05000296/LER-1996-004, :on 960717,loss of ECCS Division I & Division II Instrumentation Renders ECCS Equipment Inoperable.Caused by Loss of Inverter Output.Failed Components Replaced in Atu Inverter Circuitry & Instrumentation Logic Restored
| ML18038B870 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 04/29/1997 |
| From: | Jay Wallace TENNESSEE VALLEY AUTHORITY |
| To: | |
| Shared Package | |
| ML18038B869 | List:
|
| References | |
| LER-96-004, LER-96-4, NUDOCS 9705060101 | |
| Download: ML18038B870 (16) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2) 10 CFR 50.73(a)(2)(ii) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2961996004R00 - NRC Website | |
text
NRC FORM 366 (4-98) u.s. Nuct.EAR REeuLATQR Y coMMissioN LICENSEE EVENT REPORT'LER)
(See reverse for required number of digits/characters for each block)
APPROVED BY OMB No. 3150-0104 EXPIRES 04/30/96 ESTIMATED BURDEN PER RESPONSE TO COMPLY WITHTHIS MANDATORY INFORMATION COLLECTION REQUEST: 60.0 HRS.
REPORTED LESSONS LEARNED ARE INCORPORATED INTO THE LICENSING PROCESS AND FED BACK TO INDUSTRY.
FORWARD COMMENTS REGARDING BURDEN ESTIMATE TO T?IE INFORMATION AND RECORDS MANAGEMENT BRANCH (T-6 F33).
U.S.
NUCLEAR REGULATORY COMMISSION, WASHINGTON, DC 20868 FACIUTYNAMEIll Browns Ferry Nuclear Plant (BFN) Unit 3 oocKET NUMBER (21 05000296 PAOE ISl 1 OF 8 TITLE(4I Loss of the Emergency Core Cooling Systems (ECCS) Division I and Division II Instrumentation Renders ECCS Equipment Inoperable.
EVENT DATE (5)
LER NUMBER (6)
REPORT DATE (7)
OTHER FACILITIES INVOLVED(6)
MONTH DAY 07 17 96 SEQUENTIAL REVISION NUMBER NUMBER 96 004
02 MONTH DAY YEAR 04 29 97 FACILITYNAME N/A FACIUTYNAME N/A DOCKET NUMBER DOCKET NUMBER OPERATING MODE (9)
POWER LEVEL (10)
N 100
- 20. 2201 (b) 20.2203(a)(1)
- 20. 2203(a) (2) (I) 20.2203(a) (2) (ii) 20.2203(a) (2) (iii) 20.2203(a)(2) (v) 20.2203(a) (3) (I) 20.2203(a) (3)(ii) 20.2203(a)(4) 50.36(c) (1)
X 50.73(a)(2) 6)(B) 50.73(a)(2)(ii) 50.73(a) (2) (RI) 50,73(a) (2) (iv)
- 50. 73(a) (2) (v) 60.73(a) (2) (viii)
- 50. 73(a) (2)(x) 73.71 OTHER
~
Specify in Abstract'below r moro) (11)
UANT TO THE REQUIREMENTS OF 10 CFR 6: (Check one o THIS REPORT IS SUBMITTED PURS 20.2203(a) (2) (iv) 50.36(c) (2)
LICENSEE CONTACT FOR THIS LER (12)
X 50.73(a)(2)(vii) or In NRC Form 366A NAME TELEPHONE NUMBER Irrrclrare Area Code)
James E. Wallace, Licensing Engineer (205) 729-7874 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT (13)
CAUSE
SYSTEM COMPONENT MANUFACTURER REPORTABLE TO NPRDS
CAUSE
SYSTEM COMPONENT MANUFACTURER TO NPRDS X"
SUPPLEMENTAL REPORT EXPECTED (14)
YES (If yes, complete EXPECTED SUBMISSION DATE).
X No EXPEGTED SUBMISSION DATE (15)
MONTH DAY YEAR ABSTRACT (Limitto 1400 spaces, i.e., approximately 15 single-spaced typewritten lines) (16)
On July 17, 1996, with Units 2 and 3 at approximately 100 percent power and Unit 1 shutdown and defueled, Unit 3 operators declared ECCS Division I instrumentation inoperable, in accordance with TS 3.2.B.
This event was presumed'to be caused by a failed control board in the ECCS Division I Analog Trip Unit Inverter, which resulted in the loss of the inverter output.
The failed components were replaced, the ECCS instrumentation was declared operable.
On August 6, 1996, a second similar event occurred on the Division I instrumentation.
On November 6, 1996, a third similar.event,(LER 50-296/S6006) occurred on the Division I instrumentation, and on December 17, 1996, a fourth similar event (LER 50-296/96008) occurred on the Division II instrumentation.
After much testing/investigating, two root causes were identified:
(1) shorted silicone-controlled rectifiers and (2) a shorted commutation capacitor.
The Corrective action to minimize the impact for any future inverter failures was to install an alternate power supply to each inverter cabinet independent of the cabinet's associated ECCS ATU inverter.
This report is being submitted in accordance with 10 CFR 50.73 (a)(2)(i)(B) for a condition prohibited by Technical Specifications and 10 CFR 50.73 (a)(2)(vii) as any event where a single cause or condition caused at least one independent train or channel to become inoperable in multiple systems designed to remove residual heat or mitigate the consequences of an accident.
LERs on similar events were: 260/94001, 260/94006, 260/S4010, 2S6/96006 and 296/96008.
9705060iOi 970429 PDR ADQCK 050002'P6 8
II
NRC FORM 3CCA(445)'.S.
NUCLEAR REOULATORY COIVMSSION LICENSEE EVENT REPORT (LER)
TEXT CONTINUATION FACILITY NAME Browns Ferry Unit 3 05000296 LER NUMBER NUMBER NUMBER 2 of 8 96 004 02 moto space
>s requir, use a ooa copes orm (1 )
Z.
PLANT CONDZTZONS zz.
At the time of the initial discovery on July 17,1996, Units 2 and 3
were operating at approximately 100 percent power.
Unit 1 was shutdown and defueled.
At the time of a second event on August 6,
1996, Unit 2 aud Unit 3 were operating at approximately 36 percent power and 100 percent power, respectively and Unit 1 remained defueled.
[See LERs 50-296/96006 and 50-296/96008 for the Plane Conditions for the third and fourth events]
DESCRZPTZON OF EVENT Event INITIALEVENT On July 17, 1996, at 1300 hours0.015 days <br />0.361 hours <br />0.00215 weeks <br />4.9465e-4 months <br /> Central Daylight Time (CDT), the Unit 3, Division I Emergency Core Cooling Systems (ECCS) Analog Trip Unit (ATU) inverter power was lost as a zesult of a shorted Silicon Control Rectifiez (SCR) that cleared a fuse.
This failure affected two of the four channels of the drywell pressure and reactor water level sensors:.
These sensors feed both divisions of initiation logic for the ECCS (Residual heat removal (RHR)
[BO] system, core spray (CS)
[BG] system, high pressure coze injection (HPCI)
[BJ] system, automatic depressuzization systems (ADS)[JC], Anticipated Transient Without Scram (ATWS)
[JC], and the Unit 3 diesel generators (EDG)
[EK]). Additionally the failure affected the Reactor Coze Injection Cooling (RCIC)
[JN] system.
The above ECCS and RCIC were declared inoperable due to the loss of control power as delineated in Technical Specifications (TS)
Table 3.2.B.
Since the ADS was declared inoperable, the plant was placed in limiting condition for operations (LCO) requiring the unit to be in hot shutdown within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.
Additionally TS 3.5.A.3 and 3.5.B.8 for RHR and CS require the unit to be in cold shutdown within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
These were the most restrictive LCOs.
As a result of these LCOs, power reduction was initiated on Unit 3 at 1700 hours0.0197 days <br />0.472 hours <br />0.00281 weeks <br />6.4685e-4 months <br />.
At 1757 hours0.0203 days <br />0.488 hours <br />0.00291 weeks <br />6.685385e-4 months <br />, the ECCS ATU Invezter was repaired by the replacement of a cleared
- fuse, a shorted
- SCR, and removed an ATU Inverter control board.
At this time, the LCOs were exited.
It should be noted that HPCI was out of service for the first two and a half hours because of scheduled maintenance.
SECOND EVENT On August 6,
- 1996, a second similar event occurred at 2041 hours0.0236 days <br />0.567 hours <br />0.00337 weeks <br />7.766005e-4 months <br /> Central Daylight Time (CDT), the Unit 3, Division I ECCS ATU inverter power was lost again as a result of a shorted SCR that NRC FORM 368A (495)
0 1
NRC FORM3CCA'495)'.S.
NUCLEAR REOULATORYCOIVMSSION LICENSEE EVENT, REPORT (LER)
TEXT CONTINUATION FACILITY NAME Bzowns Ferry Unit 3 05000296 NUMBER NUMBER 96 --
004 02 3 of 8 X i more space rs reqwr, use s ons copes orm (17) cleared a fuse.
This failure also affected two of the four channels of the drywell pressure and reactor water level sensors.
These sensors feed both divisions of initiation logic for the ECCS (RHR [BO] system, CS [BG] system,
ATWS [JC], and the Unit 3 EDG [EK]). Additionally the failure affected the RCIC [JN] system.
The above
.ECCS and RCIC were declared inoperable due to the loss of logic inputs as delineated in Technical Specifications (TS)
Table 3.2.B.
Since the ADS was de"lared inoperable, the plant was placed in limiting condition for operations (LCO) requiring the unit to be in hot shutdown within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.
Additionally, TS 3.5.A.3 and 3.5.B.8 for RHR and CS require the unit to be in,cold shutdown within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
At 2200 hours0.0255 days <br />0.611 hours <br />0.00364 weeks <br />8.371e-4 months <br />, Maintenance and Technical Support personnel [utility, nonlicensed]
commenced troubleshooting the ECCS Division I ATU inverter.
At 0256 hours0.00296 days <br />0.0711 hours <br />4.232804e-4 weeks <br />9.7408e-5 months <br /> on August 7r 1996r the ATU Inverter was declared operable after the replacement of a fuse, a
- SCR, and an ATU Inverter control board.
At this time, the LCOs were exited.
THIRD AND FOURTH EVENTS
~ r [See LER 50-296/96006 and 96008 for Description of the Events.]
REPORTING REQUIREMENTS
BFN notified the NRC Operations Center of a one-hour non-emergency report due to the initiation of the plant shutdown per TSs in accordance with 10 CFR 50.72(b) (l),(i) (A).
For the second
- event, the one-hour reporting requirement was not applicable because the ECCS ATU inverter was repaired before the power reduction commenced.
Therefore, a four-hour non-emergency report to the NRC was made due to a condition that alone could have prevented the fulfillment of the safety function of systems that are needed to zemove residual heat or mitigate the consequences of an accident in accordance with 10 CFR 50.72(b)(2)(iii).
These events were submitted in accordance with 10. CFR 50.73 (a),(2)(i)(B) for a condition prohibited by Technical Specifications and 10 CFR 50.73 (a)(2)(vii) as events where a
single cause or condition caused at least one independent train or channel to become inoperable in multiple systems designed to emove residual heat or mitigate the consequences of an accident.
B.
Ino erable Structures, Co onentsr or S stems that Contributed to e Event:
None.
NRC FORM 368A (445)
0
. NRC FORM 366A" (44I5)
U.S. NUCLEAR REOULATORYCOIVIVISSION LICENSEE EVENT REPORT (LER)
TEXT CONTINUATION PACZLZTY MAME Browns Ferry Unit 3 05000296 NUMBER 96 004 02 4 of 8 E
more space rs requir
~ use s ons copes orlil 1 )
C.
Dates and A roximate Times of Ma or Occurrences:
INITIALEVENT July 17, 1996, at 1300 hours0.015 days <br />0.361 hours <br />0.00215 weeks <br />4.9465e-4 months <br />, CDT 1700 hours0.0197 days <br />0.472 hours <br />0.00281 weeks <br />6.4685e-4 months <br />, CDT 1747 hours0.0202 days <br />0.485 hours <br />0.00289 weeks <br />6.647335e-4 months <br />, CDT 1757 hours0.0203 days <br />0.488 hours <br />0.00291 weeks <br />6.685385e-4 months <br />, CDT Unit 3 ECCS ATU Inverter power was lost.
Unit 3 power reduction was initiated.
One-hour non-emergency report to the NRC was made due to the initiation of the plant shutdown pez TSs in accozdance with 10 CFR 50.72(b)(1)(i)(A).
The ECCS ATU Inverter was repaired, and LCOs were exited.
SECOND EVENT
~ f August 6,
- 1996, 2041 hours0.0236 days <br />0.567 hours <br />0.00337 weeks <br />7.766005e-4 months <br />, CDT 2200 hours0.0255 days <br />0.611 hours <br />0.00364 weeks <br />8.371e-4 months <br />, CDT August 7 r 1 996 r 0037 hours4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br />
Troubleshooting commenced.
A four-hour non-emergency report to the NRC was made due to a condition that alone could have prevented the fulfillment of the safety function of systems that are needed to remove residual heat or mitigate the consequences of an accident in accordance with 10 CFR 50.72(b)(2)(iii).
0256 hours0.00296 days <br />0.0711 hours <br />4.232804e-4 weeks <br />9.7408e-5 months <br />, CDT The ECCS ATU Inverter was repaired, and LCOs were exited.
THIRD AND FOURTH EVENTS [See LER 50-296/96006 and 96008 for Major Occurrences.]
D.
Other 8 stems or Seconda Functions Affected:
No other systems or secondary functions were affected in the four events.
Method of Discove The first two events were promptly discovered when control zoom al'arms indicated ECCS Division I instrumentation problems.
[See LER 50-296/96006 and 96008 for Method of Discovery for third and 4l
~I
, NRC FORM 366A" (485)
U.S. NUCLEAR REOULATQRYCOM4SSION LICENSEE EVENT'EPORT (LER)
TEXT CONTINUATION FACILITY NAME Browns Ferry Unit 3 05000296 NUMBER NUMBER 5 of 8 96 004 02 X
more space rs requir, use a Nona copes ofm (17)
F.
fourth events.]
erator Actions:
II In all four events, steps were taken to identify the problems, and maintenance actions were initiated as appropriate.
G.
Safet S stem Res nses:
No safety systems were required to respond in any of the four events.
zzz.
CAUSE OF THE-EVENT A.
Zmmediate
Cause
The immediate cause in the four events was a loss of invertez output.
B.
~ Root Cause:
After extensive testing, there appears-.to be two root causes for the four events.
In the first, second, and fourth events where the inverter fuse cleared and one of the SCRs shorted, the most probable root cause was a SCR failure due to a manufacturing defect.
This cause appears to be limited in scope because not all SCRs failed.
In the third event where the fuse cleared 'but a SCR did not short, the most probable root cause was a shorting of the commutation capacitor bus by contact with an aiz-core inconductor when its restraining strap failed.
C.
Contributin Factors:
None.
zv.
ANALYSZS OF THE EVENT The failed ECCS ATU Inverters caused a. loss of level and/or pressure input signals to the HPCIr CSr RHR ATWSr and ADS from their respective instrumentation logic systems.
~
The first two events resulted from a power supply failure to the instrumentation logic for Division I of the ECCS.
- However, the Division II instrumentation logic in these events remained operable for automatic initiation during the event.
Additionally, the Division I ECCS equipment was available for automatic initiation except for the following equipment:
(1)
RCIC would not automatically inject water into the reactor vessel as a result of a loss of flow controller power.
Howeverr HPCI was available to initiate and to inject water into the reactor
- vessel, and (2) Core Spray (CS) loop I would also not inject lk JNUCLEAR REOULATORY COMhlSSION LICENSEE EVENT REPORT (LER)
TEXT CONTINUATION FACILITY HAHE Browns Ferry Unit' 05000296 NUMBER NUMBER 96 004 02 6 of 8 T
more space rs reqwr, use s ons copes orm (17) water into the reactor vessel due to the loss of the inboard injection valve control circuit to sense a reactor pressure less than 450 psig.
- However, CS loop I inboard injection valve could be manually opened at the handwheel'. to allow water to inject into the reactor vessel.
~
[See LER 50-296/96006 and 96008 foz Analysis of the Event for third and fourth events.]
~
The BFN Final Safety Analysis Report (FSAR), Section 6.'5.2, states in part "...the reliability and the redundancy of the controls and instrumentation of the Emergency Core Cooling Systems show that no failure of a single initiating sensor either prevents or falsely starts the initiations of these cooling systems.
No single control failure prevents the combined cooling systems from providing the core with adequate cooling."
~
The four events did not result in any plant transient described in the BFN FSAR, Chapter 14, Accident Analyses.
Therefore, based'n the above, the four events did not result in a condition outside the design basis of the,-plant, nor did they adversely affect the health and safety of plant personnel or the public.
V.
CORRECTIVE ACTIONS
A.
Immediate Corrective Actions
B.
In the four events, failed components were replaced in the respective ECCS Division I and II ATU Inverter circuitry.
As a result of these replacements, the instrumentation logic was restored.
The existing LCOs were exited.
Corrective Actions To Prevent Recurrence:
TVA sent failed components to an independent laboratory and the vendor.
Additionally, TVA performed in-situ monitoring of the Division I circuitry to trend any electrical perturbations that might have caused any of these events.
No definitive adverse trend was identified.
TVA repaired the failed restraining strap and inspected the Division I restraining strap for heat damage that could cause its failure.'.
An alternate power supply which will minimize the impact of the loss of a single ATU Inverter has been installed in both Unit 3 ATU inverter circuits.
This modification removes one 120 VAC to 24 VDC power supply from each ECCS ATU invertez cabinet and replaces it with a 250 VDC to 24 VDC converter.
Unit 2's installation of the alternate power supplies will be implemented ig)
0(405)
U.S. NUCLEAR REOULATORY COIVMSS(ON LICENSEE EVENT REPORT (LER)
TEXT CONTINUATIGN FACILITX MAME Bzowns Ferry Unit 3 05000296 NUMBER NUMBER 7 of 8 96 004
02 mora space is raquir, usa s eris copes orm
. (17) during the next Unit 2 refueling outage.
This modification will allow uninterrupted operation of ATUs in the event of an invezter failure.
vz.
ADDZTZONAL ZNFORMMZON A.
Failed Co onents:
Jul 17, 1996 and Au st 6, 1996 Even"s
~
Silicon Controlled Rectifiezs, Part No.
TD 42 F-HDR 6841310
- 13C, manufactured by HDR Power Systems, Inc.
~
ATU Inverter control board, Part No.
2033189G REV 0 and H2033189 SN 104 Rev A, respectively, manufactured by HDR Power Systems, Inc.
~
BUSS Semiconductor
- fuses, Part No.
FWH-125A, 550v AC/DC manufactured by Bussmann, Inc.
~
[See LER 50-296/96006 and 96008 for Failed Components foz third and fourth events.]
B. Previous LERs on Similar Events:
TVA has reviewed previous BFN LERs to determine if similar events have occurred.
Three previous events have occurred:
LER (260/94001) was written as a result of an ATU DC input fuse failure.
However, the zoot cause of the event was determined to be as failure of the ECCS ATU Inverter control board.
Since this event occurzed on a different unit, the corrective actions taken in LER 260/94001 would not have precluded this event (296/96004).
LER 260/94006 described the failure of an ECCS Division I ATU
- Invezter, 250 VDC reactor motor operator valve (RMOV) breaker trip..
This ECCS RMOV breaker tripped as a result of a shorted SCR.
Therefore, corrective actions taken foz LER 260/94006 would not have precluded this event (296/96004).
LER 260/94010 addressed the failure of Cl capacitor in the Division II ATU Inverter capacitor bank.
This failed capacitor was a result of a manufacturing defect.
Therefore, corrective actions for LER 260/94010 would not have precluded this event,(296/96004).
TVA has had previous reportable events.
At that time, TVA believed that corrective actions taken to resolve those previous LERs would not have precluded this event.
Based on the extensive testing of these
- events, TVA confirmed that no prior corrective actions would have precluded these events.
J
0 NRC FORM SCCA (446)
U.S. NUCLEAR REOULATXlRYCOIUMSStON LICENSEE EVENT REPORT (LER)
TEXT CONTINUATION PACILITX NAME Browns Ferry Unit 3 05000296 NUMBER'UMBER
.PAGE 8of8 96 004
02 TEXT more apace is require, use a ooa copies orm (17)
VZI ~
COMMITMENTS
An alternate power supply will be installed for the Unit 2 ECCS ATU Inverters during the next Unit 2 refueling outage (currently scheduled
,to begin September 21, 1997).
Energy. Industry Identification System (EIIS) system and component codes are identified in the text with brackets (e.g.,
[XX]).
ig(
lt II