ML20044C919

LER 93-002-01:on 930206 & 07,Train a & Train B CR Pressurization Sys Failed 18-month Surveillance Test, Respectively.Caused by Moisture in Air Banks & Pressure Oscillations.Air Banks purged.W/930507 Ltr

ML20044C919
Person / Time
Site:	Millstone
Issue date:	05/07/1993
From:	Mcdonald R, Scace S NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-93-002, LER-93-2, MP-93-374, NUDOCS 9305140059
Download: ML20044C919 (5)

Text

1 NORTHEAST UTILITIES o--a" Sans" Benc=a*==*

3 vesf e$.Yi$$bn Y s itN. rYcomp$n[ P.O. Box 270

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oEas [d$:$~[e vihcNpany Nonneast Noear Energy Company HARTFORD. CONNECTICUT OS141-0270 (203)SE5-5000 l May 7, 1993 MP-93-374 Re: 10CFR50.73(a)(2)(vii)

U.S. Nuclear Regulatory Commission Document Control Desk Washington. D.C. 20555 t

Reference:

Facility Operating License No. NPF-49 Docket No. 50-423 Licensee Event Report 93-002-01 Gentlemen:

This letter forwards Licensee Event Report 93-002-01 required to be submitted within thirty (30) days pursuant to 10CFR50.73(a)(2)(v:.i).

Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY

/

Stephe ENeace e

Vice President - Millstone Station SES/RJM:ljs

Attachment:

LER 93-002-01 cc: T. T. Martin, Region 1 Administrator P. D. Swetland, Senior Resident Inspector, Millstone Unit Nos.1, 2 and 3 V. L. Rooney, NRC Project Manager, Millstone Unit No. 3 120cn /

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Common A1 ode Failure of Control Room Emelope Pressurization Sgstem EVENT C ATE (5; LER NUMBFA 461 l REPOAT DATE (7 p OTef A F ACtLrTIES INVOL VED (81 F ACUTY NAME S voth DAY YEAR YEAR "Y MONTH DAY YE A:,

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NO l l l ABSTRACT (Lime 11c 94D0 spaces, . e., ap;>roxamatory f.fteen singie-space type *ntten bnes) (15)

On February 7,1993, at 1938 hours0.0224 days <br />0.538 hours <br />0.0032 weeks <br />7.37409e-4 months <br /> with the plant in Afode I at 1009 power the A Train Control Room Pressunration System failed its 16 month pressurization surveillance test. On February 6th the B Train had imtially failed its surveillance test. An imestigation revealed two potential common mode failures and both trams were declared inoperable pending further invesupation.

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The first potential common mode failure is freezing of the pressure regulating vahe caused by moisture in the 4 air banks. The second common mode failure is pressure oscillations unthin the control room envelope which cause the differential pressure to period >cally fall below the .125 in wg acceptance enteria for brief periods of time.

The immediate correctne act on was to place the Control Room in filtered recirculation in accordance with i Technical Specification 3.7.8.b.1. The B Train surveillance was satisfactorily performed after a throttle vahe was opened further. The air banks were purged and refilled with dry air to reduce the dewpoint.

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NRc Form 360 (6-69)

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1. Deermtion of Event On February 7,1993, at 1936 hours0.0224 days <br />0.538 hours <br />0.0032 weeks <br />7.36648e-4 months <br /> with the plant in Mode 1 at 100cc power (2250 psia and 557 l degrees Fahrenheit), the A Trair2 Control Room Envelope Pressurization System failed the 16 month l prenurizauon suneiilance test. On February 6,1993, the B Train had imtial!y failed and then passed the same surveillance test. On February 11, 1993, both trams were declared inoperable based on a potennal common mode failure of the system.

As immediate correcthe action the controi room envelope was placed m fihered recirculation in  ;

accordance with Technical Spec:fication 3.7.S.b.1.

i Oscillations m the control buildmg envelope pressure on each train resuhed in differential pressures  !

dropping slightly below the acceptance critena dunng retests. These d:ps occurred on a cyclic basis 5 to S  !

times over the one hour duration of the test and resuhed in differential pressures slightly below the accertance criteria for penods of thiny seconds to two minutes.

After the air banks were refilled with known dry air, the same oscillations in control room pressure occurred.

11. Caute of Event The root cause of the initial B bank failure was thought to be moisture m the bottled air system which condensed and froze at the pressure regulating valve (3HVC-PCV6SB), during system discharge. Further mvestigation determined that although thi potential exists for introducing moisture into the system, it probably did not factor into the failure of the surveillance tests on either train.

The root cause of the pressure oscillations was equipment failure, specifically misoperation of another l

component. The pressure oscillations were determined to be caused by temperature changes within the control room envelope which were the result of hunting of the B train chilled water valve control loop in the air conditiomng system. The loop response to slight changes in control room temperature was a rapid and gross repositioning of the valve. The result was dramatic oscillation of the chilled water valve.

The control room pressure oscillauons mirrored the chilled water valve movement, as the valve opened the pressure decreased and as the valve closed the pressure increased.

l 111. Analvcic of Event This event is bemg reported in accordance with 10CFR50.73(a)(2)(vii) as an event where a single cause or condition caused two independent trains to become inoperable in a single system designed to mitigate the consequences of an accident. Pressure oscillations caused the control buildmg envelope differential pressure to penodically drop below the acceptance cnteria of .125 in wg. The dips were temporary and the control room pressure remains positive relative to outside.

On Febntary 7,1993 the A Train failed the surveillance test and was declared inoperable. LCO 3.7.6.a was entered for one train of Control Room Envelope Pressurizauon inoperable. On February 11, 1993, after an investigauon determined a potential common mode failure the B Train was also declared inoperable. The control building envelope was placed m the filtered recirculauon mode in accordance with Technical Specification LCO 3.7.B.b.1.

Each train of the Control Room Envelope Pressurization System is designed to pressurize the control room emelope to .125 in up or greater for one hour in the event of a Control Building Isolation (CBI) signal. After one hour the Emergency Filtration System is staned and provides filtered air under a slight positive pressure to the control building envelope.

The Safety Significance of this event is low because fihered intake was always atailable and the control room pressure remained positive in all tests of the pressurization system.

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• more soace is rease: a5e acomonat Nac Form 36E A s; (1D IV. Corrective Amnn The immediate correcure action after an investigation determined a potential common mode failure was '

to ceclare both trams of Control Room Fnvelope Pressurizauon System inoperable, enter LCO 3.7.5.b.1 '

and place the Control Building Emergency Filtration System mto operation m the fihered recirculation mode.  !

a Samples of the air in the air bottles of both trains tested at dew points around minus 40 degrees  !

i Fahrenheit at atmospheric pressure, which corresponds to approximately 70 degrees Fahrenheit at -

l 2250 psig. Excess moisture could potenually enter the system due to improper blowing down of the condensate traps or from purifying cartridges which have exceeded their useful life. The system design i does not include drying capabilities other than the moisture removal capaeny of the compressors. .

l j

The air banks were purged with dry air after the compressor punfier cartridges were replaced. The air l banks were charged wnh air which tested at better than minus 60 degrees Fahrenheit dew point. An j

, mspection was performed on one of the lower air bottles for the presence of moisture or corrosion l caused by the presence of moisture. The inspection determmed that no moisture was present in the l bottle and that only two small areas of surface discoloration were present. l Dunng testmp to determine the catise of the oscillations, the A Train pressure regulating valve  !

f 3HVC*PCV66A) failed in the open position. The valve was disassembled and n was determmed that a ,

dowel pm in the disc and stem assembly had failed. The vahe was replaced, tested and returned to l service.

j Once the B Tram chilled water control loop was identified as the cause, the loop was tuned to respond l

in a more gradual manner to temperatures off of setpomt. The A train was renfied to respond i appropriately to temperature changes. Subsequent pressurization tests showed very stable readings for  ;

control room pressure and chilled water valve position for each train of Control Building air conditioning.

Action to Prevent Recurrence The chilled water valve controller tuning will be checked during periodic calibration of the loops. The surveillance procedure has been revised to include a prerequisne step to notify Engineering prior to ,

performing this surveillance test. This will allow for comprehensive trending of system performance. i I

Target Rock, the pressure control vahe manufacturer was contacted in regards to the failure mode of the valve. No preventative maintenance is scheduled on the valve as a result of this failure due to the i infrequency of similar failures. An NPRDS search did not identify any similar failures.

l The potential for introducing moicture into the system was discovered during this investigation.

1 Dewpoints obtained during and after charging evolutions in March and April 1993 hase consistently been in the -60 to -80 degree Fahrenhen range. Dewpoint samples will be obtained during charging of the system and periodically at the bottles through the summer of 1993. An evaluation will then be performed to determine long term correctise action such as installing an air receiver with drainage capabihties or installmg a dew point analyzer.

V. Additional Information a

l Licensee Event Reports submitted which discuss related events are as follows. ,

i LER Number Title 92-004 Control Room Pressurization Valves Closed Due to improper Verification 1 f7-005 Control Room Pressurization Surveillance Failure Caused By

Mispositioned Throttle Valve Due to Personnel Error garm ass 1

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• TEXT 01 more space is rea.wed. use and tenai NRc Form 350A s) (17)

LER 3-92-004 documents an event where one train of the Control Room Envelope Pressurization system was isolated by the closure of two manual valves. The mispositioning of the valves was discovered dunng the performance of the monthly vahe hneup suneillance. The root cause was improper self verification i of valve posmon after a charging evolution. Correctise action was a program to stress the importance of self verification.

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LER 3-57-005 documents an event where the A Train of the Control Room Envelope Pressurization l system fatled the pressuriranon surveillance test because the throttle vahe downstream of the pressure  !

control vahe was opened too far. The root cause was personnel error and procedure deficiencies.

Correcure action was to place a tag on the throttle valve for each train which specifies the required ,

procedurally controlled position of the valve. The incident was also reviewed by all operating personnel.  !

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f Svnems Comnoner tc l l l l Control Room - VI Pressure Control Valve - PCV l

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FF40 Form 3b6 15-bO)

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