ML17354A984

LER 97-007-01:on 970730,automatic Reactor Trip Occurred Due to Closure of B Msiv.Caused by Failed BFD22S Relay.Six Relays on 3A,3B & 3C MSIVs Were Replaced & Implemented Plant Change to Disable Electronic Trip Function on 3 AFW Pumps

ML17354A984
Person / Time
Site:	Turkey Point
Issue date:	06/18/1998
From:	Lafleur D FLORIDA POWER & LIGHT CO.
To:
Shared Package
ML17354A982	List: ML17354A981 ML17354A984
References
LER-97-007, LER-97-7, NUDOCS 9806230040
Download: ML17354A984 (12)

Text

LICENSEE EVENT REPORT (LER}

DOCKET NUMBER (2) PAGE (3)

FACILITY NAME (1)

TURKEY POINT UNIT 3 05000250 OP TITLE (4) Automatic Reactor Trip due to Closure of the B Main Steam Isolation Valve EVENT DATE 5) LER NUMBER(6) RPT DATE (?I OTHER FACILITIES INV. 8 MON DAY YR SE ¹ R¹ MON DAY YR FACILITY NAMES DOCKET It S) 07 30 97 97 007 01 06 15 98 OPERATING MODE 9 POWER LEVBL 100 (10)

LICENSEE CONTACT FOR THIS LER (12)

TELBPHONE NUMBER D. R. Lafleur, Licensing Engineer 305-246-7150 COMPLETE ONE LINE FOR EACH COMPONENT FAILURB DESCRIBED IN THIS RBPORT 13 CAUSE SYSTEM COMPONENT MANUFACTURER EPIX? CAUSE SYSTEM EPIX?

B SB RLY W120 N BA TAC D245 N EXPECTED MONTH SUPPLEMENTAL REPORT EXPECTED (14) NO X YES SUBMISSION DATE (15)

(if yes, complete EXPECTED SUBMISSION DATB)

ABSTRACT (16)

On July 30, 1997, Florida Power 8 Light (FPL) Company's Turkey Point Unit 3 was operating in Mode 1 at 100% power. At 1506 hours0.0174 days <br />0.418 hours <br />0.00249 weeks <br />5.73033e-4 months <br />, the 3B Main Steam Isolation Valve (MSIV) failed closed.

The resultant level decrease in the 3B Steam Generator (SG) caused the reactor to trip automatically on low=low level in the 3B SG. All control rods fully inserted.

A 3B steam line Main Steam Safety Valve (MSSV) opened then reseated as expected during the initial transient, as pressure in the 3B SG exceeded the MSSV setpoint pressure of 1085 psig.

The A Auxiliary Feedwater (AFW) Pump tripped on overspeed during the event. Plant response was otherwise .as expected.

The cause of the MSIV closure was attributed to a failed BFD22S relay. The six relays on the 3A, 3B, and 3C MSIV's were replaced.

The most probable cause of the A AFW pump overspeed trip was a spurious electronic overspeed signal caused by the A AFW pump tachometer module. A plant change was implemented to disable the electronic trip function of all three AFW pumps to enhance their reliability.

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LICENSEE E~ REPORT (LER) TEXT PZINUATION FACILITY NAME DOCKET NUMBER LER NUMBER PAGE NO.

TURKEY POINT UNIT 3 05000250 97-007-01 2 OF 6 I. DESCRI'PTION OF THE EVENT On July 30, 1997, Florida Power 6 Light (FPL) Company's Turkey Point Unit 3 was operating in Mode 1 at 100% power. At 1506 hours0.0174 days <br />0.418 hours <br />0.00249 weeks <br />5.73033e-4 months <br />, the 3B Main Steam Isolation Valve (MSIV) [SB:isv] failed closed. The 3B Steam Generator (SG) [SB:sg] level decreased as a result of the MSIV closure. Upon indications of MSIV closure, the reactor operator attempted to trip the reactor manually. However, the reactor tripped automatically on low-low level in the 3B SG approximately ll seconds after MSIV closure and less than a second before the manual trip was actuated. All control rods fully inserted.

A Main Steam Safety Valve (MSSV)[SB:rv] on the 3B steam line opened as pressure in the 3B SG exceeded the 1085 psig safety valve setpoint. As pressure in the steam line decreased to below the setpoint, the MSSV reseated as expected.

The reactor operators started the 3B Charging Pump [CB:p] to maintain pressurizer [AB:pzr] level, and closed the MSIV's on the 3A and 3C SG's to stop the reactor coolant system average temperature from decreasing. Upon plant stabilization, the Emergency Operating Procedures (EOP's) were exited and General Operating Procedure, 3-GOP-103, was entered to place the plant in Hot Standby.

At 1509 hours0.0175 days <br />0.419 hours <br />0.0025 weeks <br />5.741745e-4 months <br />, the A Auxiliary Feedwater (AFW) Pump [BA:p]

(normally aligned to Train 1) tripped on mechanical overspeed.

The 3A and 3B Steam Generator Feedwater pumps and the B and C AFW pumps (normally aligned to Train 2) continued to supply feed to the SGs. Plant response was otherwise as expected.

After stabilization of the plant, the A AFW Pump (and therefore Train 1) was declared out of service, placing the plant in Technical Specification LCO 3.7.1.2, Action Statement 1. The AFW system was realigned with the C pump on Train 1 and the B pump on Train 2, to provide for dual Train operation, exiting Action Statement 1 and entering Technical Specification LCO 3.7.1.2, Action Statement 3. This Action Statement requires that the AFW pump be restored to an Operable status within 30 days. This pump was returned to Operable status after replacement of the tachometer that supplies its electronic overspeed trip signal, exiting the LCO.

The NRC operations center was notified at 1621 on July 30, 1997, in accordance with 10 CFR 50.72 (b) (2) (ii), Reactor Protection System Actuation and Engineered Safety Feature Actuation.

An Event Response Team was formed to.investigate the causes for the failure of the 3B MSIV and the overspeed trip of the A AFW Pump.

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LICENSEE E~ REPORT (LER) TEXT FZXNUATTON FACILITY NAME DOCKET NUMBER LER NUMBER PAGE NO.

TURKEY POINT UNIT 3 05000250 97-007-01 3 OF 6 II. CAUSE OF THE EVENT

't MSIV Closure The cause of the MSIV closure was determined to be the failure of a Westinghouse BFD22S relay coil (tag number 3-AX/2605A)-[SB:rly],

which is part of the 3B MSIV actuator circuitry. FPL believes that the relay was installed in November of 1983. This relay is normally energized. When this relay is energized, it actuates a solenoid valve which delivers air to the MSIV actuator. When the relay deenergizes, the solenoid allows air to vent from the MSIV actuator, allowing the MSIV to close. Each MSIV has two relays, one for each actuation train, which perform this function. The failed relay coil was open and its casing was cracked, showing signs of thermal aging.

A AFW Pump Mechanical Overspeed Trip Turkey Point plant design includes a common AFW system composed of three steam driven pumps, each designed to operate at 5900 rpm. Each pump turbine is protected by both an electronic and a mechanical overspeed trip. The electronic overspeed trip, set at 6200 RPM, uses a speed sensor to provide a trip signal to close the pump turbine steam inlet valve. The mechanical overspeed trip, set to actuate at 6500 RPM, uses a pin-type trip weight attached to the pump turbine rotor. When sufficiently displaced by centrifugal force, the trip weight will actuate a spring loaded lever to trip closed the pump turbine steam inlet valve.

When an AFW actuation signal is present, the electronic overspeed trip will automatically reset after actuation.theDuring this event, approximately 1 minute after AFW initiation, A AFW pump turbine cycled in this manner (from trip to reset) for approximately 70 seconds. Based on AFW flow recorder traces and operator reports, the A AFW pump tripped on electronic overspeed, then reset, several times. Pump speed increased due to this cycling process, eventually tripping on a mechanical overspeed trip.

The most probable cause for the electronic overspeed trip of the A AFW pump was a spurious electronic overspeed signal caused by the A AFW pump tachometer module (Dresser-Rand part g890009A03)

[BA:tac]. This style module was installed in August of 1995, replacing the original Airpax module (Airpax .part gFSS1416; Dresser-Rand part g890061A20).

A review of the circuitry was performed to compare the original Airpax module (Airpax gFSS1416; Dresser-Rand g890061A20) to the Dresser-Rand part g890009A03. The original Airpax module schematics identified an L-C filter network, which would effectively filter electric noise from its 125 Vdc vital power

Vl LICENSEE E44T REPORT (LER) TEXT FZINUATION FACILITY NAME DOCKET NUMBER LER NUMBER PAGE NO.

TURKEY POINT UNIT 3 05000250 97-007-01 4 OF 6 supply. No schematics were available for the Dresser-Rand module

¹890009AO'3. However, visual inspection of the module revealed differences in the filtering capabilities of the module circuitry.

Review of the plant's AFW pump history indicates that the A AFW pump tripped on January 7, 1997, due to a spurious electronic trip with the Dresser-Rand tachometer module ¹890009A03 installed. No spurious electronic overspeed trips have been recorded for either the B or C AFW pumps, which have the original style tachometer module (Airpax ¹FSS1416; Dresser-Rand

¹890061A20).

The A AFW pump tachometer module was replaced with an Airpax 300 Series tachometer module which is the recommended replacement for the now obsolete original module (Airpax ¹FSS1416; Dresser-Rand

¹890061A20).

Xndustry research identified that in 1993, Vogtle Nuclear Station experienced problems with spurious tripping of an AFW pump due to electric noise introduced from 125 Vdc vital control circuits, with the same Dresser-Rand part number module (Dresser-Rand

¹890009A03). A 10 CFR Part 21 notification was made for this module (LER 50-425/93-007-00). FPL has found no record of having received this notification.

Bench testing was performed on the Dresser-Rand tachometer module (Dresser-Rand ¹890009A03) to determine the overspeed trip's susceptibility to noise. During the testing, the tachometer and dc to dc power supply were subjected to various electrical noise sources. Only the inductive surge produced by a de-energizing dc relay was able to trip the tachometer's overspeed bistable. The testing did not replicate exact plant conditions. The test results were considered 'to be inconclusive.

Discussions with nineteen other plants utilizing equivalent or identical AFW pumps and/or turbines confirmed that an electronic trip is n'ot a standard installation and that such a trip is not considered neccessary. Two other plants had removed the electronic trip function following similar spurious tripping events.

A plant change was implemented to disable the electronic trip function of all three AFW pumps. By disabling the trip function, spurious trip signals will be eliminated thereby enhancing the reliability of the pumps. Each AFW pump is protected from overspeed by the Woodward mechanical governor [BA:65] and the mechanical overspeed trip device [BA:12] . The relibility of these devices is assured by regular maintenance activities and has been demonstrated through scheduled testing.

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LICENSEE EAT REPORT (LER) TEXT ITINUATION FACILITY NAME DOCKET NUMBER LER NUMBER PAGE NO.

TURKEY POINT UNIT 3 05000250 97-007-01 5 OF 6 III. ANALYSIS OF. THE EVENT A reactor trip due to a low-low level in a steam generator is a previously analyzed event. Upon closure of the 3B MSIV, the sudden loss of steam flow from the 3B SG caused an increase in pressure in that SG. The pressure. increase resulted in a decrease of water level in the 3B steam generator to the low-low level reactor trip setpoint of 10% narrow range level.

The Updated Final Safety Analysis Report (UFSAR) assumes a loss of normal feedwater to all steam generators due to the loss of the steam generator feed pumps or valve malfunction. The analysis shows that following a loss of normal feedwater, the AFW system is capable of removing the reactor's stored and residual heat assuming that only one pump is available. Since the normal feedwater flow as well as two of the three AFW pumps were available for removing stored and residual heat, the event did not compromise the health and safety of the public.

IV. CORRECTIVE ACTIONS 1 The failed Westinghouse BFD22S relay, 3-AX/2605A, as well as on each actuation train of all three

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equivalent relays MSIV's (3-AX/2605B, 3-AX/2604A,3-AX/2604B, 3-AX/2606A, and 3-AX/2606B) were replaced with Westinghouse BFD65 relays. These relays are of an improved dual-coil design which uses a lower wattage (producing a lower operating coil temperature) to hold the relay when energized.

2. Equivalent function relays on Unit 4 MSIVs were replaced during the 1997 Unit 4 outage.

3. Use of normally energized Westinghouse BFD relays were reviewed to determine if other normally energized relays required replacement. The evaluation concluded that no other BFD relays required replacement.

4. A review of the plant Preventative Maintenance (PM) program indicated that the MSIV relays were not included in the program. The relays have been scheduled for periodic replacement.

5. An investigation was performed to determine if a 10 CFR Part 21 report would be required for the Dresser-Rand part g890009A03 tachometer module. Industry. research identified that in 1993, Vogtle Nuclear Station issued a 10 CFR Part 21 notification for the same part (LER 50-425/93-007-00).

6. Bench testing was performed on the Dresser-Rand module (Dresser-Rand g890009A03) to determine the overspeed to noise. During the testing, the tachometer trips'usceptibility

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LICENSEE E+4T REPORT (LER) TEXT 1TINUATION FACILITY NAME DOCKET NUMBER LER NUMBER PAGE NO.

TURKEY POINT UNIT 3 05000250 97-007-01 6 OF 6 and dc to dc power supply were subjected to various electI.ical noise sources. Only the inductive surge produced by a de-energizing dc relay was able to trip the tachometer's overspeed bistable. The testing did not replicate exact plant conditions. The test results were considered to be inconclusive.

7. Electronic overspeed trip functions were disabled on all AFW pumps to improve their reliability by eliminating the cause of repetitive failure identified as spurious trip signals due to inductive noise.

V. ADDITIONAL INFORMATION Licensee Event Report 251/89-020, dated 1/18/90, reported a reactor trip due to an inadvertent closure of an MSIV. That closure was due to corrosion of a terminal block supplying 125 Vdc vital power to the MSIV actuator solenoids.

EIIS Codes are shown in the format [EIIS SYSTEM: IEEE component function identifier, second component identifier (if appropriate)j .

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