ML20024G970

LER 91-002-00:on 910406,operating Module Breaker in Automatic Turbine Tester Circuitry Tripped Open,Deenergizing Control Solenoid for change-over Valve.Caused by Use of Undersized Circuit Breaker.Module replaced.W/910506 Ltr

ML20024G970
Person / Time
Site:	Grand Gulf
Issue date:	05/06/1991
From:	Cottle W, Ruffin R ENTERGY OPERATIONS, INC.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
GNRO-91-00075, GNRO-91-75, LER-91-002-02, LER-91-2-2, NUDOCS 9105140289
Download: ML20024G970 (5)

Text

m _

g Entergy Operat6ons,Inc.

Operations  ;

W. T. Cottle May 6. 1991 U.S. Nuclear Regulatory Cormnission Mail Station P1-137 Washington, D.C. 20555 Attention: Document Control Desk

Subject:

Grand Gulf Nuclear Station Unit 1 Docket No. 50-416 License No. NPF-29 Reactor Scram During ATT Surveillance LER 91-002 GNRO-91/00075 Gentlemen Attached is Licenseo Event Report (LER)91-002 which is an interim report.

Yours truly.

h W C/RR/cg attachment: LER 91-002 ce: Mr. L. C. Hini.z (w/a)

Mr. R. B. McGehee (w/a)

Mr. N. S. Reynolds (w/a)

Mr. H. L. Thomas (w/c)

Mr. J. L. Mathis (w/a)

Mr. F. W. Titus (w/a)

Mr. Stewart D. Ebneter (w/a)

Regional Administrator U.S. Nuclear Regulatory Co:anission Region 11 101 Marietta St., N.W., Suite 2900 Atlanta, Georgia 30323 Mr. L. L. Kintner, Project Manager (w/a)

Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Mall Stop 11D21 Washington, D.C. 20555

((bb 13 l R.1R91002 /sCMPFLR - 1 9105140209 910506 PDR ADOCK 05000416 j[/

S PDR

_ _ ~ ~ - . - - . - - _ ~._. _ _ - - - - . - - - . - - . ~

Attachment to GNRO-91/00075

~

,c,,. . m. ui ucheA. .ioutaeon,c m io. I aernovao ous =o timeim tlCENSEE EVENT MEPORT ltER) siemes e si =

..C. Liv v . A . m oocai? v .m is, ng i Grand Gulf Nuclear Station, l' nit 1 ia h. i.. o 151 o l o I o (e i 16 h)O l4 Reactor Scram During ATT Surveillances

, e vent Dar8 'le te n tvues a .6, assomt caftsti ofwin f actLit:48 inv0Lvl0 80t e MO%fw Car vtaa vtaa $8[] *

,h $8,Q wO% t ee Dev stap

• aces Y % awls Doca g Y %gwStat ni 0 1 5l0to!o; i l

~ ~

0l4 d6 9 1 9l 1 0l 0l2 0l 0 0l5 nl6 9l1 o l5 1 0 toioi ; i

,,,,,,,,, tais menost is svowiftso evasvawt to two asovianweers ce to cea 4 < cane ... . , e+ , e new. , oli M005 rei 1 20 40iits 20 40siel Y to 73mu2Hwi ti 71661 a N 400 seilt slo 60 Mielm to 73musu.i 73 f tser ni 7 is a 40. .,n H.,

M eosisH1Hwi

.0 = .e n,,

60 t3isetttid

. ,3.u,i, ,

to 73ieH3H.adtal

_ ==g,y,,

J6848 ft A0GwH1hh.I to 73eH3Het to 184a 42 He*He r N 406isH1Het to ? Smit 3 Heel to taieM3Hel Lacthett C0% tact som TM;g kan atti Navt

't 6tP=c%g %,veta asia cxi Riley Ruffin / Lice'1 sing Specialist 610 0 4131 71-1211 1617 COMeistl 0%E Liht 80s E ACM Comepongst pa#Lyng gescaisg0 eas twis oepost visi COU'0%I%t ca wll Sv$'tv ' ' , f '[g' e#f5 Ca wlt B rit t u COM*0%8%t

[ ]'[",','

1 I J l i I I I  ! ! !  ! 1 I l l i I I I i l l ! I i l l St,'wLf wl NT AL p t *0st $ u #f Cit 0114' WO%ta Cav *Esa St 4W &aiON f ,es n,,e awe.e. as*testo soevesstor ca rri } %;, #*""

Of q\ 9l1 utaact - ,,.im .e e ,e.-e... ..,.e e,. ~ , ~. .o.,

During a routine surveillance of the' Main Turbine Mechanical Overspeed Trip Devices, an automatic reactor shutdown occurred. The test was being performed by the Automatic Turbine Tester (ATT) which uses a computerized test program. During the reset step of the computer program, a miniature circuit breaker in the ATT circuitry tripped open. This deewrgized three solenoids which were needed to prevent a turbine trip due o testing. The cause of the premature breaker trip is attributed to the c cuit breaker being undersized for its application. Due to aging and the breaker operating at loads which sometimes exceeded its rated current value, the breaker was operating in a degraded condition. The operating module which contained'the miniature breaker was replaced and the test was successfully completed. An evaluation will be performed pending vendor recommendations to develop the most adequate design to preclude recurrence.

Until an interim or final resolution has been implemented, the ATT will not be used to perform the overspeed test. The surveillance requirement will be satisfied by performance of the test from the hydraulic rack. All safety systems functioned as designed. The reactor water remained at least 164 inches above the top of active fuel.

,a,e

, . . ,n, L. ZZ"i "

~

ge

• • • ' u =veusa u:u64,o., co ne io=

smo s owe o awa UCENSEE EVENT REPORT (LER) TEXT CONTINUATION omsawm I Leftannes m Docaatsevasta ul Lan asudsta foi tact tal F *ia. "en.'; -

"'#.7 t, rand Gulf Nuclear Station o ja jo lo l01411 l 6 9 11 0 l012 -

O lo nl2 0F nI&

A. Reportable Occurrence During a routine surveillance of the Main Turbine Mechanical Overspeed Trip Devices, an actuation of the Reactor Protection System (RPS; Ells Code: JC) cccurred. The actuation was due to a Turbine Stop and Control Valve Closure. This occurrence is reported pursuant to 10CFR50.73(a)(2)(iv).

B. Initial Condition The plant was in Operational Condition 1 at approximately 745 reactor power. A biweekly surveillance to demonstrate the operability of the Main Turbine Mechanical Overspeed Trip Devices was in progress at the time of occurrence.

C. Description of Occurrence On April 6, 1991, a biweekly surveillance of the Turbine Mechanical Overspeed Trip Devices was in progress using the Automatic Turbine Tester (ATT).

The ATT uses a computerized test program to perform the overspeed test.

The devices being tested were two mechanical overs)eed bolts located in ,

the turbine shaft. During the test, test fluid, w1ich is supplied by the overspeed trip test-device (OTO), is used to force these bolts to their extended (trip) positions. To prevent a turbine trip, due to testing, the ATT energizes a solenoid valve (IN32F512) which controls the Turbine Control System's (EHC; EIIS Code: TG) chae-over valve. The change-over valve allows sufficient control fluid pressure to the EHC's trip fluid header to prevent a turbine stop/ control valve closure. Control fluid is supplied to the change-over valve through two solenoid valves (IN32F511A

& B) which are also energized by the ATT. The electrical trip signals are routed to the two supply solenoids to provide turbine protection during the test.

On April 6, 1991 at approximately 0427 an operating module breaker within the ATT circuitry tripped open, during Step 51 of the test program, deenergizing the control solenoid for the change-over valve and its associated supply valves. This caused a main turbine trip which resulted in an automatic reactor shutdown, g ... .

LER91002/SCMPFLR - 3

- ' , ,m.- , . - . .

r ~.

Attachment to GNRo-91/00075

1. 1. C *erm ABBA y 3 seUCLE&2 EtrugatDat CCm*WiSSIO8e UCENSEE EVENT REPORT (LER) TEXT CONTINUATION Amovio owe =o w-o+

EEPiet$ l'31,s NLaTV enaast na poca gy a,yegga gg, L A NUMB R tll PAGE 131

"*a " tM. O*J:

Grand Gulf Nuclear Station o ja jo lo j o l 4]l j6 9;0 _

0 [ 0l 2 0l 0 0l3 or 0 l4 turt n . w - : =ac -w nn ,

The turbine trip also caused the Reactor Recirculation Pumps (Ells Code:

AD) to shift from their normal power supply to the low frequency motor generators. The decrease in forced core flow caused a level increase in the vessel downcomer region which resulted in a decrease in feedwater system (FWS; Ells Code: SJ) flow. As a result of the FWS flow decrease, vessel water level decreased to approximately -2 inches as indicated by General Electric Transient Analysis Recordirig System (GETARS). Sensing the decrease in vessel water level, the FWS increased its flow rate to compensate for the decrease in vessel water level. The FWS restored the vessel water level to approximately 35 inches as indicated by GETAR5.

Due to the closure of the main turbine stop and control valves, reactor steam dome pressure peaked at 1054 psig. The main bypass /stop valves opened to decrease pressure and maintained vessel pressure at approximately 955 osig.

D. Apparent Cause A subsequent investigation of the occurrence determined that a miniature circuit breaker (b4) in an ATT operating module (K21A) tripped open during the test. The breaker supplied power to the change-over valve's control solenoid and its two supply valves, which were essential to maintain the turbine online during the test.

During Step 16 of the ATT test program, the program verifies the mechanical bolts extend to 1. heir trip position prior to the test fluid header pressure reaching 21 psig, which is the computed upper limit for acceptance for the ATT Test, in the event both bolts do not trip prior to the test fluid header reaching 21 psig, the ATT drives the OTD to its end position to ensure the bolts are fully exercised during the test. It takes approximately 90 seconds to drive the OTD to its end position.

Following Step 16, Step 51, which is the first step of the reset portion of the test program, would verify that the OTD was in its normal ooerating position. Therefore, the program would remain in Step 51 until tne OTD was driven from its end position to its normal operating position. During Step 51, the current load exceeded the breaker's rated current value (3 amps). The duration of the breaker having to maintain the excess current was approximately 90 seconds. Subsequent calculations indicated that the current imposed on breaker b4 during step 51 was approximately 3.8 amps.

During the testing of the b4 breaker, it repeatedly tripped at approximately 90 seconds. During the investigation, test data for the b4 breaker was compared to new breakers and a current vs time-to-trip curve.

Based on this comparison, it was concluded that the breaker should have maintained the load, imposed during Step 51, for a longer period of time (approximately 40 mins.).

e.ac poem seea l

• LER91002/SCMPFLR - 4

.', Attachment to GNRo-91/00075 1RC Pym Fu y 8 NUCLlan Rt0VLATO21 CouMTS$104 LICENSEE EVENT REPORT (LER) TEXT CONTINUATION *naovie owe ao w-cua ExPtatt 4 31 ab M 87V HAM 4 Ill Occati muusta L2) Lam huusta tea PAOS 13) v8** t!.R."  ?'J *,.B Grand Gulf Nuclear Station 0 l6 j o { 0 l0 l 411 l 6 4l1 -

010l2 -

Ol0 0 l4 0' nl4 ttXT M sese spees e aspeest aus seenw.r MC 15 mum 3estsiI1M The K21A module was replaced with a new module and the ATT overspeed test program was completed satisfactorily with the turbine at 1800 rpm.

The cause of the breaker tripping prematurely is attributed to an inadequate design in the ATT circuitry, The b4 breaker is undersized for the load imposed during step 51 of the test program.

There are two major contributing factors to the occurrence.

o The b4 breaker had degraded due to its age and its operating at current values greater than its rated value, o The overspeed bolts not extending to their trip positions prior to the test fluid pressure exceeding the pressure setpoint caused the amount of time the program would remain in step 51 to increase due to the OTD being at its end position.

E. Supplemental Corrective Actions An evaluation will be performed to develop the most adequate design to preclude recurrence. A supplemental report will be submitted by August 01, 1991 describing the final resolution.

Until an interim or final resolution has been implemented, the ATT will not be used to perform the mechanical overspeed test. The Technical Surveillance requirement will be satisfied by performance of the test from the hydraulic rack.

F. Safety Assessment The Post Trip Analysis confirmed that the safety systems functioned as designed. The reactor level remained at least 164 inches above the top of active fuel during the event. The turbine / generator trip functions were in place at all times.

g.- -. Luvtuon sc m a - t,

._. . _ _ _ __________________ - __ - _ _