ML20005E333

LER 89-026-00:on 891125,reactor Scram Occurred Due to Closure of Outboard Msivs.Caused by post-filter Media Ignition by Hot Air or Particles from Dryer.Air Dryer B Disassembled & inspected.W/891226 Ltr

ML20005E333
Person / Time
Site:	Cooper
Issue date:	12/26/1989
From:	Horn G, Reeves D NEBRASKA PUBLIC POWER DISTRICT
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
CNSS892321, LER-89-026, NUDOCS 9001050082
Download: ML20005E333 (7)

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CNSS892321-W., -December 26, 1989--

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[' U.S. NuclearLRegulatory. Commission

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Dear = Sir:

Cooper-Nuclear Station' Licensee Event Report'89-026. Kevision 0,'is_being ,

c. forwarded as an' attachment to this letter. -!

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h G. R ' Horn "

Division Manager of-Nuclear Operations Cooper Nuclear Station n

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'INPO Records Center ANI Library NRC Resident Inspector J- R. J. Singer CNS' Training CNS Quality Assurance  !

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"' Reactor > Scram Due to Main Stear Isolation Valve Closure as a Result of Low Instrument Air Pressure Caused by an Air Dryer Malfunction EVEstT DATI (6) Ltn NyaABtR tel REPORT DAff (7) OTHER F ACILITlf 8 INVOLVED 65) - ,

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AneTRACT as.,,, R, oa Na.u , mam,,.wr o.n .,ne.Nm rw. ant an.,1 oei 1 On November 25, 1989, at 10:15 P.M., with the plant at full power, a reactor scram occurred when the outbeard Main Steam Isolation Valves (MSIVs) closed as I

a. result of depressurization of the Instrument Air (IA) System. The dscrease h l in IA System pressure occurrcd when the "B" Air Dryer Post Filter assembly I ruptured. Investin tien revealed that the Post Filtet media (paper cartridges) had been ignited by either hot air or hot particles from the  ;

dryer. The burning cartridges resulted in overheating of the filter assembly j housing.-'The associated heat induced stress, coupled with internal (system) l air pressure, caused the housing to deform and rupture. l Corrective actions taken included complete disassembly, inspection and  ;

refurbishment of the "B" Air Dryer, including replacement of the Post Filter assembly, as well as an extensive inspection of the "A" Air Dryer. Filter I cartridges capable of withstanding high temperature conditions were installed  !

in both Post Filter assemblies. Approximately fifteen feet of piping downsteam of the Post Filter was replaced due to concerns about the possible effects of overheating. Checks were made of the IA System downstream of the

. Post Filters to verify system cleanliness. Additionally, components in the vicinity of the ruptured Post Filter asseml@ were inspected to ensure that debris from the event was not present.

Further corrective action includes upgrading the Air Dryer training programs, perfor, nance of a design review of the Air Dryer trains, and implementation of upgraded Air Dryer operational checks, and preventive maintenance activities.

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.4. PAPERWO K RE U T e J A ,' I OF MANAGEME NT AND OUD LYT. W ASHINGTON,0C 20603.

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'A2 : EVENT DESCRIPTION j

~t On November 25, 1989, at 10:15 P.M., with the plant in routine. j operation at_ full-power, a reactor scram occurred when the outboard l Main Steam Isolation Valves-(MSIVs) closed as.a result of l

'depressurization of the Instrument Air (IA) System. Just prior to '

.)

N the event, Control Room Operators heard a muff]cd rise. This was j immediately followed-by several Loss of Air Pressu~e alarms. IA and "

Service Air (SA) pressures were checked and determ ued to be about

~ 40 psig and decreasing. Normnl pressure ranges from 100 to 105 psig. The.b" pass around the air dryers (SA-MOV-81MV) was immediately opened in an effort to restore system pressure._ 1 IA-MOV-80MV was noted to be automatically closing to isolate 6 non-critical air system demands. Shortly thereafter,.the Control j

• Rod Drift alarm on Control Room Panel 9-5 annunciated. However, before the reactor could be manually scrammed, the automatic scram due to MSIV closure occurred. The tine interval!between receipt of the Control Rod Drift alarm and Reactor'Frotection System (RPS)  !

actuation was approximately six seconds. .

Subsequent to tle scram, the ensuing water 3avel transient resulted_

in initiation of. Groups 2,.3, and 6 Isolations (Reactor Coolant;

-Reactor Water Clean-up; and Secondary Containment, including  ;

actuation ~of the Standby Gas Treatment System). Additionally both '

i

, the High Pressure Coolant Injection (HPCI) and Reactor Core Isolation Cooling (RCIC) Systems automatically initiated as a result of the water level transient and functioned to restore and maintain  !

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Reactor water level.

As_ indicated above, the reduction in-IA System pressure resulted in -!

closure of the outboard'MSIVs. It should'be noted that during normal plant operation,.the inboard MSIV pneumatic supply is N f*

• the Nitrogen Inerting Systec.; consequently, the inboard MSIVs,2 as ,

well as the remainder of the components inside the Drywell being supplied with N , were unaffected by the TA System depressurization.

Themostimmedikteeffect,however,oftheIASystem depressurizacion was associated with the Control Rod Drive (CRD) i System Scram Air Header. As IA System pressure decreased, Scram Air Header pressure also began to decrease. As a result, CRD insertion ,

(annunciated in the Control Room by the Control Rod Drift alarm) began due to the individual CRD Scram Pilot Valves and Scram Valves repositioning to their open (fail safe) position. Prior to y actuation of the RPS due to'the MSIVs not being fully open (with the Reactor Mode switch in RUN), 125 of the 137 contral rods were partially or fully inserted in the core. As a result, at the time of RPS actuation, power level had been reduced from 100 percent to less than 10 percent.

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~Upon rupture of the "B" Air Dryer Post Filter assembly, the high flow rate of air through the drying chamber in service resulted in

. excessive differential pressure across the Air uryer chamber outlet strainer.- The strainer collapsed, allowing a small amount of ,

!' desiccant to be transported out of the chamber, through the ruptured 1 Post Filter assembly and into the Control Building basement area.

4 At.10:37:P.M.,-the "B" Air Dryer was isolated and IA System pressure p restored'to-normal. Subsequently, normal scram recovery procedures

-were implemented.

B. PLANT STATUS 0

Prior to rupture of the "B" Air Dryer Post Filter assembly, the plant was operating at approximately 100 percent power (800 MWe).

At the' time of RPS actuation, power had been reduced to less than ten;(10) percent due to Control Rod insertion as a result of-low IA System (and Scram Air Header) pressure. I C. BASIS FOR REPORT An. unplanned RPS actuation, reportable in accordance with E

10 CFR 50.73(a)(2)(iv).

D. CAUSE i Each of the Air Dryers (A and B trains) has two chambers (towers) which contain an alumina oxide desiccant mat $ rial. While one ,

chamber is on line, the other chamber is being regenerated by reversing the purge air flow and heating the deciccant with an l~ electric heater-(Pall Trinity Drawing 6AC2024-ED1). A timer controls the switching action between the two chambers of the dryer.

'A regeneration cycle is three (3) hours of heating and one (1) hour of cooling. The heating of the desiccant is controlled by.

temperature switch 2TS (Pall Trinity Drawing C3024-46T), which is designed to open at 650*F. This temperature switch failed to control the electric heater during the regeneration cycle. As a result,'when the timer shifted the two air dryer chambers and placed g this chamber on line, hot air (and possibly some hot desiccant particles) flowed into the Air Dryer Post Filter and resulted in ignition of the paper filter elements in the Post Filter assembly.

The Post Filter assembly heated and ruptured under system pressure.

Contributing to the failure of the Post Filter assembly was the use of low temperature filter elements made of paper (ignition temperature of 451'F). At the time that the Air Dryers and Post Filter assemblies were installed, the vendor design specified these filter elements. The current design uses high temperature filter l

elements which are not susceptible to ignition.

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, None. .-During normal plant operation, the pneumatically operated

'devicca inside the primary containment (ADS, Low-Low Set, inboard MSIVs and-associated accumulators, etc.)_are suppled with N,3 from the Nitrogen Inerting System that was unaffected by the event.

Therefore, operability of these components was not affected by the Ldegraded IA System pressure, It should be noted also, that d.ese devices have Class I accumulators and check valves, and are

periodically leak tested to ensure operability upon loss- of N '

2 The IA supplied air-operated devices outside of primary containment that perform isolation functions are provided with seismic Class 1 accumulators and check valves. .These components are periodically leak tested to verify their capability to perform their intended function-following loss of IA. During this event, these valves functioned as designed. Finally, the MSIVs have a " fail-safe" design in that they will close in the event of. loss of IA. The outboard MSIV's performed as intended during this event.

F. SAFETY IMPLICATIONS None. During normal plant operation, upon loss of IA, the outboard MSIVs will c1cse. As noted in the USAR.in Chapter XIV, Section 5.0, the analytical results of a MSIV closure event, considered to be an

l. abnormal operational transient, are based upon the plant being in  ;

t! operation at 105 percent of rated power. This event, then,

l. constituted the most limiting transient which, as previously noted

'in Section E. resulted in no safety concerns.

l G. CORRECTIVE ACTION

'As noted in Section A, Event Description, HPCI and RCIC l automatically actuated to restore Reactor Vessel Water level and the "B" Air Dryer was isolated providing for restoration of the IA and L

SA Systems to normal. Subsequently, normal scram recovery actions were initiated.

With regard to the "B" Air Dryer and Post Filter assembly, a detailed systematic investigation of Air Dryer operation was completed. As a result of this effort, the "B" Air Dryer was completely refurbished. Elements of the investigation and refurbishment effort included:

1. All associated instrumentation and controls were either replaced or recalibrated, and verified to be operating correctly.

2. The electric heaters and the deaiccant in both chambers of the "B" Air Dryer were replaced.

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3. 'A replacement Post Filter assembly was procured and installed. '

The replacement unit is functionally equivalent to the originally installed unit that failed, but differs in overall r dimensions and inlet and outlet configuration. As a result, .

piping configuration changes were implemented as required.

4. Filter cartridges resistant to high temperature gases were procured and installed. Pre-service testing included verification that the new post filter media does not burn (tested to.1200'F).

5. The p-efilter media was inspected and replaced with the normal

• filter media.

6. ' Fifteen feet of piping downstream of the "B" Air Dryer Post Filter assembly was replaced due to concerne regardirg the potential effects of its being overheated, s

Following this effort, a detailed post mainte.ance test was performed to verify proper operations of the "B" Air Dryer.

An inspection and refurbishment effort of the "A" Air Dryer was also conducted.- A leaking check valve was replaced; following verification of correct setpoints, replacement thermal switches for the heaters were installed; and the heaters were tested, resulting in one being repinced. The dryer outlet strainers were inspected and cleaned. The control timer was also verified to be operating-correctly. The Post Filter media was replaced with new high temperature media, and the pre-filter media was replaced. Post maintenance testieg was performed to verify correct operating sequence. Further inspection of the "A" Air Dryer is planned during the 1990 Refueling Outage.

1 l The rupture of the "B" Ait Dryer Post Filter discharged filter i debris and a small amount of desiccant from the in-service chamber -

into the Control Building basement. The motors of pumps and the air compressors located in the basement, as well as Motor Control Center o cabinets MCC-L, T, and X, were inspected to ensure they were clean L and free of any material discharged by the ruptura. Additionally, l the intake air filters of the three Station Air Compressors were l' cleaned. To ensure that desiccant and/or other debris from the in-service chamber was not carried downstream into the IA System, in-line filters installed close to components supplied with IA were inspected. No contaminants were found. Additionally, the air lines l were blown down at varicus selected locations. No evidence that i desiccant or debris had migrated downstream from the Air Dryer or Post Filter assembly from this event was found.

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G.. CORRECTIVE ACTION (Contf.nued) ]

Corrective actions currently underway or being planned include the following:'  ;

1. A revision-is being made to Operator logs to provide for more closely monitoring dryer. chamber bed temperatures. Normal operating parameter limits have been specified.

2. Appropriate training programs regarding Air Dryer operation will be reviewed and upgraded.-  ;

3. The existing design of the dryer trains will be re-evaluated.

4. Preventative Maintenance requirements and routine operational.  :

checks will be reassessed, based upon vendor manual. +

requirements and industry experience. Upgrades to existing program requirements will be implemented, as appropriate.

H. SIMILAR EVENTS None.

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