United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-11: Turbine Overspeed and Reactor Cooldown During Shutdown Evolution

                                UNITED STATES
                           WASHINGTON, D.C. 20555

                              February 8, 1994

                               DURING SHUTDOWN EVOLUTION


All holders of operating licenses or construction permits for nuclear power


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 evolution.  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 "D" control rods fully inserted and bank "C" 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 284�C [543�F] to 279�C [535�F]; and the
intermediate range nuclear instrumentation indication increased from        
1.9 x E-9 amps to 2.2 x E-9 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.  

9402020376.                                                            IN 94-11
                                                            February 8, 1994
                                                            Page 2 of 3


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 Oil 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.
.                                                            IN 94-11
                                                            February 8, 1994
                                                            Page 3 of 3

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.

                                    /S/'D BY BKGRIMES

                                    Brian K. Grimes, Director
                                    Division of Operating Reactor Support
                                    Office of Nuclear Reactor Regulation

Technical contact:  Eric J. Benner, NRR
                    (301) 504-1171

1.  Main Steam Stop and Governor Valves
2.  List of Recently Issued NRC Information Notices

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