05000423/LER-1998-039, :on 981001,TS 3.0.3 Entry Was Noted.Caused by F1 Blown Fuse Inside Vital AC Inverter 1 Due to Electrical Component Problem within Inverter.Replaced Six Circuit Boards

:on 981001,TS 3.0.3 Entry Was Noted.Caused by F1 Blown Fuse Inside Vital AC Inverter 1 Due to Electrical Component Problem within Inverter.Replaced Six Circuit Boards

ML20155H907
Person / Time
Site:	Millstone
Issue date:	11/02/1998
From:	Danni Smith NORTHEAST NUCLEAR ENERGY CO.
To:
Shared Package
ML20155H897	List: 05000423/LER-1998-039, :on 981001,TS 3.0.3 Entry Was Noted.Caused by F1 Blown Fuse Inside Vital AC Inverter 1 Due to Electrical Component Problem within Inverter.Replaced Six Circuit Boards
References
LER-98-039, LER-98-39, NUDOCS 9811100326
Download: ML20155H907 (4)

LER-1998-039, on 981001,TS 3.0.3 Entry Was Noted.Caused by F1 Blown Fuse Inside Vital AC Inverter 1 Due to Electrical Component Problem within Inverter.Replaced Six Circuit Boards

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(viii) 10 CFR 50.73(a)(2)(ii) 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
4231998039R00 - NRC Website

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'f.'HRC FORM 366 U.s. NUCLEAR REGULATORY Commission APPROYED BY OM8 NO. 3150-0104 (4-95)

EXPIRES 04/30/99 LICENSEE EVENT REPORT (LER)

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digits /charactersfor each block)

ONYE NAG E T AND U ET.W SH TO D 5

FACILITY NAME 11)

DOCKET NUMBER (2)

PAGE (3)

Millstone Nuclear Power Station Unit 3 05000423 1OF4 TITLE 141 Technical Specification 3.0.3 Entry Due to Vital inverter Failure Resulting In the "A" and "C" Recirculation Spray System (RSS) Pumps Being Inoperable While The "B" RSS Pump Was Out-of-Service For Maintenance EVENT DATE (5)

LER NUMBER (6)

REPORT DATE (7) oTHER FACILITIES INVOLVED (B)

SEQ AL RE $ N MONTH DAY YEAR YEAR MONTH DAY YEAR U

U R

C" "V""

10 01 1998 1998 -- 039 --

00 11 02 1998 OPERATING THis REPORT is SUBMITTED PURSUANT To THE REQUIREMENTS oF 1o CFR 5: (Check one or more) (11)

MODE (9) 1 20.2201!b) 20.2203(a)(2)(v)

X 50.73(a)(2)(i) 50.73(a)(2)(viii)

POWER 20.2203(a)(1) 20.2203(a)(3)(i) 50.73(a)(2)(ii) 50.73(a)(2)(x)

LEVEL (10) 100 20.2203(a)(2Hi) 20.2203(a)(3)(ii) 50.73(a)(2)(iii) 73.71 W

20.2203(aH2Hii) 20.2203(a)(4) 50.73(a)(2)(iv) oTHER 20.2203(a)(2)(iii) 50.36(c)(1) 50.73(a)(2)(v) 20.2203(aH2Hiv) 50.36(c)(2) 50.73(a)(2)(vii) gygA N rm A

LICENSEE CONTACT FoR THis LER (12)

NJ.M E TELEPHONE NUMBER (include Area Codel y<

David A. Smith, Manager Unit 3 Regulatory Compliance (860)437-5840 q

COMPLETE ONE LINE FoR EACH COMPONENT FAILURE DESCRIBED IN THis REPORT (13)

^

CAUSE

SYSTEM COMPONENT MANUFACTURER

CAUSE

SYSTEM COMPONENT MANUFACTURER O

D X

EF INVT E209 Y

SUPPLEMENTAL REPORT EXPECTED (14)

EXPECTED MONTH DAY YEAR YES submission No DATE (15)

(if yes, complete EXPECTED sUBMisSloN DATE).

ABSTRACT (Limit to 1400 spaces,i.e., approximately15 single spacedtypewrittenlines) (16)

On October 1,1998, at 18:11, with the Unit in Mode 1 at 100 percent power, entry into Technical Specification (TS) 3.0.3 occurred due to the simultaneous loss of the A and B Recirculation Spray System (RSS) trains. The "A" and "C" RSS [ Train A) pumps became incapable of automatic starting on a loss-of-power (LOP) due to a failure of vital AC Inverter 1 which supplies the "A" Emergency Generator Load Sequencer. At the time of the inverter failure, the "B" RSS pump [ Train B] was out-of-service for surveillance testing and scheduled preventative maintenance. This risulted in not meeting the requirements of TS 3.6.2.2," Containment Systems Recirculation Spray System," which requires that "Two independent Recirculation Spray Systems shall be OPERABLE." Entry into Technical Specification 3.0.3 is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B) as an operation or condition prohibited by the unit Technical Specifications.

The cause of this Technical Specification 3.0.3 entry was the F1 blown fuse inside vital AC Inverter 1 due to an clIctrical component problem (s) within the inverter.

There was minimal safety significance associated with this event. The "A' and "C" RSS pumps, while not capable of gutomatic starting on a LOP due to the Emergency Generator Load Sequencer being inoperable in accordance with TS, w re capable of being manually loaded from the associated Emergency Diesel Generator and would have been availablein the event of an accident. There were no safety consequences associated with event. RSS train B was r: stored to operable status, TS 3.0.3 was exited, and the reactor downpower stopped at 48 percent reactor power.

The six circuit boards responsible for the firing and coordination of the firing times of the silicon controlled rectifiers (SCRs) in Inverter 1 were replaced. The revision level of the circuit boards associated with the SCR firing network in sich of the vital inverters will be evaluated.

9811100326 981102 PDR ADOCK 05000423 S

PM

.U.s. NUCLEAR REGULATORY Commission (4-95)

UCENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME (1)

DOCKET LER NUMBER (6)

PA.GE (3)

SEQUENTIAL REVISloN YEAR NUMBER NUMBER 2OF4 Millstone Nuclear Power Station Unit 3 05000423 1998 - 039 -

00 TEXT tif more spaceis required use additionalcopies of NRC Form 366A) til) a 1.

Description of Event

On October 1,1998, at 18:11, with the Unit in Mode 1 at 100 percent power, entry into Technical Specification (TS) 3.0.3 occurred due to the simultaneous loss of the A and B Recirculation Spray System (RSS) [BE] trains. The "A* and "C" RSS [ Train A) pumps became incapable of automatic starting on a loss-of-power (LOP) due to a failure of vital AC Invirter 1 [lNVT] which supplies the "A" Emergency Generator Load Sequencer (EGLS). At the time of the inverter f;ilure, the "B" RSS pump (Train B] was out-of-service for surveillance testing anc scheduled preventative maintenance.

This resulted in not meeting the requirements of TS 3.6.2.2, " Containment Systems Recirculation Spray System," which requires that "Two independent Recirculation Spray Systems shall be OPERABLE." Entry into Technical Specification 3.0.3 is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B) as an operation or condition prohibited by the unit Technical Specifications.

On October 1,1998, at 18:11, Technical Specification 3.8.3.1, "Onsite Power Distribution," Action b was entered when a fuse blew in vital AC Inverter 1 (VIAC 3VBA*lNV1) and became inoperable, and the vital AC bus (EF] had to be placed on its attemate source. While on the altemate source the "A" EGLS [JE] is not capable of automatic operation on a LOP. This rendered both the "A" and "C" RSS [ Train A) pumps incapable of automatic starting on a LOP. The "B*

RSS pump [ Train B] had been previously removed from service on October 1,1998, at 05:32, for a survei!!ance testing end for a 6 month preventative maintenance sampling of motor oil.

Discussicns occurred between the Shift and managemelit personnel as to the applicability of Technical Specification 3.0.3. At 20:45, TS 3.0.3 was entered with a time of discovery of 18:11, which was when the fuse blew in vital AC Inv:rter 1.

.s At 21:20 the Unit entered AOP 3575," Rapid Downpower," and began reducing reactor power. At 21:28 notification of m

ths TS 3.0.3 required shutdown was made pursuant to 10 CFR 50.72(b)(1)(i)(A). At approximately 22:50 the "B* RSS pump was declared operable and, TS 3.0.3 was exited, and the reactor shutdown was stopped at 48 percent reactor power.

Discussion of Previous inverter 1 (VIAC-1) Problems:

Th3 200 amp fuse (F1) associated with the silicon controlled rectifier (SCR) firing network (A4) within Inverter 1 had blown on two previous occasions resulting in entry into TS 3.8.3.1 Action b for a vitalinverter out-of-service.

On August 23,1998, fuse F1 located in series with the DC input to the inverter section of the Inverter (which provides electrical protection to one of the three SCR firing networks) was blown. A visualinspection of the internals of the inv:rter was performed, the blown fuse replaced and the inverter was returned to service.

On September 23,1998, the F1 fuse again blew. Trouble shooting of the inverter SCR firing network section found no degraded or failed components with the exception of Varistor RV1. This varistor (RV1) has a rated breakdown voltage of cpproximately 175 volts. RV1 showed signs of degradation since it was breaking down at 140 volts which is close to the DC input potential seen at the F1 fuse. This could have caused excess current to flow through F1 causing the fuse to blow. The varistor was replaced along with the F1 fuse and following a monitoring period of the output wave forms of the firing network, as well as, the over all wave forms of the inverter, the inverter was returned to service.

On October 1,1998, at 18:11, the F1 fuse again blew and in conjunction with the "B" RSS pump being out-of-service resulted in the TS 3.0.3 entry. Trouble shooting of the inverter SCR firing network section found no degraded or failed components. The circuit boards responsible for the firing and coordination of the firing times of the SCRs within the

(

SCR firing network sections of the inverter were replaced, i.e., the 4 SCR driver cards (J109-112) as well as the Driver NAC FOnM 366A (4-95)

.. =

) NRC FORM 366A u.S. HuCLEAR REGULATORY CoMMISSloN (4-95)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME (1)

DOCKET LER NUMBER (6)

PAoE (3)

SEQUENTIAL REVisloN YEAR NUMBER NUMBER 3OF4 Millstone Nuclear Power Station Unit 3 05000423 1998 ~ 039 -

00 TEXT (11more space is reqaired, use additionalcopies of NRC Form 366A) (11) 1

,q Logic and Power Logic Cards (J9 and J8). The F1 fuse was replaced, and following a monitoring period of the output I

wave forms of the SCR firing network, as well as, the over all wave forms of the inverter, the inverter was recurned to service.

ll. Cause of Event i

The cause of this Technical Specification 3.0.3 entry was the blown F1 fuse inside vital AC Inverter i due to an dectrical component problem (s). This resulted in the "A" and "C" RSS [ Train A) pumps being incapable of automatic starting on a LOP (since the "A" EGLS was inoperable) while the "B" RSS pump was out-of-service for maintenance.

The cause of the F1 fuse blowing inside Inverter 1 is believed to be the miss-firing of an SCR within the associated SCR firing network (A4) caused by a problem with either the Driver Logic and/or the Power Logic Cards which are responsible for the timing and the firing of the SCRs. The problem with the circuit boards could be with a component that is degrading and its effect on the existing circuitry (and/or with the compatibility between different revision levels 7

of circuit boards) within the associated SCR firing network.

4 Discussions with the vendor, Elgar, and system engineering counterparts at other power plants identified that blowing L

the F1 fuse within Elgar inverters is a relatively common problem. A review of industry operating experience I

identified failures that could cause the F1 fuse to blow, i.e., noise, loose connections, degraded or failed solid state L

components due to heating or aging, and circuit board revision level compatibility, all centered around the miss-firing i

L of an SCR.

L' The six circuit cards within Inverter 1 that were identified as potentially causing the problems were returned to the

[

vendor for analysis. Elgar performed a Quality Assurance receipt inspection which included a visualinspection for bad connections, loose wires, bad solderjoints, and signs of overheating. The circuit boards were then submitted to Elgar's Standard Nuclear Acceptance Test Procedure at both ambient and elevated temperature (122*F) conditions to determine if there were any components that were showing signs of breaking down. This test consisted of i

. providing inputs and verifying the correct outputs characteristics for the cards. No deficiencies were identified that could be concluded would have resulted in blowing the F1 fuse if the board (s) had been installed in an operating inverter.

Ill. Analysis of Event There was minimal safety significance associated with this event. The "A' and "C" RSS pumps while not capable of automatic starting on a LOP due to the Emergency Generator Load Sequencer being inoperable in accordance with the TS, were capable of being manually loaded from the associated Emergency Diesel Generator and would have been availablein the event of an accident. RSS train B was restored to operable status, TS 3.0.3 exited, and the reactor downpower stopped at 48 percent reactor power. The timing of the applicable shutdown actions required by TS 3.0.3, which was entered when TS 3.6.2.2 could not be met were satisfied. The unit downpower was performed in cecordance with the operating procedures and no equipment problems were noted. There were no safety consequences associated with this event r----

4 NRC FCRM 366A U.S. NUCLEAR REGULATORY COMMISSloN

($95)

LICENSEE EVENT REPORT (LER) t' TEXT CONTINUATION L

FACILITY NAME (1)

DOCKET LER NUMBER (6)

PAGE (3)

SEQUENTIAL REVISloN YEAR NUMBER NUMBER 4OF4 Millstone Nuclear Power Station Unit 3 05000423 1998 ~ 039 -

00 TEXT (11more space is required, use additionalcopies of NRC Form 366A) (11)

.; +.

IV. Corrective Action

The following corrective action (s) have been completed:

1.

The six circuit boards responsible for the firing and coordination of the firing times of the silicon controlled rectifiers (SCR) within the SCR firing network sections of Invertar 1 were replaced.

The following corrective action (s) will be performed.

J

1.. Evaluate the compatibility of the existing circuit boards (revision levels) associated with the SCR firing network sections in each of the VitalInverters by January 15,1999.

J V. Additionalinformation j

The results of the vital inverter circuit board (revision lev'el) compatibility review will be evaluated as part of the Maintenance Rule Functional Failure Evaluation Program. Corrective actions developed as part of the Maintenance 1

Rule functional failure evaluation for the vital inverter failure will be tracked under the Millstone Corrective Action

(

Program, i

S j

r I

Similar Events

R LER 97-025-00 Historical Event: A Failure to Enter Technical Specification 3.0.3 Upon Loss of Vital AC Bus VlAC-1 l

Manufacturer Data l

Ells Codes Systems:

Engineered Safety Features Actuation (Instrumentation) System.,....

..J E Instrument and Uninterruptible Power System - Class 1E...EF Recirculation (Containment) Spray System..

..BE Components:

Inverter...

..INVT -

N.4C FORM 366A (4 95) t