~ccats.ER~TzD dlTRIavTrow Dz~on&R~now svs TzivrREGULATORY XNFQRMATIQN 'DlSTRIBUTIQN. SYSTEM (RIDS)

ACCESSION NBR: 9209020333 DOC. DATE: 92/08/28 NOTARIZED: NO - DOCKET 0FACIL; 50-.275 Diablo Canyon Nuclear Poeer Plant> Unit 1. Pacific Ga 05000275

AUTO. NAME AUTHOR AFFILIATIONTHIERRY> R. L. Pacific Gas 5 Electric Co.RUEGER> G. M. Pacific Gas 5 Electric Co.

RECIP. NAME RECIPIENT AFFILIATION

SUBJECT: LER 91-021-00: on 910916> during post-mod testing> shearedmotor pinion keg identified in MO for cold leg IV Caused bginadequate keg material. Operating Cc maintenance procedureseil1 be revised to prevent valve stroking. M/920828 ltr.

DISTRIBUTION CODE: IE22T COP LES RECEX VED: LTR ENCL SIZE:TITLE: 50. 73/50. 9 Licensee Event Report (LER)> Incident Rpt. etc.

NOTES:

REC IP IENTID CODE/NAME

PD5 L*RQQD> H

CQP IESLTTR ENCL

1

1 1

REC IP IENTID CODE/NAME

PD5 PD

COPIESLTTR ENCL

1 1

A

INTERNAL: ACNMAEOD/DOAAEOD/ROAB/DSPNRR/DLPG/LHFB10NRR/DOEA/OEABNRR/DST/SELB 8D

RRÃQ --SPLB8DlREG FILE 02RGN5 VILE 01

21

21

1

1

1

1

1

1

21-1

1

1

1

1

ACRSAEOD/DSP/TPABNRR/DET/EMEB 7ENRR/DLPG/LPEB10NRR/DREP/PRPB 1 1

NRR/DST/SXCB8H3NRR/DST/SRXB 8ERES/DSIR/EIB

2 21 1

1 1

. 1 1

2 21

1 1

EXTERNAL: EGSG BRYCE> J. HNRC PDRNSIC PQQRE> W.

2 21 . 11,1

L ST LOBBY NARDNSIC MURPHY> G. ANUDOCS FULL TXT

NOTE TO ALL "RIDS RECIPIENTS:

PLEASE HELP US TO REDUCE O'ASTE! CONTACT THE DOCUMENT CONTROL DESKROOh'I PI-37 (EXT. 504-2065) TO ELIMINATEYOUR NAME FROM DISTRIBUTIONLISTS FOR DOCUMENTS OU DON'T NJEEDI

FULL TEXT CONVERSION REGUI REDTOTAL NUMBER OF COPIES REQUIRED: LTTR 31 ENCL 31

Pacific Gas and Electric Company 77 Beale Street

San Francisco, CA 94106415/973-4684

Gregory M. RuegerSenior Vice President and

General ManagerNuclear Power Generation

August 28, 1992

PGLE Letter No. DCL-92-189

U.S. Nuclear Regulatory CommissionATTN: Document Control DeskWashington, D.C. 20555

Re: Docket No. 50-275, OL-DPR-80Docket No. 50-323, OL-DPR-82Diablo Canyon Units 1 and 2Licensee Event Report 1-91-021-00Failure of Hotor Pinion Keys in Limitorque SMB-3-80 Motor Operatorsdue to Inadequate Design of Material

Gentlemen:

PG&E is submitting the enclosed voluntary Licensee Event Report (LER)concerning the failure of motor pinion keys in Limitorque model SMB-3-80motor operators. This LER is submitted for information purposes only asdescribed in Item 19 of Supplement 1 to NUREG-1022.

This event has in no way affected the health and safety of the public.

Sincerely,

~g>H I~+)~Gregory M. Rueger

cc: Ann P. HodgdonJohn B. MartinPhilip J. HorrillHarry RoodCPUCDiablo DistributionINPO

DC2-91-EH-N085

Enclosure

1031S/85K/PSN/2246

02OA~ ~.9209020333 920828PDR ADOCK 05000275S PDR

LICENSEE EVENT REPORT (LER)

FAclllTYNAME(1l

DIABLO CANYON UNIT 1

DOCKET NIANB(R 2 PACE050002751"TITLE(41 FAILURE OF MOTOR PINION KEYS IN LIMITORQUE SMB-3-80 MOTOR OPERATORS DUE TO

INADEQUATE DESIGN OF MATERIALEVENT DATE (6)

MON DAY

(ER NUMBDI(Sl

SEOUENTIALNUMBER

REVINONNUMSER

REPORT DATE (II

MON DAY YR

OTHER FACILITIES INVOLVED (B)DOCKET NUMBER (6)

0 5 0 0 0 3 2 3

FACIUTYNAMES

DIABLO CANYON UNIT 2

09 16 91 91 0 2 1 — 0 0 08 28 92 0 5 0 0 0OPERATiNGHOOE (S)

THIS REPORT IS SUBMITTED PURSUANT To THE REOIIREHENTS OF lo CFR: (11)

POWERLEVEL 000 1D CFR

X OTHER - VOLUNTARY REPORT

(Specify in Abstract below and in text, NRC Form 366A)

UCENSEE CONTACT FOR YISS uR (12

RAYMOND L. THIERRY, SENIOR REGULATORY COMPLIANCE ENGINEER

COMPLETE ONE UNE FOR EACH COMPONENT FAILURE DESCIVSED IN TIES REPORT (13l

1 P NUMB R

, CAUSE SYSTEM COHPONENT HANUFAC-YVRER

REPORTABLETo NPRDS

CAUSE SYSTEM COMPONENT MANUFAC-TURER

REPORTABLE >g$%?wS?'..j;:.'c)~

BBQ0020L200 B B P 0020 L200 Y

SVPPL8VIENTALREPORT EXPECTED (14)

YES (If yes, complete EXPECTED SUBMISSIoN DATE) i X i NO

ABSTRACT (16)

EXPECTEDSUBMISS ION

DATE (15)

HONTH DAY YEAR

This voluntary LER is submitted for information purposes only as described in Item 19 ofSupplement 1 to NUREG-1022.

.On September 16, 1991, due to a miscommunication resulting from inadequateprocedures, the motor operator for cold leg isolation valve SI-2-8809B electricallyengaged while the valve was being manually stroked open during post-modificationtesting. The, motor operator electrically engaged and closed the valve ("shortstroking"). Investigation determined that this electrical "short stroking" of 'the,valve caused the motor pinion key to shear.

On October 3, 1991, other safety-related Unit 2 SMB-3-80 motor operators were inspected.The motor operators for accumulator discharge isolation valves SI-2-8808B and SI-2-8808Dwere identified as having failed keys similar to the failed key identified in SI-2-88098.Further investigation revealed small cracks emanating from both corners of the keyway onthe SI-2-8809B motor shaft.

The root cause of the sheared motor pinion key was that the key material was inadequate.The key was supplied with the motor operator by the vendor. The key material isconsidered outdated but still acceptable according to the vendor's design.

Corrective actions to prevent recurrence include: (1) replacing the motor pinion keyswith stronger material on safety-related SMB-3-80 motor operators; and (2) .revisingoper ating and maintenance procedures to prevent valve "short stroking."

1031S/85K

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION

FACILITY NAME (I) DOCKET NOSER (E) LER NWLSER 6STOVWINL;,','~, 8EVISON

MJll8QI y.',.I NLMSSR

PAGE 3

DIABLO CANYON UNIT 1

TEXT (17)

0 5 0 0 0 2 7 5 91 021-002 "7

I. 2l t C dhoti

Unit 1 was in Mode 1 (Power Operation) at 100 percent power.

Unit 2 was in Mode 5 (Cold Shutdown) at 0 percent power.

II. Descri tion of Event

A. Summary:

On September 16, 1991, during post-modification testing, a sheared motorpinion key was identified in the motor operator fo} cold leg isolationvalve (BP)(20) SI-2-8809B. Subsequent inspections identified similarfailed keys in the motor operators for accumulator (BP)(ACC) dischargeisolation valves (BP)(20) SI-2-8808B and SI-2-8808D, and also smallcracks emanating from both corners of the keyway on the SI-2-8809B motorshaft.

B. Background:

SI-2-8809B is the residual heat removal (RHR) heat exchanger (BP)(HX)outlet valve to reactor coolant system (RCS) (AB) cold legs 3 and 4.The valve was not required to be in service at the time the key sheared.

The operator for. SI-2-8809B is a Limitorque SMB-3-80, high speed(3380 RPM), 80 ft-lb start torque/16 ft-lb running torque (80/16)operator. The motor pinion key connects the motor drive shaft to thepinion drive gear. The pinion gear drives the valve stem to stroke thevalve.

C.

SI-2-8808B is the 2-2 accumulator discharge isolation valve, andSI-2-8808D is the 2-4 accumulator discharge isolation valve. Thesevalves have Limitorque SMB-3-80 80/32 ft-lb operators.

Event Description:

On September 16, 1991, post-modification testing was being performed onvalve SI-2-8809B following the implementation of a wiring design change.The valve was successfully electrically stroked several times.

The motor operator was then locally placed in manual so that a manualstroke test of the valve could be performed. However, the threeposition switch (BP)(HS) in the control room (NA) was incorrectlypositioned to the close position because the electrical maintenancetechnician did not request that the position switch in the control roombe set to neutral. As the valve was being manually opened, the springpack relaxed, which allowed the close torque switch to indicate that thevalve was no longer fully closed. The motor operator then electricallyengaged and attempted to close the valve further. This electrical

1031S/85K

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION

FACILITY NAME (I) DOCKET NUMBER (2) LER NUMBER 6

DIABLO CANYON UNIT 1

TEXT (17)0 5 0 0 0 2 7 5 91 021 0 0 3 '

engagement of the. motor operator during the manual operation ("shortstroking") resulted in application of a force higher than usual, butstill within design limits, to the motor pinion key. Further attemptsto electrically stroke the valve were unsuccessful.

The motor operator was inspected, and it was determined that the motorpinion key, had sheared. The sheared key allowed the motor drive shaftto rotate within the pinion gear, thus preventing the valve fromopening. Although it was initially believed that the sheared key wasthe result of the one-time short stroking event, the other Unit 2safety-related SHB-3-80 high start torque (80 ft-lb) operators were alsoinspected, with the following results.

On October 3, 1991, the SHB-3-80 motor operators on valves SI-2-8808Band SI-2-8808D were identified as having sheared keys. However, themotor operators were still capable of stroking the valves due tofriction caused by contact between the motor drive shaft and the piniongear, and the valves were determined to have been operable. SI-2-8808Band SI-2-8808D were last successfully stroke tested on Harch 22, 1991,with no pressure differential across the valves. These same conditionswould exist during an accident. The inspections of the other Unit 2SHB-3-80 operators with high speed motors did not identify any othersheared keys. The corresponding motor pinion keys in Unit 1 cannot beinspected with the unit at power; however, the pinion keys are scheduledto be inspected and replaced during the next refueling outage.

On October 16, 1991, and in Harch 1992, PGEE received information fromanother nuclear power plant that had experienced similar Limitorquemotor operator key shearing problems. In addition to sheared keys, thatplant noted that motor shaft deformation and cracking may occur.Following receipt of this information, inspection of the SI-2-8809Bmotor shaft identified small cracks emanating from both corners of thekeyway.

On April 29, 1992, metallurgical analysis of the SI-2-8809B sheared keyindicated that it was made of a low carbon, resulfurized and leadedsteel, such as ASTH A29, Grade 12L13. The key from the SI-2-8809A

'perator(which exhibited some plastic deformation but had not sheared)was made of a carbon steel similar to ASTH A29, Grade 1018. Both ofthese materials have a lower hardness, and therefore a lower shearstrength, than ASTH A29, Grade 4140 material, which the vendor currentlyrecommends for use in SHB-3 motor pinion keys.

D. Inoperable Structures, Components, or Systems that Contributed to theEvent:

None.

1031S/85K

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION

FACILITY NAHE (I)

DIABLO CANYON UNIT 1

TEXT (17)

DOCKET NLNSER (E)

0500027591

LER NUMBER 6SIOUWTIAL

NUMsol rcpt

021

PAGE 3

OOO 'F 7

E. Dates and Approximate Times, for Major Occurrences:

1. September 16, 1991: Event/Discovery Date. SI-2-8809B wasidentified as having a sheared motorpinion key.

2. October 3, 1991:

3. March/April 1992:

SI-2-8808B and SI-2-8808D wereidentified as having sheared keys.

Small cracks were identified on themotor shaft from SI-2-8809B.

F. Other Systems or Secondary Functions Affected:

G.

H.

None.

Method of Discovery:

The sheared key was identified by utility electrical maintenancepersonnel during the performance of scheduled post-maintenance testing.

Operator Actions:

None.

I. Safety System Responses:

A.

None.

Cause of the Event

Immediate Cause:

The immediate cause of the failure of the keys for SI-2-8809B,SI-2-8808B, and SI-2-8808D was the application of a force to the keysexceeding the yield point of the material, thereby causing the keys toshear.

B. Root Cause:

The root cause of the sheared motor pinion key was that the key materialwas inadequate. The key was supplied with the motor operator by thevendor. The key material is considered outdated but still acceptableaccording to the vendor's design.

The root cause of the motor shaft keyway cracking was also determined tobe the key material. Deformation of the key allows increased clearance

. between the key and the shaft. This increased clearance allows higher'mpact loading due to a "hammer blow".-type effect, and also shifts the

1031S/85K

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION

FACILITY NAHE (1)

DIABLO CANYON UNIT ITEXT (17)

DOCKET HUNGER (2)

05 0 0 0 2 7 5 91

LER NUHBER 6STOUCNTNl

NVMOOI c„:i

'21

PAGE 3

0 0 5 " 7

load concentration toward the outer end of the shaft keyway. Thiscauses the shaft to fracture at the sharp keyway corners.

C. Contributory Cause:

l. A contributory cause of the event was short stroking (due toelectrical engagement of the operator) while the valve was beingmanually operated. The cause of the short stroking was failure ofthe operating and maintenance procedures to specify that thecontrol room switch for. valve SI-2-8809B be placed in neutral prior,to manually operating the valve.

IV. Anal sis of the Event

S I-2-8809 B:

Valve SI-2-8809B is the RHR heat exchanger outlet isolation valve to coldlegs 3 and 4. The safety function of the valve is to be open during the coldleg recirculation phase of recovery from a loss-of-coolant accident (LOCA) toprovide a flowpath from the RHR heat exchanger back into the RCS.

During normal plant operation, valve SI-2-8809B is normally open, withelectrical power removed from the operator. Valve SI-2-8809B is required toclose during a transfer from cold leg recirculation to hot leg recirculation.

Investigation indicates that the valve had stroked properly prior to theinadvertent electrical operation of the valve while being manually operated.The force applied to the operator under this "short stroking" condition isgreater than that applied during normal operation or accident conditions.The valve would not be manually engaged during accident conditions.

Recent testing of SI-2-8809B pursuant to NRC Generic Letter 89-10 indicatesthat the initial application of force to the key in the closing direction isapproximately I/10 of the force when seating or unseating the valve. Theforce when seating or unseating the valve is greater because the valve discis torqued into .its seat when closed, but is not torqued into its backseatwhen opened. The key is most likely to fail during the initial "hammer blow"of force in the opening direction. However, if the key were to fail in theclosing direction, it would fail when the valve disc was seated but not fullytorqued shut. Nevertheless, the full seating force on this valve would notbe required for the valve to perform its safety function of diverting flowfrom the cold leg to the hot leg. Further, this safety function does notrequire the valve to open and close repeatedly.

Metallurgical analysis of the key indicates that'the key failed due to shearoverload. Since the key did not fail due to fatigue, additional cycling ofthe valve would not have sheared the key unless excessive force was appliedto the key.

1031S/85K

4 ~ % 4 ~ ~

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION

FACILTTY NAME (1) DOCKEt NIM8ER (2) LER NUM8ER 6SCOVOIIIAL;ii~~ l%YSON

NVV8OI;":.'I; NIAISBI

PAGE 3

DIABLO CANYON UNIT 1

TEXT (17)05 0 0 0 2 7 5 91 021 0 0 6 0F 7

Since the key is expected-to fail when operating the valve from theclosed-to-open direction only during excessive force conditions, the keywould not be expected to shear during operation under accident conditions inthe open-to-closed direction and therefore would perform its intended safetyfunction. Therefore, the failure of the key in valve SI-2-8809B did notadversely affect the health and safety of the public.

SI-2-8808B and SI-2-8808D:

Valves SI-2-8808B and SI-2-8808D are the= accumulator discharge isolationvalves for accumulators 2-2 and 2-4. These valves are normally open, motoroperated valves. During normal operation, power to the motor operator isremoved to prevent inadvertent closure of th'e valves. The safety function'ofthe valves is to be open to allow the accumulators to discharge into the RCSif the RCS pressure drops below approximately 600 psia. Closure of thesevalves is not a required safety function.

Although the keys on valves SI-2-8808B and SI-2-8808D were found to besheared, the valves were still capable of stroking. The valves had beensuccessfully stroked during their last stroke test on March 22, 1991.Inspections of the motor shaft and pinion key assemblies showed that whilethe keys had indeed failed, there was mechanical interference between themotor drive shaft and the pinion gear that stopped the shaft from slipping inthe pinion gear.

Because valves SI-2-8808B and SI-2-8808D were in the proper position toperform their safety function and were capable of being stroked, the functionof the valves was not impaired. In addition, failure of the keys would nothave caused the valves to close. Therefore, the failure of the keys invalves SI-2-8808B and SI-2-8808D did not adversely affect the health andsafety of the public.

V. Corrective Actions

A. Immediate Corrective Actions:

1. Following the discovery of the SI-2-8809B key failure, the othersafety-related Unit 2 SMB-3-80 motor operators were inspected todetermine if a similar problem existed. As discussed above,SI-2-8808B and SI-2-8808D were found to have similar sheared keys.The failed components were replaced, .and no other key. failures werediscovered on Unit 2.

B. Corrective Actions to Prevent Recurrence:

1. The motor pinion keys on the Unit 1. high speed SMB-3 80 ft-lb motoroperators (i.e., those on the 8808 and 8809 valves) will bereplaced with 4140 steel during the next refueling outage.

1031S/85K

tJ

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION

FACILITY HAHE (I) DOCKET HERE8ER (2)YEAR

LER HUH8ER 6SEEWWRAL v:«: l%VNOR

PAGE 3

DIABLO CANYON UNIT 1

TEXT (17)

05 0 0 0 2 7 5 91 021- 0 0 7 '

2. Operating procedures will be revised to prevent valve shortstroking.

3. Maintenance procedures will be revised to prevent valve shortstroking.

VI. dditional Information

A. Failed Components:

Limitorque motor operator model SMB-3-80 failed.

B. Previous Similar LERs:

None.

103IS/85K