United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 95-30: Susceptibility of Low-Pressure Coolant Injection and Core Spray Injection Valves to Pressure Locking

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                         WASHINGTON, D.C.  20555-0001

                                August 3, 1995


NRC INFORMATION NOTICE 95-30:  SUSCEPTIBILITY OF LOW-PRESSURE COOLANT
                               INJECTION AND CORE SPRAY INJECTION VALVES
                               TO PRESSURE LOCKING


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the potential susceptibility of boiling-water
reactor low-pressure coolant injection (LPCI) and core spray injection (CSI)
gate valves to pressure locking.  It is expected that recipients will review
the information for applicability to their facilities and consider actions, as
appropriate, to avoid similar events or problems.  However, suggestions
contained in this information notice are not NRC requirements; therefore, no
specific action or written response is required.

Description of Circumstances

From May to July 1995, Georgia Power Company, the licensee for the Edwin I.
Hatch Plant, experienced several valve failures in both units during testing
of its inboard LPCI 24-inch flexible-wedge gate valves.  These failures, which
are still under investigation for root causes, involved shearing of the motor
shaft and the pinion key, and overheating of the motor.  Surveillance testing
of the valves was conducted at power and the downstream check valve leakage
which resulted in the reactor operating pressure on one face of the valve, may
have contributed to the failures.  During the investigation of the root causes
for these failures and in responding to the NRC staff's inquiries, the
licensee realized that because of backleakage of the downstream check valves,
reactor system pressure could cause the LPCI valves to be susceptible to
pressure locking during an accident.  (See "Discussion" section of this
information notice for a description of "pressure locking.")  

The licensee reevaluated the operability of the LPCI valves and of the core
spray injection 10-inch flexible-wedge valves because of their similar valve
alignment.  The licensee determined that one LPCI valve might not be capable
of opening under pressure-locking conditions and declared this valve
inoperable.  The licensee installed a larger motor and different gearing in
this valve.  The licensee also made a similar modification to the valve
located in the other train.  



9508030050
.                                                            IN 95-30
                                                            August 3, 1995
                                                            Page 2 of 4


Discussion

Pressure locking may occur in flexible-wedge and double-disk gate valves when
fluid becomes pressurized within the valve bonnet and the actuator is not
capable of overcoming the additional thrust requirements resulting from the
differential pressure acting across both valve discs.  This situation can
occur, for example, when a check valve exposed to high reactor coolant
pressure is in series with a gate valve.  The check valve may meet leakage
requirements, but, over time, the pressure in the piping between the check
valve and the gate valve can increase.  With time, the bonnet cavity pressure
and the pipe pressure will tend to equalize at the reactor coolant pressure. 
If a large loss-of-coolant accident (LOCA) occurred, the pressure in the
piping would be rapidly reduced.  This occurrence would trap high-pressure
fluid in the bonnet cavity.  If no internal or external path is provided to
lower the bonnet pressure, the valve may become pressure locked.

Pressure locking can cause a power-operated valve to fail to open, resulting
in an inability of the associated safety train or system to perform its safety
function.  Pressure locking represents a potential common-cause failure
mechanism that can render redundant trains of multiple safety systems
incapable of performing their safety functions. 

The NRC staff and the nuclear industry have been aware of pressure-locking
problems in gate valves for many years.  The industry has issued several event
reports describing failures of safety-related gate valves to operate because
of pressure locking.  Several generic industry communications have given
guidance for identifying susceptible valves and for taking appropriate
preventive and corrective measures.

The NRC staff has provided information on pressure locking of gate valves to
the industry and has discussed the safety significance of the potential for
pressure locking of gate valves at public meetings.  

      �     In March 1993, the NRC issued NUREG-1275, Volume 9, "Operating
            Experience Feedback Report-Pressure Locking and Thermal Binding of
            Gate Valves," which contains a discussion of the potential for
            pressure locking of LPCI and CSI valves when leakage past their
            check valves pressurizes the valve bonnet.  

      �     A summary of a public workshop held by the NRC staff on February
            4, 1994, is available in the NRC Public Document Room (Accession
            Number 9403020090).  This summary contains information on
            evaluation of the potential for pressure locking and the actions
            taken in response to the identification of susceptible valves. 
            The workshop proceedings will be issued soon as NUREG/CP-0146.  

      �     In Enclosure 1 to Supplement 6 (March 8, 1994) of Generic Letter
            (GL) 89-10, "Safety-Related Motor-Operated Valve Testing and
            Surveillance," the NRC staff described an acceptable approach for
            licensees to address the potential for pressure locking of motor-
            operated gate valves as part of their GL 89-10 programs.

                                                            IN 95-30
                                                            August 3, 1995
                                                            Page 3 of 4


The power-operated valves in the LPCI and CSI lines are designed to serve a
vital safety-related function.  During a postulated large-break LOCA, as well
as under some small-break LOCA scenarios, the opening of these valves is
relied upon to provide a flow path for cooling water to the reactor core.  If
these valves are unable to open because of pressure locking (or some other
common-cause problem), reactor fuel might be damaged during a LOCA.

Entergy Operations, Inc., has developed a methodology to predict the thrust
required to overcome pressure locking of flexible-wedge gate valves.  However,
the methodology is based on limited pressure-lock testing.  The Georgia Power
Company applied this methodology in predicting the thrust required to overcome
pressure locking of their LPCI and CSI valves as the best available
information at the time.  The licensee is considering making modifications to
these valves during the next outage.  The Georgia Power Company also found
that care must be taken in adapting the Entergy methodology to valves in each
case, particularly with regard to the valve-specific forces across the hub of
flexible-wedge gate valves.    

The NRC staff will, in the near future, issue a generic letter that will ask
licensees to verify that they (1) have performed or will perform evaluations
and appropriate analyses and (2) will take appropriate corrective actions, as
necessary, to ensure that safety-related power-operated gate valves that may
be susceptible to pressure locking are capable of performing their required
safety functions.  A draft of this generic letter, "Pressure Locking and
Thermal Binding of Safety-Related Power-Operated Gate Valves," was published
in the Federal Register on April 26, 1995, (60 FR 15799) along with a request
for public comment.  The staff is issuing this information notice in advance
of the generic letter because of the high safety significance of this issue
and to indicate the susceptibility of the LPCI and CSI gate valves to pressure
locking.  

Related Generic Communications

On April 2, 1992, the NRC staff issued NRC Information Notice (IN) 92-26,
"Pressure Locking of Motor-Operated Flexible Wedge Gate Valves," which
discusses the failure of an LPCI valve at the James A. FitzPatrick Nuclear
Power Plant as a result of pressure locking following a hydrostatic pressure
test of the LPCI piping.  IN 92-26 also referenced earlier NRC staff documents
on pressure locking.  

Recently, the NRC staff issued IN 95-14 (February 28, 1995), "Susceptibility
of Containment Sump Recirculation Gate Valves to Pressure Locking," and IN 95-
18 (March 15, 1995) and Supplement 1 (March 31, 1995), "Potential Pressure
Locking of Safety-Related Power-Operated Gate Valves," to alert licensees to
potential pressure locking of pressurized-water reactor containment sump
recirculation valves and safety injection valves identified at the Millstone
Nuclear Power Station and the Haddam Neck Nuclear Plant, respectively.

  .                                                            IN 95-30
                                                            August 3, 1995
                                                            Page 4 of 4


This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.

                                    /S/'D BY DMCRUTCHFIELD


                                    Dennis M. Crutchfield, Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Thomas G. Scarbrough, NRR
                     (301) 415-2794

                     Howard J. Rathbun, NRR
                     (301) 415-2787

                     Edward H. Girard, RII
                     (404) 331-4186

Attachment:
List of Recently Issued NRC Information Notices

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