Information Notice 1994-11, Turbine Overspeed and Reactor Cooldown During Shutdown Evolution

Turbine Overspeed and Reactor Cooldown During Shutdown Evolution

ML031060633
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant, Crane
Issue date:	02/08/1994
From:	Grimes B Office of Nuclear Reactor Regulation
To:
References
IN-94-011, NUDOCS 9402020376
Download: ML031060633 (10)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555

February 8, 1994 NRC INFORMATION NOTICE 94-11:

TURBINE OVERSPEED AND REACTOR COOLDOWN

DURING SHUTDOWN EVOLUTION

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

PurDose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information

notice to alert addressees to a recent event involving the spurious opening of

turbine governor and stop valves and resultant turbine overspeed and reactor

coolant system cooldown due to a malfunctioning control system during a

shutdown evoluation. It is expected that recipients will review the

information for applicability to their facilities and consider actions, as

appropriate, to avoid similar problems. However, suggestions contained in

this information notice are not NRC requirements; therefore, no specific

action or written response is required.

Description of Circumstances

On September 12, 1992, the Diablo Canyon Power Plant, Unit 1, was in Mode 2 at

0 percent power. The Pacific Gas and Electric Company (the licensee) was

shutting down the reactor for a refueling outage. The main turbine had been

manually tripped and removed from service. The unit was subcritical, with

bank OD" control rods fully inserted and bank KCT rods at approximately

50 steps. To prevent excessive reactor coolant system cooldown, an operator

was dispatched to re-latch the main turbine. This causes the stop valve trip

pilot valves to close which isolates a main steam leakage path to the

condenser. However with the re-latching of the main turbine, the turbine

speed rapidly increased from about 1100 rpm to 1870 rpm; reactor coolant

temperature decreased from 284C [5430F] to 2790C [535'F]; and the

intermediate range nuclear instrumentation indication increased from

1.9 x 10P amps to 2.2 x 10

amps. At the governor valve overspeed setpoint

of 103 percent (1854 rpm), the governor valves began to close. In response to

these indications, the operators again tripped the main turbine. The

operators observed that reactor coolant system temperature continued to

decrease and tripped the reactor to prevent an inadvertent return to

criticality from the cooldown.

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February 8, 1994 Discussion

During reactor operations at Diablo Canyon, turbine speed is regulated by the

positions of four governor valves (see Attachment 1) which are controlled by

the P2000 computer. When the turbine is tripped, pressure switch PS-22B (Low

Auto Stop 01 Pressure) is designed to provide a contact closure signal to the

P2000 computer. This signal is used by the P2000 computer to reset the main

turbine speed reference signal to 0 rpm. To prevent excessive cooldown

following a plant shutdown, the licensee re-latches the main turbine while the

stop valve equalizing valves are closed. Re-latching the turbine in this

manner closes the stop valve trip pilot valves, which isolates a steam leakage

path to the condenser.

During this event, PS-22B failed to close due to corrosion of internal switch

components. The failure of PS-22B allowed the P2000 main turbine speed

reference signal to remain at 1800 rpm.

As the main turbine deviated from

1800 rpm during coastdown, the P2000 speed controller integrated the deviation

which resulted in a maximum speed-increase demand signal. Upon re-latching

the main turbine, governor valve FCV-141 responded to the P2000 demand signal

and opened in an attempt to return main turbine speed to 1800 rpm. The main

turbine accelerated rapidly to 103 percent (1854 rpm), at which point the

governor valve overspeed system closed the governor valves. At approximately

the same time that the governor valves began to close, the licensee tripped

the turbine. The maximum speed attained of 1870 rpm was below the main

turbine overspeed trip setpoint of 1980 rpm. The turbine trip system remained

operable throughout the event.

The mechanical components of PS-22B include a plunger rod, a bushing, and a

case. The plunger rod was fabricated of austenitic stainless steel, and the

bushing and case were fabricated of aluminum. These two materials are far

apart on the galvanic series. The resulting potential difference could result

in the formation of a galvanic cell, creating aluminum oxide buildup between

the plunger and the bushing. Corrosion product buildup, combined with a

static pressure system, is believed to have caused PS-22B to stick.

A similar event occurred at the James A. Fitzpatrick Nuclear Power Plant. On

December 28, 1992, during a refueling outage, the main turbine stop valves

spuriously opened when the main turbine trip system was reset. Testing of the

turbine electro-hydraulic control system revealed a failed Agastat time delay

relay in the primary trip/reset logic circuitry. The time delay relay allows

the turbine to be reset without opening the main turbine stop valves. This

failure of the relay allowed the electro-hydraulic control system speed

circuit to change from 0 rpm to 1800 rpm spontaneously, creating a demand

signal to open the main turbine stop valves. In this event, the main steam

isolation valves closed, which prevented an actual turbine overspeed.

I

.

bA 94-11 Yebruary 8, 1994 The events discussed above are examples of how turbine control system failures

can result in unanticipated challenges to turbine overspeed protection systems

during all modes of operation.

This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

the technical contact listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contact:

Eric J. Benner, NRR

(301) 504-1171 Attachments:

1. Main Steam Stop and Governor Valves

2. List of Recently Issued NRC Information Notices

FROM MAIN STEAM

FCV-146 FCV-1 45 FCV-1 41

142

...... .....................................

..................

................................................................ ..

FCV-1 39, -140, -141, AND

(

-142 ARE CONTROL VALVES

FCV-143, -144, -145, AND

-146 ARE STOP VALVES

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FCV-1

139 FCV-1

43

Attacdment 2

IN 94-11

February 8, 1994 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information

Date of

Notice No.

Subject

Issuance

Issued to

94-10

94-09

94-08

93-26, Supp. 1

94-07

94-06

93-85, Rev. 1 Failure of Motor-Operated

Valve Electric Power

Train due to Sheared or

Dislodged Motor Pinion

Gear Key

Release of Patients with

Residual Radioactivity

from Medical Treatment and

Control of Areas due to

Presence of Patients Con- taining Radioactivity

Following Implementation

of Revised 10 CFR Part 20

Potential for Surveil- lance Testing to Fail

to Detect an Inoperable

Main Steam Isolation Valve

Grease Solidification

Causes Molded-Case

Circuit Breaker Failure

to Close

Solubility Criteria for

Liquid Effluent Releases

to Sanitary Sewerage Under

the Revised 10 CFR Part 20

Potential Failure of

Long-Term Emergency

Nitrogen Supply for the

Automatic Depressurization

System Valves

Problems with X-Relays

in DB- and DHP-Type

Circuit Breakers Manu- factured by Westinghouse

02/04/94

02/03/94

01/01/94

01/31/94

01/28/94

01/28/94

01/20/94

All holders of OLs or CPs

for nuclear power reactors.

All U.S. Nuclear Regulatory

Commission medical

licensees.

All holders of OLs or CPs

for nuclear power reactors.

All holders of OLs or CPs

for nuclear power reactors.

All byproduct material and

fuel cycle licensees with

the exception of licensees

authorized solely for

sealed sources.

All holders of OLs or CPs

for boiling water reactors.

All holders of OLs or CPs

for nuclear power reactors.

OL = Operating License

CP = Construction Permit

IN 94-11 February 8, 1994 The events discussed above are examples of how turbine control system failures

can result in unanticipated challenges to turbine overspeed protection systems

during all modes of operation.

This information notice requires no specific action or written response.

If

you have any questions about the information in this notice, please contact

the technical contact listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.

Oginal sined by

Brian K. Grimes

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contact:

Eric J. Benner, NRR

(301) 504-1171 Attachments:

1. Main Steam Stop and Governor Valves

2. List of Recently Issued NRC Information Notices

• See previous concurrences

OFC

OEAB:DORS

SC/OEAB:DORS

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NAME

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DATE

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NAME

DKirkpatrick*

GMarcus*

l DATE

04/08/93

04/09/93

102/17 /94

[OFFICIAL RECORD COPY]

DOCUMENT NAME:

94-11.IN

IN 94-XX

February xx, 1994 The events discussed above are examples of how turbine control system failures

can result in unanticipated challenges to turbine overspeed protection systems

during all modes of operation.

This information notice requires no specific

action or written response. If you have any questions about the information

in this notice, please contact the technical contact listed below or the

appropriate Office of Nuclear Reactor Regulation (NRR) project manager.

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contact:

Eric J. Benner, NRR

(301) 504-1171 Attachments:

1. Main Steam Stop and Governor Valves

2. List of Recently Issued NRC Information Notices

• See previous concurrences

OFC

OEAB:DORS

SC/OEAB:DORS

PUB:ADM

C/OEAB:DORS

NAME

EBenner*

EGoodwin*

Tech Ed*

AChaffee*

DATE

03/01/93

03/05/93

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03/31/93 OFC

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NAME

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DATE

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[OFFICIAL RECORD COPY]

DOCUMENT NAME: S:\\DORSSEC\\DCTURBIN.IN

IN 93-XX

April xx, 1993 This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

the technical contact listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.

Brian K. Grimes, Director

Division of Operating Reactor Support

Eric J. Benner, NRR

(301) 504-1171 Attachments:

1. Main Steam Stop and Governor Valves

2. List of Recently Issued NRC Information Notices

• See previous concurrences

OFC

OEAB:DORS

SC/OEAB:DORS

PUB:ADM

C/OEAB:DORS

NAME

EBenner*

EGoodwin*

Tech Ed*

AChaffee*

DATE

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DOCUMENT NAME: S:\\DORS SEC\\DCTURBIN.IN

IN 93-XX

April XX, 1993 This information notice requires no specific action or written response.

If

you have any questions about the information in this notice, please contact

the technical contact listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.

Brian K. Grimes, Director

Division of Operating Reactor Support

Eric J. Benner, NRR

(301) 504-1171 Attachments:

1.

Main Steam Stop and Governor Valves

2.

List of Recently Issued NRC Information Notices

OEAB:DORS*

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