ML19325C199

LER 89-015-00:on 890905 & 23,special Ventilation Sys of Control Rooms 122 & 121 Actuated Automatically.Caused by False High Chlorine Signal & Broken Chlorine Sensitive Paper Tape,Respectively.Detectors repaired.W/891005 Ltr

ML19325C199
Person / Time
Site:	Prairie Island
Issue date:	10/05/1989
From:	Hunstad A, Parker T NORTHERN STATES POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM), Office of Nuclear Reactor Regulation
References
LER-89-015-01, LER-89-15-1, NUDOCS 8910120202
Download: ML19325C199 (6)

Text

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414 Nicollet Mall -

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October'5; 1989 10 CFR Part 50 Section 50.73-F Director of' Nuclear. Reactor Regulation

.U S Nuclear Regulatory Commission Attn:. Document Control. Desk

' Washington,- DC 20555' PRAIRIE ISIAND NUCLEAR GENERATING PLANT Docket Nos. 50-282 License Nos. DPR-42 50 306 DPR-60 3 Automatic Control Room Isolation and Start of Control Room Cleanuo Fan Due to Failure of a Chlorine Gas Monitor The Licensee Event Report for this occurrence'is attached.

-This event was reported via the Emergency Notification System in accordance with 10 CFR Part 50, Section 50.72, on September 5, 1989 and September 23,

=1989. Please contact us if you require additional information related to this event.

Thomas M Parker

-Manager Nuclear Support Services c: Regional Administrator - Region III, NRC NRR Project Manager, NRC Senior Resident Inspector, NRC MPCA Attn: Dr J W Ferman Attachment (

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This event consists of two similar occurrences. On September 5, 1989, the automatic actuation of 122 control room special ventilation system, was caused by a spurious high chlorine signal. On September 23, 1989 the automatic actuation of 121 control room special ventilation system was caused by a spurious high chlorine signal. Two different chlorine detectors were

- involved.

Thts cause of the first occurrence was determined to be a false high chlorine signal generated when its optic block malfunctioned. The optics block was replaced and the monitor was returned to service. The cause of the second occurrence was determined to be from a broken chlorine sensitive paper tape.

The tape was replaced with a new one and the monitor was returned to service.

Corrective action to be taken include the installation of additional monitors and a modification of the actuation logic which initiates the automatic actuation.

The control room ventilation system entered its safeguards mode of operation as designed.

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EVENT DESCRIPTION ,

On September 5, 1989, the " chlorine gas present" alarm was received, the 122 control room clean up filter fan (EIIS Identifier FAN) started,-and the outside air to the control room automatically isolated. An operator,was sent >

'to invastigate. It was determined that there was no chlorine present and the chlorine monitor (EIIS Identifier MON) was placed in bypess. The optics block a and gate assembly was replaced and the monitor was returned to service ,

September 11, 1989. 1 On September 23, 1989, the " chlorine gas present" alarm was received, the 121 control room clean up filter fan started,-and the outside air to the control room automatically isolated. An operator was sent to Anvestigate. It was determined that there was no chlorine present and the chlorine monitor was' placed in bypass. The chlorine monitor's chlorine sensitive tape was found a -torn. . The tape was replaced, and the monitor was returned to service September 29, 1989.

CAUSE OF THE EVENT The auto starts of a control room special ventilation system were caused in each case by erroneous high chlorine gas signals from a control room chlorine monitor. ~The signal in the first occurrence was caused by a faulty optics block, similar to that reported in LER 1-89-12. The high chlorine signal in the second occurrence was caused when the chlorine sensitive tape was torn, causing the remaining tape to become stationary. When stationary, the tape eventually becomes discolored by the continuous sample flow past it, causing the monitor's optics system to interprer the discoloration as an indication of the presence of chlorine.

! ANALYJJS OF THE EVENT g

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The functional response of the automatic actuation of the control room clean up fan and isolation of the outside air to the control room was according to 1 design. The purpose of this isolation is to protect control room personnel from chlorine. Since neither of these occurrences was triggered by the presence of chlorine in the atmosphere, there was no threat to the operation

l. of the plant. In both occurrences, the affected chlorine monitor failed to the safe condition. Therefore, this event did not affect the health and safety of the public.

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CORRECTIVE ACTION The two chlorine detectors were repaired and returned to service as neted, n

The logic is designed such that a spurious high chlorine' signal on any'one'of=

the four detectors will. actuate engineered safeguards features.. An interim mod.tfication to the control room ventilation system will be made to reduce the L probability of spurious actuation of these engrneered safeguards features.

1 Two additional chlerine detectors, AE X and AE-Y, will be added to the #122 control room ventilation air supply duct (Figure 1). The logic changes are E identified below:

For Actuation 3 to occur, it will require both AE-28346 and AE-X detectors to trip in order to initiate the actuation logic. j For Actuation 4 to occur, it will require both AE 28347 and AE-Y detectors to trip in order to initiate the actuation logic.

This change will reduce the automatic isolations of #122 control room

-ventilation system and automatic initiations of the #122 control room cleanup.

' filter fan caused by spurious high chlorine signals. Two chlorine detectors will have to trip before an actuction signal is generated'(c 2 out of 2 y; logic). However, a single detector failure will not prevent automatic r l- isolation because the two detectors in-the other 2 out of 2 logic are' independent and can initiate the isolation if there is in fact a high 1 concentration of chlorine present. If one of the detectors becomes inoperable, the inoperable detector will be placed in the trip position and L that channel would then remain operable with a 1 out of 1 logic.

L Since we haven't yet determined the best long term solution and there is a L lack of detectors available in the near future, the same modification will not j .be made to #121 control room ventilation system . In order to prevent '

spurious isolations of #121 control room ventilation system and automatic 1 initiations of the #121 control room cleanup filter fan, we intend to remove from service the chlorine detection channels associated with #121 control room ventilation system and place its chlorine detectors in " bypass". This will prevent either of the detectors from init!2 ting an actuation signal. It is 1 l; recognized that we will be entering the Limiting Condition for Operation l action statement in Technical Specif.ication 3.13.E by placing the chlorine  !

l: detectors in " bypass". This will, however, nor decrea:e the protection of the I l.

control room from a chlorine incident, because the outside air intake dampers associated with #121 control room ventilation system will be closed.

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With the exception of the chlorine detection being in " bypass", #121 control room ventilation system will be.in its normal configuration when #122 control ,

room ventilation system is in use. If it becomes necessary to put #121  ;

control room ventilation system into service, its chlorine detectors will be taken out of " bypass". .

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It is intended to complete this interim modification under 10 CFR Part 50, Section 50.59 by January 1, 1990, ,

To improve the reliability of the chlorine detection system for the long term, we will perform the following investigations:  :

Investigate the removal of the onsite liquid / gas chlorine source and determine the effect of that change on the necessity of chlorine detection capability.

Investigt w the availability of qualified detectors with higher reliability than those presently installed.

Investigate the installation of a system which is capable of sampling chlorine from either #121 or #122 control room ventilation systems. This would allow the use of only four detectors and a 2 out of 4 logic. This requires reanalysis of the response times-involved to ensure timely isolation of the control room.

COMPONENT IDENTIFICATION MDA Scientific Chlorine Detection System Model 7040 FAN f

PREVIOUS SIMIIAR EVENTS

-Previous similar events were reported as Unit 1 LERs 89 006,89-009 and 89-012, L

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