ML20042E223

LER 90-007-00:on 900311,control Bldg Ventilation Sys Isolated & Filtration Unit Initiated.Caused by Severe Electrical Transient.Performance of Radiation Monitors Improved Prior to Implementation of procedures.W/900409 Ltr

ML20042E223
Person / Time
Site:	River Bend
Issue date:	04/09/1990
From:	England L, Odell W GULF STATES UTILITIES CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-007, RBG-32683, NUDOCS 9004200443
Download: ML20042E223 (4)

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GULF STA TES UTILXTIES COMPANY ttyt f: ht P.D it Ah ?N PO$1 Of hcl &DX 2D f.1 f fANT $ W,'.L LOJ:OAM N711 Ant. A CM'T 6N Lib tiy>4 M b(10f1 April 9, 1989 RBG- 32683 File Nos. G9.5, G9.25.1.3 U.S. Nuclear Regulatory Commission -

Document Control Desk Washington, D.C. 20555 l

Gentlemen:

River Bend Station - Unit 1 Docket No. 50-458 Please find enclosed Licensee Event Report No.90-007 for i River Bend Station - Unit 1. This report is being subnitted pursuant to 10CFR50.73.

Sincerely,  ;

2 Manager-Oversight River Bend Nuclear Group i

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TFP/WH0/PDG/es Pow cc: U.S. Nuclear egulatory Connission 611 Ryan Plaza Drive, Suite 1000 '

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l NRC Resident inspector P.O. Box 1051 St. Francisville, LA 70775 INPO Records Center 1100 Circle 75 Parkway '

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At 1248 on 3/11/90 with the unit at 100 percent power the Division II 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 1RMS*RE13B. 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 susceptibility to electrical transients. 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 and transients. ,

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 l which actuated placed the plant in a more conservative configuration '

by filtering the supply air to the main control room.

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REPORTED CONDITION At 1248 on 3/11/90 with the unit at 100 percent power a transformer at a local industrial facility failed. Consequently a severe transient on the electrical distribution system occurred which resulted in the Division II control building ventilation system (*VI*) isolating and its respective filtration unit (IHVC* PLT 3D) (*FLT*) automatically initiating. The initiation was caused by a spurious high alarm signal from the control building intake radiation monitor 1RMS*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 safety feature (ESP) actuation. Therefore, this event is reportable under 10CFR50.73 (a) (2) (iv) .

INVESTIGATION The severity of the transient was such that the voltage on switchgear IENS*SWGID dropped to 3193V. As a result, 1RMS*RE13B went into a high alarm state causing the actuation of the Division II control building filter trains, per design. In addition, all inverters swapped to battery supply during the transient and provided appropriate alarms to control room annunciators. The ventilation system and radiation monitor were returned to a normal configuration after the high radiation alarm was determined to be invalid. Inverter alarms were reset following the event and the response was verified to be in accordanco with the design.

Previously submitted LERs concerning events in which ESF actuations occurred due to spurious actuations of radiation monitors have been reviewed. In seven cases radiation monitors have shown susceptibility to electrical noise / transients. Four of these incidents occurred in 1986, one in 1987, one in 1988, and one in 1989. These events have been reported in LERs86-020, 86-040, 86-052, 86-062, 87-008, 88-008 and 89-037. Four of these previous occurrences were caused by 1RMS*RE138.

Electrical transients of this severity have an impact on downstream equipment fed from unregulated power supplies. This is evidenced by 1RMS*RE13B going into high alarm along with 19 other monitors which momentarily lost sample flow plus two other monitors entering high ,

alarm conditions due to the electrical transient.

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 and light g,.o= == . u a o.o i. +, . m 4 e

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010 0 13 0' O 13 wt . . e w.om circuits, etc. MR86-1669 has changed preamplifier discriminator settings to make the detector circuitry less sensitive to electrical noise. These modifications have been very effective 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, 1988, and 1989.

Radiation monitor 1RMS*RE13B is apparently much more sensitive to electrical noise than other radiation monitors. However, further '

filtering, voltage suppression or discrimination against electrical noise for 1 RMS

• RE13 B is not feasible 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 subject electrical transient to which the monitor was subiected, no further corrective action is necessary.

SAFETY ASSF;SSMENT 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 tiltering the air entering the main control room.

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

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