Text

Response to Follow-up Request for Additional Information Regarding NRC Bulletin 2012-01: Design Vulnerability in Electric Power System
	ML14163A006
Person / Time
Site:	Brunswick
Issue date:	05/22/2014
From:	Hamrick G; Duke Energy Progress
To:	; Document Control Desk, Office of Nuclear Reactor Regulation
References
	BL-12-001, BSEP 14-0057
	Download: ML14163A006 (7)
	v • d • e

George T. Hamrick DUKE ENERGY.

Vice President Brunswick Nuclear Plant Southport, NC 28461 o: 910.457.3698 May 22, 2014 Serial: BSEP 14-0057 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001

Subject:

Brunswick Steam Electric Plant, Unit Nos. 1 and 2 Renewed Facility Operating License Nos. DPR-71 and DPR-62 Docket Nos. 50-325 and 50-324 Response to Follow-up Request for Additional Information Regarding NRC Bulletin 2012-01: Design Vulnerability in Electric Power System

References:

1. NRC Bulletin 2012-01: Design Vulnerability in Electric Power System, dated July 27, 2012, ADAMS Accession Number ML12074A115

2. Letter from Michael J. Annacone (Duke Energy Progress, Inc.), to U.S. Nuclear Regulatory Commission (Serial: BSEP 12-0114), Response to NRC Bulletin 2012-01:

Design Vulnerability in Electric Power System, dated October 24, 2012, ADAMS Accession Number ML12310A060

3. Letter from Michele G. Evans (U.S. Nuclear Regulatory Commission) to Michael J.

Annacone (Duke Energy Progress, Inc.), Request for Additional Information Regarding Response to Bulletin 2012-01, Design Vulnerability in Electric Power System, dated December 20, 2013, ADAMS Accession Number ML13351A314

4. Letter from George T. Hamrick (Duke Energy Progress, Inc.), to U.S. Nuclear Regulatory Commission (Serial: BSEP 14-0011), Response to Request for Additional Information Regarding NRC Bulletin 2012-01: Design Vulnerability in Electric Power System, dated January 31, 2014, ADAMS Accession No. ML14043A169

5. Electronic Mail from Siva Lingam (U.S. Nuclear Regulatory Commission) to William R.

Murray (Duke Energy Progress, Inc.), Brunswick I and 2 - Bulletin (BL) 2012-01 Request for Additional Information (RAI) Response Follow-up RAI, dated April 25, 2014, ADAMS Accession No. ML14115A444 Ladies and Gentlemen:

By letter dated October 24, 2012 (i.e., Reference 2), Duke Energy Progress, Inc., responded to NRC Bulletin 2012-01 (i.e., Reference 1) for the Brunswick Steam Electric Plant (BSEP), Unit Nos. 1 and 2. On January 31, 2014 (i.e., Reference 4), Duke Energy Progress, Inc., responded to a request for additional information (RAI) (i.e., Reference 3) regarding Duke Energy'sBulletin 2012-01 response. Subsequently, on April 25, 2014 (i.e., Reference 5), the NRC provided an electronic, follow-up RAI. The response to the follow-up RAI is enclosed.

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U.S. Nuclear Regulatory Commission Page 2 of 2 This document contains no new regulatory commitments.

I declare, under penalty of perjury, that the foregoing is true and correct. Executed on May 22, 2014.

Sincerely, George T. Hamrick MAT/mat

Enclosure:

Response to NRC Bulletin 2012-01 Follow-up Request for Additional Information cc:

U. S. Nuclear Regulatory Commission, Region II ATTN: Mr. Victor M. McCree, Regional Administrator 245 Peachtree Center Ave, NE, Suite 1200 Atlanta, GA 30303-1257 U. S. Nuclear Regulatory Commission ATTN: Mr. Andrew Hon (Mail Stop OWFN 8G9A) (Electronic Copy Only) 11555 Rockville Pike Rockville, MD 20852-2738 U. S. Nuclear Regulatory Commission ATTN: Ms. Michelle P. Catts, NRC Senior Resident Inspector 8470 River Road Southport, NC 28461-8869 Chair - North Carolina Utilities Commission P.O. Box 29510 Raleigh, NC 27626-0510

Enclosure Page 1 of 5 Response to NRC Bulletin 2012-01 Follow-up Request for Additional Information

Background

By letter dated October 24, 2012, Duke Energy Progress, Inc., responded to NRC Bulletin 2012-01 for the Brunswick Steam Electric Plant (BSEP), Unit Nos. 1 and 2. On January 31, 2014, Duke Energy Progress, Inc., responded to a request for additional information (RAI) regarding Duke Energy'sBulletin 2012-01 response. Subsequently, on April 25, 2014, the NRC provided an electronic, follow-up RAI. The response to the follow-up RAI follows.

NRC RAI Duke Energy Progress, Inc. (the licensee) response to NRC staffs Request for Additional Information (RAI) regarding response to Nuclear Regulatory Commission (NRC)

Bulletin 2012-01, "Design Vulnerability in Electric Power System," dated January31, 2014, in response to interim corrective action taken to ensure that plant operators can promptly diagnose and respond to open phase conditions on the offsite power circuits for Class-1 E vital buses until permanent corrective actions are completed, stated the following:

" During Operating Modes 4 and 5, daily walk-downs were performed in the Transformer Yard and Switchyard (e.g., refueling outage March 2013, and maintenance outage May 2013) to identify any open phase conditions (i.e., broken 230kV connections). The walkdowns continue to be procedurally required during Operating Modes 4 and 5. During walk downs, Operations personnel look for any anomalies or out of normal conditions and take appropriate actions.

During Operating Modes 4 and 5, phase-to-phase voltage readings were obtained daily, and are evaluated for acceptability. The voltage readings continue to be procedurally required during Operating Modes 4 and 5.

Operating instructions and training curriculum were reviewed to ensure operators can diagnose and respond to an open phase condition.

" Annunciator procedures for each 4kV motor were revised to include an open phase condition as a possible cause for a motor trip.

" Operations procedures were revised to include recognition of equipment problems caused by an open phase condition, and appropriate responses.

" Annual infrared (i.e., thermography) inspections of yard equipment have been performed and are ongoing.

The NRC staff notes that the licensee's response dated October 24, 2012, states that the installed relays were not designed to detect single-phase open circuit conditions. Please explain why taking the above actions in Mode 4 and 5 will ensure that plant operators can promptly diagnose and respond to open phase conditions on the credited offsite power circuits for Class-1 E vital buses. Also, please explain the specific interim actions that are implemented during Mode 1, 2, and 3 when the onsite and offsite power systems are required to be operable in accordance with plant Technical Specification 3.8.1 to address the open phase design vulnerability.

Enclosure Page 2 of 5

Response

General The currently installed protective relaying was not specifically designed to detect and respond to single-phase open circuit conditions on the credited offsite sources. However, they are capable of detecting the degraded voltage downstream of the affected transformer caused by an open phase condition under certain operating loads as discussed below. Refer to Attachment 1 (i.e.,

Page 5 of 5) for a simplified One-Line Diagram.

Modes 1, 2. and 3 The degraded voltage relay (DVR) consists of three single-phase relays (27DV), each connected between two different phases (A-B, A-C, B-C) wired in a two-out-of-three trip logic.

Note that this configuration is more robust than that employed by the Byron Plant which monitors only two phases in a two-out-of-two trip logic. This relay has a 10 (+/-1) second time delay and its function is to trip the Emergency Bus (E bus) incoming line breaker when voltage drops below its setting (i.e., approximately 90% of nominal). Therefore, opening of any phase feeding the transformer aligned to the E-bus would be detected by the DVR, if the E bus voltage on the affected phase drops to the DVR setting. With one phase open, the current in the remaining phases of running motors will increase. Since the DVR requires approximately 10 seconds to isolate the E bus, it is necessary to validate that overcurrent and overload relays do not actuate to lock out the motors while the relay is timing out. Actuation of motor protective devices would require manual resetting by the Operator at the switchgear or the motor control center.

Per calculation BNP-E-8.01 0, AC CoordinationStudy, safety related 4 kV motor overcurrent relays have pickup settings in the range of 158% to 193% of motor full load amperes (FLA).

These settings ensure that relay actuation will not occur as a result of the increased motor current while the DVR is timing out. Furthermore, the safe stall time of 4kV motors is greater than 11 seconds, suggesting that the motors can withstand the increased running currents with no risk of damage (i.e., motor current draw under open single-phase operation will be well below locked rotor current). Similarly, 480 V safety-related motors should be capable of operating through such an event with no overcurrent device trips or motor damage, however, this has not been confirmed with a formal review.

The Reactor Recirculation (RCR) Pump variable frequency drives (VFDs) are equipped with (1) Input Low Voltage detection, (2) Input Phase Imbalance detection, and (3) Input Phase Loss detection. The low input voltage detection alarms at 90% and 70% of rated voltage and trips the VFD at 55% of rated voltage. The input phase imbalance detection trips the VFD at 40% current imbalance. The input phase loss detector alarms if one of the input phases to the drive is not available. Based on these protective features, it can be concluded that, with the unit on line, an open phase on the startup auxiliary transformer (SAT) will result in either an alarm in the control room or in a unit trip due to the VFDs/RCR pumps tripping.

During normal plant operation (i.e., Modes 1, 2 and 3) the E buses are powered from the unit auxiliary transformer (UAT). An open phase on the main power transformer (MPT) 230kV side would have no effect on the E bus voltage. Since the UAT is tied directly to the generator terminals, it will continue to receive three-phase voltage on its primary side for as long as the generator remains online. If the generator trips on negative sequence due to the open MPT

Enclosure Page 3 of 5 phase, the E buses and the upstream balance-of-plant (BOP) buses C and D will automatically transfer to their alternate source (i.e., the SAT). Therefore, an open phase on the MPT high side while the plant is in normal operation is not of concern.

An open phase on the UAT primary side while the isophase generator and MPT connections remain intact is not credible due to the isophase bus connection arrangement, which makes it highly unlikely that a phase would open without shorting to ground and tripping the generator.

As unlikely as it may be, an open phase on the UAT primary during Modes 1, 2 or 3 will be detected by the DVR which will cause the E buses to be transferred to the emergency diesel generators (EDGs).

Engineering judgment dictates that the E bus voltages will drop well below the DVR setting due to heavy transformer loading during normal plant operation. Therefore, safety-related equipment operation under an open single primary phase will be limited to a maximum 11 second duration.

During normal plant operation (i.e., Modes 1, 2 or 3), an open single-phase on the SAT primary has no effect on safety bus voltage since the E buses are powered from the UAT. The SAT supplies the RCR pump VFDs, which are equipped with low input voltage protection (i.e.,

alarm/trip), input current imbalance protection (i.e., trip) and input open phase alarm. Therefore, an open single-phase on the SAT primary will result in either a VFD alarm or, more likely, a VFD trip. In either case, Operators will be informed of the specific cause based on control room VFD electronic data display indication. If both VFDs trip, the reactor will be manually shut down, followed by a generator lockout and the subsequent transfer of BOP buses C and D and the associated E buses to the SAT.

Since the E buses are now connected to the SAT, the open phase on the primary side will be detected by the DVR, which will isolate the E buses from the offsite source and transfer them to their respective EDGs. This conclusion utilizes engineering judgment, based on the SAT loading being sufficiently high to cause E bus voltage to drop below the DVR relay setting. As described previously, the maximum DVR response time of 11 seconds is sufficiently low to preclude 4 kV motor protective device actuation or motor damage due to the increased current associated with motor single-phasing. The same should be true for 480 V motors. Therefore, it is concluded that the safety buses are protected from SAT open phase condition during plant trips. Running loads on BOP buses C, D, and Common will continue to operate single-phased until manually secured.

Based on the above discussion, no interim actions are required while a unit is in Modes 1, 2, or 3.

Modes 4 and 5 During shutdown conditions, safety buses can be powered from either source (i.e., the UAT in the backfeed mode or the SAT) with typically lightly loaded transformers. For the SAT source, preliminary calculations indicate that for very light loading conditions, the E bus voltage may not decrease to the DVR trip setting. This is due to transformer magnetically reproducing the missing phase on the secondary side under low load or no load conditions. Since there are no similar calculations available for the UAT, it will be assumed that detection of open single-phase conditions is not available during light load conditions as well. Therefore, during unit shutdown conditions interim compensatory actions will be imposed on both offsite sources whether loaded or not to ensure that this condition would not go undetected for an extended period of time.

Enclosure Page 4 of 5 Operating procedures were revised to require Operators to walkdown the high voltage lines from each transformer to the switchyard to identify any open phase conditions. In addition, voltage measurements are taken at each 4 kV Bus that receives power from the two transformers to determine if any abnormal condition exists. Walkdowns are performed daily and voltage checks are performed once per shift while the unit is in Modes 4 or 5. Operating procedures were revised to add instructions for the operators to isolate any offsite power transformer found to be degraded and to place the emergency buses fed by that transformer on the other source of offsite power, if available, or on their associated emergency diesel generators. In addition, annunciator procedures for each 4 kV motor were revised to include an open phase condition as a possible cause for a motor trip. These procedure changes, as well as changes to an Abnormal Operating Procedure provide additional direction for Operation's personnel to follow in determining whether an open phase condition exists.

Operators were trained on recognizing an open phase condition and implementing the interim actions. Follow-up simulator testing was then performed to ensure that the training was effective.

These interim actions are multi-layered and will ensure that an open phase condition will not go undetected while a unit is in Mode 4 or 5.

Enclosure Page 5 of 5 Attachment 1 Simplified One-Line Diagram (Unit 1 distribution system is shown. Unit 2 is similar)

FOUR TRANSMISSION LINES PER UNIT

-3 t 230KVBUSIB I

I -

NOTES:

1. DASHED LINE ENCLOSURES P ROTECTIO N ZO NES Y

1_1 ) 1 230oXv Bus tA 31 I - -- 2. I-O PROTECTIVE RELAY L..... I I r. 2TtS Ef): SINGLE PHASE UN N L.CJMPTBVG l OCEAN CBT , REIAYCONNBCTED BETWEEN GENirrvi DBICHARFGE - rJOT OPFE PUUPING 4. 2TDV QOC.20): THREE SINGLE NSTATION P HAS E UAI RELAYS UON RO RING

-- :- -- 59N - --- ---- ALL @ PHASESW)ITHA2..OUT-OF I:..PH.SE.UN TRIP LOG IC UAT, SAT

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4.16 5 BUS E U O 1r4 27M50E 270V 27/50E 27DV RRP RRP (AB) (ABC. 24O) cAB) (ABC. 2.0) IA 1I Note: GDC-17 credited off-site circuits are: (1) SAT- immediately available, (2) UAT Backfeed via MPT -delayed availability (i.e., 1 hour1.157407e-5 days 2.777778e-4 hours 1.653439e-6 weeks 3.805e-7 months )