United States Nuclear Regulatory Commission - Protecting People and the Environment

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

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

                              March 15, 1995


NRC INFORMATION NOTICE 95-18:  POTENTIAL PRESSURE-LOCKING OF SAFETY-RELATED
                               POWER-OPERATED GATE VALVES


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 (IN) to alert addressees to a recent analysis demonstrating the
potential susceptibility of safety injection valves 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.

Background

IN 95-14, "Susceptibility of Containment Sump Recirculation Gate Valves to
Pressure Locking," dated February 28, 1995, was written 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 circumstances at
Haddam Neck discussed in this supplement were identified as a result of
licensee action in response to this previous Millstone determination.

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
that the operability of valves may have been jeopardized.  These
susceptibilities were detailed as follows:







9503140317.                                                            IN 95-18
                                                            March 15, 1995
                                                            Page 2 of 3


1.    Four high-pressure safety injection admission valves (SI-MOV-861A
      through 861D; see Attachment 1).  

      These normally closed valves are susceptible to pressure-locking
      following a postulated LOCA in which the pressure in the reactor coolant
      system (RCS) drops substantially before the safety injection actuation
      signal initiates the opening of the valve.  These valves are also
      susceptible to pressure locking caused by the heating of fluid captured
      in the valve bonnet when the valves are stroked during the startup of
      the plant. 

2.    Two low-pressure safety injection admission valves (SI-MOV-871A and
      871B; see Attachment 2).  

      These normally closed safety injection valves are susceptible to
      pressure-locking following a postulated LOCA where the pressure in the
      RCS drops substantially before the safety injection actuation signal
      initiates the opening of the valve.

3.    One common low-pressure safety injection isolation valve (SI-MOV-873;
      Attachment 2).  

      This normally open valve would be closed during transfer to the
      recirculation mode following a LOCA if either SI-MOV-871A or B failed to
      close.  Once this isolation valve was closed, it would be susceptible to
      pressure-locking as a result of temperature increase of fluid in the
      bonnet caused by high containment temperature.  This condition could
      prevent the reopening of the valve should it be required for entrance
      into the  two-path long term recirculation mode.

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.  A modification being considered includes venting both the bonnet space
and the packing gland seal leakoff back to the RCS to prevent pressure-
locking.  In addition, the licensee is considering procedural changes to
minimize valve unseating forces. 

Discussion

The licensee had previously evaluated these valves for possible pressure-
locking and thermal-binding and had concluded that the valves were not
susceptible to these problems.  However, recent diagnostic testing has shown
that the friction coefficients, the unseating forces, and the methodology
previously used were nonconservative.  The reexamination of the analysis by
the licensee, using the test results, indicated that these valves may become
inoperable due to pressure-locking problems.

As noted in IN 95-14, the NRC staff and the nuclear industry have been aware
of disk 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
															 
															 IN 95-18
                                                            March 15, 1995
                                                            Page 3 of 3


disks.  Several generic industry communications have given guidance for both
identifying susceptible valves and performing appropriate preventive and
corrective measures.  In Enclosure 1 to Supplement 6 of Generic Letter 89-10,
"Safety-Related Motor-Operated Valve Testing and Surveillance," dated March 8,
1994, the NRC staff discussed pressure-locking and thermal-binding of motor-
operated gate valves.

Pressure-locking may occur in flexible-wedge and parallel disk gate valves
when fluid entrapped in the bonnet becomes pressurized and the actuator is
incapable of overcoming the additional thrust requirements needed to overcome
the increased friction resulting from the differential pressure on both valve
disks from the pressurized fluid.  IN 95-14 discusses several of ways in which
fluid may enter the valve bonnet.  Thermal binding of gate valves can result
from contraction of a valve body as a result of cooling after a gate valve has
seated.  Like pressure locking, thermal binding can increase the forces that
are necessary to unseat the valve.  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.

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

                     Donald Kirkpatrick, NRR
                     (301) 415-1849

Attachments:
1.  Haddam Neck High Pressure Safety Injection 
2.  Haddam Neck Low Pressure Safety Injection and Heat Removal System

(See File IN95018.WP1 for Figures 1 and 2)
Page Last Reviewed/Updated Monday, November 18, 2013