ML18039A807

LER 99-003-00:on 990515,automatic Reactor Scram Due to Turbine Trip Was Noted.Caused by Failure of Mechanical Trip Cylinder to Latch When Hydraulically Reset.Operations Crew Stabilized Reactor Following Scram

ML18039A807
Person / Time
Site:	Browns Ferry
Issue date:	06/14/1999
From:	Roger A TENNESSEE VALLEY AUTHORITY
To:
Shared Package
ML18039A806	List: ML18039A805 ML18039A807
References
LER-99-003-02, LER-99-3-2, NUDOCS 9907010262
Download: ML18039A807 (10)

Text

NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB NO. 3150.0104 (4.9S) EKPIRES 04/30/se ESTIMATED BURDEN PER RESPONSE To COMPLY WITH THIS MANDATORY INFORMATION COLLECTION REQUEST So.o HRS. REPORTED LESSON'3 LICENSEE EVENT REPORT {LER) LEARNED ARE INCORPORATED INTO THK UCENSINQ PROCESS ANO FEO BACK To INDUSTRY, FORWARD COMMENTS REQAROINQ BVROEN ESTIMATK TO THE INFORMATION ANO RECORDS MANAQEMENT BRANCH IT.S F33), V.S.

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FACIUTY NAME l)) oocKKT.NUMBER LE) PAOE (3)

Browns Ferry Unit 2 05000260 1 OF 5 Automatic reactor scram due to 6 turbine trip

'EVENT DATE (5) LER NUMBER (6) REPORT DATE f7) OTHER FACILITIES INVOLVED IB)

MONTH DAY YEAR, YEAR SEQUENTIAL REVISION MONTH DAY YEAR FACIUTY NAME DOCKET NUMBER NUMBER NVMBER FACIUTY NAME DOCKET NUMBER 05 15 1999 1999 003 00 06 'l4 1999 NA OPERATING THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR (I: (Check one or more) (11)

MODE (6) 1, . 20.2201(b) 20.2203(a) (2) (v) 50.73(a) (2)(i) 50.73(a) (2) (viii)

POWER 20.2203(a) (1) 20.2203(n)(3) (i) 50.73(s) (2)(ii) 50.73(a)(2)(x)

LEVEL (10) 1 pp 20.2203(nl(2)(i) 20.2203(s) (3) (ii) 50.73(a) (2)(iii) 73.71

20. 2203 la) (2) (ii) 20.2203(s)(4) X 50.73(a)(2)(iv) OTHER 20,2203(n) (2) (iii) 50.36(c)(1) 50.73(s) (2) (v) Saecily In Abstract below 20.2203(s) (2) (iv) 50.36(c)(2) 50.73(a) (2)(vii) or )n NRC Form 366A LICENSEE CONTACT FOR THIS LER (12)

NAME TELEPHONE NUMBER C rex/)e A/es Code)

A. T. Rogers, Senior Licensing Project Manager (256) 729-2977 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT (13)

SYSTEM COMPONENT MANUFACTURER REPORTABLK TO SYSTEM COMPONENT'ANVFACTVROI REPOR TABLE NPROS TO NPROS SUPPLEMENTAL REPORT EXPECTED ('(4) EXPECTED 'ONTH OAY YEAR YEs SUBMISSION llf yas, complete EXPECTED SUBMISSION DATE). X DATE (15)

ABSTRACT (Limit to 1400 spaces, i.a., approximately 15 single spacod typewritten lines) (16)

On May 15, 1999, at 1456 COT, Unit 2 received an automatic scram from 100 percent'reactor power due to a turbine trip that occurred during routine turbine overspeed testing. The reactor scram caused reactor water level to go below the low level setpoint (level 3) which generated a redundant scram signal and initiated the Primary Containment Isolation System, as expected. The low reactor water level signal also initiated the Standby Gas Treatment and Control Room Emergency Ventilation Systems. All systems responded as expected and all control rods fully,inserted.

The cause was of the turbine trip was failure of the mechanical trip cylinder to latch when hydraulically reset.

TVA'is reporting this event'in accordance with 10 CFR 50.73 (a)(2)(iv) as an event that resulted in an automatic actuation of an engineered safety feature, including. the reactor protection system.

This report was initially submitted on June 14, 1999, as 50-260/1 999001. The number has been corrected and this report is. being re-submitted as 50-260/1999003.

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i NRC FORM 366A U.S. NLICLEAR REGULATORY COMMISSION I4.96)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME DOCKET LER NUMBER 6 PAGE 3 YEAR SEQUENTIAL REVISION NUMBER 3OF5 Browns Ferry Unit 2 05000260 1999 003 00 TEXT llfmore space is required, use addi rional copies of NRC Form 366Ai I17),

D. Other S stems or Seconda Functions Affected:

None.

E. Method of Discove Operators received alarms and indications of the turbine trip and subsequent reactor scram.

F. O erator Actions:

Operations personnel responded to the event in accordance with applicable plant procedures.

G. Safe S stem Res onse:

All required safety systems operated as designed.

III. CAUSE OF THE EVENT A. Immediate Cause:

The immediate cause of this event was a turbine trip while performing routine overspeed testing.

B. Root Cause:

The cause of the of the turbine trip was systematically evaluated to determine the cause of the failure.

causes were evaluated using a failure modes and effects analysis and all but one failure cause All'ossible was eliminated. As a result of this evaluation, the most probable cause was determined to be failure of the mechanical trip mechanism to relatch. The latching mechanism was inspected to the extent possible prior to restart and tested repeatedly without failure.

C. Contributin Factors:

None.

IV. ANALYSIS OF THE EVENT The scram was the result of a turbine trip and was initiated by the closure of the Main'Steam Turbine Stop/Control Valves which occurred during routine overspeed testing. The overspeed test is.a routine functional test, of the overspeed trip device and the mechanical trip valve. The test does not actually overspeed the turbine and is performed while the turbine is at rated speed and should not cause an actual turbine trip. However, the turbine tripped and resulted in closure of the turbine valves and a pressure transient within the Main Steam piping. The transient was mitigated by the automatic opening of nine Main Steam Turbine Bypass Valves and five Main Steam Safety Relief Valves. Equipment performance data was collected from the Plant Engineering Display System and from. the Transient Recorder Analysis feature of the Integrated Computer System. This data was analyzed and reviewed for appropriate equipment response.

NRC FORM 366 I4-95)

i

)s

NRC FORM 366A U.S. NUCLEAR REGULATORY, COMMISSION I4-96I LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITYNAME 1 DOCKET LER NUMBER 6 PAGE 3 YEAR SEOUENTIAL RE>>SION NUMBER 4OF5

, Browns Ferry Unit 2 05000260 1999> 003 00 TEXT lllmore speceis required, use eddidonel copies of NRC Form 366Ai l17)

IV. ANALYSIS OF THE EVENT (continued)

Each possible failure mode which could have caused the turbine trip has been evaluated. Through, personnel interviews, extensive troubleshooting, and physical inspections, the cause of the turbine trip has been determined to be failure of the trip cylinder to latch when hydraulically reset. All other potential causes,

.including improper procedure performance, low Electro-Hydraulic Control Nitrogen accumulator pressure, a stuck relay contact, solenoid valve failure, throw-out disk sticking, improper linkage tolerances, and incorrect mechanical trip valve settings were systematically evaluated during the investigation. An evaluation of the trip cylinder revealed that if it was hydraulically returned to the reset position, but not mechanically latched,

'the turbine would. trip when the reset was released and the lockout dropped out which appears to be what happened to initiate this event. The trip cylinder is mechanically latched by resting on a 125 mil landing on the trip arm. A failure of this type would not necessarily be repeatable, which accounts for the fact that

.troubleshooting activities tripped and reset the turbine numerous times with no failures noted. Therefore,

based upon the intermittent nature of this failure mechanism and the elimination of the other possible causes as delineated above, this failure mode is considered the most likely cause of the event. Preliminary discussions with the vendor indicate the possible need for a PM to inspect the overspeed trip mechanism on a routine basis.

V. ASSESSMENT OF SAFETY CONSEQUENCES The evaluation of plant system and component responses to the event concluded that responses were as designed and within the time-frames expected. Personnel performance was also evaluated and found to be timely, appropriate, and met expectations for performance during an event of this type.

The overspeed test is a functional test of the overspeed trip'device and mechanical trip valve. The test does not'actually cause the turbine to overspeed but exercises the trip linkage and mechanical trip valve by simulating an overspeed condition. Therefore, there,was no actual malfunction of the turbine control system.

There were no equipment failures during:or following the scram that complicated recovery. In addition, there were no radioactive material released and no actual or potential safety consequences as a result of this event.

Therefore,'this event did not adversely affect the safety of plant personnel or the public.

VI. CORRECTIVE ACTIONS A. Immediate Corrective Actions:

The Operations crew stabilized the reactor following the scram using the appropriate operating instructions.

NRC FORM 366 l4-95)

ii NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (4.95I LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME 1 DOCKET LER NUMBER 6 PAGE 3 YEAR SEOUENTIAL REVISION NUMBER SOF5 Browns Ferry Unit 2 05000260 1999 003 00 TEXT (if more spece is required, use eddidonef copies of NRC Form 366Al I17)

B. Corrective Action to Prevent Recurrence:

Work orders.-were issued to perform an inspection of the mechanical trip mechanism during the next available refueling outage on Unit 2 and Unit 3.

and Engineering will determine if

'Maintenance a periodic inspection of the trip latch mechanism is appropriate.

testing methods will be reviewed to determine

'xisting if alternate techniques are available to reduce or eliminate the risk of turbine trip.

'II.

ADDITIONALINFORMATION A. Failed Com onents:

None.

B. Previous Similar Events:

None.

C. Additional information:

None.

D. Safe S stem Functional Failure:

This event did not result in a safety system functional failure in accordance with NEI 99-02.

VIII. COMMITMENTS None.

'TVA does not consider this corrective action a regulatory commitment. The completion of this item will be tracked in TVA's Corrective Action Program.

NRC FORM 366 I4.95)

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