ML20029C159

LER 91-004-00:on 910212,offsite Power Low Signal Outside Design Basis.Caused by Inadequate Mod Design at Time of Performance of Original Degraded Voltage Analysis. Engineering Analysis EA-FC-91-017 performed.W/910321 Ltr

ML20029C159
Person / Time
Site:	Fort Calhoun
Issue date:	03/21/1991
From:	Gates W, Molzer D OMAHA PUBLIC POWER DISTRICT
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-91-004, LER-91-4, LIC-91-0010L, LIC-91-10L, NUDOCS 9103260320
Download: ML20029C159 (6)

Text

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Omaha Pubuc Power Distdct 444 South 16th Street Mall Omaha, Nebraska G8102-2247 402/636 2000 March 21, 1991 LIC-91-0010L U. S. Nuclear Regulatory Comission Attnt Document Control Desk Mail Station P1-137 Washington, DC 20555

Reference:

Docket No., 50-285 Gentlemen:

Subject:

Licensee Event Report 91-04 for the Fort Calhoun Station Please find attached Licensee Event Report 91-04 dated March 21, 1991. This report is being submitted pursuant to 10 CFR 50.73(a)(2)(ii)(B). A 7 day extensior, request for submittal of this report was approved by NRC Region IV personnel on March 14, 1991.

If you should have any questions, please contact me.

Sincerely,

n. 2 2.6 W. G. Gates Division Manager Nuclear Operations

> WGG/djm Attachment ,

c: R. D. Martin, NRC Regior.31 Administrator W. C. Walker, NRC Project Manager R. P. Mullikin, NRC Senior Resident Inspector INP0 Records Center q

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• n6! i The-Offsite Power Low Sign (0PLS)providesdegradedvoltageprotectionto safeguards equipment;-when a degraded vtage condition exists concurrent with a  :

Safety Injection Actuation Signal, the UPLC signal isolates safeguards buses 1A3. 1 and 1A4 from their offsite power supply (161 iW) and initiates automatic actions to load.the safeguards equipment onto the emergency diesei generators.

Engineering analysis revealed that, during a postulated accident, the voltage- '

-supplied to some 480 V safeguards loads could degrade to as low as 87.5% of rated voltage without-OPLS being actuated. Since the pcssibility existed for voltage to be lower than the recommended 90% of rated voltage for certain 480 V

-safeguards loads without an OPLS actuation, managenient determined on February 12, 1991 that the plant was outside of its design basis.

Corrective actions include administrative controls of equipment configurations U,d bus' loadings,. as well as rest.tting of the OPLS setpoints. A long term

. corrective action plan is-being developed.

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010 012 F 0l5 At Fort Calhoun Station the electrical distribution system is designed with foui separate 4160 Y buses, two of which (1A3 and 1A4) are designated as safeguards buses. These 4160 V safeguards buses are normally fed from the 161 KV system which constitute the offsite power supply. The two safeguards buses and their associated 480 casessuplypowertoEngineeredSafetyFeature(ESF) components that are required for safe reactor shutdown and removal of decay heat. To ensure that adequate voltage exists from the offsite power sys+ m to these buses during an accident, the electrical distribution system has been equipped with a degraded voltage protection system. Thedegradedvolta protectionsystemisreferredtoastheOffsitePowerLowSignal(0PLS)ge . The OPLS provides protection to safeguards equipment when an undervoltage confition is sensed on buses 1A3 or 1A4, in the presence of a Safety Injection Actuation Signal (SIAS). When a degraded voltage condition exists concurrent ith a SIAS, the OPLS signal isolates buses 1A3 u.. lA4 from their offsite power supply (161 KV) and Mitiates automatic actioas to load the safeguards equipment nto the emergency diesel generators. The OPLS setpoints were originally designed to

-ensure ths voltage being supplied to the C 60 V and 480 V loads does not d..

below 90s of nameplate voltage during a accident. The motor manufacturer recommends that terminal voltage be maintained at a minimum of 90% of motor nameplate rating to assure _long-term motor protection fro. excessive current.

Omaha Public Power bistrict (0 PPD) has been involved in a Jesign Basis Reconstitution Project to verify or reconstruct the design basis calculations and analyses of Fort Calhoun Station systems including the electricai distribution system. In 1989, the first revision of-the Design Basis Document for the electrical distribution syscam was issued. An open item in this document-identified the lack of supporting documentation to verify that certain loads could be assured of operating above their minimum voltage rating. In an effort to resolve this open item 3 computer software program, " Electrical Load Monitoring System" (ELMS), was utilized to model the plant electrical distribution system for purposes of reconstituting the original Degraded Voltage Analysis. The ELMS revealed that during an accident in which safeguard equipment would be recuired to operate, the voltage supplied to some 480 V safeguards loads coulc degrade to as low as 87.5% of rated voltage without OPLS being actuated. Since the possibility existed for voltage to be lower than the recommended 90t Of rated voltage for certain 480 V safeguards loads without an OPLS actuation, the Pla M Review Committee (PRC) determined on February 12, 1991 that the )lant was outside of its design basis. The PRC concluded that OPLS was operable )ased upon engineering judgement that the safeguards loads would continue to perform their intended functions at the degraded voltage. The plant l

-was at 75% power and mode 1 at the time of the determination.

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-o m a Nne m m nn A.one hour report was subsequently made to the NRC at 1615 hours0.0187 days <br />0.449 hours <br />0.00267 weeks <br />6.145075e-4 months <br /> on February 12, 1991 in accordance with 10 CFR 50.72(b)(1 This written report is

submittedpursuantto10CFR50.7;(a)(2)()(ii)(B). ii)(B) for the plant being outside its design basis. A 7 day extension request for submittal of this report was approved by.NRC Region IV pe,;onnel on March 14, 1991.

As a result of OPPD's effort to reconstitute the original Degraded Voltage Analysit, several modifications were identified that have contributed to degrading the plant uMervoltage protection capabilities. Initially, the OPLS degraded voltage protection system n installed in 1970 under modification MR-FC-77-064 in response to a NRC generic letter or degraded voltage, dated June 2, 1977. During a recent review of design basis d 'nents, it was discovered

-that the original analysis performed in support M .ne modification did not

-assume the worst case loading that would occur during a Loss of Coolant Accident

-(LOCA) scenario. The load model used in the original analysis was not representative of these worst case operating conditions and thus did not cor,servatively assume the loads that would be present. This potentially could

-allow a condition in which voltage on the 480 V buses could decrease below 90%

of rated' voltage without having reached the original OPLS actuation setpoints.

Another concern identified was modification MR-FC-84-105, com leted in 1985, which r9placed t % 4160/480-V transformers with those of a hi her-impedance. ,

The hignerL1mpedance transformers were selected in order to 1 mit ground fault currents available:to the 480 V switchgear. This also resulted in lower 480 V 1 system voltages due to the higher im3edances. Although the analysis concluded

, that voltage levels were acceptable.)ased on operator action to trip various '

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.large41oads during a accident,-these assumptions were invalid since they took credit for 9anual action to reduce system loads during post !.0CA operation.

3 Recent-reviewoftheoriginallicensingbasisdegradedvoltage(OPLS) submittal to the F.C on August 30, 1978, indicates:that these assumptions do not meet design criteria for " automatic" protection of ESF loads for undervoltage

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l :In 1986,'an engineering consultant was-contracted to update the electrical E "distribui.ien-system's design. basis documents for future modifications and verify r the-0PLS operating setpoints.- On November 3, 1988, plant management was

notified of the results of the report. It was identified that the-possibility

. existed for the 4160 V-buses to trip from their offs' 'ower supply (161KV) during accident conditions. It appeared tnat during 4..ident conditione N

assu.:ing safeguards bus 1A4 at its heaviest normt.1 loading,161 KV griu at its lowest normal. expected voltage, and ESF equipment sequenced onto the buses _

ifollowing a Safety udection Actuation Signat OPLS actuation would occur while offsite power was in a normal voltage range. LER 88-31 was submitted to the NRC h documenting the condition and corrective action.

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-To correct the problem, Modification MR-FC-88-110 was installed in 1988 to increase various safeguards equi > ment sequencer starting times until system starting loads had decreased. Tie analysis performed in support of the >

modification took credit for operator action to reduce certain 4160 V loads in

-order to increase system voltage. Although it wasn't realized at the time, -

l credits:ould not be taken for reducing system loads by means of non-automatic protection of.ESF equipment. As previously stated, this type of load reduction does~not meet design criteria for " automatic" protection of ESF loads for undervoltage conditions-in a post-LOCA condition.

Because the OPLS'was determined to be outside its design basis on February 12, 1991, an engineering evaluation was performed to determine the effects of degraded voltage on the' safeguards equipment, specifically the 480.V, motors. -By considering-the-time overcurrent protection curves for safeguards loads, it was concluded that sufficient mergin exists such that the safeguard equipment would be able to operate without tripping during a Design Basis Accident with an undervoltage condition on the 161 KV system. The estimated motor overload currents would not be expected to generate sufficient heating of the motor internals over the-required cporating times that would preclude acceptable ,

equipment performance. This justification is based on engineering judgement.

.The primary cause of this event can be attributed to an inadequate modification design at the time'the-original Degraded Voltage Analysis was performed. The -

assumption made-in the original analysis was nonconservative and has propagated throughout subsequent _ revisions to the t ' lysis.. A-contributing cause was lack of procedural guidance for the performanci. W documentation of-r.ssumptions used in the calculations. Another contributing use was lack of comprehensive '

DesignBasisDocumentationpriortoJuly,)! N This resulted in inadequate documentation of the original 0PLS design ci aria which led to credit being -

taken in subsequent modifications for non-automatic' protection of-safeguards loads. These primary and contributing causes have already been corrected Lthroughlthe development of upgraded procedures for the control of calculations and documentation _ requirements. Additionally, the development and issuance of L  ; plant-Design Basis Documents have provided the corrective action necessary for resolution of. incomplete design basis documentation.

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.The'following ccmpensatory measures were t'aken to ensure the operability of~the degraded voltage _ system (OPLS) and thus assure the satisfactory performance of 4 sthe safeguards motors:  !

(1) Engineering analysit EA-FC-91-017:was performed to derive new OPLS setpoints Lfor-current bus loading configurations. The new setpoints'are more conservative than the setpoints currently listed in the-Technical Specifications. The OPLS actuation relays have been recalibrated with the  ;

new setpoints-designated-in-EA-FC-91-017. The new OPLS setpoints in  !

conjunction with the= 1oadire;; configuration specified in the analysis will  !

ensure safeguards-motors.arr capable of satisfying their design function during accident conditions.  :

(2)- Administrati'?" restrictions were placed on equipment rotation =' bus loading configurations through Operations Memorandum FC-91-02. iiie loading restrictions are necessary to ensure bus alignment is consistent with that H assumed in the analysis for the new setpoints. Administrative bus loading restrictions will remain in effect until long term' corrective action has-been completed. If an electrical configuration-arises-due to plaat operating requirements which are outside the permitted restrictions, plant

-staff will contact Engineering, evaluate the situation, and-determine '

e operability and reportability.

e(3)- As an interim measure until the new OPLS setpoints wer a installed, a  ;

dedicated operator was stationed in the control room to monitor voltage j

. levels _on 4160 V buses 1A3 and 1A4 and manually actuate OPLS if voltages

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dropped below the new OPLS alarm.setpoints1 installed on the plaret computer.

'A plan for long-term corrective action is being developed to remove administrative operating restrictions. Highlights of this. plan will be provided b in a supplement to this LER:by June 21,:1991.

1. t0ther Licensce Event' Reports which have been submitted addressing design deficiencies are LERs 90-03, 90-05, 90-07, 90-09,- 90-16, 90-20, 90-23, 90-25,-

t89-09,-89-14, 89-15, 89-24,.88-09, 88-19,.88-20, 88;32,-and'88-33.

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