ML17228A185

LER 93-007-00:on 930521,observed Indication of Multiple Dropped CEA on Core Mimic Display Resulting to Manual Reactor Trip.Caused by Equipment Failure.Llrt Conducted to Verify Containment Vessel integrity.W/930621 Ltr

ML17228A185
Person / Time
Site:	Saint Lucie
Issue date:	06/21/1993
From:	Sager D, Galen Smith FLORIDA POWER & LIGHT CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
L-93-160, LER-93-007, LER-93-7, NUDOCS 9306290152
Download: ML17228A185 (7)

Text

ACCELERA D DOCU1VIENT DISTRIBUTION SYSTEM REGUD RY INFORMATION DISTRIBUT . SYSTEM (RIDE)

ACCESSION NBR:9306290152 DOC.DATE: 93/06/21 NOTARIZED: NO DOCKET FACIL:50-389 St. Lucie Plant, Unit 2, Florida Power & Light Co. 05000389 AUTH. NAME AUTHOR AFFILIATION SMITH,G.D. Florida Power & Light Co.

SAGER,D.A. Florida Power & Light Co.

RECIP.NAME RECIPIENT AFFILIATION

SUBJECT:

LER 93-007-00:on 930521,observed indication of multiple dropped CEA on core mimic display resulting to manual reactor trip. Caused by equipment failure.LLRT conducted to verify containment vessel integrity.W/930621 ltr.

DISTRIBUTION CODE: IE22T COPIES RECEIVED:LTR ENCL SIZE:

TITLE: 50.73/50.9 Licensee Event Report (LER), Incident Rpt, etc.

NOTES:

RECIPIENT COPIES RECIPIENT COPIES ID CODE/NAME LTTR ENCL ID CODE/NAME LTTR ENCL PD2-2 LA 1 1 PD2-2 PD 1 1 NORRIS F J 1 1 INTERNAL: ACNW 2 2 ACRS 2' AEOD/DOA 1 1 AEOD/DSP/TPAB 1 1 AEOD/ROAB/DSP 2 2 NRR/DE/EELB 1 1 NRR/DE/EMEB 1 1 NRR/DORS/OEAB 1 1 NRR/DRCH/HHFB 1 1 NRR/DRCH/HICB 1 1 NRR/DRCH/HOLB 1 1 NRR/DRIL/RPEB 1 1 NRR/DRSS/PRPB 2 2 NRR~ SA/SPLB 1 1 NRR/DSSA/SRXB 1 1 EG FIL 02 1 1 RES/DSIR/EIB 1 1 RGN2 FILE 01 1 1 EXTERNAL 'G&G BRYCE F J ~ H 2 2 L ST LOBBY WARD 1 1 NRC PDR 1 1 NSIC MURPHY,G.A 1 1 NSIC POOREFW. 1 1 NUDOCS FULL TXT 1 1 NOTE TO ALL'RIDS" RECIPIENTS:

PLEASE HELP US TO REDUCE WASTEl CONTACI'HE DOCUMENT CONTROL DESK, ROOM Pl-37 (EXT. 504-2065) TO EUMINATBYOUR NAME FROM DISTIUBUTION LISTS FOR DOCUMENTS YOU DON'T NEEDl FULL TEXT CONVERSION REQUIRED TOTAL NUMBER OF COPIES REQUIRED: LTTR 32 ENCL 32

P.O. Box 128, Ft. Pierce, FL 34954.0128 I

June 21, 1993 L-93-160 FPL 10 CFR 50.73 U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D. C. 20555 Re: St. Lucie Unit 2 Docket No. 50-389 Reportable Event: 93-07 Date of Event: May 21, 1993 Manual Reactor Trip After the Simultaneous Dropping of Control Element Assemblies due to E ui ment Failure The attached Licensee Event Report is being submitted pursuant to the requirements of 10 CFR 50.73 to provide notification of the subject event.

Very truly yours, D. A. ger Vice r sident St. L e Plant DAS/JJB/kw Attachment cc: Stewart D. Ebneter, Regional Administrator, USNRC Region Senior Resident Inspector, USNRC, St. Lucie Plant II DAS/PSL 8938-93 9306290152 930621

~ 5giO y(pP PDR ADOCK 05000389 S PDR an FPL Group company

F PL Fscsirrils of U.S. NUCLEAR REGULATORY COMMISSION APPITOYEOOIAT NO STICSIIA trrprpts.AO00 0 NRC Form SSS t ST IAATE0 duret N Pt N IESPONSE TO CCAAPTY YIITH 0 ISS 00 OISAATON CCUE CION LICENSE%VENT REPORT (LER) INCAN ST: $ 0 0 TITS IaTIIAIS0COAANNTS IE ONEANO TAAIEN ESTINATE TO TIE IE CaeS AIO INParr 0 IAANACENENT IPIAHCHIPSSCA IAS, IWQE NT IE OSATOTY OOANASOat YIAONIOIOI.OC $ 0ISE AtD TO THE PAPEITN0$ TA IEOVCTTTN PIYAECT Oi tc oioI0 ar a ax NANACEAENTN0$ Ttoctt, TNASNIOT0$0 oc PINCE FACILITYNAME (1) DOCKET NUMBER (2) PAGE 3 4 St. Lucie Unit 2 050003891 0 5 E( ) Manual Reactor Trip After the Simultaneous Dropping of Control Element Assemblies due to Equipment Failure EVENT DATE (5) LER NUMBER (6) REPORT DATE (7) OTHER FACILITIES INVOLVED(8)

MONTH DAY YEAR YEAR S IAL DAY YEAR FACILITYNAMES DOCKET NUMBER(S)

N/A 05 000 0 5 2 1 9 3 9 3 0 0 7 0 0 0 6 2 1 9 3 N/A 05000 THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR OPERATING Check one or more of the followin (11)

MODE (9) 20.402(b) 20.405(c) X 50.73(a)(2)(iv) 73.71(b)

POWER 20.405(a)(1)(i) 50.36(c)(1) 50.73(a)(2)(v) 73.71(c)

LEVEL (10) 0 7 2 20.405(a)(1)(ii) 50.36(c)(2) 50.73(a)(2)(vii) 0'iHER (Specify in Abstract 20.405(a)(1)(iii) 50.73(a)(2)(i) 50.73(a)(2)(viii)(A) below and in Text 20.405(a)(1)(iv) 50.73(a)(2)(viii)(B) NRC Form 366A) 50.73(a)(2)(ii) 20.405(a)(1)(v) 50.73(a)(2)(iii) 50.73(a)(2)(x)

LICENSEE CONTACT FOR THIS LER 12 NAME TELEP ONE NUMBER AREACODE Galen D. Smith, Shift Technical Advisor 4 0 7 4 6 5 - 3 5 5 0 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT 13 MANUFAC- REPORTABLE MANUFAC- REPORTABLE CAUSE SYSTEM COMPONENT TURER TO NPRDS CAUSE SYSTEM COMPONFNT TO NPRDS A A C 0 N C 5 1 5 I I I SUPPLEMENTAL REPORT EXPECTED 14 EXPECTED MONTH DAY YEAR X YES (Ifyes, complete EXPECTED SUBMISSION DATE)

SUBMISSION DATE (15) 053094 ABSTRACT (Limit to 1400 spaces.i.e. approximately fifteen single-space typewritten lines) (16)

On May 21, 1993, St. Lucie Unit 2 was manually tripped from 72% power by utility-licensed operators approximately ten seconds after seven Control Element Assemblies (CEA's) fully inserted into the core. The dropped CEAs were confirmed by the presence of rod bottom lights, CEA reed switch position transmitter indication, and decreasing reactor coolant system average coolant temperature. Standard Post Trip Actions were carried out and the unit was placed in a stable condition in Mode 3.

Initial investigation revealed that several CEA power supply breakers were open and/or fuses blown which was indicative of an electrical fault. Subsequently, two grounded CEA cables associated with CEA's 54 and 61 were discovered in electrical penetration D-1 to the containment shield building. The root cause of the electrical grounds will be investigated during the next Unit 2 refueling outage.

Corrective Actions for this event: 1) Isolated grounded conductors in electrical penetration D-1 and relanded affected CEA cables to available spare penetration modules in penetration D-1, 2) Replaced the subgroup breaker associated with subgroup sixteen and individual disconnect breakers for CEA's 8, 54, 60, and 61, 3)

Performed an Engineering Evaluation on multiple CEA drops and effects on Departure from Nucleate Boiling Ratio and Local Power Density which concluded that no Specified Acceptable Fuel Design Limits (SAFDL) were exceeded, 4) During the next Unit 2 refueling outage, penetration D-1 will be inspected, 5) A local leak rate test on penetration D-1 was performed with satisfactory results, and 6) The addition of a ground detection circuit to the Control Element Drive Mechanism Control System as a system enhancement is being evaluated.

FPL Facsimile of NRC Form 366 (6-89)

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FPL Fscsimfo oI U.S. NUCLEAR REGUlATORY COMMISSION CA@I NIL O'IT0010I PPTSPfroAPPLES:

NRC Form 366 f VNRT

~ t6 fsf LICENSEE EVEN PORT (LER) 0 ST 0 TAPNRN PT flfNSPONSS TO CTSNTT PATN TITS SPOfSAAllON CCATTCTCN INOIRSl; 00 0 IPTS tORNAICICCANRIR0 INCANONS TAPSNN 0 STIR ATE TO TIN INSORTS INST INACR I0 VANACTIR Nl SRANCH lP TATA V S IAICLSARINCASATORT TEXT CONTINUATION TI ATINICT CN . OC PTS NA AfffTO TIN PAPT PNITPN PS OVCTICPI PROITCT I PI To 0I PIT of fICE Of NANACTASNl AN) TAOCTT,TPATPNICTTPA OC PORTA FACILITYNAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)

YEAR EOUENTIAL :j REVISION NUMBER '",.; NUMBER St. Lucie Unit 2 0500038993 0 0 7 0 0 0 2 0 5 TEXT (Ifmore spaceis required, use additional NRC Form 366A's) (>7)

F EVE On May 21, 1993, while Llnit 2 was at 72'/o power steady state operation, control room operators observed the indication of multiple dropped Control Element Assemblies (CEA's) (EIIS: AA) on the Core Mimic Display and the CEA Display Panel accompanied by the simultaneous annunciation of alarm K-23 "CEDMCS (Control Element Drive Mechanism Control System) (EIIS: AA) Trouble".

Subsequently, a decease in Tave was observed. The Digital Data Processing System indicated that the seven dropped CEAs were numbers 8, 54, 60, 61, 53, 65 and 67. Approximately 10 seconds after the CEA's dropped, operators manually tripped the reactor and the turbine in accordance with the immediate operator actions of the CEA Off-Normal procedure, ONOP 2-0110030. Emergency Operating Procedure (EOP) One, "Standard Post Trip Actions",was implemented immediately post-trip. The Steam Bypass Control System (EIIS: Jl) operated properly to reduce Tave to 532 degrees Fahrenheit. The Steam Generators (SG's) (EIIS: AB) were supplied post trip via the Steam Generator Feed Pumps (SGFP) (EIIS: SJ) and the 15/o feedwater bypass valve. After the successful implementation of standard post trip actions by two Reactor Control Operators, an uncomplicated reactor trip was diagnosed and the Senior Reactor Operator directed the crew to exit EOP-1 and enter EOP-2, "Reactor Trip Recovery". After the completion of EOP-2, the unit was maintained in Mode 3 for the post trip review and event investigation. Subsequent examination of the CEDMCS revealed the following conditions for the seven dropped CEA's: (See Figure One for an abbreviated schematic of the CEA power system)

- CEA 8, Regulating Group 5 - disconnect breaker was not tripped, no Subgroup fuses blown.

- CEA 54, Shutdown Group A - disconnect breaker was tripped.

- CEA 60, Regulating Group 3 (Subgroup 15) - disconnect breaker not tripped, no Supgroup fuses blown.

- CEA 61, Regulating Group 3 (Subgroup 16) - A8 B phases of disconnect breaker tripped, 2 Subgroup fuses blown.

- CEA 63, Regulating Group 3 (Subgroup 16) - disconnect breaker closed, 2 Subgroup fuses blown.

- CEA 65, Regulating Group 3 (Subgroup 16) - disconnect breaker closed, 2 Subgroup fuses blown.

- CEA 67, Regulating Group 3 (Subgroup 16) - disconnect breaker closed, 2 Subgroup fuses blown.

VEN The CEA power distribution system utilizes a 240 VAC three phase system with an ungrounded neutral line. This three phase AC is then rectified and conditioned by the CEDMCS to a nominal 50 VDC which is then supplied to each Control Element Drive Mechanism (CEDM) (EIIS: AA) coil stack inside of containment. During steady state conditions, the control room CEDMCS operating panel is switched to "Off", as its was during this event. This results in only the Upper Gripper Coil of each CEA being energized. If the Automatic CEDM Timer Module (ACTM) for a CEA reads an abnormal current condition for the Upper Gripper coil, the CEA will be maintained withdrawn by the ACTM energizing the Lower Gripper Coil. Under normal conditions, this DC power arrangement with an ungrounded neutral line is tolerant of at least one conductor short to ground. Furthermore, as long as the conductor grounds are of the same polarity, multiple conductor grounds on different coil stacks may not affect the system's operability and may go undetected. When a second conductor ground develops on a conductor of different polarity a short circuit develops which may cause CEDM coils to deenergize.

FPL Facsimile of NRC Form 366 (6-89)

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FPL FacslrTT$ 4 Dr U.S. NUCLEAR REGULATORY COMMrSSION AATRDITDCASTNCk$ 1$ 04ISI NRC Forrrr 6M SSh%$ : ID44t (6 69I LICENSEE EVENT REPORT (LER) I T4 THATTD DIIHTTN AT R KSAOHTS 0 CTHHkT THTH THS NSOTRIATTON CTSISCTlON I%CANST: $ 0 ~ I44$ fORWAKICCARNNTS fHCARDHC rRTITN TSTTHATT TO TIN TH CCITTS HO RT ACRT 4 HAHAIHIC NT SRAHCH rA ITCI IIS IlfrklAR RT OIAATOIW TEXT CONTINUATION WASINICTON, DC TOIHk AAATTO TIN TAATRWCTW INDUCTION TRCIS CT r SI $ 44 I ffkCf fHS Cf HANACTHT NT ANT SIDOT T, WAIHHOTON OC TIHTk FACILITYNAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)

YEAR kgb EQUENTIAL REVISION St. Lucie Unit 2 NUMBER NUMBER 0 500 0389 9 3 0 0 7 0 0 0 3 0 5 TEXT (Ifmore spaceis required, use additional NRC Form 366A's) (17)

EVEN The most probable cause of the drop of seven CEAs is attributable to grounds in the 50 VDC CEDM power system located in an electrical penetration. Testing by the l&C Department revealed that five grounds were present in the Shield Building side of electrical penetration D-1. These conductor to ground shorts resulted in an overcurrent condition that opened the disconnect breaker for CEA 54, several contacts in the disconnect breaker for CEA 61, and/he 2 fuses blown on subgroup 16.

The two CEAs which did not observe a condition that would permanently interrupt power to the Upper Gripper and Lower Gripper Coils were CEAs 8 and 60. (CEDM 60 was found to have a ground on the neutral phase of its load transfer coil; this condition alone would not cause the CEA to drop.

CEDM 8 had no faults identified). The most likely cause for CEA's 8 and 60 to drop during this event was due to a transient induced on the CEDMCS 240 VAC power system when the conductor to ground faults developed. Output monitoring of the 2A Motor Generator (MG) set with a temporary power line monitor during testing of CEA 61 prior to the repairs showed that the grounds were reflected back throughout the CEDMC power system. This disturbance probably resulted in the dropping of CEA's 8 and 60.

The root cause of the conductor grounds in electrical penetration D-1 could not be determined immediately after the event since inspection of the penetration would require electrical penetration disassembly. Comprehensive testing of the CEDMCS, CEDM conductors, and the containment penetration provided a high degree of confidence in the operability of the systems required to support unit restart. A detailed analysis of the conductor grounding will be performed at the next refueling outage.

NAL VEN This event is reportable pursuant to 10CFR50.73(a)(2)(iv) as a manual actuation of the Reactor Protection System (RPS) (EIIS: JE). Having seven CEA's fully insert into the core during power operation is not specifically analyzed in the St. Lucie Unit 2 Final Updated Safety Analysis Report (FUSAR). Section 15.4.2.3.8 of the FUSAR analyzes a single dropped CEA as well as a dropped CEA subgroup. For a CEA subgroup insertion from 100/o power, a calculated minimum Departure from Nucleate Boiling Ratio (DNBR) of 1.28 is reached in approximately 4 minutes with no operator action.

The Nuclear Engineering Department analyzed the seven dropped CEA's scenario and concluded that no DNBR or Fuel Design Limits were exceeded during this event. This evaluation was conservative in that it did not credit the immediate manual trip inserted by the operators. Additionally, no incore neutron detector alarms were received prior to the manual reactor trip; providing further assurance of remaining within core design limitations. The reactor trip was observed to be a routine manual trip. The resulting plant transient was well enveloped by the St. Lucie Unit 2 FUSAR.

Visual inspection and testing of electrical penetration D-1 did not reveal any indication of containment integrity being affected by the conductor grounds. Testing indicated that the faults were in a cable splice in the shield building penetration, several feet removed from the containment penetration. A precautionary local leak rate test was performed on penetration D-1 with satisfactory results, thus assuring that containment vessel integrity was not affected during this event.

FPL Facsimile of NRC Form 366 (6-89)

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FPL Focslrtito OI U.S. NUCLEAR REGULATORY COMMISSION ~PtAOVT0 (RDI NO. SI 5005 OI NRC Form 868 IS.IOI fSPUN X AO005 LICENSEE EVENT REPORT(LER) f0 TAIAT5 0 CURST N Pf R INSPORTS 5 0 CIRPT T IR IN TITS PPTDNAARCN Cot tfCIION INOUfSTIIODIPIS toRWNOCONNTNTSINCARTANOOJRXN fSTINATS Toll@ INCCSRS TEXT CONTINUAT1ON If NO IN PORTS NANAIX Nl SRANCH TP 500A U 0 NUCtfNt IXOIAATCRT TIATtANCION,OCT055S NAI TOTIC PAPTIINOIPIIROUCTONPROJTCTTSIIO\NNTOIIICf OI NANACfNfNT NAT TAAICf5, WAN DNC TCPA OC TIROS FACILITYNAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) 5IEAR

"'N EQUENTIAL 4 RFVISION St. Lucie Unit 2 NUMBER $ NUMBER 0 500 0389 9 3 0 0 7 0 0 0 4 0 5 TEXT (Ifmore spaceis required, use additional NRC Form 366A's) (17)

1) All CEDM cables were meggered through their respective penetrations which identified the five (three on CEA 54, one on CEA 61, and one on CEA 60) ground faults in penetration D-1 and to confirm that no other CEDM penetrations were affected.

2) Spare conductors in modules 20 and 24 of penetration D-1 were meggered to ensure that no faults existed prior to placing those spares in service. The grounded conductors in modules 11 and 17 of penetration D-1 were isolated and the affected CEA cables were reterminated to leads on the five available spares in modules 20 and 24.

3) Pin to pin meggering was performed on the conductors in modules 11 and 17 of penetration D-1.

This was to confirm that there were no conductor to conductor shorts within those modules for the conductors which were kept in service.

4) CEA Subgroup 16 breaker and the individual CEA disconnect breakers associated with CEAs 8, 54, 60, and 61 were replaced.

5) Using a power line condition monitor with the trip circuit breakers closed, satisfactory CEDM motor generator bus phase to phase and phase to voltage traces were recorded and reviewed to ensure that no faults existed on that power supply.

6) Nuclear Engineering has performed an analysis of multiple rod drops as experienced by this event to confirm that DNBR or Local Power Density safety limits were not violated.

7) To confirm the integrity of the fuel, iodine levels in the RCS were monitored by Reactor Engineering and compared with previous values. No abnormal levels were noted.

8) During the next Unit 2 refueling outage, electrical penetration D-1 will be inspected by Electrical Maintenance to determine the root cause of the electrical grounds.

9) A satisfactory Local Leak Rate test on penetration D-1 verified containment vessel integrity.

10) The addition of a ground detection circuit to the Control Element Drive Mechanism Control System as a system enhancement is being evaluated.

D L IN Component: Low Voltage Electrical Penetration Manufacturer: CONAX Corporation Part Number: 7310-10004-07 rvi imilr v n See LER ¹389-89-007 and LER ¹335-80-050 (manual reactor trips due to multiple dropped CEA's)

FPL Facsimile of NRC Form 366 (6-89)

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FPL FacsNTTIC ot U.S. NUCLEAR REGULATORY COMMISSION ~ ftfttftCESO Nrt SI ICCI OI NRC Form 366 tftNNaIC44t I6.69)

LICENSEE EVENT REPORT (LER) t4TIAATtCIASICINttNINSTONTE TOCOANATTNTNTITS44ISNAATIONCCAIECTQN SEISEST: SO 4 Ifra IONNAKICONNE NI4 INCANONAT TASTXN 4 SNNATE TO TIN INCOENI NO IEIESTT 4 NANACE IE NT CNANCN IS ISa ua NNE SAN INIAAATTSW TEXT CONTINUATION WAN SIC TON, CC t00ta NO TO TIE 0AIENWQTN ICCCCTON ICONCT ISI f04 >INIOf IICE Cf NANACENt NI NO OSTCET. WANANCTISAOC TTSITA FACILITYNAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)

YEAR EQUENTIAL REVISION N NUMBER NUMBER St. Lucie Unit 2 0 500 0389 9 3 0 0 7 0 0 0 5 0 5 TEXT (Ifmore spaceis required, use additional NRC Form 366A's) (17) 0 QM3 QMS O. I 3 MG output I I disconnect breakers 240 ( 3 Trip Circuit VAC I I Breakers 3 phase CEDMCS Bus CEA subgroup fuses Sub Grp 2

( Sub Grp 12

( Sub Grp 15

( Sub Grp 16 CEA subgroup breakers CEA 0 O ~ O disconnect breakers 38 38 38 38 SCR power power power power switches switches switches assemblies switches Containment B1 D1 D1 D1 9 D6 Electrical Penetrations CEA CEA CEA CEA CEA CEA CEA 8 54 60 61 63 65 67 coils coils coils coils coils coils coils FIGURE ONE - CEA POWER DISTRIBUTION FPL Facsimile of NRC Form 366 (6-69)