ML17284A663

LER 98-006-00:on 980520,discovered Discrepancies in Low Voltage Bus Calculations During Review of App R Calculations for High Impedance Fault Analysis.Caused Indeterminate. Implemented Procedural Changes Involving Operator Action

ML17284A663
Person / Time
Site:	Columbia
Issue date:	06/19/1998
From:	Tamara Powell WASHINGTON PUBLIC POWER SUPPLY SYSTEM
To:
Shared Package
ML17284A662	List: ML17284A661 ML17284A663
References
LER-98-006, LER-98-6, NUDOCS 9806250276
Download: ML17284A663 (6)

Text

LICENSEE EVl&TRIPORT gZR)

FACILITY NAME (1) DOCKET NUMBER (2) PAGE (3)

Washin ton Nuclear Plant - Unit 2 50-397 1 OF 4 TITLE (4)

DISCREPANCIES IN APPENDIX R HIGH IMPEDANCE FAULT ANALYSIS EVENT DATE (5) LER NUMBER (6) REPORT DATE (> I OTHER FACILITIES INVOLVED (8)

MONTH DAY SEQUENTIAL REV. Momlt DAY YEAR FACILIIYNAME NUMBER NUMBER FACILITYNAME DOCKET NUMBER 05 20 98 98 006 00 06 19 98 OPERATING MODE (91 THIS REPORT IS SUBMITTED PVRSVANT TO THE REQVIREHENTS OF 10 CFR 5I (Check one or more) Ill) 20.402(b) 20.405(c) 50.73(a)(2)(iv) 73.71(b)

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

LEVEL (10) 20.405(a)(1)(ii) 50.36(c) (2) 50.73(a)(2)(vii) OTHER 0

20.405(a)(1)(iii) 50.73(a)(2)(i) 50.73(s)(2)(viii)(A) 20.405(a)(1)(iv) 50.73(a)(2)(ii) 50.73(s)(2)(viii)(B) 20.405(s)(1)(v) 50.73(a)(2) (iii) 50.73(a)(2)(x)

LICENSEE CONTACT FOR THIS LER (12)

NAME TELEPHONE NUMBER (Include Area Code)

T.J. Powell, Licensing Technical Specialist (509) 377-4161 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT (13)

CAUSE svsrKM COMPONENT ItwwfAcnxtsII REPORTABLE svsrsM CAUSE COMPONENT MANUFACTURER REPORTABLE TO EPIX TO EPIX SUPPLEMENTAL REPORT EXPECTED (14) EXPECTED MONTH DAY YES 01 yes, completed EXPECTED SUBMISSION DATE).

Y NO 07 20 98 ABSTRACT Discrepancies were discovered in low voltage bus calculations during a review of the 10 CFR Part 50 Appendix R calculations for high impedance faults. These deficiencies required procedural changes involving operator actions to monitor safe shutdown buses and the removal non-safe shutdown loads in the event that certain low voltage buses become overloaded due to fire induced electrical faults in specified fire areas.

9806250276 98061's)

PDR ADOCK 05000S'r)7 8 POR

LXCENSEE EVENT REPORT (LER)

TEXT CONTINUATION

, FACILITY NAME (I) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)

SEQUENTIAL REVISION NUMBER NUMBER Washington Nuclear Plant - Unit 2 50-397 98 006 00 2 OF 4 TEXT (If more space is required, use additional copies of NRC Form 366A) (17)

Even Descri tion A deficiency with the existing Appendix R high impedance fault (HIF) analysis using a new contractor provided methodology was discovered February 26, 1998. This discovery was not determined to be reportable at the time because, in lieu of using the new contractor HIF analysis methodology (HIF - Rev 3),

WNP-2 electrical and fire protection engineers reconimended using'he original WNP-2 kIIF analysis methodology (HIF - Rev 2), which was considered to have enough conservative margin to bound any plant modifications since its last update.

The original WNP-2 Appendix R HIF analysis methodology was taken from NRC Generic Letter 86-10 and subsequent NRC safety evaluations of the WNP-2 FSAR Fire Protection Program description. This methodology requires that the safe shutdown power supply buses remain available during fire conditions assuming that all unprotected loads located in the fire area are affected by the fire in such a way that each circuit faults and loads the safe shutdown bus to the rated value of the branch circuit protective fuse/breaker.

These fire induced loads are assumed to be in conjunction with all normal bus loads not affected by the fire.

Considering this, the total load on the safe shutdown bus must be maintained below the rating of the upstream bus supply fuse/breaker. This ensures the bus protective device does not trip resulting in a loss of power to the associated safe shutdown bus, A review of the HIF analysis was conducted as a corrective action resulting from the deficiency discovered on February 26, 1998. This review revealed discrepancies with the original WNP-2 HIF calculation that could lead to overloading certain low voltage buses [EC] due to postulated fire induced faults in specified fire areas. The deficiencies resulted from underestimated or omitted loads that could contribute to the total HIF load on a post-fire safe shutdown (PFSS) bus.. These deficiencies required procedural changes to PPM 4.12.4.1 'F'r 'nd PPI(~ 4.'2.1.1 'Control Room Evacuation and Remote Cooldown'ncludin~ the addition of new or revision of existing operator actions to monito'r safe shutdown buses or remove non-PFSS loads in the event that certain low voltage buses become overloaded due to fire induced HIFs in specified fire areas.

Further Evaluation This event is being reported per the requirement of 10CFR50.73(a)(2)(ii) as a condition found while the reactor is shutdown that involves degradation of a principal safety barrier or unanalyzed condition that could significantly compromise plant safety. The original high impedance fault analysis issued in 1988 was consistent with methodology provided in NRC Generic Letter 86-10; however, it was not updated since Revision 2 (1994).

A 20% sample of the PFSS distribution and power panel buses was conducted to determine the extent and effects of omitted cables from the HIF calculation. From this review, 9 cables (about 8% of the omitted

LlCENSEE EVENT REPORT (LER)

TEXT CONTINUATION i fACILITY NAME- (I) DOCKET NUMBER I 2) LER NUMBER I6) PAGE (3)

YEAR SEQVENTIAL REvrszoN NUMBER NVNBEA Washington Nuclear Plant - Unit 2 50-397 98 006 pp 3 OF 4 TEXT (if more space is required, use additional copies of NRC Form 366A) (17) cables) were found to have contributing HIF loads and should have been included in the analysis. Each of the 9 cables were reviewed to determine the impact of those additional loads on the calculational results.

Since considerable load margin exists within the calculation for most of the safe shutdown buses, there was no impact to the calculational results by the inclusion of these new loads. Because of available bus load margins, it is not likely that any other omitted cables that might have HIF contributing loads will affect the analysis or result in the necessity for additional operator actions to reinove these loads to protect the integrity of the PFSS buses.

Root Cause The determination of root cause for this event involves an extensive review of calculations performed over a period of several years. The root cause has not been determined at the time of issuance of this LER. A supplemental report with an identified root cause will be issued by July 20, 1998.

Further rrective Action Changes have been made in PPM 4.12.4.1 'Fire'nd PPM 4.12.1.1 'Control Room Evacuation and Remote Cooldown'hat added new or revised existing operator actions to monitor safe shutdown buses or remove non-safe shutdown loads, in the event-that certain low voltage buses become overloaded due to fire induced faults.

Hourly fire tours have been initiated in affected areas as a compensatory measure to reduce the potential for fire indu"=" able faulh.

A 20% sample of the PFSS distribution and power panel buses was conducted to determine the possible extent and effects of omitted cables (loads).

Due to the identification of input data errors (omission of cable/loads) from the results of the 20% sample, a review of all safe shutdown bus load lists, prioritized by bus susceptibility of HIF induced failure, will be conducted. This review will verify the accuracy of cables listed in the HIF calculation. A new revision to the HIF calculation will,then be issued using the methodology of Generic Letter 86-10 which incorporates the results of this HIF cable list review. The estimated completion date for these tasks is January 15, 1999.

Completion of these tasks is being tracked by corrective actions (¹8 and ¹9) approved for closure of Problem Evaluation Request ¹298-0173.

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION

~FACILITY NAME fi) DOCKET NUMBER i2) LER NUMBER i6) PAGE i3)

SEQUENTIAL REVISION NUMBER WlQBR Washington Nuclear Plant - Unit 2 50-397 006 00 4 OF 4 TEXT (If more space is required, use additional copies of NRC Form 366A) (17)

Assessment of Safet Cons uences The WNP-2 HIF calculation errors could have resulted in loss of safe shutdown capability during a design basis fire event due.to loss of power to safe shutdown equipment. However, with'hourly fire tours to compensate for HIF calculation deficiencies and the addition of PPM 4;12.4.1 and PPM 4.12,1.1 operator actions, PFSS buses are capable of performing their safety function by supplying'safe shutdown loads.

The HIF analysis is an Appendix R requirement as specified by Generic Letter 86-10. The purpose of this analysis is to assure that electrical power to fire safe shutdown equipment remains available throughout the fire event even when the safe shutdown power supply buses are subjected to multiple, fire induced HIFs. To preclude such power supply failures, Generic Letter 86-10 allows written procedures that contain operator actions to remove non-safe shutdown loads and thus reduce the total load on the PFSS buses such that adequate power is continuously available to required PFSS equipment (

Reference:

GL 86-10, Section 5.3.8).

The HIF requirement is only applicable to Appendix R fire events and does not apply to any other design basis events. Fires need not be postulated concurrent with other design basis events (except LOOP).

As stated above, available bus load margin should preclude the need for identification of any further operator actions. However, the sample set of omitted cables'escribed above did contain some loads that reduced the available margin in the calculation. For this reason, until all omitted loads have been identified and individually reviewed for their impact on the PFSS bus availability, fire tours will be implemented in vital plant areas most susceptible to HIF. This compensatory measure will help to ensure the potential for plant fires is minimized.

Similar Events There have been no recent LERs involving potential equipment inoperability as a result of deficient bus load calculations.

The underlying cause or reason for the HIF calculational deficiency will be determined by the root cause review and documented in a supplemental report due by July 20, 1998. Recent events that may share a similar root cause will be identified following the root cause determination and included in the supplemental report.

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