United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 95-18, Supplement 1: Potential Pressure-Locking of Safety-Related Power-Operated Gate Valves

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                            WASHINGTON, D.C.  20555

                                March 31, 1995

                                             SAFETY-RELATED POWER-OPERATED
                                             GATE VALVES


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice (IN) supplement to alert addressees to additional analyses identifying
the potential susceptibility of multiple safety injection flow paths to a
common mode failure due to pressure-locking.  It is expected that recipients
will review this additional 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.


The NRC staff issued IN 95-14, "Susceptibility of Containment Sump
Recirculation Gate Valves to Pressure Locking," on February 28, 1995, in
response to the determination by the licensee of the Millstone Nuclear Power
Station that both of the Unit 2 containment sump recirculation motor-operated
gate valves may experience pressure-locking during a design-basis loss-of-
coolant accident (LOCA) and fail to open.  The failure of both of these valves
would make a water source for the emergency core cooling system and the
containment spray unavailable during the recirculation phase of the LOCA.

The NRC staff issued IN 95-18, "Potential Pressure-Locking of Safety-Related
Power-Operated Gate Valves," on March 15, 1995, in response to the
determination by the licensee of the Haddam Neck Nuclear Power Plant that all
of its high-pressure and low-pressure safety injection valves were susceptible
to pressure-locking.  The licensee subsequently concluded that both low-
pressure safety injection valves and two of the four high-pressure safety
injection valves were inoperable.  

Description of Circumstances

On March 9, 1995, the Connecticut Yankee Atomic Power Company reported that
seven motor-operated gate valves in the safety injection systems at the Haddam
Neck Nuclear Power Plant were susceptible to pressure-locking to the extent 

9503290256.                                                            IN 95-18, Supp. 1
                                                            March 31, 1995
                                                            Page 2 of 4

that the operability of valves may have been jeopardized.  This report was
made after factoring in more accurate friction coefficients and unseating
forces that were determined from in-situ diagnostic testing during the current
refueling outage.  The circumstances surrounding this report were described in
IN 95-18.  

On March 15, 1995, and in later reviews the licensee identified more valves
that were susceptible to pressure-locking.  Their susceptibilities were
described as follows:

      1.    Two charging system injection valves (CH-MOV-292B and 292C). 
            These normally open valves may be required to close to prevent
            residual heat removal (RHR) pump runout.  Once closed, the valves
            would be susceptible to pressure-locking due to heatup of the
            fluid in the bonnet caused by high containment temperature.  This
            could prevent the valve from reopening for subsequent accident
            mitigation.  (See Attachment 1)

      2.    Two cross connect valves from RHR to high-pressure safety
            injection (SI-MOV-901 and 902).  These normally shut valves could
            become pressurized from the discharge of the low-pressure safety
            injection pumps and fail to open when required for high-pressure
            recirculation.  (See Attachment 2)

      3.    The flex wedge shutdown cooling valves connected to the reactor
            coolant system could be overpressurized due to normal plant heat
            up.  The plant heat up after shutdown cooling isolation could
            raise the pressure in the valve bonnet area to levels beyond code
            allowable limits.  The licensee is currently inspecting the valves
            for damage. 

The Haddam Neck plant has been shut down for refueling since January 28, 1995. 
The licensee plans to modify the susceptible valves before restarting the
plant.  Proposed modifications consist of drilling a hole in the valve disc or
venting the bonnet area to relieve internal pressure.


Pressure-locking could affect valves in a variety of safety related
applications.  IN 95-14 addressed how containment sump valves and steamline
valves could be affected.  IN 95-18 addressed the susceptibility of reactor
coolant system boundary valves in the safety injection system to pressure-
locking.  This information notice addresses the pressure locking phenomenon
for more valves that could become pressure-locked during accident mitigation
and further clarifies the pressure-locking phenomenon for the valves
identified in IN 95-14. 

Pressure-locking occurs in flexible-wedge and double-disc gate valves when
fluid becomes pressurized within the valve bonnet, creating a large internal
															IN 95-18, Supp. 1
                                                            March 31, 1995
                                                            Page 3 of 4

pressure, and the actuator is not capable of overcoming the additional thrust
required while there is differential pressure across both valve discs. 
Susceptible valves are normally closed valves that need to open for accident
mitigation, or valves that are closed during an accident and need to open for
subsequent accident mitigation.  Two primary mechanisms were identified which
can cause the large differential pressures across the valve discs.  

The first mechanism can occur when the valve is being exposed to high pressure
fluid, usually from the reactor coolant system (RCS), and the bonnet is
pressurized by leakage past the valve seat.  If the high pressure side of the
valve is subsequently depressurized, as during a LOCA, the high pressure fluid
remains trapped in the valve bonnet and retains a high pressure in the bonnet
and produces additional resistance for valve opening.  

The second mechanism is thermally induced pressure-locking; fluid is trapped
in the valve bonnet and subsequently experiences a thermal heatup that causes
a large increase in bonnet pressure and creates a significantly higher
resistance to valve opening along with differential pressure across the valve
discs.  For example, this could occur if these valves were exercised at lower
temperature and the cooler water in the bonnet later expanded as a result of
an RCS heatup or in a post-accident containment environment.  The Haddam Neck
licensee estimates that the pressure increase could exceed code allowable for
shutdown cooling isolation valves.  This problem could apply to any valves
independent of its operator type.  This failure mechanism would not be
revealed in dynamic testing since the valves are not subjected to significant
temperature transients during this testing.

The licensee had evaluated these valves in 1990 for possible pressure-locking
and thermal-binding on the basis of existing test data and industry
information and had concluded that the valves were not susceptible to these
problems.  This evaluation had concluded that any temperature rise in the
bonnet area would be dissipated to the pipe and environment.  However, recent
diagnostic testing has shown that the friction coefficients, the unseating
forces, and the methodology previously used were nonconservative.  The
licensee performed a reanalysis to calculate thrust requirements using more
conservative stem friction factors based on recent industry experience, and
more accurate unseating forces using modern test equipment and concluded that
these valves might not operate as a result of pressure-locking problems.

As noted in IN 95-14 and IN 95-18, the NRC staff and the nuclear industry have
been aware of disc binding problems of gate valves for many years.  The
industry has issued several event reports describing the failure of safety-
related gate valves to operate because of pressure-locking or thermal-binding
of the valve discs.  Several generic industry communications have given
information on identifying susceptible valves and performing appropriate
preventive and corrective measures.  These mechanisms represent potential
common cause failure modes that can render redundant trains of safety-related
emergency core cooling systems incapable of performing their safety functions. 
The NRC staff published a draft generic letter for public comment on 
March 27, 1995 titled, "Pressure Locking and Thermal Binding of Safety-Related
Power-Operated Gate Valves," which addresses this issue in greater detail. 
.                                                            IN 95-18, Supp. 1
                                                            March 31, 1995
                                                            Page 4 of 4

Additional Related Generic Communications

On February 26, 1992, the NRC staff issued IN 92-17, "NRC Inspections of
Programs Being Developed at Nuclear Power Plants in Response to Generic 
Letter 89-10."

On April 2, 1992, the NRC staff issued IN 92-26, "Pressure-Locking of Motor-
Operated Flexible Wedge Gate Valves."

In Enclosure 1 to Supplement 6 of Generic Letter 89-10, "Information on
Schedule and Grouping, and Responses to Additional Public Questions," dated
March 8, 1994, the NRC staff discussed pressure-locking and thermal-binding of
motor-operated gate valves.  The staff also 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.

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 BKGRIMES

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

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

                     John Tappert, NRR
                     (301) 415-1167

1.  Haddam Neck Charging System
2.  Haddam Neck ECCS Injection Lineup
(For Attachment 1 and 2 see File IN9518S1.WP1)
Page Last Reviewed/Updated Monday, November 18, 2013