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)

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)

ML18313A214
Person / Time
Site:	Watts Bar
Issue date:	11/09/2018
From:	Henderson E Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
CNL-18-121, EPID L-2018-LLA-0050
Download: ML18313A214 (18)

Text

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 unit's 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.

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 <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> 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 <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> and 16 hours1.851852e-4 days <br />0.00444 hours <br />2.645503e-5 weeks <br />6.088e-6 months <br /> 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 unit's AC electrical power subsystems to be operable, the opposite unit's AC electrical power subsystems supplying power to a required shared load are also required to be operable.

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 unit's 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 unit's 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.

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.

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.

The proposed TS Condition C would allow 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> 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 unit's vital bus would be inoperable for reasons other than planned maintenance, the proposed new Condition D would require 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> 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 unit's 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 unit's 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." 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 unit's 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.