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LER 98-003-00:on 981001,Unit 2 Received Full Automatic Scram from 100% Reactor Power.Caused by Failure of Normally Open Isolation Valve Which Blocked Flow of Stator Cooling Water Through Associated Heat Exchangers.Replaced Failed Valve
	ML18039A597
Person / Time
Site:	Browns Ferry
Issue date:	10/28/1998
From:	Rogers A; TENNESSEE VALLEY AUTHORITY
To:	;
Shared Package
ML18039A596	List: ML18039A595; ML18039A597;
References
	LER-98-003-03, LER-98-3-3, NUDOCS 9811090171
	Download: ML18039A597 (10)
	v • d • e

NRC FORM 366 U.s. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB NO. 3150 0104 EXPIRES 04I30/SS (4-9SI ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH THIS MANDATORY INFORMATION COLLECTIOIJ REOUEST: So,o HRS. REPORTED LESSONS LICENSEE EVENT REPORT (LER) LEARNED ARE INCORPORATED INTO THE LICENSINO PROCESS AND FED BACK TO INDUSTRY, FORWARD COMMENTS REOAROINO BVROEN ESTIMATE TO THE INFORMATION ANO RECORDS MANAOEMENT BRANCH IT.S F331, V.S.

(See reverse for required number of NUCLEAR REGULATORY COMMISSION. WASHINOTON, DC 20SSS%001, AND TO THE PAPERWORK REDUCTION PROJECT (3I 50%104I, OFFICE OF digits/characters for each block) MANAGEMENTANO BUDGET, WASHINOTON, DC 20803.

FACIUTY NAME III DOCKET NUMBER I2l PAOENE I3I Browns Ferry Unit 2 05000260 1 OF 5 Reactor Scram From Turbine Trip Due To Failed Isolation Valve In Stator Cooling System EVENT DATE (5) LER NUMBER (6I REPORT DATE (7) OTHER FACILITIES INVOLVED (6)

MONTH DAY YEAR SEQVENTIAL REVISION MONTH OAY YEAR FACIUTY NAME DOCKET NUMBER NVMBER NUMBER FACIUTY NAME DOCKET NUMBER 1998 003 00 10 28 1998 NA 10 01 1998 OPERATING THIS REPORT IS SUBMITTED PURSUAN T TO THE REQUIREMENTS OF 10 CFR 5: (Check one or. moro)

MODE (9) 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(a)(3)(i) 50.73(s)(2) (iil 50.73(a) (2) (x)

LEVEL (10) 20. 2203(a) (2) (i) 20 2203(a) (3) (iil 50.73(s) (2) (iii) 73.71 100

20. 2203 (a) (2) (ii) 20.2203(a) (4) 50.73(a)(2) (iv) OTHER

20. 2203(s) (2) (iii) 50.36(c)(1) 50.73(a)(2) (v) Specify In Abstract below

20. 2203(s) (2) (iv) 50.36(c)(2) 50.73(a) (2) (vii) or in NRC Form 366A LICENSEE CONTACT FOR THIS LER (12)

NAME TELEPHONE NVMBER (Include Area Cocci A. T. Rogers, Senior Licensing Project Manager (256) 729-2977 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT (13) coMpoNEN'r MANUFACTVRER REPORTABLE TO CAUSE SYSTEM COMPONENT MANUFACTURER REPORTABLE CAUSE SYSTEM NPRDS TO NPRDS LA e ISV A200 EXPECTED MONTH OAY YEAR SUPPLEMENTAL REPORT EXPECTED (14)

No SUBMISSION YES DATE (15l (If yss, complete EXPECTED SUBMISSION DATE).

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

On October 1, 1998, at 0047 CDT, Unit 2 received a full automatic scram from 100 percent reactor power.

The scram was generated by a main turbine trip which was the result of high generator stator cooling water outlet temperature. 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 root cause was failure of a normally open manual isolation valve which blocked the flow of stator cooling water through the associated heat exchangers. The failure resulted from the separation of the valve plug from the valve stem, which allowed the loose plug to be forced against the valve seat by system flow thus blocking the flow path to the heat exchangers. The failed valve was replaced. TVA will initiate preventative maintenance for periodic inspections of the normally open stator cooling water valves.

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.

98 i i0'r)0i7i 'P8i028 PDR ADOCK 05000260 8 PDR

NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (4-9 5)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME 1 DOCKET LER NUMBER 6 PAGE 3 YEAR SEQUENTIAL REVISION NUMBER 2OF5 Browns Ferry Unit 2 05000260 1 998 003 00 TEXT flfmore space is required, use addi donal copies of NRC Form 366A) 1171 I. PLANT CONDITIONS At the time of the event, Units 2 was at-100 percent power and Unit 3 was in Mode 5 with the reactor cavity flooded. Unit 1 was shutdown and defueled.

II. DESCRIPTION OF EVENT A. Event:

On October 1 1998, at 0047 CDT, Unit 2 received a full automatic scram and recirculation pump. trip from

~

100 percent reactor power. The scram and recirculation pump trip were generated by a main turbine trip which was the result of high generator stator cooling water outlet temperature. 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 turbine trip was caused by the main generator stator cooling water [TJ] outlet temperature reaching the high temperature trip setpoint of 81, degrees C for the designed 70 second time delay, making up the two out of three logic for the turbine trip circuitry. The stator cooling water temperature exceeded its setpoint du'e to failure [B] of. a manual isolation valve [ISV] which blocked the stator cooling water through the associated heat exchangers.

The scram resulted in the automatic actuation or isolation of the following PCIS [JE] systems and components:

~ PCIS group 2, Shutdown cooling mode of Residual Heat Removal (RHR) [BO] system; drywell floor drain isolation valves; drywell equipment drain isolation valves [WP].

~ PCIS group 3, Reactor Water Cleanup (RWCU) system [CE].

~ PCIS group 6, primary containment purge and ventilation [JM], Unit 2 reactor zone ventilation [VB];

refuel zone ventilation [VA]; Standby Gas Treatment (SGT) [BH] system; Control Room Emergency Ventilation (CREV) [Vl] system.

~ PCIS group 8, Traversing Incore Probe (TIP) [IG].

This event is reportable 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.

B. Ino erable Structures Com onents or S stems that Contributed to the Event:

None.

NRC FORM 366 14-951

4t V

NRC FORM 366A .u.s. NucLEAR REGut.AToRY COMMissiON I4.96l LICENSEE EVENT REPORT,(LER)

TEXT CONTINUATION

FACILITY NAME 1 DOCKET LER NUMBER 6 PAGE 3 YEAR SEQUENTIAL REvrBION NUMBER 3OF5 Browns Ferry Unit 2 05000260 1998 003 00 TEXT Illmore space is required, use addirional copies or NRC Form 366A/ I17I lI C. Dates and A roximate Times of Ma or Occurrences:

October 1, 1998 0040 hours4.62963e-4 days 0.0111 hours 6.613757e-5 weeks 1.522e-5 months CDT Stator cooling water inlet temperature started to increase

,(from 110 degrees F) and outlet temperature starts to increase (from 140 degrees F).

October 1, 1998 0042 hours4.861111e-4 days 0.0117 hours 6.944444e-5 weeks 1.5981e-5 months CDT 'Operations received abnormal alarm in Control Room when stator cooling water inlet temperature reached alarm setpoint (117 degrees F).

October 1, 1998 0046 hours5.324074e-4 days 0.0128 hours 7.60582e-5 weeks 1.7503e-5 months CDT Stator cooling water outlet temperature reached trip setpoint 81 degrees C (178 degrees F).

October 1, 1998 0047 hours5.439815e-4 days 0.0131 hours 7.771164e-5 weeks 1.78835e-5 months CDT Turbine tripped on high stator cooling water temperature.

Reactor scram and recirculation pumps tripped on turbine stop valve closure.

October 1, 1998 0054 hours6.25e-4 days 0.015 hours 8.928571e-5 weeks 2.0547e-5 months CDT Operations restarted a recirculation pump.

October 1, 1998 0115 hours0.00133 days 0.0319 hours 1.901455e-4 weeks 4.37575e-5 months CDT Operations reset PCIS and secured SGT and CREV.

October 1, 1998 0223 hours0.00258 days 0.0619 hours 3.687169e-4 weeks 8.48515e-5 months CDT A four-hour non-emergency report is made to the NRC pursuant to 10 CFR 50.72 (b) (2) (ii).

D. Other S stems or Seconda Functions Affected:

None.

E. Method of Discove The operating crew observed an automatic reactor scram and recirculation pump trip due to the turbine trip.

F. 0 erator Actions:

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

G. Safet S stem Res onse:

All required safety systems operated as designed.

NRC FORM 366 I4-95)

C; tl

NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION I4 95)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME I1 DOCKET LER NUMBER 6 PAGE 3 YEAR SEQUENTIAL REVISION NUMBER 4OF5 Browns Ferry Unit 2 05000260 1998 003 00 TEXT (ff more spaceis required, use additional copies of fIIRC Form 366AJ l17)

III. CAUSE OF THE EVENT A. Immediate Cause:

The main generator stator cooling water outlet temperature reached the high temperature trip setpoint of 81 degrees C (178 degrees F) for the designed 70 second time delay, making up the two out of three'logic for the turbine trip circuitry. The turbine trip caused the reactor scram and recirculation pump trip due to turbine stop valve closure.

B. Root Cause:

The root cause of this event was determined to be the failure of the inlet valve to the stator cooling water heat exchangers which is a normally open, angle globe valve. The valve plug separated from the stem and blocked:the stator cooling water through the associated heat exchangers.

C;, Contributin Factors:

This valve is configured such that the inlet flow enters perpendicular to the valve stem and exits around the valve plug resulting in flow pressure pushing the valve plug away from the valve stem. This is the only valve on the stator cooling water skid in this configuration and piping obstructions on the skid prevent mounting this valve in the normal orientation. The normal orientation of an angle valve is with Inlet flow parallel to the valve stem resulting in flow pressure pushing the plug onto the valve stem and less turbulence across the valve.

IV. ANALYSIS OF THE EVENT The generator stator high cooling water temperature was caused by the. failure of a manual isolation valve which blocked the stator cooling water flow through the associated heat exchangers. The stator cooling water system is configured such that a portion of the stator cooling water flows through the a pair of series heat exchangers while the remainder bypasses the heat exchangers. A temperature control valve downstream of the'heat exchangers maintains the desired stator cooling water temperature. The heat exchangers can be aligned for either series or parallel operation, but are normally aligned in series.

The event occurred when the normally open angle globe valve on the inlet to the first series cooler failed, blocking the stator cooling water flow path to the heat exchangers. The valve plug is threaded onto the valve stem and then secured in place with a retaining'pin. The failure resulted from the separation of the valve plug from the valve stem, and the loose plug was swept against the valve seat by the system flow. Inspections indicate the pin failed due to long'term wear caused by differential plug to stem movement which also damaged the threads on both the plug and stem. The flow path around the heat exchangers remained open, and thus the majority of system flow bypassed the heat exchangers. Stator cooling water temperature immediately began a steady increase, and the high temperature trip occurred approximately seven minutes after the failure. The failed valve was replaced. TVA will'initiate preventative maintenance for periodic inspections of the normally open stator cooling water valves which should identify deterioration prior to a similar valve failure.

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 for the event and met expectations for performance during an event of this type. There were no equipment failures during or following the scram that complicated recovery. As a result, there were no threats to ublic health or safet .

NRc FoRM 366 I4-95I

Ik NRC FORM 366A . NUCLEAR REGULATORY COMMISSION (4.95l LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME (1 DOCKET LER NUMBER 6 PAGE 3 SEQUENTIAL REYISION NUMBER SOFS Browns Ferry Unit 2 05000260 1998 003 00 TEXT (If more space is required, use addj'rional copies of NRC Form 366A/ (17l V. CORRECTIVE ACTIONS A. Immediate Corrective Actions:

The Operations crew stabilized the, plant in Mode 3 (Hot Shutdown) using the appropriate operating instructions. No immediate actions could be implemented to correct the stator cooling water failure.

B. Corrective Action to Prevent Recurrence:

TVA will evaluate either modifying or replacing the normally open stator cooling water valves on Unit 2.

TVA will initiate preventative maintenance for periodic inspections of the Unit 2 and Unit 3 normally open stator cooling water valves.

'VA will evaluate the possibility of aligning the stator cooling water system heat exchangers for parallel operation.

'l.

ADDITIONALINFORMATION A. Failed Cpm onents:

The failed valve is an ALOYCO 6-inch angle globe valve; This valve is part of a General Electric supplied generator cooling water package, and the valve appears to have been in service for the life of the plant. The valve plug is threaded onto the valve stem and then secured in place with a retaining pin.

Inspections indicate the pin failed due to long term wear caused by differential plug and stem movement which also damaged the threads on both the plug and stem. After the pin failed, flow forces caused the plug to separate from the stem and then lodge in the valve seat.

B. Previous Similar Events:

LER 50-260/9401301 was similar in that it was caused by turbine trip due to stator cooling water Itigh.

temperature and was reported pursuant to 10 CFR 50.73(a)(2)(iv). However, the root cause was a mechanical degradation. of the stator cooling water temperature switch which resulted in a setpoint drift in the conservative direction thus premature operation of the switch. No corrective actions in the previous LER would have prevented the occurrence of the condition described in'this report.

No other.LERs have been issued which involve a failure of a manual valve in this system.

Vll. 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 (4-9SI

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ML18039A597

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