United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 95-14: Susceptibility of Containment Sump Recirculation Gate Valves to Pressure Locking

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                         WASHINGTON, D.C.  20555-0001

February 28, 1995



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 the potential susceptibility of containment sump
recirculation gate valves to pressure locking at nuclear plants.  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

Description of Circumstances

On January 26, 1995, Northeast Nuclear Energy Co., the licensee for Unit 2 of
the Millstone Nuclear Power Station, determined that both containment sump
recirculation motor-operated gate valves may experience pressure locking
during a design-basis loss-of-coolant accident (LOCA) and fail in the closed
position.  This condition was discovered by the licensee after re-evaluation
of all Unit 2 valves to address weaknesses in their previous evaluation
criteria identified by the NRC Motor-Operated Valve Inspection at Millstone
Unit 1 in March 1994.  At Millstone Unit 2, failure of both of these valves
would make a water source for the emergency core cooling system (ECCS) and
containment spray unavailable during the recirculation phase of the LOCA (the
recirculation phase occurs not sooner than about 45 minutes after the start of
an event).                                                                     
Millstone Unit 2 is a pressurized water reactor with two containment sump
recirculation valves (one in each of two parallel paths).  The sump piping
paths join piping from the refueling water storage tank (RWST) leading to low
pressure safety injection pump suction.  The Millstone Unit 2 containment sump
recirculation valves are parallel-wedge gate valves that are normally closed,
leaving the containment sump side of the valve dry and exposed to the
containment.  The pump side of each valve is normally pressurized to
approximately 345 kPa [35 psig] due to the static head of the RWST.  The RWST
water may leak past the pumpside gate valve disc and may fill the valve bonnet
with water.   

9502270218.                                                             IN 95-14         
                                                             February 28, 1995
                                                             Page 2 of 4      

During a postulated LOCA at Millstone Unit 2, the sump side of each
containment sump recirculation valve could be exposed to sump water (reactor
coolant) with temperatures as high as 143 �C [289 �F] for some time prior to
the valve being required to open to allow ECCS and containment spray
recirculation cooling.  Heating of the valve could heat the water trapped in
the water-filled valve bonnet.  A pressure increase of 410 kPa per �C [33 psi
per �F] is predicted.  The licensee has determined that a bonnet pressure of
1138 kPa [150 psig] (requiring only about a 3 �C [5 �F] temperature rise in a
water solid bonnet) may prevent the valve from opening.  Therefore, it might
not be possible to establish recirculation cooling following a LOCA.           
Millstone Unit 2 has been in a cold shutdown condition since October 1994. 
The licensee is considering various modifications to the valves to prevent
pressure locking to be implemented prior to restart.


Pressure locking may occur in flexible-wedge and parallel-wedge 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 created across both valve discs by the pressurized
fluid.  For example, the fluid may enter the valve bonnet (1) during normal
open and close valve cycling, (2) when a fluid differential pressure across a
disc causes the disc to move slightly away from the seat, creating a path to
either increase the fluid pressure or fill the bonnet with fluid, or (3) for a
steamline valve, when differential pressure exists across the disc and the
valve orientation permits condensate to collect and enter the bonnet. 
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 mode
that can render redundant trains of certain safety-related systems or multiple
safety systems incapable of performing their safety functions. 

The industry has issued several event reports describing safety-related gate
valves failing to operate because of pressure locking.  Several generic
industry communications have given guidance for identifying susceptible valves
and for performing appropriate preventive and corrective measures.

In March 1993, the NRC issued NUREG-1275, Volume 9, "Pressure Locking and
Thermal Binding of Gate Valves."  Further, the NRC staff held a public
workshop on February 4, 1994, to discuss the subject.  A summary of the public
workshop is available in the NRC Public Document Room and contains both
information on evaluating the potential for pressure locking and actions taken
in response to the identification of susceptible valves.  The NRC staff
presentation at the workshop identified containment sump recirculation valves
as potentially susceptible to pressure locking.  In addition to pressure
locking events at U.S. nuclear power plants, the French experience was
documented in NUREG/CP-0137, "Proceedings of the Third NRC/ASME Symposium on
Valve and Pump Testing," in July 1994.  Related NRC generic communications are
listed later in this notice..                                                             IN 95-14         
                                                             February 28, 1995
                                                             Page 3 of 4      

The two primary safety issues related to the pressure locking scenario
described above are the potential loss of access to a water source for long-
term reactor core cooling and the potential loss of a water source for
containment spray.  During a postulated large-break LOCA or under some small-
break LOCA scenarios, the opening of the sump valves is relied on to set up a
flow path from the sump to the safety injection and containment spray pumps. 
In this alignment, the safety injection pumps recirculate the water between
the sump and the reactor for long-term core cooling and the containment spray
pumps draw water from the sump to provide containment spray for the control of
containment pressure, temperature, and fission products.  If the sump valves
are unable to open due to pressure locking, the core may be damaged in a short
time.  The containment pressure and temperature may also increase upon loss of
containment spray.  This could lead to containment failure.  The loss of
containment spray would also cause reduction of fission product scrubbing
inside the containment.  Additionally, the safety injection and containment
spray pumps may be damaged by pump cavitation if the sump valves remain closed
and the RWST is emptied.

Related Generic Communications

On April 2, 1992, the NRC staff issued Information Notice (IN) 92-26,
"Pressure Locking of Motor-Operated Flexible Wedge Gate Valves."  IN 92-26
referenced earlier NRC staff documents on pressure locking.

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
alerted licensees to the issue of pressure locking of 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.  .                                                             IN 95-14         
                                                             February 28, 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.

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

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

                     Chu Y. Liang, NRR
                     (301) 415-2878

                     Eugene M. Kelly, Region I
                     (610) 337-5111

Attachment:  List of Recently Issued NRC Information Notices

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