not have to be applied. Action b is not intended to provide actions for inoperable inverters, which is addressed by the operability requirements for the boards, and is included only for relief from the 8-hour action of Action a when only one inverter is affected. More than one inverter inoperable will result in the inoperability of the associated 120 Volt A.C. Vital Instrument Power Board(s) in accordance with Action a.

With more than one inverter inoperable entry into the actions of TS 3.0.3 is not applicable because Action a includes provisions for multiple inoperable inverters as attendant equipment to the boards.

The Surveillance Requirements for demonstrating the OPERABILITY of the diesel generators are in accordance with the recommendations of Regulatory Guides 1.9 "Selection of Diesel Generator Set Capacity for Standby Power Supplies", March 10, 1971, 1.108 "Periodic Testing of Diesel Generator Units Used as Onsite Electric Power Systems at Nuclear Power Plants," Revision 1, August 1977, and 1.137 "Fuel-Oil Systems for Standby Diesel Generators," Revision 1, October 1979. The surveillance requirements for the diesel generator load-run test and the 24-hour endurance and margin test are in accordance with Regulatory Guide 1.9, Revision 3, July 1993, "Selection, Design, Qualification, and Testing of Emergency Diesel Generator Units Used as Class 1E Onsite Electric Power Systems at Nuclear Power Plant." During the diesel generator endurance and margin surveillance test, momentary transients outside the kw and kvar load ranges do not invalidate the test results. Similarly, during the diesel generator load-run test, momentary transients outside the kw load range do not invalidate the test results.

SEQUOYAH - UNIT 2 B 3/4 8-1 d Amendment No. XXX

3/4.8 ELECTRICAL POWER SYSTEMS BASES Where the SRs discussed herein specify voltage and frequency tolerances, the following is applicable. 6800 volts is the minimum steady state output voltage and the 10 second transient value.

6800 volts is 98.6% of nominal bus voltage of 6900 volts and is based on the minimum voltage required for the diesel generator supply breaker to close on the 6.9 kV Shutdown Board. The specified maximum steady state output voltage of 7260 volts is based on the degraded over voltage relay setpoint and is equivalent to 110% of the nameplate rating of the 6600 volt motors. The specified minimum and maximum frequencies of the diesel generator are 58.8 Hz and 61.2 Hz, respectively. These values are equal to +/- 2% of the 60 Hz nominal frequency and are derived from the recommendations given in regulatory Guide 1.9.

Where the SRs discuss maximum transient voltages during load rejection testing, the following is applicable. The maximum transient voltage of 8880 volts represents a conservative limit to ensure the resulting voltage will not exceed a level that will cause component damage. It is based on the manufacturer's recommended high potential test voltage of 60% of the original factory high potential test voltage (14.8 kV). The diesel generator manufacturer has determined that the engine and/or generator controls would not experience detrimental effects for transient voltages < 9000 volts. The maximum transient voltage of 8276 volts is retained from the original technical specifications to ensure that the voltage transient following rejection of the single largest load is within the limits originally considered acceptable. It was based on 114% of 7260 volts, which is the Range B service voltage per ANSI-C84.1.

The Surveillance Requirement (SR) to transfer the power supply to each 6.9 kV Unit Board from the normal supply to the alternate supply demonstrates the OPERABILITY of the alternate supply to power the shutdown loads. The 18 month Frequency of the Surveillance is based on engineering judgment, taking into consideration the unit conditions required to perform the Surveillance, and is intended to be consistent with expected fuel cycle lengths. Operating experience has shown that these components usually pass the SR when performed at the 18 month Frequency. Therefore, the Frequency was concluded to be acceptable from a reliability standpoint. This SR is modified by two Notes. The reason for Note # is that, during operation with the reactor critical, performance of this SR for the Unit 1 Unit Boards could cause perturbations to the electrical distribution systems that could challenge continued steady state operation and, as a result, unit safety systems. Note # specifies that transfer capability is only required to be met for 6.9 kV Unit Boards that require normal and alternate power supplies. When both load groups are being supplied power by the USSTs, only the 6.9 kV Unit Boards associated with one load group are required to have normal and alternate power supplies. Therefore, only one CSST is required to be OPERABLE and available as an alternate power supply. Additionally, manual transfers between the normal supply and the alternate supply are not relied upon to meet the accident analysis. Manual transfer capability is verified to ensure the availability of a backup to the automatic transfer feature.

The Surveillance Requirement for demonstrating the OPERABILITY of the Station batteries are based on the recommendations of Regulatory Guide 1.129 "Maintenance Testing and Replacement of Large Lead Storage Batteries for Nuclear Power Plants," February 1978, and IEEE Std 450-1980, "IEEE Recommended Practice for Maintenance, Testing and Replacement of Large Lead Storage Batteries for Generating Stations and Substations."

SEQUOYAH - UNIT 2 B 3/4 8-1 e Amendment No. XXX

ATTACHMENT 5 SQN 6.9 kV AC Electrical Distribution System Diagram

ATTACHMENT 6 Probabilistic Risk Assessment of Proposed Changes

645 120516 001 PRA Evaluation Response: I SQN-0-12-078 R1 Page 1 of 19 NPG Plant(s) and Unit(s) SQN Unit 1 & Unit 2 Department Requesting Evaluation TVA Corporate Licensing Brief Description of Condition (including UNIDs out of service and start/end times)

Modification to replace the SQN Unit I and Unit 2 USSTs with new higher impedance USSTs with auto-tap changers and install new Generator Circuit Breakers (GCBs) to isolate each unit's main generator from the main transformer and USSTs. This modification provides the capability to backfeed offsite power through each unit's main transformer and power the USSTs from the offsite power grid. The Unit 1 USSTs will be powered from the 500kV offsite power system and the Unit 2 USSTs will be powered from the 161kV offsite power system. During normal operation, the USSTs will be aligned to power station loads. On a plant trip, the capability for auto-transfer of station loads from the USSTs to the CSSTs will be provided on UV and protective relaying.

PRA Model Used Including Model Type I SQN PRA CAFTA Revision 0 Initial CDF Unit I 3.0253E-05 Initial LERF Unit 1 4.3899E-06 Initial CDF Unit 2 3.5901E-05 Initial LERF Unit 2 4.6332E-06 Description of Changes made to plant model to evaluate described condition See attached.

Final CDF See attached. Final LERF See attached.

Additional Metric Used CCDPIACDFIColorl Value of Metric Risk Management Actions:

Required: N/A Suggested: N/A Conclusion / Discussion / Description of Application of Risk Insights See attached.

Revision 1: Made editorial corrections, changed unit board(s) to Unit Board(s).

Prepared By Monica Kalal 5/16/2012 x4603 Print Na'me / Signature / Date / Extension Reviewed By Michael J. Walker -, ,-5/16/2012 x8210 Print Nam"/ Signature / Date / Extension TVA 41121 [06-20-20101 Page 1 of 1 NEDP-26-4 P6-03-20101

B45 120516 001 PRA Evaluation Request NPG Plant(s) and Unit(s) Sequoyah Nuclear Plant (SQN), Units 1 and 2 Department Requesting Evaluation TVA Corporate Licensing Description of Condition to be Evaluated Modification to replace the SQN Unit 1 and Unit 2 USSTs with new higher impedance USSTs with auto-tapchangers and install new Generator Circuit Breakers (GCBs) to isolate each unit's main generator from the main transformer and USSTs. This modification provides the capability to backfeed offsite power through each unit's main transformer and power the USSTs from the offsite power grid. The Unit 1 USSTs will be powered from the 500 kV offsite power system and the Unit 2 USSTs will be powered from the 161 kVoffsite power system. The CSSTs will continue to be powered from the 161 kV offsite power system. During normal operation, the USSTs will be aligned to power station loads. On a plant trip, the capability for auto-transfer of station loads from the USSTs to the CSSTs will be provided on UV and protective relaying.

Comprehensive List of Specific Equipment Impacted Include if component is Unable to perform its design basis function Install USSTs 1A, 1B, 2A, and 2B, includes protective relaying, tapchangers, controls.

Install Unit 1 and 2 GCBs, includes trip relaying, instrumentation, controls, redundant DC power.

Add/modify Unit 1 and 2 main control room GCB and USST controls and indication.

Add Unit 1 VCT/RWST valve interlocks to address Appendix R potential for CCP damage.

Install Unit 1 and 2 6.9 kV Unit Board normal and alternate interlocks for auto/manual transfer.

Provide higher heat load and lower temperature limits for iso-phase bus duct cooling system.

Ar.tivp- finr.tinn for LIAST tancrhannrir to *di j.t frnm nnrmal tn *cr.idpnt vnitana ram dir*m~nt; Date/Time Evaluation Needed 02/03/2012 Requested By W Print Name I Signature / Date I Extension Supervisor's Initials Approved By (Corporate PRA Manager) / /.. .- ,

Print Name / Sigfiature / Date TVA 41117 Page 1 of 1 NPG-SPP-09.11-2 [01-07-2011]

B45 120516 001 1.0 Technical Adequacy of the PRA Model The current Probabilistic Risk Assessment (PRA) model for Sequoyah Nuclear Plant (SQN) was issued on May 27, 2011. The previous model was updated from RISKMAN to CAFTA, which included a detailed internal flooding analysis, and a more in depth analysis of all plant systems to represent the as built, as operated plant. A peer review of the model was performed in January 2011 on the internal events and internal flooding supporting requirements of the American Society of Mechanical Engineers/American Nuclear Society (ASME/ANS) PRA Standard. The peer review team endorsed the model as being compliant with the ASME/ANS PRA standard with 77 facts and observations identified. All of the Facts and Observations were resolved to meet capability category II prior to issue of the model of record.

For the addition of the Unit Station Service Transformer (USST) for Unit 1 and Unit 2, all postulated accidents and system alignments are covered in the PRA model. The PRA model is based on the as built, as operated plant design and can be used to evaluate a possible USST failure event by applying specific modeling changes. The PRA model was adjusted according to DCN 22616 (Reference 1).

2.0 References

1. DCN 22616 (Unit 1) and DCN 22617 (Unit 2) Packages.

2. 1,2-15E500-1, Key Diagram Station Aux. Power System, R34

3. MDN-000-000-2010-0200, "SQN Probabilistic Risk Assessment - Summary Notebook,"

Revision 0.

4. Regulatory Guide 1.174, "PRA in Risk-Informed Decisions on Changes to Licensing Basis," Revision 2.

5. NUREG/CR 6928, "Industry-Average Performance for Components and Initiating Events," Table 5-1.

3.0 Assumptions

1. The 6.9kV Unit Boards will be normally aligned to the USSTs, with the appropriate Common Station Service Transformer (CSST) as an alternate alignment.

2. The circuit breakers being aligned from the USSTs to the Unit Boards were not included within the data population for the Bayesian Updating of the circuit breaker type codes in the PRA model. Since there is no test data for these circuit breakers, new type codes that are separate from the model type codes need to be created for circuit breakers failing to open, failing to close, and spurious operation. New type codes were used for these circuit breakers (CKBFO, CKBFC and CKBSO) using only generic data from NUREG/CR 6928 Table 5-1 (Reference 5).

3. Once all of the alignment changes are complete, with Unit 1 using the 500kV grid and Unit 2 using the 161kV grid, there exists a potential to have a loss of offsite power that would only affect either the 161 kV grid and not the 500kV grid or vice versa. However, it is assumed that a grid loss of offsite power will affect both units, no matter the grid that is affected.

4. The USST will require power feed from the Main Bank Transformers 1A, 1B, & 1C for Unit 1 and 2A, 2B, & 2C for Unit 2. A failure of any 1 of the 3 Main Bank transformers for

B45 120516 001 a Unit will fail the offsite power feed from the USST to the Unit Boards. A failure of either the A or B USST for a Unit will also fail the offsite power feed from the USST.

5. A failure of the generator output breaker to open on a reactor or turbine trip will fail the offsite power feed from the USST.

6. A generator trip due to high buss temperature will not allow the generator output breaker to open, which will result in a failure of the offsite power feed from the USST. This was accounted for in the failure probability of the breaker failing to open.

4.0 Probabilistic Risk Assessment Modeling For DCN 22616 and DCN 22617, the postulated changes to the model concern the change in alignment of the 6.9kV Unit Boards from the CSSTs to the new USSTs. New logic has been added to the PRA model of record to represent the new alignment.

4.1 Changes to Plant Alignments 4.1.1 Intermediate Alignment The following changes are going to be implemented for the intermediate alignment where only Unit 2 will have the new USST alignment applied to the 6.9kV Unit Boards (Reference 1):

6.9kV Unit Boards 2A and 2B will be aligned to the 2A USST, with the A and B CSST as an alternate alignment.

6.9kV Unit Boards 2C and 2D will be aligned to the 2B USST, with the B and C CSST as an alternate alignment.

Appropriate flag file alignments were added to the master flag file (Reference 3) for the Unit 2 changes to the model, see below. The alignment flags allow either the USSTs or the CSSTs to be the primary source of power and the other a backup supply. For this configuration, the Unit 2 USSTs are initially aligned and the CSSTs are the backup supply power.

USST DCN FLG USST2A INIT INSERVICE EQU .T.

FLG USST2B INIT INSERVICE EQU .T.

FLG CSSTA INIT INSERVICE EQU .F.

FLGCSSTCINITINSERVICE EQU .F.

B45 120516 001 New logic was added to the PRA to represent the changes made in DCN 22617 for the intermediate alignment.

Shown below, alignment logic was added under 6.9UNIT2A (gates 6.9UNIT2B, 6.9UNIT2C, and 6.9KVUNIT2D are not shown, but are similar).

I INSERVICE 69UI2AG0021 Page23

B45 120516 001 Shown below, gate 6.9UNIT2A_G001 was added to represent the new USST alignments (gates 6.9UNIT2B_GO01, 6.9UNIT2CG001, and 6.9UNIT2DGO01 are not shown, but are similar). With the USST initially in service, the USST failures were modeled along with the CSST back-up alignment. Flag events, FLGUSST2AINITINSERVICE and FLGUSST2BINITINSERVICE are used to determine which alignment is being used.

The flag events are set to either TRUE or FALSE in the master flag file applied when quantifying the model (see above).

B45 120516 001 Shown below, gate 6.9UNIT2AG008 was added to represent the new USST dependencies (gates 6.9UNIT2B_G008, 6.9UNIT2C_G008, and 6.9UNIT2D_G008 are not shown, but are similar). The dependencies on the main bank transformers, generator circuit breaker, offsite power and the other USST are modeled as failing the start bus.

2.17E-05

B45 120516 001 Shown below, gate 6.9UNIT2A_G004 was added to represent the CSST back-up alignment to the USST (gates 6.9UNIT2B_G004, 6.9UNIT2CG004, and 6.9UNIT2D_G004 are not shown, but are similar). With the USST initially in service, the CSST provides an alternate alignment to provide power to the start busses.

B45 120516 001 Shown below, gate 6.9UNIT2AG002 was added to represent the Unit Boards aligned to the CSSTs initially (gates 6.9UNIT2B_G002, 6.9UNIT2C_G002, and 6.9UNIT2DG002 are not shown, but are similar). With the CSST initially in service, the CSST failures were modeled along with the USST back-up alignment. Flag events, FLGCSSTAINITINSERVICE and FLGCSSTCINITINSERVICE are used to determine which alignment is being used.

POWERFRO OS USST2A BACKUPFAILS FLAGOR THECST FAILS INITIALLY INSERVICE IFLG CSSTA INITINSERVICE 0.00E+00 tr-ats .:,ailso f"t Breaker'1532 for 6SgkVUnIM Board 2A SpuriousOpinrg R.OU[T2 GO .OU!TOAGPDCBKSO2BCTAO02CG/21 I2:55)

Poge 23 69.

P-age23 4.10E-06 Pa23 LOOPinti..o. or CSSTA Eoadtrl NormalSupply Urni2 Loss of Off0ee Unt LOs. OfOffsite POnwe Board 2A QcurhtBrealua PC t.3E.05

B45 120516 001 Shown below, gate 6.9UNIT2A_G006 was added to represent the USST back-up alignment to the CSST (gates 6.9UNIT2B_G006, 6.9UNIT2C_G006, and 6.9UNIT2D_G006 are not shown, but are similar). With the CSST initially in service, the USST provides an alternate alignment to provide power to the start busses.

'*4,"IE-06

B45 120516 001 4.1.2 Final Alignment The following changes are going to be implemented for the final alignment where both Unit 1 and Unit 2 will have the new USST alignment implemented (Reference 1):

6.9kV Unit Boards 2A and 2B will be aligned to the 2A USST, with the A and B CSST as an alternate alignment.

6.9kV Unit Boards 2C and 2D will be aligned to the 2B USST, with the B and C CSST as an alternate alignment.

6.9kV Unit Boards 1A and 1B will be aligned to the 1A USST, with the A and B CSST as an alternate alignment.

6.9kV Unit Boards 1C and 1D will be aligned to the 1B USST, with the B and C CSST as an alternate alignment.

Additional flag file alignments were added to the master flag file (Reference 3) for the Unit 1 changes to the model, see below. The alignment flags allow either the USSTs or the CSSTs to be the primary source of power and the other a backup supply. For this configuration, the USSTs are initially aligned for both units and the CSSTs are the backup supply power.

USST DCN FLG USST1A INIT INSERVICE EQU .T.

FLG USSTIB INIT INSERVICE EQU .T.

FLG USST2A INIT INSERVICE EQU .T.

FLG USST2B INIT INSERVICE EQU .T.

FLG CSSTA INIT INSERVICE EQU .F.

FLG CSSTC INIT INSERVICE EQU .F.

New logic was added to the PRA to represent the changes made in DCN 22616 for the final alignment.

The logic changes included in Section 4.1.1 were also applied to the final alignment model changes.

Shown below, alignment logic was added under 6.9UNIT1A (gates 6.9UNIT1B, 6.9UNIT1C, and 6.9UNIT1D are not shown, but are similar).

B45 120516 001 Shown below, gate 6.9UNIT1A_G001 was added to represent the new USST alignments (gates 6.9UNIT1 B_GO01, 6.9UNIT1 C_GO01, and 6.9UNIT1 D_GO01 are not shown, but are similar). With the USST initially in service, the USST failures were modeled along with the CSST back-up alignment. Flag events, FLGUSST1A_INITINSERVICE and FLGUSST1 B_INITINSERVICE are used to determine which alignment is being used.

The flag events are set to either TRUE or FALSE in the master flag file applied when quantifying the model (see above).

Page 21

B45 120516 001 Shown below, gate 6.9UNIT1A_G008 was added to represent the new USST dependencies (gates 6.9UNIT1B_G008, 6.9UNIT1C_G008, and 6.9UNIT1D_G008 are not shown, but are similar). The dependencies on the main bank transformers, generator circuit breaker, offsite power and the other USST are modeled as failing the start bus.

3A3E-O'VY '* 2.56E*-03 1.94F-OIY

B45 120516 001 Shown below, gate 6.9UNIT1AG004 was added to represent the CSST back-up alignment to the USST (gates 6.9UNIT1 B_G004, 6.9UNIT1 C_G004, and 6.9UNIT1 D_G004 are not shown, but are similar). With the USST initially in service, the CSST provides an alternate alignment to provide power to the start busses.

2.73E-03

B45 120516 001 Shown below, gate 6.9UNITIA_G002 was added to represent the Unit Boards aligned to the CSSTs initially (gates 6.9UNITIB_G002, 6.9UNITIC_G002, and 6.9UNIT1D_G002 are not shown, but are similar). With the CSST initially in service, the CSST failures were modeled along with the USST back-up alignment. Flag events, FLGCSSTAINITINSERVICE and FLGCSSTCINITINSERVICE are used to determine which alignment is being used.

B45 120516 001 Shown below, gate 6.9UNIT1A_G006 was added to represent the USST back-up alignment to the CSST (gates 6.9UNIT1B_G006, 6.9UNITIC_G006, and 6.9UNIT1 D_G006 are not shown, but are similar). With the CSST initially in service, the USST provides an alternate alignment to provide power to the start busses.

2.72E-06

B45 120516 001 4.2 Truncation Analysis The model results were truncated at the limits specified in the Summary Document (Reference

3) for Unit 1, 1E-1 2 for Core Damage Frequency (CDF) and 1 E-1 3 for Large Early Release Frequency (LERF) and for Unit 2, 1E-12 for CDF and 5E-14 for LERF.

5.0 Results 5.1 Intermediate Alignment During the intermediate alignment stage, only Unit 2 will have the new USST alignment applied to the 6.9kV Unit Boards.

5.1.1 Unit 1 Table 1 - Change in Risk with New USST Alignment on Unit 2 Only Risk Metric Model of Record Value Alignment Change Value Change in Value CDF 3.0253E-05 3.0253E-05 None LERF 4.3899E-06 4.3899E-06 None The change in CDF and LERF are presented in Table 1. There is no change in CDF or LERF, which shows there is no risk impact on Unit 1 from Unit 2 having the new USST alignment.

5.1.2 Unit2 Table 2 - Change in Risk with New USST Alignment on Unit 2 Only Risk Metric Model of Record Value Alignment Change Value Change in Value CDF 3.5901 E-05 3.5901E-05 None LERF 4.6332E-06 4.6332E-06 None The change in CDF and LERF are presented in Table 2. The change to the model adds more reliability to the offsite power supplies. The Loss of Offsite Power (LOOP) initiator is not a dominant contributor to the overall CDF and LERF (Reference 3). Therefore, there is no change in CDF or LERF.

In this case, CDF and LERF showed no changes, so the alignment changes to the start buses with the addition of the USSTs are considered risk neutral and acceptable.

B45 120516 001 5.2 Final Alignment During the final alignment stage, both Unit 1 and Unit 2 will have the new USST alignment implemented.

5.2.1 Unit I Table 3 - Change in Risk with New USST Alignment on Both Units Risk Metric Model of Record Value Alignment Change Value Change in Value CDF 3.0253E-05 3.0252E-05 -1.OOE-09 LERF 4.3899E-06 4.3898E-06 -1.OOE-1 0 The change in CDF and LERF are presented in Table 3. The change to the model adds more reliability to the offsite power supplies. The LOOP initiator is not a dominant contributor to the overall CDF and LERF (Reference 3). Therefore, there is a minor risk reduction with the addition of the USSTs.

Regulatory Guide 1.174 (Reference 4) states that if an application shows a decrease in CDF or LERF, the change will be considered to have satisfied the relevant principle of risk-informed regulation with respect to CDF or LERF. In this case, CDF and LERF were reduced, so the changes to the start bus alignments with the addition of the USSTs are acceptable.

5.2.2 Unit 2 Table 4 - Change in Risk with New USST Alignment on Both Units Risk Metric Model of Record Value Alignment Change Value Change in Value CDF 3.5901 E-05 3.5900E-05 -1.OOE-09 LERF 4.6332E-06 4.6331 E-06 -1.OOE-10 The change in CDF and LERF are presented in Table 4. The change to the model adds more reliability to the offsite power supplies. The LOOP initiator is not a dominant contributor to the overall CDF and LERF (Reference 3). Therefore, there is a minor risk reduction with the addition of the USSTs.

Regulatory Guide 1.174 (Reference 4) states that if an application shows a decrease in CDF or LERF, the change will be considered to have satisfied the relevant principle of risk-informed regulation with respect to CDF or LERF. In this case, CDF and LERF were reduced, so the changes to the start bus alignments with the addition of the USSTs are acceptable.

5.2.3 Surveillance Testinq Surveillance testing will require the manual and automatic transfer function of the 6.9kV Unit Boards to be tested. If the surveillance is being performed on the 6.9kV Unit Board, its associated Shutdown Board loads will be transferred to alternate (another Unit Board that is not being tested) prior to performing the surveillance. Since the Unit Board surveillance is performed on an unloaded board, it has no impact on risk.

B45 120516 001 5.3 Conclusion DCN 22616 (Unit 1) and DCN 22617 (Unit 2) provide both units with additional offsite power sources. In the intermediate alignment phase, the USSTs are implemented on Unit 2 only. The USSTs 2A and 2B are realigned as the normal power source to the 6.9kV Unit Boards 2A, 2B, 2C and 2D. With the addition of appropriate logic to the SQN PRA model to alter the power alignments to the 6.9kV Unit Boards on Unit 2, there was no risk impact to Unit 2 and no risk impact to Unit 1 during this alignment. The LOOP initiator is not a dominant contributor to the overall CDF and LERF (Reference 3). Therefore, the intermediate alignment is risk neutral.

For the final alignment phase, the USSTs are implemented on Unit 1 as well. Unit 1 is realigned so the USSTs 1A and 1B are the normal power source to the 6.9kV Unit Boards 1A, 1B, 1C and 1D. Changes to the SQN PRA model of record were incorporated to alter the power alignments for the 6.9kV Unit Boards for Unit 1. The new alignments slightly reduced risk by adding redundancy to the Unit Boards on both units, which is considered risk neutral and is acceptable by the Regulatory Guide 1.174 (Reference 4).

DCN 22616 and DCN 22617 are being implemented as a response to the dual unit trip in March of 2009. A loss of CSST C caused a loss of power to two 6.9kV Unit Boards on each unit that feed the RCPs. CSST B continued to supply offsite power to the Train B safety-related 6.9kV Shutdown Boards for both units. The emergency diesel generators started and powered the Train A safety-related 6.9kV Shutdown Boards for both units until offsite power was restored.

Implementation of the proposed USST modifications will eliminate this single point vulnerability for a dual unit trip. The PRA model does not model dual unit trip initiating events. Qualitatively, the new power alignments for the 6.9kV Unit Boards with the USSTs installed will provide added redundancy (back-up power from the CSSTs), and allow a different offsite power supply to be utilized for each unit. Reducing the potential for a dual unit trip will reduce the overall plant risk to CDF and LERF by reducing reactor trip frequency, and reducing challenges to safety equipment and Operator actions during mitigation. The ability to power equipment from the either the 161kV or the 500kV switchyards will also reduce the risks to the plant from external events.

v t e Letter Sequence Request
TAC:ME8772, Modify Section 3.8 Mode Restriction Notes (Approved, Closed)
Initiation Request Acceptance... Administration Meeting, Meeting, Meeting Questions 31 July 2012 3 August 2012 Answers 23 August 2012 Results Approval Other: ML12177A000
MONTHYEAR2012-07-16 [Table View]

ML12146A385

Text

Subject:

Enclosure:

Subject:

SUMMARY

SUMMARY

Background

3.0 TECHNICAL EVALUATION

3.1 System Description

4.0 REGULATORY EVALUATION

5.0 ENVIRONMENTAL CONSIDERATION

6.0 REFERENCES

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