ML17312A950

Summary of 960711 Meeting in Rockville,Md W/Aps to Discuss Double Sequencing of Safety Loads Under Degraded Grid Voltage Conditions.List of Attendees & Viewgraphs Encl

ML17312A950
Person / Time
Site:	Palo Verde
Issue date:	09/09/1996
From:	Clifford J NRC (Affiliation Not Assigned)
To:	NRC (Affiliation Not Assigned)
References
NUDOCS 9609120256
Download: ML17312A950 (32)

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UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 2055&4001 September 9,

1996 LICENSEE:

Arizona Public Service Company FACILITY:

Palo Verde Nuclear Generating Station, Units 1, 2, and 3

SUBJECT:

HEETING

SUMMARY

The NRC staff met with representatives of the Arizona Public Service Company (APS) on July ll, 1996, at NRC Headquarters in Rockville, Maryland, to discuss various topics of management interest.

The licensee identified the meeting as a management "drop-in" visit with NRC managers.

The licensee provided a

detailed presentation of a specific technical issue on which they had been working for all three Palo Verde units.

The issue relates to plant responses that result from transients initiated during degraded grip voltage conditions.

An attendance list is provided as Attachment 1.

The viewgraphs used by the licensee in their discussion are provided in Attachment 2.

The licensee briefly discussed that they had originally identified a concern in 1993 with the plant design for degraded grid voltage.

The concern, delineated in LER 93-011-00, was for the potential either that degraded grid voltage would be low enough to result in actuation of the safety buses'ndervoltage trips, or that double sequencing of safety loads would occur following a unit trip.

Double sequencing would be postulated in this case when the plant trip resulted in fast bus transfer of safety loads to the alternate supply with sequencing of safety loads back onto the safety<buses, and a subsequent loss'of the alternate power supply on undervoltage due to high loading of the alternate transformers and persistence of the original degraded grip voltage condition, with resequencing of safety loads onto the emergency diesel generators.

At the time of the original concern, the licensee administratively controlled bus voltage, and during degraded voltage conditions had procedures that blocked fast bus transfer of one train within one hour, and started and loaded the emergency diesel generator in the other train within two hours, to assure that double sequencing did not occur.

The licensee later codified these actions in theit technical specifications.

During the meeting with NRC management on July ll, 1996, the licensee presented information related to a specific plant response

that, under postulated transient conditions, could result in a condition outside the

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plant's design basis.

The condition was identified during a licensee program that involved an expert team that investigated the potential that components in the unblocked train may be affected by double-sequencing in the blocked train.

The specific postulated scenario that the licensee discussed could lead to a condition with auxiliary feedwater (AFW) feeding both steam generators, but the motor operators for the isolation valves for the one AFW train that are AC powered trip on thermal overload caused by the degraded voltage conditions.

This would result in the inability to operate the AC powered AFW valves remotely from the control room until the thermal overloads were reset locally at the valves.

Thus, if one steam generator were faulted (e.g., for a, main steam line break or steam generator tube rupture),

the

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operators would not be able to remotely isolate the faulted steam generator from the unfaulted'steam generator.

The licensee stated that they had modified the instructions to their operators to block fast bus transfer on both trains while this issue is studied further.

This action would reduce the likelihood that the degraded voltage condition would initiate double sequencing of safety loads onto their safety buses as described above by ensuring that the non-safety buses that provide power to the reactor coolant pumps are isolated from the alternate power supply following a plant trip.

This condition would result, h'owever, in loss of reactor coolant pumps following a plant trip under degraded voltage conditions.

The operators would then have to control plant cooldown under natural circulation conditions, a condition covered by current procedures and training but is not the preferred operating condition.

The NRC staff requested that the licensee document in a letter the condition described and current actions the licensee is taking to deal with the postulated scenario.

The staff also indicated that they would look into this situation in more detail upon receipt of the licensee's letter.

Ja es W. Cliffor, Senior Project Manager P oject Directorate IV-2 Division of Reactor Projects III/IV Office of Nuclear Reactor Regulation Docket Nos.

STN 50-528, STN 50-529 and STN 50-530 Attachments:

I.

Attendance List 2.

Handout cc w/atts:

See next page

I,

operators would not be able to remotely isolate the faulted steam generator from the unfaulted steam generator.

The licensee stated that they had modified the instructions to their operators to block fast bus transfer on both trains while this issue is studied further.

This action would reduce the likelihood that the degraded voltage condition would initiate double sequencing of safety loads onto their safety buses as described above by ensuring that the non-safety buses that provide power to the reactor coolant pumps are isolated from the alternate power supply following a plant trip.

This condition would result,

however, in loss of reactor coolant pumps following a plant trip under degraded voltage conditions.

The operators would then have to control plant cooldown under natural circulation conditions, a condition covered by current procedures and training but is not the preferred operating condition.

The NRC staff requested that the licensee document in a letter the condition described and current actions the licensee is taking to deal with the postulated scenario.

The staff also indicated that they would look into this

'situation in more detail upon receipt of the licensee's letter.

Original signed by:

James W. Clifford,.Senior Project Manager

'roject Directorate IV-2 Division of 'Reactor Projects III/IV Office of Nuclear Reactor Regulation Docket Nos.

STN 50-528, STN 50-529 and STN 50-530 Attachments:

1.

Attendance List 2.

Handout cc w/atts:

See next page

• Hard Copies

.DISTRIBTION (w/encls 1 and 2):

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KPerkins, WCFO*

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DISTRIBUTION (w/encl 1):

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EPeyton EJordan

WKropp, EDO DOCUMENT NAME:

7-11-96.SUM OFC PDIV-2 LA PDIV-2 PM DRPW:D ADPR NAME e

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operators would not be able to remotely isolate the faulted steam generator from the unfaulted steam generator.

The licensee stated that they had modified the instructions to their operators to block fast bus transfer on both trains while this issue is studied further.

This action would reduce the likelihood that the degraded voltage condition would initiate double sequencing of safety loads onto their safety buses as described above by ensuring that the non-safety buses that provide power to the reactor coolant pumps are isolated from the alternate power supply following a plant trip.

This condition would result,

however, in loss of reactor coolant pumps following a plant trip under degraded voltage conditions.

The operators would then have to control plant cooldown under natural circulation conditions, a condition covered by current procedures and training but is not the preferred operating condition.

The NRC staff requested that the licensee document in a letter the condition described and current actions the licensee is taking to deal with the postulated scenario.

The staff also indicated that they would look into this situation in more detail upon receipt of the licensee's letter.

Original signed by:

James W. Clifford,.Senior Project Manager Project Directorate IY-2 Division of Reactor Projects III/IV Office of Nuclear Reactor Regulation Docket Nos.

STN 50-528, STN 50-529 and STN 50-530 Attachments:

l.

Attendance List 2.

Handout cc w/atts:

See next page

• Hard Copies DISTRIBTION (w/encls 1 and 2):

Docket File*

PUBLIC

~

PDIV-2 Reading*

JClifford*

KPerkins, WCFO*

DISTRIBUTION (w/encl 1):

WRussel 1 /FMiragl i a RZimmerman JRoe EGA1 WBateman OGC*

ACRS*

EPeyton EJordan

WKropp, EDO DOCUMENT NAME:

7-11-96.SUM OFC PDIV-2 LA NAME etn DATE 8

6 DRPW:D PDIV-2 PM JCQfford:

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96 OFFICIAL RECORD COPY ADPR RZ'rman 96 cc w/atts:

Mr. Steve Olea Arizona Corporation Commission 1200 W. Washington Street Phoenix, Arizona 85007 Douglas Kent Porter Senior Counsel Southern California Edison Company Law Department, Generation Resources P.O.

Box 800

Rosemead, California 91770 Senior Resident Inspector

USNRC, P. 0.

Box 40 Buckeye, Arizona 85326 Regional Administrator, Region IV U. S. Nuclear Regulatory Commission Harris Tower 5 Pavillion 611 Ryan Plaza Drive, Suite 400 Arlington, Texas 76011-8064

Chairman, Board of Supervisors ATTN:

Chairman 301 W. Jefferson, 10th Floor

Phoenix, Arizona 85003 Mr. Aubrey V. Godwin, Director Arizona Radiation Regulatory Agency 4814 South 40 Street Phoenix, Arizona 85040 Ms. Angela K. Krainik, Manager Nuclear Licensing Arizona Public Service Company P.O.

Box 52034 Phoenix, Arizona 85072-2034 Mr. John C. Horne, Vice President Power Supply Palo Verde Services 2025 N. Third Street, Suite 220 Phoenix, Arizona 85004 Mr. William L..Stewart

'xecutive Vice. President, Nuclear Arizona Public Service Company

'ost Office Box 53999 Phoenix, Arizona 85072-3999

Attachment 1

July ll, 1996 MEETING WITH ARIZONA PUBLIC SERVICE COMPANY PALO VERDE NUCLEAR GENERATING STATION UNITS 1, 2, AND 3 MANAGEMENT MEETING AND DISCUSSION OF DOUBLE SEQUENCING OF SAFETY LOADS UNDER DEGRADED GRID VOLTAGE CONDITIONS ATTENDEES Name Roy Zimmerman Jack Roe Jim Clifford Jack Bailey Angie Krainic Affiliation USNRC USNRC USNRC APS APS

Attachment 2

MEETING WITHARIZONAPUBLIC SERVICE COMPANY PALO VERDE NUCLEAR GENERATING STATION UNITS 1, 2, AND 3 MANAGEMENTMEETING AND DISCUSSION OF DOUBLE SEQUENCING OF SAFETY LOADS UNDER DEGRADED GRID VOLTAGE CONDITIONS July 11, 1996 E6&lDOUTMATERIALUSED AT THE MEETING

Response To Double Sequencing

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Double Sequencing i Self Identified LER 93-011 Supplement 1

i Extensive Design Basis Calculation 526 Kvto 120 VAC o Tech Spec for Offsite Voltage Levels

Palo Verde Unit 1 El'ectrical AuxiliarySystem Simplified Single Line Diagram (LOCA Condition)

- NANX03.

42/56/TO MVAT UNIT 3 LOCA Lump I oad Ap rox. 5.6 MVA)

NANS05 13.8 KV MANX01 13.8KV NANX02 42/56/70 MVA UNIT2 LOCA Lump Load (Approx. 5.5 MVA)

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Common Load NANS03

~ 13.8 KV NANS01 13.8 KV MAN X02'3.8 K$

Approx.

2 MVA)

Common Load NANS02 NBNX03 4.16 KV House Load (Approx.

42 MVA)

House Load (Approx.

42 MVA) 4.16 KV Soutce; Engtneettng Study 13-ES-A24 Page A2, 750 KVA PEAG01 Emergency Diesel Generator PEBG02 Emergency 750 KVA Oiesel Generator

VOLTAGE REGULATION IMPROVEMENT PROJECT 4200 4100 4000 S

0 3900 3800 Pre-LOCA voltage (varies depending on system loading)

Voltage drop due to fast bus transfer L

Degraded voltage relay range of potential operation (ref. 13-EC-PB-202)

Reset Dropout Yr l>bo

'>>>(~.>'744 3700 3600

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>>448i Y>>tk'(4'~>/ ~

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Minimum voltages per 13-EC-MA-221 Table 5-4 5

10 15 20 25 30 Fast bus transfers occurs Sequencer Steps (seconds)

VNQVOLQNSTPPf (COA)

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1995 Tech Spec Submittal o T.S. Action Statement sought to balance risk of complicated plant trip (natural circ) with risk of double sequencing o Assumed blocking FBT resulted in operability of blocked train t Actions focused on quick response

(< 1Hr) o 'Interim T.S. change actions specified For sustained low voltages of < 100'io

~ Block Fast Bus Transfer in one train within 1 hr.

~ Start and load DG, isolate offsite power opposite train

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~ Block Fast Bus Transfer in opposite train within 2 hrs

1.40E46 Gomparison of Hourly GDF 1AOE46 1.20E46 1.00E46 8.00E47 lLx LL Cl 6.00E47 4.00E47 2.00E47 0.00E+00 5.42E49 Base Case No Action 3.60E48 Block 1 Train 6.70E49 Block 2 Trains 7.10E49 Block1/EDG 1

~ Base Case a No AcUon a Block 1 Train a Block 2 Trains a Block1/EDG 1

Concern I Is blocked train free from adverse system interactions from unblocked train?

I PVNGS review of T.S. submittal assumed unblocked train could be disregarded Accepted industry standards for safety related train separation adequate justification for not analyzing further Opposite train not "credited" for safe shutdown/accident mitigatiori

Cross Discipline Team Evaluation

e. Assessment of potential adverse system

'nteractions

'.Table top review applies best engineering judgment to identify potential interactions

~ Low failure probability not excluded from review

~ Catastrophic failure of unblocked equipment assumed

~ Single Failure exclusions disregarded Multiplefailures could lead to adverse system interaction

STEAM GEHERATOR N0.1 STEAM GEHERATOR NON CONDENSATE STORAGE TAHK M

M TURBINE DRIVEN PUMP V13T V015 M

M MOTOR V138 V02i HV31 UV35 M

M HV30 UV34 DJL AUXlLIARYFEEDWATER SCHEMATlC

( ESSENTlAL TRA)NS )

Safety Significance I AFW scenario contributes an additional 1.5/o to the total CDF above those in the 1995 Tech Spec submittal

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t Revised CDF still well under the risk of a forced shutdown.

2.50E46 Probability of Core Melt During a 72 Hour Period vs. Forced Shutdown (Revised) 2.40E46 1.50E46 5.00E47 T. S. Submittal Revised Anatyata Forced Shutdown

1995-96 Degraded Voltage Frequency o 1995-1996 events short duration 02/95 2.50 min 51 8.0 kV (98.67'/o) 07/95 0.16 min 523.6 kV (99.73/0) 10/95 3.00 min 523.5 kV (99.71 /o) 07/96 0.18 min 521.9 kV (99.40/o)

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Interim Corrective Actions I

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o Maintain switchyard voltage range 100/o-1024/o

1t'or sustained voltages of <100'/o..

Block Fast Bus Transfer in both trains as soon as possible, but within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />

- THEN-Start and load the DG and isolate from offsite power in the selected train

Reportability Determination Change in system operational interaction assumption i 50.73 (a)(2)(v)

Reported 06/1 7/96

"...alone could have prevented..."

w 50.73 (a)(2)(vi)

Included in 06/17/96 report a 50.9 Complete and accurate information Not 60.9-information was accurate at the time Change does not have "significant implication.for public health and safety"

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VOLTAGE REGULATION IMPROVEMENT PROJECT 1EEORE PLANT MODS Anticipated range of voltage delivered to bus by upstream distribution system DESIRED CONDITION 4160

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Degraded voltage relay potential range of operation Minimum voltage at 4KV bus to satisfy downstream equipment requirements

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