ML20042G798

LER 90-016-00:on 900414,insulator Fault on Local Grid Resulted in Trip of 500 Kv Breaker.Caused by Spurious Alarm Signal from Control Room Local Intake Radiation Monitor 1RMS*RE13B.Sys Returned to Normal operation.W/900510 Ltr

ML20042G798
Person / Time
Site:	River Bend
Issue date:	05/10/1990
From:	England L, Odell W GULF STATES UTILITIES CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-016, LER-90-16, RBG-32815, NUDOCS 9005160108
Download: ML20042G798 (4)

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GULF STATE'S UTILITIES ' COMPANY '

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May 10,c 1990 ; 'I RBG-32815 n File' Nos. G9.5. G9.25.1J3'.-

'U.S. Nuclear Regulatory Commission! 'I Document Control-Desk'. -

Washington, D.C. 20555- '<

Gentlemen: _

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River Bend Station .U' nit 1 .

~ Docket No.'50-458'- >

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-Please find enclosedELicensee : Event Report-No T90-016Lfor- l River Bend Station. :: Unit 1. This report ist being? submitted 0

-pursuant to.10CFR50.73.-

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W. . Odell .

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Manager-0versight

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No l l l Le$TR ACT time re f *00 soeces e acomera*ere v 4'rsen s"'f e seece tree.rr tiss ionest 06i At 1350 on 04/14/90 with the unit at 100 percent power (Operational Condition 1), an insulator fault on the local grid' resulted in_ the trip of a 500 KV breaker. The associated voltage transient resulted in'the Division II control building ventilation-system isolating and its respective filtration unit (1HVC*FLT3B) automatically initiating.

.The initiation was caused by a spurious high alarm signal from the control room local intake radiation monitor 1RMS*RE138. The operators returned the system to a normal configuration after verifying that a

• high radiation condition did not exist.

The spurious high alarm signal from the radiation monitor and subsequent Division II control building ventilation system isolation with auto-initiation of th~e filtration unit constitutes an engineered safety feature (ESP) actuation. Therefore, this event ~ is reportable pursuant 10CFR50. 73 (a) (2) (iv) .

- There was no adverse impact on the safe operation of the plant or to the health and safety of the public as a result of this event since  ;

the ventilation system isolation and filter initiation placed the -

plant in a more conservative configuration by filtering the main control room air supply.

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REPORTED CONDITION At 1350 on 04/14/90 with the unit a' 100 percent power (Operational Condition.1),.an-insulator (*EB*) fault on the local grid (*EC*)~ l resulted in the trip of a 500 KV breaker (*BKR*). The associated voltage'. transient resulted in the Division II control building ventilation (*VI*) system isolating and its respective filtration 1 unit (lHVC*FLT3B) automatically initiating. The-initiation-was caused by a j spurious high alarm signal from the control room local intake-radiation monitor. (*45*) 1RMS*RE13B. The operators returned the system to a normal' configuration after verifying that a high radiation condition did not exist.

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'The -spurious high . alarm signal from the radiat' ion monihor and- l subsequent Division II control building-ventilation system isolation  !

with auto-initiation of the filtration unit constitutes an engineered  !

safety feature' (ESP) actuation. Therefore, this event-is reportable '

pursuant 10CFR50.73 (a) (2) (iv) . i

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INVESTIGATION  !

The voltage transient resulting from the trip of-the. 500 KV breaker was such that several radiation monitors were affected. 1RMS*RE13B went into a high alarm state causing the actuation of tho' Division II .

control building filter train, per design. .The ventilation system and  !

radiation monitor were returned to a normal configuration- after the high radiation alarm was determined to be invalid.

Electrical transients of this severity have en impact on downstream j equipment fed from unregulated power supplies. . This is evidenced by a momentary loss of-sample flow of l'4 other monitors,,and.a high alarm  ;

condition cn1 two monitors in addition to 1RMS*RE138. An electrical  ;

transient is considered the root cause of this event.  !

Previously submitted LERs concerning events in which ESF actuations  ;

have occurred due'to spurious actuations of radiation. monitors have '

been reviewed. This event is similar in nature to that reported in-LER 90-007, in which a severe voltage transient resulted in an ESF  ;

actuation generated from 1RMS*RE13B. t Radiation monitor problems caused by electrical noise / transients have been'previously addressed'by electrical noise studies. .As a result of those studies, modifications MR 86-1113 which installed RC networks (voltage suppressors) across monitor noise sources and MR 86-1669 which revised preamplifier discriminator. settings to reduce the sensitivity of the detector circuitry to noise, were implemented. The modifications were successful in significantly reducing spurious radiation monitor actuations due to electrical noise /trannients.

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- ten n . w.e. sm.an=ac w maa m m Radiation monitor 1RMS*RE13B is. apparently more sensitive to

• electrical voltage transients than other radiation monitors. However, further filtering, voltage suppression. or discrimination against electrical noise for 1RMS*RE13B is not feasible without adversely affecting. detector sensitivity.

CORRECTIVE ACTION Based on the improved' performance of the radiation monitors subsequent to the implementation of modifications. MR 86-1113 and MR 86-1669, along with the severity of- the electrical transient to which the monitor was subjected, no further corrective action is necessary.

. SAFETY ASSESSMENT ,

, The safe operation of the plant and the health and safety of the public were not affected as a result of this event. The ' continued operation of the plant will have no impact on safety since the'syst m placed the plant in a more conservative configuration by filtering the ,

air entering the. main control room.

NOTE: Energy Industry Identification System Codes are identified in the text as (*XX*).

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