ML19351A464

LER 89-037-00:on 891108,Div II Control Bldg Ventilation Sys Isolated & Filtration Unit Automatically Initiated.Caused by Spurious Signal from Control Bldg Intake Radiation Monitor. Voltage Suppressors Installed on monitors.W/891208 Ltr

ML19351A464
Person / Time
Site:	River Bend
Issue date:	12/08/1989
From:	Booker J, England L GULF STATES UTILITIES CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-89-037, RBG-31923, NUDOCS 8912180053
Download: ML19351A464 (4)

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l December B. 1989 '

RBG- 31923 File Nos. G9.5, G9.25.1.3 U.S. Nuclear Regulatory Commission Document Control Desk Washington, D.C. 20555 .

Gentlemen:

River Bend Station - Unit 1 Docket No. 50-458 P3 ease find enclosed Licensee Event Report No.89-037 for River Bend Station - Unit 1. This report- is being submitted '

pursuant to 10CFR50.73. ,

Sincerely,

, F. J.9-endp J. E. Booker Manager-River Bend Oversight River Bend Nuclear Group <

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SEp /TFP/RGW/DCH/JWM/ch cc U.S, Nucleat ,egulatory Commission 611 Ryan Plaza Drive, Suite 1000 Arlington, TX 76011 NRC Resident Inspector P.O. Box 1051 St. Francisville, LA 70775 ,

IIPO Records Center 1100 Circle 75 Parkway Atlanta, GA 30339-3034 89121G0053 891208 3 PDR ADOCK 05000458 6 PDC i

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.setaact,$- ,.,> = -.,.. ,. . .. ~ .,. .. w ~.. . n e i At 1416 on 11/8/89 with the unit at 96 percent power the Division Il control building ventilation system isolated and its respective filtration unit automatically initiated. The initiation was caused by ,

a spurious signal from the control building intake radiation monitor 3RMS*RE13D. The operators, determined that no actual high radiation condition existed and returned the system to its normal configuration.

Radiation monitor 1RMS*RE13B and other radiation monitors have previously shown succeptibility to electrical noise. Therefore, two modifications were implemented prior to this event in order to reduce spurious radiation monitor trips on a programmatic basis by reducing radiation monitor / detector susceptibility to electrical noise.

The safe operation of the plant and the health and safety of the i public were not affected as a result of this event. The continued

( operation of the plant will have no impact on safety since the system which actuated placed the plant in a more conservative configuration by filtering the supply air to the main control room.

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At 1416 on 11/8/89 with the unit at 96 percent power transformer #9291

(*XFMR*) at Grant Substation experienced an internal fault to ground and caused the 230KV switch $26532 to cpen and lockout. Consequently a severe electrical transient on the electrical distribution systern occurred which resulted in the Division II control building ventilation tystem (* VI*) isolating and its respective filtration unit (IHVC* PLT 3Bi (* PLT *) automatically initiating. The initiation was caused by a spurious high alarm signal from the control building intake radiation monitor 3RMS*RE13B (

• RA * ) . 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 the filtration unit constitutes an engineered safet'; feature (ESP) actuation. Therefore, this event is reportable under 10CFR50. 73 (a) (2) (iv) .

INVESTIGATION .

Approximately four (4) seconds after the voltage transient on the elect.rical distribution system occurred, 1RMS*RE13B went into a high alarm state resulting in the following correct and complete actions:

1HVC* PLT 3B's booster fan started, air-operated dampers IHVC*AOD51B, 1HVC*AOD52B and motor-operated valve 1HVC*MOV1B closed, fann 1HVC*FN4 and 1HVC*FN5 stopped and air-operated dampers 1HVC*AOD19D and 1HVC*AOD19F opened. The ventilation system and radiation monitor were returned to a normal configuration after the high radiation alarm was determined to be invalid.

Previously submitted LERs concerning events in which ESF actuations occurred due to spurious actuations of radiation monitors have been reviewed. In six cases radiation monitors have shown susceptibility to electrical noise / transients. Four of these incidents occurred in 1986, one in 1987 and one in 1988 These events have been reported in LERs86-020, 86-040,86-052, 86-062,87-008 and 88-008. Three of these previous occurrences were caused by 1RMS*RE13B, The problems with electrical noise / transients resulted in two major modifications following electrical noise studies on the radiation monitors. MR86-1113 installed RC networks (voltage suppressors) at each radiation monitor noise source such as manual switches, light l circuits, etc. MR86-1669 has changed preamplifier discriminator settings to make the detector circuitry less sensitive to electrical ,

noise. These modifications have been very offective as evidenced by spurious radiation monitor actuations due to electrical noise / transients being reduced from four per year in 1986 to one per year in 1987 and 1988.

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010 01 3 0F 0 13 m ,, . ,nac w mievnn Radiation monitor 1RMS*kE13B is apparently much more nensitive to electrical noise than other radiation monitors. However, further t filtering, voltage suppression or discrimination against electrical noise for 1RNS*RE13B is not plausible without adversely affecting detector sensitivity.

CORRECTIVE ACTION ,

Based on the improved performance of radiation monitors subsequent to the implementation of MR86-1113 and MR86-1669, along with the severity of the aubject 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 system which actuated 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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