ML20045D904

LER 93-012-00:on 930524,reactor Trip Occurred Due to Turbine Trip & Malfunction Caused Feedwater Regulating Valve a to Stay Approx 56% Open.Caused by Lack of Heat Removal Capabilities.Hand Wheels secured.W/930623 Ltr

ML20045D904
Person / Time
Site:	Millstone
Issue date:	06/23/1993
From:	Hoagland Z, Scace S NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-93-012, LER-93-12, MP-93-503, NUDOCS 9306300196
Download: ML20045D904 (5)

Text

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NORTHEAST UTILITIES oenerei cm'ca seicen street. Berkn connecticut I WYsfe$"U$l[s'acNsettIE1e r$c n P.O. BOX 270

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' Nutheast Nue ear Energy Company HARTFORD. CONNECTICUT 06141-0270-(203)665-5000 June 23, 1993 MP-93-503 Re: 10CFR50.73(a)(2)(iv)

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

Reference:

Facility Operating License No. DPR-65 '

Docket No. 50-336 Licensee Event Report 93-012-00 .

Gentlemen:

This letter forwards Licensee Event Report 93-012-00 required to be submitted within thirty (30) days pursuant to 10CFR50.73(a)(2)(iv).

Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY FOR:

V e, hen E. ScaceStep ' President - Millstone Station BY: red R. Dacimo . -

Millstone Unit 3 Director SES/ZH:ljs

Attachment:

LER 93-012-00 cc: T. T. Martin, Region 1 Administrator P. D. Swetland. Senior Resident Inspector, Millstone Unit Nos.1, 2 and 3 G. S. Vissing, NRC Project Manager, Millstone Unit No. 2 t90036 ffp/h /V// +6DSbV $

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e .j NRC korm 366 U. S NUCLE AR REGULATORY COMM:SS:ON APPROVED OMB NC 3150-0104 56-89:

• E XPRES: 4 /30/g2 Estimated buroen per response to comptr w:tn this informat9on conection reauest 50.0 hrs. Forward Comments and Reportsregarcirtg bu'oen Managernent ef,ttmate Bra to treu.s.

cn (p-530). Records Naciear LICENSEE EVENT REPORT (LER)

Regulatory Commission Washington DC 20555. and to the Pape work Recuetron Profect (3150-0104) . Office of Managernert and Buoget Wasnmoton. DC 20503 F AC1UTY NAME (1) DQCMT NUMWtR (2) W

Millstone Nuclear Power Station Umt 2 ol st oj ol ol3 l3 ls 1jod0l4 TfiL6 44)

Turbme and Reactor Trip During Mussel Cook (Thermal Backwash)

, EVENT DATE (5) LE4 NUMBEA ffh REPC AT DATE (7) OTHE A FACfLITIES INVOLVED 88:

MONTF day YEAR YEAR MONTH DAY YEAR F AC!UTY NAMES of si ol ol of I I

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opER AT!uG THis REPORT IS BEING SUBMITTED PUASUANT To THE RECutREMENTS On 10 CFR l- (Check one or more of tne folfowing)(11)

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UCENSEE CONTACT FOR THIS LER (12)

NAME TELEPHONE NUMBER ARE A CODE Zan Hoagland, Engmeer, X 4725 2l0l3 4l 4l 7l-l 1l 7l 9l 1 COMRETE ONE UNE FOR E ACH COMDONENT F AILURE DE SCR: BED IN THIS REPORT (131 OAUSE SYSTEU COMPONENT Nkh peh CAUSE SYSTEM COMPONENT pph i II I I I I I II I I I I I II Ii1 1 I I ii l i I l SUPPLEMENT AL REPOAT EXPECTED l1di MONTH DAY YEAR EXPECTED SUBM SSiON YE S M ves. corap>ete E XPECTED SUBM!SS:ON DATE1 NO DATE (15) l l ADSTR ACT (L'mit to 1400 spaces i e.. eporommatsty t:ticen str>gie-space t pewntten y imes) (16 )

On May 24,1993, at 0943 hours0.0109 days <br />0.262 hours <br />0.00156 weeks <br />3.588115e-4 months <br />, with the plant in Mode 1 at 100% power, a reactor trip occurred due to a turbme trip. The turbine trip was caused by high generator stator cooling water temperature resulting from mussel cookinc (thermal backwash) operations on the 'B' condenser waterbox. During the subsequent transient, a malfunction caused the 'A' feedwater reculating vahe to stay at approximately 56% open which caused the #1 steam generator water level to increase to the high level setpoint. The reactor operator tripped the running feedwater pump based on the high steam generator water level. Steam generator water levels were subsequently controlled using the auxihary feedwater system. Operators performed Emergency Operatine Procedure 2525,

" Standard Post Trip Actions." All safety related equipment. with the exception of one of the main feedwater regulaung vahes, responded as expected and the unit was placed in a stable condition.

These esents are bemp reported pursuant to the requirements of Paragraph 50.73(a)(2)(iv), reporting any event or condiuon that resulted m manual or automatic actuation of any Engineered Safety Feature System.

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• r FAc Form 366A U.S. NUoLEAR REGULATORY coMMcSSioN APPRoVEo OMB No. 3150-0K4 (6 -89 ) EXPlRES: 4 /30 G2 Estimated burden per response in comply witn tnes Woma" m"*:t'* r*cw so e nrs. Forware LICENSEE EVENT REPORT (LER) cornments regarding burce'st:

n estimate to the Recorcs TEXT CONTINUATION ,,o senorts Manage nent Branen (p-saci. U S. Nuclear Regwatory Commission. Wasnmgton. Dc 20555. and to the Paperworn Recschon Project (3150-01CW. Othce of Management and Budget. Washincten. DC 20503 F AcUTY N AVE 0, cocKET NUMSER (21 TEA qMpgp @ PAGE131 YEAR NN -

N mem em Niillstone Nuclear Power Station Urut -

ol5l0l0lol3l3l6 9l3 0l1l2 0l0 0l 2 OF 0l4 Terr e mue space is ree, wee vs. acewai NRc or , x6A si nn I. Deerinnon of Event On May 24.1993. at 0920. with the plant at 1009 power, mussel cooking (thermal backwash) operanons on the 'IP waterbox were mitiated. At approximately 0940 hours0.0109 days <br />0.261 hours <br />0.00155 weeks <br />3.5767e-4 months <br />, the time hmit for elevated temperature in the 'B' waterbox was satisfied and restoration of the waterbox was underway.

Durmg the course of the restoration, the elevated temperature in the 'B' circulating water intake bay was affecung the service water temperature and Turbine Building Closed Cooling Water temperature. It was determmed that the service water inlet isolation valves for the Turbme Building Closed Cooling Water (TBCCW) beat exchangers were restricting the flow and heat removal capabihties. The service water inlet tse'ation valves were then opened from 30 degrees to 40 degrees open in response to elevated TBCCW temperatures.

Approximately 0940 hours0.0109 days <br />0.261 hours <br />0.00155 weeks <br />3.5767e-4 months <br />, a common alarm for the main generator stator coohng cabinet was annunciated in the Control Room. A Plant Equipment Operator was dispatched to investigate this alarm.

The main generator stator windmg inlet temperature was found to be 51 degrees Celsius, which was above the alarm setpoint of 47 degrees Celsius. This condiuon was reponed to the Shift Supervisor by the Plant Equipment Operator. When the Plant Equipment Operator returned to the main generator stator cochne cabmet, the " Generator Protection Trip Circuit Enercized" alarm was observed to be annunciated followed shortly by the turbme trip. When the alarm energized. the process computer indicated that the stator cooline water heat exchanger outlet temperature was 75 degrees Celsius. The setpoint for the " Generator Protection Trip Circuit Energized" is 81 degrees Celsius.

The feedwater regulatmg valves receive a ramp close signal as pan of the normal trip response. During the subsequent transient, a malfunction caused the 'A' feedwater regulating valve to stay at approximately 569 open causmg the #1 steam generator water lesel to mcrease to the high level setpoint. In response to this candluon, the reactor operator closed the bypass valve and the blocking valve for the 'A' f eedwater regulating valves. Then the 'B' steam generator level went high. The 'B' feedwater regulating valve indicated 109 open. The bypass valve and the blocking valve for the 'B' feedwater regulating valves were closed. Additionally, the reactor operator tripped the running feedwater pump based on the tugh stearn generator water level. Steam generator water levels were subsequently controlled using the auxiliary f eedwater system. Operators performed Emergency Operating Procedure 2525 " Standard Post Trip ActionsJ All safeti related equ pment responded as expected with the exception of one of the mam feedwater regulating vahes and the unit was placed in a stable condition.

11 Cause of Esent

1. The reactor trip was caused by a turbine trip. The root cause of the turbine trip was the lack of heat removal capabihties from the Main Generator Stator Water Cochng System. The warm water beine discharged through the 'D' circulating water pump irito the 'B' intake bay was being recirculated to the ' A' and 'C' mtake bays, causing an increased service water temperatures. The tide was a significant factor m the high service water temperature. The low incommg tide allowed the 'B' intake ha>'s high temperature to cause an aggravated effect on the adjacent bays. Some temperature stratification m the adiacent bays has been observed on previous mussel cooks. The low ude placed the higher temperature water at the suction of the Service Water pumps.

During the mussel cooking operations, all components cooled by the TBCCW System experienced elevated temperatures. The Main Generator Stator Cooling Water outlet temperature increased greater than the temperature switch setpoint, 81 degrees Celsius, for greater than 70 seconds which resuhed m a Turbme tnp.

2. The root cause of the 'A' feedwater regulating valve was vibration which caused the vahe to go into local manual control. The 'B' feedwater regulatmg valve was actually completely closed with the indication showmg the valve 10% open.

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'n'orn aton centon reauest: so o n . Forward LICENSEE EVENT REPORT (LER) Comments regardag burgen estimate to the RecorCs TEXT CONTINUATION ane neoorts vanagement ara,ch WS3On U S. Nuclear Reputatory Commission. Wasmngton. DC 20655 and to the Paperwork Recuction Project (3150-0104) Office of Management and Burmet. Washingtort DC 20603.

F AcMTV NAMc t t) DOCKET NUMBER (2) L E A NUMPFra e PAGE (3)

N YEAR NJJBER NUMIER 1 Millstone Nuclear Power Station Unit ' OF 0l 5l 0l 0l 0l3 l3 l6 9l3 0l1l2 0l0 0l 3 0l4 TEXT 01 more spat.e is reprec use acortior at NRc Form 366A s) (17) 111. Zwalvm of Event These events are bemg reported pursuant to the requirements of Paragraph 50.73(a)(2)fiv), reporting any event or condition that resuhed m manual or automatic actuation of any Engmeered Safety Feature System, including the Reactor Protection System.

There were no safety consequences from this reactor trip event. All safety equipment with the exception of one of the main feedwater regulating valves, responded as expected and plant operators executed apphcable Emergency Operaung Procedures accordingly.

IV. Corrective Action

1. Post trip data review identified that the Main Generator Stator Cochng Water outlet temperature, as monitored by a Resistar:ce Temperature Device (RTD), only reached a maximum of the equivalent of 75 degrees Celsius, below its computer alarm setpoint and the trip setpoint value of 81 degrees Celsius. The trip is mitiated by a separate temperature switch. During the subsequent shutdown, the Main Generator Stator Water Cooling water outlet temperature switch calibration was checked and was found to operate within allowable ranges. Addittonally, the stator water temperature RTD cahbration was checked. It was found acceptable. It was determined that the clearance between the RTD and the inner diameter of thermowell was excessive. This would cause a time delay for the indicated temperature to read the actual temperature. A thermoconductivity filler was installed in the thermowell, and the RTD was reinstalled. The presence of this gap explained the temperature discrepancy observed between the stator cooling outlet temperature temperature switch, trip setpoint and the stator cochng temperature, RTD, indication recorded by the process computer.

To prevent recurrence of this event, procedure enhancements are being evaluated to address the Service Water System and the closed cochng water system temperature issues associated with mussel cooking operations. These additional controls will be placed in the mussel cooking operation prior to the next mussel cook.

2. The feedwater regulating vahes hand wheels were mechanically secured to prevent the large handwheel from vibrating to the manual engagement point. A design change is being evaluated for the next refuel outage.

Aho, on the 'B' feedwater regulating vahe, a cabbranon of the positioner and the position mdication circuit was performed. The valve was stroked from the Control Room to veiify the stroke and the posioon indicator. The valve wa: verified satisfactory.

V. Adchuanal Information Snmlar LERs: None Ells Codes for referenced components:

- Main Feedwater Regulating Valves: SJ-FCV-C635

+ Main Feedwater Regulating Bypass Valves: SJ-FCV-C635

- Stator Cooling Water Outlet Temperature Switch: TJ-TS-G054

+ Turbine Buildmg Closed Coohng Water Heat Exchangers KB-HX-$30B

- Main Feedwater Blocking Valve: SJ-ISV-C654

+ Auxihary Feedwater Pump: B A-P- 1075

• Main Feedwater Pump: SJ-P- 1075

• Main Generator Stator Cochng Water Heat Exchanger: TJ-H X-GO S4

• Main Generator Stato Cooling Water RTD TJ-23-G084 vg orm au

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FACIUTY NAME (1) DOCKET NUMBER. (2) MA NUMBEA 16) PAGE (3)

YEAR ~NN .

EE Millstone Nuclear Power Station Unit 4 ol $l ol ol ol3 l3 l6 9l3 0l1l2 0l0 0l 4 OF 0l4 TEXT itf more space is required. use additional NRC Form 366A $) (17) r MUSSEL COOK (THERMAL BACKWASH) LINEUP Traveling Screens Traveling Screens Traveling Screens Travel!ag Screens O O O  ;

C SW Pump B SW Pump A SW Pump ' +

D C B A DC Pump- CCW ump BC Pump AC Pump A

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X11E { 11F { 11G {11H D Condenser C Condenser B Condenser A Condenser ,

Waterbox Waterbox Waterbox Waterbox 12B A 12A' .

U w r3 A' y V

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-l To Discharge Canal

• Throttle 11 A to maintain inlet temperature of 'B' ,

I Circulating waterbox between 110 - 120 F.

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