05000336/LER-1998-001, :on 980108,ongoing Engineering Evaluations of Fault Clearing & Fuse Coordination for Vital 120 Volt AC Sys Occurred.Caused by Incomplete Engineering Evaluation of Fault Clearing.Changes Will Be Implemented

:on 980108,ongoing Engineering Evaluations of Fault Clearing & Fuse Coordination for Vital 120 Volt AC Sys Occurred.Caused by Incomplete Engineering Evaluation of Fault Clearing.Changes Will Be Implemented

ML20202C729
Person / Time
Site:	Millstone
Issue date:	02/06/1998
From:	Joshi R NORTHEAST NUCLEAR ENERGY CO.
To:
Shared Package
ML20202C697	List: 05000336/LER-1998-001, :on 980108,ongoing Engineering Evaluations of Fault Clearing & Fuse Coordination for Vital 120 Volt AC Sys Occurred.Caused by Incomplete Engineering Evaluation of Fault Clearing.Changes Will Be Implemented
References
LER-98-001, LER-98-1, NUDOCS 9802130015
Download: ML20202C729 (3)

LER-1998-001, on 980108,ongoing Engineering Evaluations of Fault Clearing & Fuse Coordination for Vital 120 Volt AC Sys Occurred.Caused by Incomplete Engineering Evaluation of Fault Clearing.Changes Will Be Implemented

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) 10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition
3361998001R00 - NRC Website

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NRC iORM 36V U.s. NUCLE AR RE GULAVORY CoMMtSSlote APPnovED sy oMe NO. 311,0-0t04 (O 9 51 ExPinEs 04tsoiss

' UCENSEE EVENT REPORT (LER)

'JEMMM'Mid RN"Whio'WE

=%'"44f."! "^ab"S o 'a4Mr*..tTA',A ^Ni:

(See teverse for required number of EE"v7 E0a'"a'iEfaTavEIUIs%" '*"a'020'5"$

digits /charactersfor each block)

EE S' F" aN E

EY A

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r Acnny mAus m oocarTwumstatai PAos asi Millstone Nuclear Power Station Unit 2 05000336 1OF3 TITtt 108 Vital 120 volt AC System Fault Clearing Coordinationinadequate EVENT DATE (5)

LER NUMBER (6)

REPORT DATE (71 OTHER FActLITIEsINVolvED(8)

SEQ

^L S

MONTH DAY YEAR YEAR W NTH DAY YEAR U

NU 01 08 98 98

-- 001 00 02 06 98 OPERATING THis REPORT IS SU8MITTED PURSUANT TO THE REQUlHEMENTS OF 10 CFR li (Check one or more) (11)

MODE (9)

N 20.2201(b) 20.2203(a)(2)(v) 50.73(a)(2)(i) 50.73(a)(2)(viii) power 20.2203(aH1) 20.2203(a)(3Hi)

X 50.73(a)(2)(u) 50.73(aH2)(x)

- LEVEL (10) 000 20.2203(a)(2Hi)

- 20.2203(a)(3Hu) 50.73(aH2Hui) 73.71 20.2203(aH2)(ii) 20.2203(a)(4) 50.73(a)(2)(iv)

OTHER 20.2203(aH2)(ni) 50.36(cH1) 50.73(a)(2Hv)

Specify in Abstract below 20.2203(aH2)(iv) 50.36(cH2) 50.73(aH2)(vii) in NRC Form 366A LICENSEE CONTACT FOR THis LER (12)

NoMt TEl.EPHONE NUMBER (include Area Codel R. G. Joshi, MP2 Regulatory Compliance Manager (860)440-2080 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT (13)

CAUSE

SY$ TEM COMPCNENT MANUFACTURER

CAUSE

SYSTEM COMPONENT MANUFACTURER 0 PRD PD sVPPLEMENTAL REPORT EXPECTED (14)

EXPECTED MONTH DAY YEAR YEs sUBMISsloN lif yes, complete EXPECTED submission DATE).

X NO DATE (15)

ABSTRACT (Limit to 1400 spaces,i.e., approximatelv15 single-spacedtypewrittenlines) (16)

On January 8,1998, at about 1630 hours0.0189 days <br />0.453 hours <br />0.0027 weeks <br />6.20215e-4 months <br />, during the ongoing engineering evaluations of fault clearing and fuse coordination for the Vital 120 volt AC system, it was postulated that during a High Energy Line Break (HELB) or seismic event, the system may not provide adequate isolation of the vital buses. Certain Class 1E loads located in HELB harsh environments may fault during accident conditions after they have performed their required safety function. A circuit fault could result in the loss of the associated vital bus since the breakers and fuses used in these circuits do not adequately coordinate with the transfer feature of the vitalinverter transfer switch. In addition, non-

. Class 1E loads are supplied from the vital bus. Certain of the non-Class 1E circuit fuses, similarly are not adequately coordinated and may fail to isolate the downstream circuit, resulting in the potential loss of a vital bus.

t The cause of this condition was an incomplete engineering evaluation of the fault clearing and fuse coordination study for the vital 120 VAC buses.

As a result of this condition, the ongoing review of vital bus fault clearing and fuse coordination will be completed and design changes will be implemented, as required. This work will be completed prior to entry into Mode 4 from the current outage.

9002130015 900206 ADOCK050003]6 PDR 8

NRC FORM 366 14 951

,e NRC4 GRM 366A u.s. NUCLE AR REGulAT oHY CoMMISsloN (4-9M UCENSEE EVENT REPORT (LER)

TEXT CONTINUATION F ACILITY NAME 11)

DOCKET LER NUMBER (6)

PAGE (3)

SEQUENTIAL REVISloN YEAR NUMBER NUMBER 2OF3 Millstone Nuclear Power Station Unit 2 05000330 98

- 001 -

00 TEXT fil more spece ss required use additionalcopies of NRC form 366Al (11) 1.

Description of Event

On January 8,1998, at about 1630 hours0.0189 days <br />0.453 hours <br />0.0027 weeks <br />6.20215e-4 months <br />, during the ongoing engineering evaluations of fault clearing and fuse coordination for the Vital 120 volt AC (VAC) [EF) system, it was pos'ulated that during a High Energy Line Break (HELB) or seismic event, the system may not provide adequate isolation of the vital buses. Certain Class 1E loads located in HELB harsh environments may fault during,

dent conditions after they have performed their required safety function. A circuit fault could result in the los:,r the associated vital bus since the breakers and fuses used ir, these circuits do not adequately coordinate with the transfer feature of the vital inverter transfer switch. In addition, non Class 1E loads are supplied from the vital bus. Certain of the non-Class 1E circuit breakers or fuses, similarly are not ade~ately coordinated and may fail to isolate the downstream circuit, resulting in the potentialloss of a vital bus. At -

me of discovery the plant was defueled.

This issue was previously reported in L ~

1-026. Design char:ges to power supplies and control circuitry fuses were implemented to coi ect the deficiencies that had been identified. Further engineering evaluations found that additional Class 1E circL ts could be susceptible to HELB harsh environments, and certain connected non-Class 1E circuits had riot been fully evaluated for failures resulting from a seismic event.

The four Vital 120 VAC buses, VA10, VA20, VA30, and VA40, provide reliable power sources to the Reactor Protection System (RPS)[JC), Engineered Safeguards Actuation System (ESAS)[JE] instrumentation and other l

vitalloads. Two of these buses, VA10 and VA20, provide power to the redundant ESAS actuation cabinets and Emetre/ Diesel Generator (EDG)[EK) load sequencers. Each vital bus receives power from a primary inverter j

(that includes a static switch) which is fed from a 125 volt DC (VDC) [EJ) bus. The Facility I (Z1) 125 VDC bus provides power via inverters to VA10 and VA30, and the Facility 11 (Z2) 125 VDC bus provides power via inverters to VA20 and VA40. It is postulated that a circuit fault on a connected load may cause the primary inverter to reach the current limit which would result in instantaneous transfer of the vital bus to the altemate source. For a l

sustained fault on an inverter output, the current limit controls cause the output voltage to drop to zero followed by a voltage buildup to the point at which current limit is achieved. For VA10 and VA20, the attemate source is a similar inverter that does not inriude a static switch. Upon the static switch transfer, the alternate source inverter would also current limit and reduce its output voltage until the circuit protective device clears the faulted circuit.

During this sustained undervoltage condition, the ESAS and EDG load sequencers may not function. Once the faulted circuit is isolated from the vital bus, by either the circuit breaker tripping or the fuse blowing, the static switch transfers back to the primary inverter and the vital bus voltage is restored to normal. Properly coordinated circuit breakers or fuses would isolate the faulted circuit before the current limit controls cause the static switch to transfer.

in the single failure analysis, if a failure is a consequmtlat failure as a result of the accident, then an additional single failure must be assumed. ' t s, if one Facility vital bus has a sustained undervoltage due to a circuit fault (resulting from harsh environment or a seismic event), the other Facility vital bus must be assumed to fail. During operations in Modes 1 through 4 all four vital buses must be operable. During Modes 5 and 6 there are no analyzed events where this condition could result in the coincident loss of the two vital buses which are required to be operable by Technical Specifications.

This condition is being reported pursuant te 10 CFR 50.73(a)(2)(ii)(B), a condition that was outside of the design basis of the plant. Failure to isolate the vital bus from a circuit fault, which may occur during a design basis event, may render the vital bus degraded and result in operation outside the design basis of the plant.

NRC FORM 366 (4 9M I

O

NHC4 G5tM 366A U.s. NUCLE AR REouLAToRY COMMisslON (4 %

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACluTY NAME (1)

DOCKET LER NUMBER (6)

PAGE (3)

SEOuENTIAL REVislON YEAR NUMBER NUMBER 3OF3 Millstone Nuclear Power Station Unit 2 05000336 98

- 001 00 TEKT III more space is required. use additionalcopoes of NRC form 366A) til}

II.

Cause of Event

The cause of this condition was an incomplete engineering evaluation of the fault clearing and fuse coordination study for the vital 120 VAC buses.

Analysis of Eve".

The four Vital 120 VAC buses provide reliable power sources to the RPS, ESAS instrumentatior

, other vital loads. The vital bus system was designed to preclude failures when connected Class 1E circuits are exposed to a harsh environment, and when interactions occur with connected non-Class 1E circuits.

During a HELB, the failure would occur after the ESAS actuation system had performed its safety function. During a seismic event, a HELB is not assumed, so ESAS actuation would not be required. Based on this information, this condition is not safety sigr4ficant.

4

IV. Corrective Action

As a result of this condition, the ongoing review of vital bus fault c! earing and fuse coordination will be completed and design changes will be implemented, as required. This work will be completed prior to entry into Mode 4 from the current outage.

V.

Additionallnformation

Similar Events

LER 94-026:

This LER reported that dunng an HELB the Hydrogen Analyzers could short circuit causing failure of both vital buses VA10 and VA20. A design change was implemented to move the Hydrogen analyzers to VA30 and VA40 and provide properly coordinated fuses. Additionally, properly coordinated fuses were provided for four enclosure building dampers circuits and auxiliary circuits for two charging flush puryps.

Energy industry identification System (EllS) codes are identified in the text as [XX].

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