Text

Revised Response to Request for Additional Information Regarding Application to Modify Technical Specifications 3.8.9 Regarding Alternating Current (AC) Vital Buses (WBN-TS-17-19)
	ML19098A857
Person / Time
Site:	Watts Bar
Issue date:	03/21/2019
From:	Henderson E; Tennessee Valley Authority
To:	; Document Control Desk, Office of Nuclear Reactor Regulation
References
	CNL-19-033, EPID L-2018-LLA-0050, WBN-TS-17-19
	Download: ML19098A857 (43)
	v • d • e

Tennessee Valley Authority, 1101 Market Street, Chattanooga, Tennessee 37402 CNL-19-033 March 21, 2019 10 CFR 50.90 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20555-0001 Watts Bar Nuclear Plant, Unit 1 Facility Operating License No. NPF-90 NRC Docket No. 50-390 Watts Bar Nuclear Plant, Unit 2 Facility Operating License No. NPF-96 NRC Docket No. 50-391

Subject:

Revised Response to Request for Additional Information Regarding Application to Modify Watts Bar Nuclear Plant, Units 1 and 2 Technical Specifications 3.8.9 Regarding Alternating Current (AC) Vital Buses (WBN-TS-17-19) (EPID L-2018-LLA-0050)

References:

1. TVA Letter to NRC, CNL-17-119, Application to Modify Watts Bar Nuclear Plant, Units 1 and 2 Technical Specifications 3.8.9 Regarding Alternating Current (AC) Vital Buses (WBN-TS-17-19), dated February 28, 2018 (ML18060A337)

2. NRC Electronic Mail to TVA, WATTS BAR Units 1 and 2 - Modify TS 3.8.9 Completion Time for Inoperable 120 VAC Vital Buses (L-2018-LLA-0050), dated September 26, 2018 (ML18270A236)

3. NRC Electronic Mail to TVA, RE: Extension for the RAI response on the 120V AC LAR, dated October 29, 2018

4. TVA Letter to NRC, CNL-18-121, Response to Request for Additional Information Regarding Application to Modify Watts Bar Nuclear Plant, Units 1 and 2 Technical Specifications 3.8.9 Regarding Alternating Current (AC) Vital Buses (WBN-TS-17-19) (EPID L-2018-LLA-0050),

dated November 9, 2018 (ML18313A214)

In Reference 1, Tennessee Valley Authority (TVA) submitted for Nuclear Regulatory Commission (NRC) approval, a request for an amendment to Facility Operating License Nos. NPF-90 and NPF-96 for Watts Bar Nuclear Plant (WBN) Units 1 and 2, respectively, to revise the WBN Units 1 and 2 Technical Specification (TS) 3.8.9 to add a new Condition C with an eight-hour completion time for performing maintenance on the opposite units vital bus when the opposite unit is in Mode 5, Mode 6, or defueled.

U.S. Nuclear Regulatory Commission CNL-19-033 Page 2 March 21, 2019 In Reference 2, the Nuclear Regulatory Commission (NRC) transmitted a request for additional information (RAI) and requested a response by October 31, 2018. In Reference 3, the NRC granted an extension of the due date for the RAI response to November 9, 2018. In Reference 4, TVA provided a response to the RAI. As a result of an audit with NRC on February 26, 2019, regarding References 1 and 4, TVA is providing a revised response to EEOB RAl-1 that was provided in Reference 4. EEOB RAl-2 and EEOB RAl-3 are also being revised for consistency with the revised response to EEOB RAl-1. Enclosure 1 to this letter provides the revised RAI responses and replaces in its entirety the responses that were provided in Reference 4.

As noted in Enclosure 1 to this letter, the response to EEOB RAl-1 requires a revision to the proposed changes to WBN Units 1 and 2 TS 3.8.9, "Distribution Systems - Operating," that were provided in References 1 and 4. Accordingly, Enclosure 2 to this letter provides the WBN Units 1 and 2 TS 3.8.9 pages marked up to show the proposed changes. Enclosure 3 to this letter provides the existing WBN Units 1 and 2 TS 3.8.9 pages retyped to show the changes incorporated. Enclosure 4 provides copies of the WBN Units 1 and 2 TS 3.8.9 Bases markups for information only. Enclosures 2 and 3 supersede the proposed WBN Units 1 and 2 TS 3.8.9 pages provided in References 1 and 4 and the proposed WBN Units 1 and 2 TS 3.8.9 Bases pages provided in Reference 1.

The enclosures to this letter do not change the no significant hazards consideration or the environmental considerations contained in Reference 1. Additionally, in accordance with 10 CFR 50.91 (b)(1 ), TVA is sending a copy of this letter and the enclosures to the Tennessee Department of Environment and Conservation. provides the new regulatory commitment associated with this submittal. Please address any questions regarding this request to Michael A. Brown at 423-751-3275.

I declare under penalty of perjury that the foregoing is true and correct. Executed on this 21st day of March 2019.

E. K. Henderson Director, Nuclear Regulatory Affairs Enclosures cc (see Page 3)

U.S. Nuclear Regulatory Commission CNL-19-033 Page 3 March 21, 2019

Enclosures:

1. Revised Response to Request for Additional Information EEOB RAI-1 Regarding Application to Modify Watts Bar Nuclear Plant, Units 1 and 2 Technical Specifications 3.8.9 Regarding Alternating Current (AC) Vital Buses (WBN-TS-17-19)

(EPID L-2018-LLA-0050)

2. Proposed TS 3.8.9 Changes (Markups) for WBN Units 1 and 2

3. Proposed TS 3.8.9 Changes (Final Typed) for WBN Units 1 and 2

4. TS 3.8.9 Bases Markups for WBN Units 1 and 2 (For Information Only)

5. New Regulatory Commitment cc (Enclosures):

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

CNL-19-033 E1-1 of 7 Revised Response to Request for Additional Information EEOB RAI-1 Regarding Application to Modify Watts Bar Nuclear Plant, Units 1 and 2 Technical Specifications 3.8.9 Regarding Alternating Current (AC) Vital Buses (WBN-TS-17-19) (EPID L-2018-LLA-0050)

By letter dated February 28, 2018 (Agencywide Documents Access management System (ADAMS) Accession No. ML18060A337), Tennessee Valley Authority (TVA, the licensee),

requested an amendment to Operating Licenses NPF-90 and NPF-96 for Watts Bar Nuclear Plant (WBN), Units 1 and 2. The proposed amendment would revise the Technical Specifications (TS) 3.8.9, Distribution Systems - Operating, with regard to the completion times (CTs) for restoring one or more inoperable alternate current (AC) vital bus(es) in one channel of the opposite unit to operable status. Specifically, the proposed changes would add to TS 3.8.9 one new Condition C with an eight-hour CT for performing maintenance on the opposite units AC vital bus when the opposite unit is in Modes 5-6 and defueled; and one new Condition D with a two-hour CT for restoring to operable status the opposite unit AC vital bus when it is inoperable for reasons other than planned maintenance.

The Electrical Engineering Operating Branch (EEOB) staff has determined that the following additional information is needed to complete the review of the WBN license amendment request (LAR).

Regulatory Requirements Title 10 of the Code of Federal Regulations (10 CFR) Part 50.36, Technical Specifications, requires, in part, that the operating license of a nuclear production facility include TS. 10 CFR 50.36 (c)(2) requires that the TS include limiting conditions for operation (LCOs) which are the lowest functional capability or performance levels of equipment required for safe operation of the facility. When an LCO of a nuclear reactor is not met, the licensee shall shut down the reactor or follow any remedial action permitted by the technical specifications until the condition can be met.

EEOB RAI - 1 The licensee proposed Conditions C and D that would be applicable when the AC vital bus(es) in the opposite unit are inoperable. The proposed Required Action C.1 for Condition C would require the restoration of the inoperable opposite unit AC vital bus to operable status within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months during planned maintenance of the opposite unit AC vital bus when the opposite unit is in Mode 5, 6, or defueled. The proposed Required Action D.1 for Condition D would require the restoration of the inoperable opposite unit AC vital bus(es) to operable status within 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months and 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months from discovery of failure to meet LCO when the opposite unit AC vital bus is inoperable for reasons other than planned maintenance.

LAR Section 3.1, System Description of Electrical Power Distribution System states:

The AC sources for [the safety-related systems shared between the units] loads are supplied from A Train and B Train AC electrical power subsystems from either one or both units. Therefore, in addition to requiring the associated units AC electrical power subsystems to be operable, the opposite units AC electrical power subsystems supplying power to a required shared load are also required to be operable.

CNL-19-033 E1-2 of 7 The NRC staff notes that for the AC vital electrical power distribution system, the Train A consists of AC vital buses channels 1-I, 2-I, 1-III, and 2-III, and the Train B consists of AC vital buses channels 1-II, 2-II, 1-IV, and 2-IV. This makes a total of 8 channels of AC vital bus electrical power distribution subsystems for both Train A and Train B. In addition, the proposed Conditions C and D with associated Required Actions would imply that the operability of the AC vital buses in the opposite unit would be required by the operable units LCO 3.8.9 in Modes 1, 2, 3 and 4. However, the WBN current TS LCO 3.8.9 identifies only 4 channels of AC vital bus electrical power distribution subsystems to be operable for one unit (e.g., WBN-1) in Modes 1 - 4 and does not identify the AC vital bus electrical power distribution subsystems from the opposite unit (e.g., WBN-2).

10 CFR 50.36 (c)(2) requires the TS to include LCOs which are the lowest functional capability or performance levels of equipment required for safe operation of the facility. Since the opposite units channels of AC vital bus electrical power distribution subsystems supplying power to safety-related shared systems are required for the safe operation of the WBN units during Modes 1 - 4, as implied in Conditions C and D, LCO 3.8.9 would require 8 channels of AC vital bus electrical power distribution subsystems, for both Train A and B to be operable, instead of the 4 channels of AC vital bus electrical power distribution subsystems identified in the current LCO 3.8.9.

Provide a discussion that addresses this discrepancy.

TVA Response to EEOB RAI-1 Watts Bar Nuclear Plant (WBN) Units 1 and 2 Updated Final Safety Analysis Report (UFSAR)

Section 1.2.2.7 states that the 120V ac [alternating current] Distribution System receives power from eight independent inverters and distributes it to safety-related loads of both units.

WBN Units 1 and 2 UFSAR Section 8.3.2.4 states, Four channel 120V ac instrument power is supplied from eight (four per unit) uninterruptible power supplies. WBN UFSAR Section 8.3.1.1, Section Subheading 120 V Vital AC System - System Reliability, states, There are two unit inverters and a spare inverter for each channel... In addition, WBN Units 1 and 2 Technical Specifications (TS) 3.8.7, Inverters - Operating, require two inverters in each of the four [vital] channels to be operable.

Therefore, the 120V AC vital distribution system is divided into four channels with eight inverters (and four spare inverters) that supply power to the 120V AC vital instrumentation system for both units. Furthermore, the WBN Units 1 and 2 TS LCO 3.8.9, Distribution Systems -

Operating, states, Train A and Train B AC, four channels of vital DC, and four channels of AC vital bus electrical power distribution subsystems shall be OPERABLE. However, TS Bases Table B 3.8.9-1, AC and DC Electrical Power Distributions Systems, lists eight AC vital channels for each unit. These statements appear to be inconsistent, due to the labeling of the eight vital AC buses as channels in TS Bases Table B 3.8.9-1. Therefore, as indicated in, WBN Units 1 and 2 TS Bases Table B 3.8.9-1 is being revised to indicate that there are four AC vital channels with two AC vital buses in each channel.

Because the eight 120V AC vital buses are shared between the units there is some ambiguity regarding the AC vital buses to which the new WBN Units 1 and 2 TS 3.8.9, Condition C, which was provided in References 1 and 2, applies. Therefore, new Condition C is being revised to specify the 120V AC vital buses to which the new Condition C applies (see Enclosures 2 and 3).

Specifically, for WBN Unit 1 TS 3.8.9, proposed Condition C applies when planned maintenance is performed on AC vital bus 2-I, 2-II, 2-III, or 2-IV. For WBN Unit 2 TS 3.8.9, proposed

CNL-19-033 E1-3 of 7 Condition C applies when planned maintenance is performed on AC vital bus 1-I, 1-II, 1-III, or 1-IV. A note modifying Condition C specifies that it only applies when the opposite unit is in Mode 5, Mode 6, or defueled. These notes allow the plant configuration to be aligned to minimize the features that will be inoperable when one of the AC vital buses is made inoperable and limits the applicability of Condition C to when the opposite units systems supported by the associated AC vital bus are not required to be operable.

By providing additional clarity to the wording of TS 3.8.9 new Condition C, the proposed changes to existing TS 3.8.9 Condition B, as was proposed in References 1 and 2, can be simplified to apply to one or more AC vital buses that are inoperable for reasons other than Condition C. The change to Condition B obviates the need for a new Condition D, as was proposed in References 1 and 2; therefore, the proposed new Condition D has been removed.

Enclosures 2 and 3 supersede the proposed WBN Units 1 and 2 TS 3.8.9 pages provided in References 1 and 2 and the proposed WBN Units 1 and 2 TS 3.8.9 Bases pages provided in Reference 1.

As a result of an audit with NRC on February 26, 2019, regarding this RAI response, TVA recognized that the description of the 120V AC vital system also requires clarification.

Therefore, TVA will revise the WBN UFSAR to reflect that the 120V AC vital system is shared between WBN Units 1 and 2 and that the 120V AC vital system is composed of four channels with two vital inverters and two vital buses per channel. Enclosure 5 provides the new regulatory commitment associated with this submittal.

References

1. TVA Letter to NRC, CNL-17-119, Application to Modify Watts Bar Nuclear Plant, Units 1 and 2 Technical Specifications 3.8.9 Regarding Alternating Current (AC) Vital Buses (WBN-TS-17-19), dated February 28, 2018 (ML18060A337)

2. TVA Letter to NRC, CNL-18-121, Response to Request for Additional Information Regarding Application to Modify Watts Bar Nuclear Plant, Units 1 and 2 Technical Specifications 3.8.9 Regarding Alternating Current (AC) Vital Buses (WBN-TS-17-19)

(EPID L-2018-LLA-0050), dated November 9, 2018 (ML18313A214)

EEOB RAI-2 Table 1 of the LAR shows that the 120 V vital inverters are shared between the two units.

Note 8 of Table 1 indicates that the loads associated with the shared vital inverters 1-I, 2-I, 1-II, 2-II assigned to Unit 1 are provided in Table 2 of the LAR. Table 2 of the LAR provides the TS and CTs that are applicable to the safety-related common systems. All of the applicable common systems TS CTs are greater than the proposed 8-hour CT for Condition C. In addition, the LAR states that the safety-related common loads fed from the 120 V AC vital boards are encompassed by the common systems shown in Table 2.

The NRC staff notes that Table 1 provides no indication that there are common systems fed from the shared vital inverters 1-III, 2-III, 1-IV and 2-IV. Hence, the safety-related common loads provided in Table 2 of the LAR, which TS CTs are greater than the 8-hour CT proposed for Condition C, would be fed from the 120 V AC vital boards associated with the shared vital inverters 1-I, 2-I, 1-II, 2-II. But, the UFSAR Figure 8.1-3 shows that the shared vital inverters 1-III, 2-III, 1-IV and 2-IV supply power to the AC vital channels 1-III, 2-III, 1-IV and 2-IV, respectively.

CNL-19-033 E1-4 of 7 Clarify whether the AC vital channels 1-III, 2-III, 1-IV and 2-IV are required to be operable to support common loads shared by the WBN Units 1 and 2. If so, provide the common systems that are fed from the AC vital channels 1-III, 2-III, 1-IV and 2-IV and the applicable TS CTs for these common systems.

TVA Response to EEOB RAI-2 As noted in the referenced letter, TVA requested an LAR to revise the WBN Units 1 and 2 TS 3.8.9 to add a new Condition C to support maintenance of the 120-volt (V) AC vital buses when one unit is operating and the opposite unit is in Mode 5, Mode 6 or defueled. Enclosure 1, Table 1, of the LAR provided a list of major safety-related systems powered from the AC electrical power distribution system. Table 1 showed the unit and train relationship of the overall electrical power to the listed safety-related systems, including the safety-related common systems shared between the units. Table 2 showed the Technical Specification applicable to the safety-related common systems. Table 1, Note 8 stated, The loads associated with the vital inverters apply to TS 3.3.7, 3.3.8, and 3.7.4, in Table 2. Note 8 was applied to 120V Vital Inverters 1-I, 2-I, 1-II, and 2-II.

During development of the response to EEOB RAI-2, TVA identified that the information provided in Enclosure 1, Table 1 to the LAR, required clarification. Specifically, Note 8 in, Table 1 to the LAR should have applied to inverters 1-I and 1-II. In addition, the list of TS associated with the shared loads fed from these inverters should have also included LCO 3.6.9, Emergency Gas Treatment System (EGTS), LCO 3.7.10, Control Room Emergency Ventilation System (CREVS), LCO 3.7.11, Control Room Emergency Air Temperature Control System (CREATCS), and 3.7.12, Auxiliary Building Gas Treatment System (ABGTS). Additionally, inoperable AC Vital buses 1-I and 1-II also affect the Auxiliary Air Control System (ACAS). ACAS is a non-TS support system that provides essential air to components in EGTS, ABGTS, CREVS, and CREATCS. In addition, ACAS also provides essential air to TS-required components associated with LCO 3.7.4, Atmospheric Dump Valves (ADV), and LCO 3.7.5, Auxiliary Feedwater System (AFW).

The following table indicates the TS required common equipment loads supplied by AC vital buses 1-I and 1-II when the opposite unit is in operation. Currently, there are no TS required common loads fed from AC vital buses 1-III, 1-IV, 2-I, 2-II, 2-III, and 2-IV.

CNL-19-033 E1-5 of 7 AC Vital Bus System loads Opposite Unit LCO and CT with one train inoperable 1-I ACAS (affects ADVs, AFW, and systems below)

EGTS ABGTS CREVS CREATCS LCOs 3.7.4 and 3.7.5 - 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months LCO 3.6.9 - 7 days LCOs 3.3.8 and 3.7.12 - 7 days LCOs 3.3.7 and 3.7.10 - 7 days LCO 3.7.11 - 30 days 1-II ACAS (affects ADVs, AFW, and systems below)

EGTS ABGTS CREVS CREATCS LCOs 3.7.4 and 3.7.5 - 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months LCO 3.6.9 - 7 days LCOs 3.3.8 and 3.7.12 - 7 days LCOs 3.3.7 and 3.7.10 - 7 days LCO 3.7.11 - 30 days Reference TVA Letter to NRC, CNL-17-119, Application to Modify Watts Bar Nuclear Plant, Units 1 and 2 Technical Specifications 3.8.9 Regarding Alternating Current (AC) Vital Buses (WBN-TS-17-19), dated February 28, 2018 (ML18060A337)

EEOB RAI-3 The proposed TS Condition C would allow 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months to restore one opposite unit AC vital bus in one channel to operable status during planned maintenance of the opposite unit AC vital bus in Mode 5, Mode 6, or defueled. In addition, if the opposite units vital bus would be inoperable for reasons other than planned maintenance, the proposed new Condition D would require 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months for restoration of the inoperable opposite unit AC vital bus to operable status.

LAR Section 3.2.1, Assessment of Maintenance of the 120V AC Vital Buses, states:

With one unit operating, the proposed new Condition C establishes an eight-hour completion time when performing planned maintenance on the opposite units 120V AC vital buses when the opposite unit is in Modes 5 or 6 or defueled.

LAR Section 3.2.3, Safety Assessment of the Proposed Technical Specification Changes, states the following as a justification for the proposed changes:

The opposite unit's AC vital buses are not as critical to the operating unit (fewer operating unit loads) as the operating unit's AC vital buses.

The NRC staff notes that the opposite units AC vital buses are required to power the safety-related common systems that are shared with the operating unit, and the impact of not having the common systems required by the operating unit during entry into Condition C is not clearly discussed in the LAR. To allow the NRC staff to evaluate the technical adequacy of the 8-hour CT for Condition C based on the above statements, provide the following information:

a-Explain why the opposite unit's AC vital buses are considered as not as critical to the operating unit as the operating unit's AC vital buses.

CNL-19-033 E1-6 of 7 b-Provide a discussion about the impact of having one opposite unit AC vital bus in one channel inoperable (proposed Condition C) on the operating units safety-related systems and its response if the following conditions would occur:

The worst case design basis event in the operating unit The worst case design basis event concurrent with the failure of another AC vital bus in the operating unit. If there would be a potential loss of safety functions, provide a discussion of the compensatory measures, equipment alignment, and procedures that are or will be in place to address the potential consequences to the operating unit.

TVA Response to EEOB RAI-3

a. The proposal to allow an opposite units vital AC bus to be inoperable for eight hours with the opposite unit in Mode 5, Mode 6, or defueled will allow for the performance of maintenance that will ensure continued system reliability. The proposed completion time of eight hours takes into account that the opposite units AC vital buses only feed shared systems associated with the operating unit and that the shared systems are fully redundant, as indicated in Enclosure 1, Table 1 to the referenced letter. In addition, as indicated in the response to EEOB RAI-2, the proposed completion time of eight hours for an inoperable AC vital bus is bounded by the completion time for each common load that would be affected when AC vital bus 1-I or 1-II is inoperable. Currently, there are no TS required common loads fed from AC vital buses 1-III, 1-IV, 2-I, 2-II, 2-III, and 2-IV.

b. As shown in Enclosure 1, Table 1, to the referenced letter (as revised by the response to EEOB RAI-2), the loads powered from an opposite unit AC vital bus consist of systems common to both units. There are no common system loads powered across the units, such that a loss of an operating unit AC vital bus during maintenance on the opposite units AC vital bus (when in Mode 5, Mode 6 or defueled) would result in a loss of safety function.

However, when either AC vital bus 1-I or 1-II is inoperable for maintenance, one train of ACAS is inoperable, thereby affecting the operability of one train of CREVS, CREATCS, EGTS, ABGTS, ADV and AFW on the operating unit.

The worst case design basis event (DBE) in the operating unit is a loss of coolant accident.

If a DBE were to occur in the operating unit during maintenance of an opposite unit AC vital bus, the remaining operable trains of the affected shared systems are capable of performing their respective safety functions and mitigate the DBE.

If, in addition to the above scenario, another AC vital bus were to fail in the opposite train of the operating unit, a loss of safety function could occur for each of the above systems.

The potential for occurrence of the above scenario is mitigated by the following:

During maintenance on AC vital bus 1-I or 1-II, the respective opposite train AC vital buses will be protected for the operating unit by site-specific administrative barriers (e.g.,

signage) in accordance with site and corporate procedures.

The maintenance activity on AC vital buses results in loss of the seismic qualification of the 120V AC vital panel while the panel cover is removed. During maintenance on the AC vital buses, the 120V AC vital panel under maintenance will be continuously manned and can be restored to an operable status, if a seismic event or design basis event occurs.

CNL-19-033 E1-7 of 7 In addition, the 120V AC vital panels are supplied by inverters that are uninterruptible power supplies. The system incorporates features that serve to increase the overall reliability.

Each inverter has access to four power sources: two 480V AC sources, a 125V DC source, and a 120V AC regulated transformer bypass source. There are two unit inverters and a spare inverter for each channel capable of receiving power from either the 480V AC source or the 125V DC source. Each inverter has an auctioneered solid-state transfer switch between the 480V AC and 125V DC sources. The unit inverter output has an automatic make-before-break solid-state transfer to the regulated bypass source in the event of inverter failure or overload. An automatically synchronized manual transfer between the output of the inverter and the 120V AC regulated transformer bypass source and between the unit inverter system output and the spare inverter is provided, so that the inverters may be taken out of service for maintenance without interrupting power to the loads. The current limiting feature of the inverter provides self-protection from load faults. The inverters and instrumentation power boards are monitored to alert the operator of abnormalities. The distribution bus is sectionalized with coordinated fuses to prevent losing the entire board due to failure of a single branch circuit breaker. Therefore, the occurrence of a loss of an additional AC vital bus during an 8-hour maintenance activity on AC vital buses 1-I or 1-II is unlikely.

Reference TVA Letter to NRC, CNL-17-119, Application to Modify Watts Bar Nuclear Plant, Units 1 and 2 Technical Specifications 3.8.9 Regarding Alternating Current (AC) Vital Buses (WBN-TS-17-19), dated February 28, 2018 (ML18060A337)

CNL-19-033 E2-1 Proposed TS 3.8.9 Changes (Markups) for WBN Units 1 and 2

Distribution Systems-Operating 3.8.9 Watts Bar-Unit 1 3.8-41 Amendment XX 3.8 ELECTRICAL POWER SYSTEMS 3.8.9 Distribution Systems - Operating LCO 3.8.9 Train A and Train B AC, four channels of vital DC, and four channels of AC vital bus electrical power distribution subsystems shall be OPERABLE.

APPLICABILITY:

MODES 1, 2, 3, and 4.

ACTIONS CONDITION REQUIRED ACTION COMPLETION TIME A.

One or more AC electrical power distribution subsystems inoperable.

A.1 Restore AC electrical power distribution subsystem to OPERABLE status.

8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months AND 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months from discovery of failure to meet LCO B.

One or more AC vital buses in one channel inoperable for reasons other than Condition C.

B.1 Restore AC vital bus(es) to OPERABLE status.

2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months AND 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months from discovery of failure to meet LCO

NOTES-----------------

1.

Only applicable during planned maintenance of AC vital bus 2-I, 2-II, 2-III, or 2-IV.

2.

Only applicable when Unit 2 is in MODE 5, MODE 6, or defueled.

C.

AC vital bus 2-I, 2-II, 2-III, or 2-IV inoperable.

C.1 Restore AC vital bus to OPERABLE status.

8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months (continued)

Distribution Systems-Operating 3.8.9 Watts Bar-Unit 1 3.8-42 Amendment XX ACTIONS (continued)

CONDITION REQUIRED ACTION COMPLETION TIME CD.

One or more vital DC electrical power distribution buses inoperable.

CD.1 Restore DC electrical power distribution bus to OPERABLE status.

2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months AND 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months from discovery of failure to meet LCO DE.

Required Action and associated Completion Time not met.

DE.1 Be in MODE 3.

AND DE.2 Be in MODE 5.

6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months 36 hours EF.

Two trains with one or more inoperable distribution subsystems that result in a loss of safety function.

EF.1 Enter LCO 3.0.3.

Immediately SURVEILLANCE REQUIREMENTS SURVEILLANCE FREQUENCY SR 3.8.9.1 Verify correct breaker alignments and voltage to required AC, vital DC, and AC vital bus electrical power distribution subsystems.

7 days

Distribution Systems - Operating 3.8.9 Watts Bar - Unit 2 3.8-37 Amendment XX 3.8 ELECTRICAL POWER SYSTEMS 3.8.9 Distribution Systems - Operating LCO 3.8.9 Train A and Train B AC, four channels of vital DC, and four channels of AC vital bus electrical power distribution subsystems shall be OPERABLE.

APPLICABILITY:

MODES 1, 2, 3, and 4.

ACTIONS CONDITION REQUIRED ACTION COMPLETION TIME A. One or more AC electrical power distribution subsystems inoperable.

A.1 Restore AC electrical power distribution subsystem to OPERABLE status.

8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months AND 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months from discovery of failure to meet LCO B. One or more AC vital buses in one channel inoperable for reasons other than Condition C.

B.1 Restore AC vital bus(es) to OPERABLE status.

2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months AND 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months from discovery of failure to meet LCO

NOTES--------------

1. Only applicable during planned maintenance of AC vital bus 1-I, 1-II, 1-III, or 1-IV.

2. Only applicable when Unit 1 is in MODE 5, MODE 6, or defueled.

C. AC vital bus 1-I, 1-II, 1-III, or 1-IV inoperable.

C.1 Restore AC vital bus to OPERABLE status.

8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months (continued)

Distribution Systems - Operating 3.8.9 Watts Bar - Unit 2 3.8-38 Amendment XX ACTIONS (continued)

CONDITION REQUIRED ACTION COMPLETION TIME CD. One or more vital DC electrical power distribution buses inoperable.

CD.1 Restore DC electrical power distribution bus to OPERABLE status.

2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months AND 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months from discovery of failure to meet LCO DE. Required Action and associated Completion Time not met.

DE.1 Be in MODE 3.

6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months AND DE.2 Be in MODE 5.

36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months EF. Two trains with one or more inoperable distribution subsystems that result in a loss of safety function.

EF.1 Enter LCO 3.0.3.

Immediately SURVEILLANCE REQUIREMENTS SURVEILLANCE FREQUENCY SR 3.8.9.1 Verify correct breaker alignments and voltage to required AC, vital DC, and AC vital bus electrical power distribution subsystems.

7 days

CNL-19-033 E3-1 Proposed TS 3.8.9 Changes (Final Typed) for WBN Units 1 and 2

Distribution Systems-Operating 3.8.9 Watts Bar-Unit 1 3.8-41 Amendment XX 3.8 ELECTRICAL POWER SYSTEMS 3.8.9 Distribution Systems - Operating LCO 3.8.9 Train A and Train B AC, four channels of vital DC, and four channels of AC vital bus electrical power distribution subsystems shall be OPERABLE.

APPLICABILITY:

MODES 1, 2, 3, and 4.

ACTIONS CONDITION REQUIRED ACTION COMPLETION TIME A.

One or more AC electrical power distribution subsystems inoperable.

A.1 Restore AC electrical power distribution subsystem to OPERABLE status.

8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months AND 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months from discovery of failure to meet LCO B.

One or more AC vital buses in one channel inoperable for reasons other than Condition C.

B.1 Restore AC vital bus(es) to OPERABLE status.

2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months AND 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months from discovery of failure to meet LCO

NOTES-----------------

1.

Only applicable during planned maintenance of AC vital bus 2-I, 2-II, 2-III, or 2-IV.

2.

Only applicable when Unit 2 is in MODE 5, MODE 6, or defueled.

C.

AC vital bus 2-I, 2-II, 2-III, or 2-IV inoperable.

C.1 Restore AC vital bus to OPERABLE status.

8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months (continued)

Distribution Systems-Operating 3.8.9 Watts Bar-Unit 1 3.8-42 Amendment XX ACTIONS (continued)

CONDITION REQUIRED ACTION COMPLETION TIME D.

One or more vital DC electrical power distribution buses inoperable.

D.1 Restore DC electrical power distribution bus to OPERABLE status.

2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months AND 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months from discovery of failure to meet LCO E.

Required Action and associated Completion Time not met.

E.1 Be in MODE 3.

AND E.2 Be in MODE 5.

6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months 36 hours F.

Two trains with one or more inoperable distribution subsystems that result in a loss of safety function.

F.1 Enter LCO 3.0.3.

Immediately SURVEILLANCE REQUIREMENTS SURVEILLANCE FREQUENCY SR 3.8.9.1 Verify correct breaker alignments and voltage to required AC, vital DC, and AC vital bus electrical power distribution subsystems.

7 days

Distribution Systems - Operating 3.8.9 Watts Bar - Unit 2 3.8-37 Amendment XX 3.8 ELECTRICAL POWER SYSTEMS 3.8.9 Distribution Systems - Operating LCO 3.8.9 Train A and Train B AC, four channels of vital DC, and four channels of AC vital bus electrical power distribution subsystems shall be OPERABLE.

APPLICABILITY:

MODES 1, 2, 3, and 4.

ACTIONS CONDITION REQUIRED ACTION COMPLETION TIME A. One or more AC electrical power distribution subsystems inoperable.

A.1 Restore AC electrical power distribution subsystem to OPERABLE status.

8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months AND 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months from discovery of failure to meet LCO B. One or more AC vital buses in one channel inoperable for reasons other than Condition C.

B.1 Restore AC vital bus(es) to OPERABLE status.

2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months AND 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months from discovery of failure to meet LCO

NOTES--------------

1. Only applicable during planned maintenance of AC vital bus 1-I, 1-II, 1-III, or 1-IV.

2. Only applicable when Unit 1 is in MODE 5, MODE 6, or defueled.

C. AC vital bus 1-I, 1-II, 1-III, or 1-IV inoperable.

C.1 Restore AC vital bus to OPERABLE status.

8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months (continued)

Distribution Systems - Operating 3.8.9 Watts Bar - Unit 2 3.8-38 Amendment XX ACTIONS (continued)

CONDITION REQUIRED ACTION COMPLETION TIME D. One or more vital DC electrical power distribution buses inoperable.

D.1 Restore DC electrical power distribution bus to OPERABLE status.

2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months AND 16 hours1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months from discovery of failure to meet LCO E. Required Action and associated Completion Time not met.

E.1 Be in MODE 3.

6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months AND E.2 Be in MODE 5.

36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months F. Two trains with one or more inoperable distribution subsystems that result in a loss of safety function.

F.1 Enter LCO 3.0.3.

Immediately SURVEILLANCE REQUIREMENTS SURVEILLANCE FREQUENCY SR 3.8.9.1 Verify correct breaker alignments and voltage to required AC, vital DC, and AC vital bus electrical power distribution subsystems.

7 days

CNL-19-033 E4-1 TS 3.8.9 Bases Markups for WBN Units 1 and 2 (For Information Only)

Distribution Systems - Operating B 3.8.9 (continued)

Watts Bar-Unit 1 B 3.8-89 Revision 67, 75, 76, 77, 78, 125 Amendment 103 B 3.8 ELECTRICAL POWER SYSTEMS B 3.8.9 Distribution Systems - Operating BASES BACKGROUND The onsite Class 1E AC electrical power distribution system is divided by train into two redundant and independent AC electrical power distribution subsystems.

The AC electrical power subsystem includes four 6.9 kV shutdown boards. Each 6.9 kV shutdown board has access to the two separate and independent preferred offsite sources of power as well as a dedicated onsite diesel generator (DG) source. Usually, one of the two offsite sources will be the normal power source for a 6.9 kV shutdown board, and the other offsite source will be the alternate power source. Transfers from the normal source to the alternate source may be manual or automatic. Automatic transfers only occur when the relay logic is tripping a transmission line and the associated common station service transformers. Only manual transfers are permitted from alternate to normal. Additionally, the maintenance source to the 6.9 kV shutdown board can be used as an offsite power source. Transferring the 6.9 kV shutdown board to the maintenance power source is done manually from the normal or alternate power source. For a loss of offsite power 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 breakers is supplied from the Class 1E batteries.

Additional description of this system may be found in the Bases for LCO 3.8.1, "AC Sources-Operating," and the Bases for LCO 3.8.4, "DC Sources -

Operating."

The AC Distribution System includes the 480 V shutdown boards and associated supply transformers, load centers, and protective devices shown in Table B 3.8.9-1.

The 120 VAC vital buses are arranged in four load groupschannels as follows:

Channel I - vital buses 1-I and 2-I; Channel II - vital buses 1-II and 2-II; Channel III - vital buses 1-III and 2-III; Channel IV - vital buses 1-IV and 2-IV.

and The 120 VAC vital buses are normally powered from the unit inverters or spare inverters and DC Boards I, II, III, and IV. An alternate power supply for the vital buses is a regulated transformer bypass source powered from the same train as the associated unit inverter, and its use is governed by LCO 3.8.7, "Inverters - Operating."

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. The list of all required distribution buses is presented in Table B 3.8.9-1.

Distribution Systems - Operating B 3.8.9 BASES (continued)

(continued)

Watts Bar-Unit 1 B 3.8-90 APPLICABLE The initial conditions of Design Basis Accident (DBA) and transient analyses SAFETY ANALYSES in the FSAR, Section 6 (Ref. 1), and in the FSAR, Section 15 (Ref. 1), assume ESF systems are OPERABLE. The AC, vital DC, and AC vital bus electrical power distribution systems are designed to provide sufficient capacity, capability, redundancy, and reliability to ensure the availability of necessary power to ESF systems so that the fuel, Reactor Coolant System, and containment design limits are not exceeded. These limits are discussed in more detail in the Bases for Section 3.2, Power Distribution Limits; Section 3.4, Reactor Coolant System (RCS); and Section 3.6, Containment Systems.

The OPERABILITY of the AC, vital DC, and AC vital bus electrical power distribution systems is consistent with the initial assumptions of the accident analyses and is based upon meeting the design basis of the plant. This includes maintaining power distribution systems OPERABLE during accident conditions in the event of:

a.

An assumed loss of all offsite power or all onsite AC electrical power; and

b.

A worst case single failure.

The distribution systems satisfy Criterion 3 of the NRC Policy Statement.

LCO The required power distribution subsystems listed in Table B 3.8.9-1 ensure the availability of AC, vital DC, and AC vital bus electrical power for the systems required to shut down the reactor and maintain it in a safe condition after an anticipated operational occurrence (AOO) or a postulated DBA. The AC, vital DC, and AC vital bus electrical power distribution subsystems are required to be OPERABLE.

Distribution Systems - Operating B 3.8.9 BASES (continued)

Watts Bar-Unit 1 B 3.8-91 Revision 67, 75, 76, 77, 78 LCO Maintaining the Train A and Train B AC, four channels of vital DC, and (continued) four channels of AC vital bus electrical power distribution subsystems OPERABLE ensures that the redundancy incorporated into the design of ESF is not defeated. Therefore, a single failure within any system or within the electrical power distribution subsystems will not prevent safe shutdown of the reactor.

OPERABLE AC electrical power distribution subsystems require the associated buses, load centers, motor control centers, and distribution panels to be energized to their proper voltages. OPERABLE DC electrical power distribution subsystems require the associated buses to be energized to their proper voltage from either the associated battery or charger. OPERABLE vital bus electrical power distribution subsystems require the associated buses to be energized to their proper voltage from the associated unit or spare inverter via inverted DC voltage, unit or spare inverter using internal AC source, or the regulated transformer bypass source.

In addition, tie breakers between redundant safety related AC, vital DC, and AC vital bus power distribution subsystems, if they exist, must be open. This prevents any electrical malfunction in any power distribution subsystem from propagating to the redundant subsystem, that could cause the failure of a redundant subsystem and a loss of essential safety function(s). If any tie breakers are closed, the affected redundant electrical power distribution subsystems are considered inoperable. This applies to the onsite, safety related redundant electrical power distribution subsystems. It does not, however, preclude redundant 6.9 kV shutdown boards from being powered from the same offsite circuit.

APPLICABILITY The electrical power distribution subsystems are required to be OPERABLE in MODES 1, 2, 3, and 4 to ensure that:

a.

Acceptable fuel design limits and reactor coolant pressure boundary limits are not exceeded as a result of AOOs or abnormal transients; and

b.

Adequate core cooling is provided, and containment OPERABILITY and other vital functions are maintained in the event of a postulated DBA.

Electrical power distribution subsystem requirements for MODES 5 and 6 are covered in the Bases for LCO 3.8.10, "Distribution Systems - Shutdown."

Distribution Systems - Operating B 3.8.9 BASES (continued)

(continued)

Watts Bar-Unit 1 B 3.8-92 ACTIONS A.1 With one or more required AC shutdown boards, load centers, motor control centers, or distribution panels, except AC vital buses, in one train inoperable, the remaining AC electrical power distribution subsystem in the other train is capable of supporting the minimum safety functions necessary to shut down the reactor and maintain it in a safe shutdown condition, assuming no single failure. The overall reliability is reduced, however, because a single failure in the remaining power distribution subsystems could result in the minimum required ESF functions not being supported. Therefore, the required AC shutdown boards, load centers, motor control centers, and distribution panels must be restored to OPERABLE status within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months .

Condition A worst scenario is one train without AC power (i.e., no offsite power to the train and the associated DG inoperable). In this Condition, the plant is more vulnerable to a complete loss of AC power. It is, therefore, imperative that the plant operator's attention be focused on minimizing the potential for loss of power to the remaining train by stabilizing the plant, and on restoring power to the affected train. The 8 hour9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months time limit before requiring a plant shutdown in this Condition is acceptable because of:

a.

The potential for decreased safety if the plant operator's attention is diverted from the evaluations and actions necessary to restore power to the affected train, to the actions associated with taking the plant to shutdown within this time limit; and

b.

The potential for an event in conjunction with a single failure of a redundant component in the train with AC power.

The second Completion Time for Required Action A.1 establishes a limit on the maximum time allowed for any combination of required distribution subsystems to be inoperable during any single contiguous occurrence of failing to meet the LCO. If Condition A is entered while, for instance, a DC bus is inoperable and

Distribution Systems - Operating B 3.8.9 BASES (continued)

Watts Bar-Unit 1 B 3.8-93 Revision 67, 75, 76, 77, 78 ACTIONS A.1 (continued) subsequently restored OPERABLE, the LCO may already have been not met for up to 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months . This could lead to a total of 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months , since initial failure of the LCO, to restore the AC distribution system. At this time, a DC circuit could again become inoperable, and AC distribution restored OPERABLE. This could continue indefinitely.

The Completion Time allows for an exception to the normal "time zero" for beginning the allowed outage time "clock." This will result in establishing the "time zero" at the time the LCO was initially not met, instead of the time Condition A was entered. The 16 hour1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months Completion Time is an acceptable limitation on this potential to fail to meet the LCO indefinitely.

B.1 With one or more AC vital buses in one channel inoperable for reasons other than Condition C, the remaining OPERABLE AC vital buses are capable of supporting the minimum safety functions necessary to shut down the plant and maintain it in the safe shutdown condition. Overall reliability is reduced, however, since an additional single failure could result in the minimum required ESF functions not being supported. Therefore, the required AC vital bus must be restored to OPERABLE status within 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months by powering the bus from the associated unit or spare inverter via inverted DC, unit or spare inverter using internal AC source, or regulated transformer bypass source.

Condition B represents one or more AC vital buses in one channel without power; potentially both the DC source and the associated AC source are nonfunctioning. In this situation, the plant is significantly more vulnerable to a complete loss of all noninterruptible power. It is, therefore, imperative that the operator's attention focus on stabilizing the plant, minimizing the potential for loss of power to the remaining vital buses and restoring power to the affected vital bus.

Distribution Systems - Operating B 3.8.9 BASES (continued)

Watts Bar-Unit 1 B 3.8-94 ACTIONS B.1 (continued)

This 2 hour2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months limit is more conservative than Completion Times allowed for the vast majority of components that are without adequate vital AC power. Taking exception to LCO 3.0.2 for components without adequate vital AC power, that would have the Required Action Completion Times shorter than 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months if declared inoperable, is acceptable because of:

a.

The potential for decreased safety by requiring a change in plant conditions (i.e., requiring a shutdown) and not allowing stable operations to continue;

b.

The potential for decreased safety by requiring entry into numerous Applicable Conditions and Required Actions for components without adequate vital AC power and not providing sufficient time for the operators to perform the necessary evaluations and actions for restoring power to the affected train; and

c.

The potential for an event in conjunction with a single failure of a redundant component.

The 2 hour2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months Completion Time takes into account the importance to safety of restoring the AC vital bus to OPERABLE status, the redundant capability afforded by the other OPERABLE vital buses, and the low probability of a DBA occurring during this period.

The second Completion Time for Required Action B.1 establishes a limit on the maximum allowed for any combination of required distribution subsystems to be inoperable during any single contiguous occurrence of failing to meet the LCO. If Condition B is entered while, for instance, an AC bus is inoperable and subsequently returned OPERABLE, the LCO may already have been not met for up to 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months . This could lead to a total of 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months , since initial failure of the LCO, to restore the vital bus distribution system. At this time, an AC train could again become inoperable, and vital bus distribution restored OPERABLE. This could continue indefinitely.

This Completion Time allows for an exception to the normal "time zero" for beginning the allowed outage time "clock." This will result in establishing the "time zero" at the time the LCO was initially not met, instead of the time Condition B was entered. The 16 hour1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months Completion Time is an acceptable limitation on this potential to fail to meet the LCO indefinitely.

Distribution Systems - Operating B 3.8.9 BASES (continued)

Watts Bar-Unit 1 B 3.8-95 ACTIONS C.1 (continued)

With AC vital bus 2-I, 2-II, 2-III, or 2-IV inoperable for planned maintenance, the remaining OPERABLE AC vital buses are capable of supporting the minimum safety functions necessary to shut down the plant and maintain it in the safe shutdown condition. Overall reliability is reduced, however, since an additional single failure could result in the minimum required ESF functions not being supported. Therefore, the inoperable AC vital bus must be restored to OPERABLE status within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months . The 8-hour time limit is acceptable because the unit's reactor protection and core cooling and containment cooling actuation systems are powered from AC vital buses 1-I, 1-II, 1-III, or 1-IV.

This 8 hour9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months limit is more conservative than Completion Times allowed for the majority of components that are without adequate vital AC power. Taking exception to LCO 3.0.2 for components without adequate vital AC power that would have the Required Action Completion Times shorter than 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months if declared inoperable, is acceptable because:

a.

The potential for decreased safety by requiring a change in unit conditions (i.e., requiring a shutdown) and not allowing stable operations to continue;

b.

The potential for decreased safety by requiring entry into numerous Applicable Conditions and Required Actions for components without adequate vital AC power and not providing sufficient time for the operators to perform the necessary evaluations and actions for restoring power to the affected train; and

c.

The potential for an event in conjunction with a single failure of a redundant component.

Condition C is modified by two notes that limit the conditions and parameters that allow entry into Condition C. The first note states that Condition C is only applicable during planned maintenance of AC vital bus 2-I, 2-II, 2-III, or 2-IV. This note allows the plant configuration to be aligned to minimize features being inoperable when one of these vital buses is made inoperable. The second note limits the applicability of Condition C to the time period when the opposite unit is either MODE 5, MODE 6, or defueled. This note limits the time period allowing Condition C to be entered, minimizing when the allowance can be utilized. The allowance for Condition C is acceptable based on the following:

a.

AC vital buses 2-I, 2-II, 2-III, and 2-IV do not power the unit's reactor protection and core cooling and containment cooling actuation systems.

b.

Performing maintenance on these components will increase the reliability of the Class 1E AC Electrical Power Distribution System.

Distribution Systems - Operating B 3.8.9 BASES (continued)

Watts Bar-Unit 1 B 3.8-96 ACTIONS C.1 (continued)

c.

The limited opportunities that allow the planned maintenance to occur.

During the planned maintenance of AC vital bus 2-I, 2-II, 2-III, or 2-IV, if a condition is discovered on the board requiring corrective maintenance, this maintenance may be performed under Condition C.

CD.1 With one or more vital DC bus inoperable, the remaining DC electrical power distribution subsystems are capable of supporting the minimum safety functions necessary to shut down the reactor and maintain it in a safe shutdown condition, assuming no single failure. The overall reliability is reduced, however, because a single failure in the remaining DC electrical power distribution subsystem could result in the minimum required ESF functions not being supported. Therefore, the required DC buses must be restored to OPERABLE status within 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months by powering the bus from the associated battery or charger.

Condition CD represents one or more trains without adequate DC power; potentially both with the battery significantly degraded and the associated charger nonfunctioning. In this situation, the plant is significantly more vulnerable to a complete loss of all DC power. It is, therefore, imperative that the operator's attention focus on stabilizing the plant, minimizing the potential for loss of power to the remaining train(s) and restoring power to the affected train(s).

This 2 hour2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months limit is more conservative than Completion Times allowed for the vast majority of components that would be without power. Taking exception to LCO 3.0.2 for components without adequate DC power, which would have Required Action Completion Times shorter than 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months , is acceptable because of:

a.

The potential for decreased safety by requiring a change in plant conditions (i.e., requiring a shutdown) while allowing stable operations to continue;

b.

The potential for decreased safety by requiring entry into numerous applicable Conditions and Required Actions for components without DC power and not providing sufficient time for the operators to perform the necessary evaluations and actions for restoring power to the affected train; and

c.

The potential for an event in conjunction with a single failure of a redundant component.

Distribution Systems - Operating B 3.8.9 BASES (continued)

Watts Bar-Unit 1 B 3.8-97 ACTIONS CD.1 (continued)

The 2 hour2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months Completion Time for DC buses is consistent with Regulatory Guide 1.93 (Ref. 2).

The second Completion Time for Required Action CD.1 establishes a limit on the maximum time allowed for any combination of required distribution subsystems to be inoperable during any single contiguous occurrence of failing to meet the LCO. If Condition CD is entered while, for instance, an AC bus is inoperable and subsequently returned OPERABLE, the LCO may already have been not met for up to 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months . This could lead to a total of 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months , since initial failure of the LCO, to restore the DC distribution system. At this time, an AC train could again become inoperable, and DC distribution restored OPERABLE. This could continue indefinitely.

This Completion Time allows for an exception to the normal "time zero" for beginning the allowed outage time "clock." This will result in establishing the "time zero" at the time the LCO was initially not met, instead of the time Condition CD was entered. The 16 hour1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months Completion Time is an acceptable limitation on this potential to fail to meet the LCO indefinitely.

DE.1 and DE.2 If the inoperable distribution subsystem cannot be restored to OPERABLE status within the required Completion Time, the plant must be brought to a MODE in which the LCO does not apply. To achieve this status, the plant must be brought to at least MODE 3 within 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and to MODE 5 within 36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months . The allowed Completion Times are reasonable, based on operating experience, to reach the required plant conditions from full power conditions in an orderly manner and without challenging plant systems.

EF.1 With two trains with one or more inoperable distribution subsystems that result in a loss of safety function, adequate core cooling, containment OPERABILITY, and other vital functions for DBA mitigation would be compromised, and immediate plant shutdown in accordance with LCO 3.0.3 is required.

Distribution Systems - Operating B 3.8.9 BASES Watts Bar-Unit 1 B 3.8-98 SURVEILLANCE SR 3.8.9.1 REQUIREMENTS This Surveillance verifies that the required AC, vital DC, and AC vital bus electrical power distribution systems are functioning properly, with the correct circuit breaker alignment. The correct breaker alignment ensures the appropriate separation and independence of the electrical trains is maintained, and the appropriate voltage is available to each required bus. The verification of proper voltage availability on the buses ensures that the required voltage is readily available for motive as well as control functions for critical system loads connected to these buses. The 7 day Frequency takes into account the redundant capability of the AC, vital DC, and AC vital bus electrical power distribution subsystems, and other indications available in the control room that alert the operator to subsystem malfunctions.

REFERENCES

1.

Watts Bar FSAR, Section 6 "Engineering Safety Features," Section 8 "Electric Power," and Section 15 "Accident Analysis."

2.

Regulatory Guide 1.93, "Availability of Electric Power Sources," U.S.

Nuclear Regulatory Commission, December 1974.

Distribution Systems - Operating B 3.8.9 Watts Bar-Unit 1 B 3.8-99 Revision 33, 124 Table B 3.8.9-1 (page 1 of 1)

AC and DC Electrical Power Distribution Systems TYPE VOLTAGE TRAIN A*

TRAIN B*

AC safety buses 6900 V 480 V Shdn Bd 1A-A, 2A-A Shdn Bd 1A1-A, 1A2-A 2A1-A, 2A2-A Rx MOV Bd 1A1-A, 1A2-A 2A1-A,** 2A2-A C & A Vent Bd 1A1-A, 1A2-A 2A1-A, 2A2-A Diesel Aux Bd 1A1-A, 1A2-A 2A1-A, 2A2-A Rx Vent Bd 1A-A, 2A-A**

Shdn Bd 1B-B, 2B-B Shdn Bd 1B1-B, 1B2-B 2B1-B, 2B2-B Rx MOV Bd 1B1-B, 1B2-B 2B1-B,** 2B2-B C & A Vent Bd 1B1-B, 1B2-B 2B1-B, 2B2-B Diesel Aux Bd 1B1-B, 1B2-B 2B1-B, 2B2-B Rx Vent Bd 1B-B, 2B-B**

AC vital buses 120 V Channel I Vital channel bus 1-I Vital channel bus 2-I Channel III Vital channel bus 1-III Vital channel bus 2-III Channel II Vital channel bus 1-II Vital channel bus 2-II Channel IV Vital channel bus 1-IV Vital channel bus 2-IV DC buses 125 V Board I Board III Board II Board IV Each train of the AC and DC electrical power distribution systems is a subsystem.

For WBN Unit 1, 480V Reactor MOV Boards 2A1-A and 2B1-B and 480V Reactor Vent Boards 2A-A and 2B-B are available for economic and operational convenience. The boards contain no Unit 1 Technical Specification (TS) Required loads. The boards are considered part of the Unit 1 / Unit 2 Electrical Power Distribution System and meet Unit 1 TS Requirements and testing only while connected. WBN Unit 1 is designed to be operated, shutdown, and maintained in a safe shutdown status without any of these boards or their loads. As such, the boards may be disconnected from service without entering a Unit 1 LCO provided their loads are not substituting for a Unit 1 TS Required load.

Distribution Systems - Operating B 3.8.9 Watts Bar - Unit 2 B 3.8-86 (continued)

B 3.8 ELECTRICAL POWER SYSTEMS B 3.8.9 Distribution Systems - Operating BASES BACKGROUND The onsite Class 1E AC electrical power distribution system is divided by train into two redundant and independent AC electrical power distribution subsystems.

The AC electrical power subsystem includes four 6.9 kV shutdown boards. Each 6.9 kV shutdown board has access to the two separate and independent preferred offsite sources of power as well as a dedicated onsite diesel generator (DG) source. Usually, one of the two offsite sources will be the normal power source for a 6.9 kV shutdown board, and the other offsite source will be the alternate power source. Transfers from the normal source to the alternate source may be manual or automatic. Automatic transfers only occur when the relay logic is tripping a transmission line and the associated common station service transformers. Only manual transfers are permitted from alternate to normal. Additionally, the maintenance source to the 6.9kV shutdown board can be used as an offsite power source. Transferring the 6.9 kV shutdown board to the maintenance power source is done manually from the normal or alternate power source. For a loss of offsite power 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 breakers is supplied from the Class 1E batteries. Additional description of this system may be found in the Bases for LCO 3.8.1, AC Sources -

Operating, and the Bases for LCO 3.8.4, DC Sources - Operating.

The AC Distribution System includes the 480 V shutdown boards and associated supply transformers, load centers, and protective devices shown in Table B 3.8.9-1.

The 120 VAC vital buses are arranged in four load groups channels as follows: Channel I - vital buses 1-I and 2-I; Channel II - vital buses 1-II and 2-II; Channel III - vital buses 1-III and 2-III; Channel IV - vital buses 1-IV and 2-IV.and The 120 VAC vital buses are normally powered from the unit inverters or spare inverters and DC Boards I, II, III, and IV. An alternate power supply for the vital buses is a regulated transformer bypass source powered from the same train as the associated unit inverter, and its use is governed by LCO 3.8.7, Inverters - Operating.

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.

The list of all required distribution buses is presented in Table B 3.8.9-1.

Distribution Systems - Operating B 3.8.9 BASES (continued)

(continued)

Watts Bar - Unit 2 B 3.8-87 APPLICABLE SAFETY ANALYSES The initial conditions of Design Basis Accident (DBA) and transient analyses in the FSAR, Section 6 (Ref. 1), and in the FSAR, Section 15 (Ref. 1), assume ESF systems are OPERABLE. The AC, vital DC, and AC vital bus electrical power distribution systems are designed to provide sufficient capacity, capability, redundancy, and reliability to ensure the availability of necessary power to ESF systems so that the fuel, Reactor Coolant System, and containment design limits are not exceeded. These limits are discussed in more detail in the Bases for Section 3.2, Power Distribution Limits; Section 3.4, Reactor Coolant System (RCS); and Section 3.6, Containment Systems.

The OPERABILITY of the AC, vital DC, and AC vital bus electrical power distribution systems is consistent with the initial assumptions of the accident analyses and is based upon meeting the design basis of the plant. This includes maintaining power distribution systems OPERABLE during accident conditions in the event of:

a.

An assumed loss of all offsite power or all onsite AC electrical power; and

b.

A worst case single failure.

The distribution systems satisfy Criterion 3 of 10 CFR 50.36(c)(2)(ii).

LCO The required power distribution subsystems listed in Table B 3.8.9-1 ensure the availability of AC, vital DC, and AC vital bus electrical power for the systems required to shut down the reactor and maintain it in a safe condition after an anticipated operational occurrence (AOO) or a postulated DBA. The AC, vital DC, and AC vital bus electrical power distribution subsystems are required to be OPERABLE.

Maintaining the Train A and Train B AC, four channels of vital DC, and four channels of AC vital bus electrical power distribution subsystems OPERABLE ensures that the redundancy incorporated into the design of ESF is not defeated. Therefore, a single failure within any system or within the electrical power distribution subsystems will not prevent safe shutdown of the reactor.

OPERABLE AC electrical power distribution subsystems require the associated buses, load centers, motor control centers, and distribution panels to be energized to their proper voltages. OPERABLE DC electrical power distribution subsystems require the associated buses to be energized to their proper voltage from either the associated battery or charger. OPERABLE vital bus electrical power distribution subsystems

Distribution Systems - Operating B 3.8.9 BASES (continued)

Watts Bar - Unit 2 B 3.8-88 LCO (continued) require the associated buses to be energized to their proper voltage from the associated unit or spare inverter via inverted DC voltage, unit or spare inverter using internal AC source, or the regulated transformer bypass source.

In addition, tie breakers between redundant safety related AC, vital DC, and AC vital bus power distribution subsystems, if they exist, must be open. This prevents any electrical malfunction in any power distribution subsystem from propagating to the redundant subsystem that could cause the failure of a redundant subsystem and a loss of essential safety function(s). If any tie breakers are closed, the affected redundant electrical power distribution subsystems are considered inoperable. This applies to the onsite, safety related redundant electrical power distribution subsystems. It does not, however, preclude redundant 6.9 kV shutdown boards from being powered from the same offsite circuit.

APPLICABILITY The electrical power distribution subsystems are required to be OPERABLE in MODES 1, 2, 3, and 4 to ensure that:

a.

Acceptable fuel design limits and reactor coolant pressure boundary limits are not exceeded as a result of AOOs or abnormal transients; and

b.

Adequate core cooling is provided, and containment OPERABILITY and other vital functions are maintained in the event of a postulated DBA.

Electrical power distribution subsystem requirements for MODES 5 and 6 are covered in the Bases for LCO 3.8.10, Distribution Systems -

Shutdown.

ACTIONS A.1 With one or more required AC shutdown boards, load centers, motor control centers, or distribution panels, except AC vital buses, in one train inoperable, the remaining AC electrical power distribution subsystem in the other train is capable of supporting the minimum safety functions necessary to shut down the reactor and maintain it in a safe shutdown condition, assuming no single failure. The overall reliability is reduced, however, because a single failure in the remaining power distribution subsystems could result in the minimum required ESF functions not being

Distribution Systems - Operating B 3.8.9 BASES (continued)

Watts Bar - Unit 2 B 3.8-89 ACTIONS A.1 (continued) supported. Therefore, the required AC shutdown boards, load centers, motor control centers, and distribution panels must be restored to OPERABLE status within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months .

Condition A worst scenario is one train without AC power (i.e., no offsite power to the train and the associated DG inoperable). In this Condition, the plant is more vulnerable to a complete loss of AC power. It is, therefore, imperative that the plant operator's attention be focused on minimizing the potential for loss of power to the remaining train by stabilizing the plant, and on restoring power to the affected train. The 8 hour9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months time limit before requiring a plant shutdown in this Condition is acceptable because of:

a.

The potential for decreased safety if the plant operator's attention is diverted from the evaluations and actions necessary to restore power to the affected train, to the actions associated with taking the plant to shutdown within this time limit; and

b.

The potential for an event in conjunction with a single failure of a redundant component in the train with AC power.

The second Completion Time for Required Action A.1 establishes a limit on the maximum time allowed for any combination of required distribution subsystems to be inoperable during any single contiguous occurrence of failing to meet the LCO. If Condition A is entered while, for instance, a DC bus is inoperable and subsequently restored OPERABLE, the LCO may already have been not met for up to 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months . This could lead to a total of 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months , since initial failure of the LCO, to restore the AC distribution system. At this time, a DC circuit could again become inoperable, and AC distribution restored OPERABLE. This could continue indefinitely.

The Completion Time allows for an exception to the normal time zero for beginning the allowed outage time clock. This will result in establishing the time zero at the time the LCO was initially not met, instead of the time Condition A was entered. The 16 hour1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months Completion Time is an acceptable limitation on this potential to fail to meet the LCO indefinitely.

Distribution Systems - Operating B 3.8.9 BASES (continued)

Watts Bar - Unit 2 B 3.8-90 Amendment XX ACTIONS (continued)

B.1 With one or more AC vital buses in one channel inoperable for reasons other than Condition C, the remaining OPERABLE AC vital buses are capable of supporting the minimum safety functions necessary to shut down the plant and maintain it in the safe shutdown condition. Overall reliability is reduced, however, since an additional single failure could result in the minimum required ESF functions not being supported.

Therefore, the required AC vital bus must be restored to OPERABLE status within 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months by powering the bus from the associated unit or spare inverter via inverted DC, unit or spare inverter using internal AC source, or regulated transformer bypass source.

Condition B represents one or more AC vital buses in one channel without power; potentially both the DC source and the associated AC source are nonfunctioning. In this situation, the plant is significantly more vulnerable to a complete loss of all non-interruptible power. It is, therefore, imperative that the operator's attention focus on stabilizing the plant, minimizing the potential for loss of power to the remaining vital buses and restoring power to the affected vital bus.

This 2 hour2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months limit is more conservative than Completion Times allowed for the vast majority of components that are without adequate vital AC power.

Taking exception to LCO 3.0.2 for components without adequate vital AC power, that would have the Required Action Completion Times shorter than 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months if declared inoperable, is acceptable because of:

a.

The potential for decreased safety by requiring a change in plant conditions (i.e., requiring a shutdown) and not allowing stable operations to continue;

b.

The potential for decreased safety by requiring entry into numerous Applicable Conditions and Required Actions for components without adequate vital AC power and not providing sufficient time for the operators to perform the necessary evaluations and actions for restoring power to the affected train; and

c.

The potential for an event in conjunction with a single failure of a redundant component.

The 2 hour2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months Completion Time takes into account the importance to safety of restoring the AC vital bus to OPERABLE status, the redundant capability afforded by the other OPERABLE vital buses, and the low probability of a DBA occurring during this period.

Distribution Systems - Operating B 3.8.9 BASES (continued)

Watts Bar - Unit 2 B 3.8-91 Amendment XX ACTIONS B.1 (continued)

The second Completion Time for Required Action B.1 establishes a limit on the maximum allowed for any combination of required distribution subsystems to be inoperable during any single contiguous occurrence of failing to meet the LCO. If Condition B is entered while, for instance, an AC bus is inoperable and subsequently returned OPERABLE, the LCO may already have been not met for up to 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months . This could lead to a total of 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months , since initial failure of the LCO, to restore the vital bus distribution system. At this time, an AC train could again become inoperable, and vital bus distribution restored OPERABLE. This could continue indefinitely.

This Completion Time allows for an exception to the normal time zero for beginning the allowed outage time clock. This will result in establishing the time zero at the time the LCO was initially not met, instead of the time Condition B was entered. The 16 hour1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months Completion Time is an acceptable limitation on this potential to fail to meet the LCO indefinitely.

C.1 With AC vital bus 1-I, 1-II, 1-III, or 1-IV inoperable for planned maintenance, the remaining OPERABLE AC vital buses are capable of supporting the minimum safety functions necessary to shut down the plant and maintain it in the safe shutdown condition. Overall reliability is reduced, however, since an additional single failure could result in the minimum required ESF functions not being supported. Therefore, the inoperable AC vital bus must be restored to OPERABLE status within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months . The 8-hour time limit is acceptable because the unit's reactor protection and core cooling and containment cooling actuation systems are powered from AC vital buses 2-I, 2-II, 2-III, or 2-IV.

This 8 hour9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months limit is more conservative than Completion Times allowed for the majority of components that are without adequate vital AC power.

Taking exception to LCO 3.0.2 for components without adequate vital AC power that would have the Required Action Completion Times shorter than 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months if declared inoperable, is acceptable because:

a.

The potential for decreased safety by requiring a change in unit conditions (i.e., requiring a shutdown) and not allowing stable operations to continue;

b.

The potential for decreased safety by requiring entry into numerous Applicable Conditions and Required Actions for components without adequate vital AC power and not providing sufficient time for the operators to perform the necessary evaluations and actions for restoring power to the affected train; and

Distribution Systems - Operating B 3.8.9 BASES (continued)

Watts Bar - Unit 2 B 3.8-92 Amendment XX ACTIONS C.1 (continued)

c.

The potential for an event in conjunction with a single failure of a redundant component.

Condition C is modified by two notes that limit the conditions and parameters that allow entry into Condition C. The first note states that Condition C is only applicable during planned maintenance of AC vital bus 1-I, 1-II, 1-III, or 1-IV. This note allows the plant configuration to be aligned to minimize features being inoperable when one of these vital buses is made inoperable. The second note limits the applicability of Condition C to the time period when the opposite unit is either MODE 5, MODE 6, or defueled. This note limits the time period allowing Condition C to be entered, minimizing when the allowance can be utilized. The allowance for Condition C is acceptable based on the following:

a.

AC vital buses 1-I, 1-II, 1-III, and 1-IV do not power the unit's reactor protection and core cooling and containment cooling actuation systems.

b.

Performing maintenance on these components will increase the reliability of the Class 1E AC Electrical Power Distribution System.

c.

The limited opportunities that allow the planned maintenance to occur.

During the planned maintenance of AC vital bus 1-I, 1-II, 1-III, or 1-IV, if a condition is discovered on the board requiring corrective maintenance, this maintenance may be performed under Condition C.

CD.1 With one or more vital DC buses inoperable, the remaining DC electrical power distribution subsystems are capable of supporting the minimum safety functions necessary to shut down the reactor and maintain it in a safe shutdown condition, assuming no single failure. The overall reliability is reduced, however, because a single failure in the remaining DC electrical power distribution subsystem could result in the minimum required ESF functions not being supported. Therefore, the required DC buses must be restored to OPERABLE status within 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months by powering the bus from the associated battery or charger.

Condition CD represents one or more trains without adequate DC power; potentially both with the battery significantly degraded and the associated charger nonfunctioning. In this situation, the plant is significantly more vulnerable to a complete loss of all DC power. It is, therefore, imperative

Distribution Systems - Operating B 3.8.9 BASES (continued)

Watts Bar - Unit 2 B 3.8-93a Amendment XX ACTIONS CD.1 (continued) that the operator's attention focus on stabilizing the plant, minimizing the potential for loss of power to the remaining train(s) and restoring power to the affected train(s).

This 2 hour2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months limit is more conservative than Completion Times allowed for the vast majority of components that would be without power. Taking exception to LCO 3.0.2 for components without adequate DC power, which would have Required Action Completion Times shorter than 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months , is acceptable because of:

a.

The potential for decreased safety by requiring a change in plant conditions (i.e., requiring a shutdown) while allowing stable operations to continue;

b.

The potential for decreased safety by requiring entry into numerous applicable Conditions and Required Actions for components without DC power and not providing sufficient time for the operators to perform the necessary evaluations and actions for restoring power to the affected train; and

c.

The potential for an event in conjunction with a single failure of a redundant component.

The 2 hour2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months Completion Time for DC buses is consistent with Regulatory Guide 1.93 (Ref. 2).

The second Completion Time for Required Action CD.1 establishes a limit on the maximum time allowed for any combination of required distribution subsystems to be inoperable during any single contiguous occurrence of failing to meet the LCO. If Condition CD is entered while, for instance, an AC bus is inoperable and subsequently returned OPERABLE, the LCO may already have been not met for up to 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months . This could lead to a total of 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months , since initial failure of the LCO, to restore the DC distribution system. At this time, an AC train could again become inoperable, and DC distribution restored OPERABLE. This could continue indefinitely.

This Completion Time allows for an exception to the normal time zero for beginning the allowed outage time clock. This will result in establishing the time zero at the time the LCO was initially not met, instead of the time Condition CD was entered. The 16 hour1.851852e-4 days 0.00444 hours 2.645503e-5 weeks 6.088e-6 months Completion Time is an acceptable limitation on this potential to fail to meet the LCO indefinitely.

Distribution Systems - Operating B 3.8.9 BASES Watts Bar - Unit 2 B 3.8-93b Amendment XX ACTIONS (continued)

DE.1 and DE.2 If the inoperable distribution subsystem cannot be restored to OPERABLE status within the required Completion Time, the plant must be brought to a MODE in which the LCO does not apply. To achieve this status, the plant must be brought to at least MODE 3 within 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and to MODE 5 within 36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months . The allowed Completion Times are reasonable, based on operating experience, to reach the required plant conditions from full power conditions in an orderly manner and without challenging plant systems.

EF.1 With two trains with one or more inoperable distribution subsystems that result in a loss of safety function, adequate core cooling, containment OPERABILITY, and other vital functions for DBA mitigation would be compromised, and immediate plant shutdown in accordance with LCO 3.0.3 is required.

SURVEILLANCE REQUIREMENTS SR 3.8.9.1 This Surveillance verifies that the required AC, vital DC, and AC vital bus electrical power distribution systems are functioning properly, with the correct circuit breaker alignment. The correct breaker alignment ensures the appropriate separation and independence of the electrical trains is maintained, and the appropriate voltage is available to each required bus.

The verification of proper voltage availability on the buses ensures that the required voltage is readily available for motive as well as control functions for critical system loads connected to these buses. The 7 day Frequency takes into account the redundant capability of the AC, vital DC, and AC vital bus electrical power distribution subsystems, and other indications available in the control room that alert the operator to subsystem malfunctions.

REFERENCES

1.

Watts Bar FSAR, Section 6 Engineering Safety Features, Section 8 Electric Power, and Section 15 Accident Analysis.

2.

Regulatory Guide 1.93, Availability of Electric Power Sources, U.S. Nuclear Regulatory Commission, December 1974.

Distribution Systems - Operating B 3.8.9 Watts Bar - Unit 2 B 3.8-94 Revision 1 Table B 3.8.9-1 (page 1 of 1)

AC and DC Electrical Power Distribution Systems TYPE VOLTAGE TRAIN A*

TRAIN B*

AC safety buses 6900 V Shutdown Board 1A-A, 2A-A Shutdown Board 1B-B, 2B-B 480 V Shutdown Board 1A1-A, 1A2-A 2A1-A, 2A2-A Rx MOV Board 1A1-A**, 1A2-A 2A1-A, 2A2-A C & A Vent Board 1A1-A, 1A2-A 2A1-A, 2A2-A Diesel Aux Board 1A1-A, 1A2-A 2A1-A, 2A2-A Rx Vent Board 1A-A**, 2A-A Shutdown Board 1B1-B, 1B2-B 2B1-B, 2B2-B Rx MOV Board 1B1-B**, 1B2-B 2B1-B, 2B2-B C & A Vent Board 1B1-B, 1B2-B 2B1-B, 2B2-B Diesel Aux Board 1B1-B, 1B2-B 2B1-B, 2B2-B Rx Vent Board 1B-B**, 2B-B AC vital buses 120 V Channel I Vital channel bus 1-I Vital channel bus 2-I Channel III Vital channel bus 1-III Vital channel bus 2-III Channel II Vital channel bus 1-II Vital channel bus 2-II Channel IV Vital channel bus 1-IV Vital channel bus 2-IV DC buses 125 V Board I Board III Board II Board IV Each train of the AC and DC electrical power distribution systems is a subsystem.

For WBN Unit 2, the 480V Reactor MOV Boards 1A1-A and 1B1-B and 480V Reactor Vent Boards 1A-A and 1B-B are available for economic and operational convenience. The boards contain no Unit 2 Technical Specification (TS) Required loads. The boards are considered part of the Unit 1 / Unit 2 Electrical Power Distribution System and meet Unit 2 TS Requirements and testing only while connected. WBN Unit 2 is designed to be operated, shutdown, and maintained in a safe shutdown status without any of these boards or their loads. As such, the boards may be disconnected from service without entering an Unit 2 LCO provided their loads are not substituting for an Unit 2 TS required load.

CNL-19-033 E5-1 of 1 Regulatory Commitments Commitment Due Date/Event TVA will revise the WBN UFSAR to reflect that the 120V AC vital system is shared between WBN Units 1 and 2 and that the 120V AC vital system is composed of four channels with two vital inverters and two vital buses per channel.

Prior to implementation of the approved TS 3.8.9 license amendment (WBN-TS-17-19).

v t e Letter Sequence Response to RAI
EPID:L-2018-LLA-0050, (Approved, Closed)
Initiation Acceptance Administration Questions 26 September 2018 Answers 9 November 2018 21 March 2019 Results Approval Other:
MONTHYEAR2018-09-26 [Table View]

CNL-19-033, Revised Response to Request for Additional Information Regarding Application to Modify Technical Specifications 3.8.9 Regarding Alternating Current (AC) Vital Buses (WBN-TS-17-19)

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CNL-19-033, Revised Response to Request for Additional Information Regarding Application to Modify Technical Specifications 3.8.9 Regarding Alternating Current (AC) Vital Buses (WBN-TS-17-19)

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