[Federal Register Volume 60, Number 58 (Monday, March 27, 1995)]
[Notices]
[Pages 15799-15804]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 95-7431]



-----------------------------------------------------------------------


NUCLEAR REGULATORY COMMISSION

Proposed Generic Letter; Pressure Locking and Thermal Binding of 
Safety-Related Power-Operated Gate Valves

AGENCY: Nuclear Regulatory Commission.

ACTION: Notice of issuance.

-----------------------------------------------------------------------

SUMMARY: The Nuclear Regulatory Commission (NRC) is proposing to issue 
a generic letter regarding pressure locking and thermal binding of 
safety-related power-operated gate valves. This proposed generic letter 
is intended to ensure that addressees have performed or will perform 
evaluations, and as appropriate, analyses and/or corrective actions to 
ensure that safety-related power-operated gate valves that may be 
susceptible to pressure locking or thermal binding are capable of 
performing their required safety functions. The NRC is seeking comment 
from interested parties regarding both the technical and regulatory 
aspects of [[Page 15800]] the proposed generic letter presented under 
the Supplementary Information heading. This generic letter and 
supporting documentation were discussed in meeting number 268 of the 
Committee to Review Generic Requirements (CRGR) on January 24, 1995. 
The staff incorporated the changes requested by CRGR plus information 
concerning two recent events and obtained CRGR endorsement. The 
relevant information that was sent to the CRGR to support their review 
of the proposed generic letter will be placed in the Public Document 
Room. The NRC will consider comments received from interested parties 
in the final evaluation of the proposed generic letter. The NRC's final 
evaluation will include a review of the technical position and, when 
appropriate, an analysis of the value/impact on licensees. Should this 
generic letter be issued by the NRC, it will become available for 
public inspection in the Public Document Room.

DATES: Comment period expires April 26, 1995. Comments submitted after 
this date will be considered if it is practical to do so, but assurance 
of consideration cannot be given except for comments received on or 
before this date.

ADDRESSES: Submit written comments to Chief, Rules Review and 
Directives Branch, U.S. Nuclear Regulatory Commission, Washington, DC 
20555. Written comments may also be delivered to 11545 Rockville Pike, 
Rockville, Maryland, from 7:30 am to 4:15 pm, Federal workdays. Copies 
of written comments received may be examined at the NRC Public Document 
Room, 2120 L Street, NW. (Lower Level), Washington, DC.

FOR FURTHER INFORMATION CONTACT:
Thomas G. Scarbrough, (301) 415-2794.

SUPPLEMENTARY INFORMATION: NRC Generic Letter 95-XX: Pressure 
Locking and Thermal Binding of Safety-Related Power-Operated Gate 
Valves.

Addresses

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

Purpose

    The U.S. Nuclear Regulatory Commission (NRC) is issuing this 
generic letter to request that addressees perform, or confirm that they 
already have performed, (1) evaluations of operational configurations 
of safety-related power-operated (including motor, air, and hydraulic-
operated) gate valves for susceptibility to pressure locking and 
thermal binding, and (2) further analyses, and any needed corrective 
actions, to ensure that safety-related power-operated gate valves that 
are susceptible to pressure locking or thermal binding are capable of 
performing their required safety functions.
    NRC previously provided guidance on an acceptable approach for 
addressing pressure locking and thermal binding of MOVs in Supplement 6 
to Generic Letter (GL) 89-10, ``Safety-Related Motor-Operated Valve 
Testing and Surveillance,'' but did not request specific actions by 
licensees to address these problems at that time. This letter confirms 
(as was indicated earlier in Supplement 6) that licensees are expected 
under existing regulations to take actions to ensure that safety-
related power-operated gate valves susceptible to pressure locking or 
thermal binding are capable of performing their required safety 
functions. The guidance in Attachment 1 to this letter is derived 
directly from (and is intended to be the same as) the guidance provided 
in Enclosure 1 to GL 89-10, Supplement 6; except, in this generic 
letter, (1) the guidance is being issued as an approved generic NRC 
staff position for implementation by licensees who have not already 
satisfactorily addressed pressure locking and thermal binding of MOVs 
by implementing the guidance in Supplement 6 (or equivalent industry 
methods); and (2) the guidance is also intended for adaptation and 
implementation by all licensees, to address the pressure-locking and 
thermal-binding phenomena in other types of power-operated (i.e., air 
and hydraulic-operated) gate valves, as well as MOVs. Finally, for both 
MOVs and other power-operated valves, this letter requires that 
licensees submit for staff review summary information regarding any 
actions taken to ensure that valves susceptible to pressure locking or 
thermal binding are capable of performing their required safety 
functions, including (a) actions taken by licensees on their own 
volition to implement the guidance provided in Supplement 6 (or 
equivalent industry methods), as well as (b) actions taken in response 
to this letter. (No response was required from licensees in Supplement 
6 to GL 89-10 regarding pressure locking and thermal binding.)
    In this generic letter, the NRC staff is requesting a preliminary 
evaluation of pressure locking and thermal binding of safety-related 
power-operated gate valves, and, subsequently, a more detailed 
evaluation and resolution of the issue.

Background

    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 failure of safety-related gate valves 
to operate due to pressure locking or thermal binding of the valve 
discs. Several generic industry communications have given guidance for 
both identifying susceptible valves and performing appropriate 
preventive and corrective measures. Despite industry awareness of the 
problem, pressure locking and thermal binding events continue to occur. 
In addition to events at U.S. nuclear power plants, French experience 
with pressure locking events was recently documented in NUREG/CP-0137 
(July 1994), ``Proceedings of the Third NRC/ASME Symposium on Valve and 
Pump Testing.''
    In GL 89-10 (June 28, 1989), the staff asked holders of operating 
licenses and construction permits to provide additional assurance of 
the capability of safety-related MOVs and certain other MOVs in safety-
related systems to perform their safety-related functions by reviewing 
MOV design bases, verifying MOV switch settings initially and 
periodically, testing MOVs under design-basis conditions where 
practicable, improving evaluations of MOV failures and necessary 
corrective action, and trending MOV problems. In Enclosure 1 to 
Supplement 6 to GL 89-10 (March 8, 1994), the NRC staff described one 
acceptable approach for licensees to address pressure locking and 
thermal binding of motor-operated gate valves.
    In March 1993, the NRC issued NUREG-1275, Volume 9, ``Pressure 
Locking and Thermal Binding of Gate Valves.'' This NUREG give the 
history of pressure locking and thermal binding events, describes the 
phenomena, discusses the consequences of locking or binding on valve 
functionality, summarizes preventive measures, and assesses the safety 
significance of the phenomena. Pressure locking or thermal binding 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 and thermal binding represent potential 
common-cause failure modes that can render redundant trains of certain 
safety-related systems or multiple safety systems incapable of 
performing their safety function. Such failures may not be self-
revealing through existing surveillance tests or normal operating 
cycles. [[Page 15801]] 

Description of Circumstances

    After issuing Volume 9 of NUREG-1275, the NRC staff discussed 
pressure locking and thermal binding with several licenses (1) to 
gather information on the technical issues related to generic and 
plant-specific valve and system characteristics, and (2) to determine 
the implementation status of prior industry guidance for identification 
of susceptible valves and application of preventive and corrective 
measures. NRC surveys indicated that some licensees have performed 
multiple reviews of pressure locking and thermal binding. However, the 
staff found only limited instances of valves being modified to 
alleviate the effects of pressure locking and thermal binding.
    In Enclosure 1 to Supplement 6 of GL 89-10, the NRC staff reminded 
licensees that they are expected under existing regulations to take 
actions to ensure that safety-related power-operated gate valves 
susceptible to pressure locking or thermal binding are capable of 
performing their required safety functions, and described an acceptable 
approach for licensees and permit holders to address pressure locking 
and thermal binding of motor-operated gate valves as part of their GL 
89-10 programs. The information on pressure locking and thermal binding 
of motor-operated gate valves provided in Enclosure 1 to Supplement 6 
of GL 89-10 was intended as timely notification of operating experience 
feedback. During inspections of GL 89-10 programs, the staff found the 
actions taken by licensees to address pressure locking and thermal 
binding of motor-operated gate valves to be varied. Although many 
licensees had conducted some level of review of the potential for 
pressure locking and thermal binding of their motor-operated gate 
valves, few licensees had either (1) thoroughly evaluated the 
capability of the motor actuators to overcome the phenomena, or (2) 
taken corrective action to prevent the phenomena as discussed in 
Supplement 6. In view of these inspection results, the NRC staff has 
determined that further action (i.e., this generic letter) is now 
warranted to ensure that safety-related power-operated gate valves 
susceptible to pressure locking or thermal binding are capable of 
performing their required safety functions.
    Most licensees are nearing completion of their GL 89-10 programs. 
In meetings with industry representatives and licensees, the staff 
stated that, during its closure review, it will assess the progress 
being made by licensees in addressing pressure locking and thermal 
binding of motor-operated gate valves. The staff also stated that 
licensees need not complete their response to the pressure locking and 
thermal binding issue at the time that the verification of the design-
basis capability of MOVs within the scope of GL 89-10 is completed 
because the staff would evaluate the acceptability of addressee 
resolution to pressure locking and thermal finding of all safety-
related power-operated gate valves, including MOVs, in a consolidated 
effort (via this generic letter). Finally, the staff stated that this 
generic letter would address the schedule for completing the licensees' 
response to the pressure locking and thermal binding issue.
    The NRC staff held a public workshop on February 4, 1994, to 
discuss pressure locking and thermal binding of gate valves, including 
prioritization of susceptible valves for corrective action. A summary 
of the public workshop is available in the NRC Public Document Room and 
contains information on evaluation of pressure locking and thermal 
binding, and actions taken in response to the identification of 
susceptible valves.
    On February 28, 1995, NRC issued Information Notice (IN) 95-14, 
``Susceptibility of Containment Sump Recirculation Gate Valves to 
Pressure Locking.'' This information notice alerted licensees to a 
report from Northeast Nuclear Energy Company, the licensee for 
Millstone Nuclear Power Station, Unit 2, that both containment sump 
recirculation motor-operated gate valves might experience pressure 
locking during a design-basis loss-of-coolant accident and fail in the 
closed position. On March 15, 1995, NRC issued IN 95-18, ``Potential 
Pressure-Locking of Safety-Related Power-Operated Gate Valves.'' This 
information notice alerted licensees to a report from Connecticut 
Yankee Atomic Power Company, the licensee for Haddam Neck Nuclear Power 
Plant, that seven motor-operated gate valves in the safety injection 
systems were susceptible to pressure-locking to the extent that the 
operability of the valves may have been jeopardized.

Discussion

    The pressure locking and thermal binding phenomena are based on 
well-known concepts. The identification of susceptible valves and the 
determination of when the phenomena might occur requires a thorough 
knowledge of components, systems, and plant operations. Pressure 
locking occurs in flexible-wedge and double-disc 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 in the valve bonnet. 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. Surveillance testing can cause a valve to experience pressure 
locking or thermal binding. For example, an inboard isolation MOV in 
the reactor core isolation cooling (RCIC) system steamline at a 
boiling-water reactor (BWR) plant failed in the closed position 
following routine surveillance testing. Pressure locking and thermal 
binding may occur in varying degrees but may not, in all cases, render 
a valve incapable of operating, though a delay in valve stroke time or 
valve damage may occur.
    Various plant operating conditions can introduce pressure locking. 
Valve bonnet pressure might be higher than anticipated, causing 
pressure locking under certain conditions. For example, when (1) the 
gate valve is in a line connected to a high-pressure system and 
isolated only by check valves (which may transmit pressure even when 
passing leak-tightness criteria) and (2) bonnet volume temperature 
increases, pressurization results from thermal expansion of the 
confined fluid. Valve bonnet temperature might increase in response to 
heatup during plant operation, ambient air temperature rise due to 
leaking components or pipe breaks, or thermal conduction or convection 
through connected piping. Over time, bonnet pressure could decay by 
leakage past the seating surfaces or stem packing. However, the 
depressurization time may be longer than the system response time to 
initiate valve actuation to perform its safety function. Also, valve 
actuator operation at locked rotor conditions for a few seconds could 
degrade the motor torque capability of a motor-operated gate valve.
    Thermal binding is generally associated with a wedge gate valve 
that is closed while the system is hot and then allowed to cool before 
attempting to open the valve. Mechanical interference occurs because of 
different expansion and contraction characteristics of the valve body 
and [[Page 15802]] disc materials. Thus, reopening the valve might be 
prevented until the valve and disc are reheated. Solid-wedge gate 
valves are most susceptible to thermal binding. However, flexible-wedge 
gate valves with a high temperature gradient across the discs may 
experience thermal binding.
    Pressure locking or thermal binding occurs as a result of the valve 
design characteristics (wedge and valve body configuration, 
flexibility, and material thermal coefficients) when the valve is 
subjected to specific pressures and temperatures during various modes 
of plant operation. Operating experience indicates these situations 
were not always considered as part of the design basis for valves in 
many plants.

Requested Actions

    Within 60 days of the date of this generic letter, each addressee 
of this generic letter is requested to perform and complete the 
following actions:
    1. Evaluate (in at least a preliminary manner) the operational 
configurations of all safety-related power-operated (i.e., motor-
operated, air-operated, and hydraulic-operated) gate valves to identify 
those valves that are potentially susceptible to pressure locking or 
thermal binding; and
    2. Document a basis for the operability of the potentially 
susceptible valves or, where operability cannot be supported, take 
action in accordance with the Technical Specifications.
    Within 180 days of the date of this generic letter, each addressee 
of this generic letter is requested to implement and complete the 
guidance provided in Attachment 1 to perform the following actions:
    1. Evaluate the operational configurations of safety-related power-
operated (i.e., motor-operated, air-operated, and hydraulic-operated) 
gate valves in its plant to identify valves that are susceptible to 
pressure locking and thermal binding;
    2. Perform further analyses as appropriate, and take needed 
corrective actions (or justify longer schedules), to ensure that the 
susceptible valves identified in 1 are capable of performing their 
intended safety function(s) under all modes of plant operation, 
including test configuration.

    Note: If a licensee has already performed an evaluation of 
operational configurations to identify motor-operated gate valves 
susceptible to pressure locking and thermal binding, and has 
performed additional analyses and taken needed corrective actions 
for identified valves, in a manner that satisfactorily implements 
the guidance in Supplement 6 to GL 89-10 (or equivalent industry 
methods) so that the identified valves are capable of performing 
their required safety functions, the licensee need not perform any 
additional action under 1 and 2 above for MOVs.

50.54(f) Information Request

1. Requested Information
    All addressees, including those who have already satisfactorily 
addressed pressure locking and thermal binding for MOVs by implementing 
the guidance in Supplement 6 to GL 89-10 (or equivalent industry 
methods), are requested to provide a summary description of the 
following:
    a. The susceptibility evaluation of operational configurations 
performed in response to (or consistent with) 180-day Requested Action 
1, and the further analyses performed in response to (or consistent 
with) 180-day Requested Action 2, including the bases or criteria for 
determining that valves are/are not susceptible to pressure locking or 
thermal binding;
    b. The results of the susceptibility evaluation and the further 
analyses referred to in (a) above, including a listing of the 
susceptible valves identified;
    c. The corrective actions, or other dispositioning, for the valves 
identified as susceptible to pressure locking or thermal binding, 
including: (i) Equipment or procedure modifications completed and 
planned (including the completion schedule for such actions); and (ii) 
justification for any determination that particular safety-related 
power-operated gate valves susceptible to pressure locking or thermal 
binding are acceptable as is.
    The staff believes that a corrective action schedule (if corrective 
actions are needed) may be based on risk significance, including 
consideration of common cause failure of multiple valves. However, the 
time schedules for completing corrective action in response to pressure 
locking or thermal binding concerns do not supersede the requirements 
of the NRC regulations and individual plant Technical Specifications in 
the event that a safety-related valve is determined to be incapable of 
performing its safety function. An addressee's schedule for completing 
corrective action in response to this generic letter will be considered 
independent from GL 89-10.
2. Required Response
    All addressees are required to submit the following written 
response to this generic letter:
    a. Within 30 days from the date of this generic letter, a written 
response indicating whether or not the addressee will implement the 
action(s) requested above. If the addressee intends to implement the 
requested action(s), provide a schedule for completing implementation. 
If an addressee chooses not to take the requested action(s), provide a 
description of any proposed alternative course of action, the schedule 
for completing the alternative course of action (if applicable), and 
the safety basis for determining the acceptability of the planned 
alternative course of action;
    b. Within 180 days from the date of this generic letter, a written 
response to the information request specified above in Requested 
Information Items 1.a, 1.b, and 1.c;
    All addressees shall submit the required written responses and 
report specified in item 2 above to the U.S. Nuclear Regulatory 
Commission, ATTN: Document Control Desk, Washington, DC 20555, under 
oath or affirmation under the provisions of section 182a, Atomic Energy 
Act of 1954, as amended, and 10 CFR 50.54(f). In addition, a copy shall 
be submitted to the appropriate Regional Administrator.
Backfit Discussion

    10 CFR part 50, appendix A, Criteria 1 and 4, and plant licensing 
safety analyses, require and/or commit that the addressees design and 
test safety-related components and systems to provide adequate 
assurance that those systems can perform their safety functions. Other 
individual criteria in appendix A to 10 CFR part 50 apply to specific 
systems. In accordance with those regulations and licensing 
commitments, and under the additional provisions of 10 CFR part 50, 
appendix B, Criterion XVI, licensees are expected to take actions to 
ensure that safety-related power-operated gate valves susceptible to 
pressure locking or thermal binding are capable of performing their 
required safety functions. Supplement 6 to GL 89-10 alerted licensees 
to the problems with pressure locking and thermal binding in MOVs, and 
described an acceptable approach for addressing these phenomena for 
MOVs but did not request any specific actions or response form 
licensees.
    The actions requested in this generic letter are considered 
compliance backfits, under the provisions of 10 CFR 50.109 and existing 
NRC procedures, to ensure that safety-related power-operated gate 
valves that are susceptible to pressure locking and thermal binding are 
capable of performing their intended safety functions. In accordance 
with the provisions of Sec. 50.109 regarding compliance backfits, a 
full backfit analysis was not performed for this proposed action; but a 
documented [[Page 15803]] evaluation was performed, including a 
statement of the objectives of and reasons for the requested actions 
and the basis for invoking the compliance exception. A copy of this 
evaluation will be made available in the public document room.

Attachment 1--Guidance for Addressing Pressure Locking and Thermal 
Binding of Power-Operated Gate Vales

    The following summarizes an acceptable approach to addressing 
pressure locking and thermal binding of gate valves within the scope of 
this generic letter:
    1. Perform an evaluation of the safety-related power-operated gate 
valves having operational configurations that may be susceptible to 
pressure locking or thermal binding. Document the basis for determining 
whether valves (a) are susceptible to pressure locking or thermal 
binding or (b) can be removed from further consideration. For example, 
solid wedge disk gate valves might not be susceptible to pressure 
locking. Double disk gate valves are not likely to be susceptible to 
thermal binding.
    The evaluation should include consideration of the potential for 
gate valves to undergo pressure locking or thermal binding during 
surveillance testing.
    The evaluation also should include review of generic studies for 
site-specific applicability, such as in the areas of thermal effects 
and design-basis depressurization.
    Examples of unacceptable reasons for eliminating valves from 
consideration of pressure locking or thermal binding are (1) leakage 
rate, (2) engineering judgement without justification, and (3) lack of 
event occurrence at the specific plant.
    Several plants have experienced either pressure locking or thermal 
binding. These cases are discussed in NUREG-1275, Volume 9. Examples of 
gate valves involved in pressure locking events are:
    * Low-pressure coolant injection (LPCI) and low-pressure core spray 
(LPCS) system injection valves;
    * Residual heat removal (RHR) system hot-leg crossover isolation 
valves;
    * RHR containment sump and suppression pool suction valves;
    * High-pressure coolant injection (HPCI) steam admission valves;
    * RHR heat exchange outlet valves;
    * Emergency feedwater isolation valves; and
    * RCIC steamline isolation valve.
    Examples of gate valves involved in thermal binding events are:
    * Reactor depressurization system isolation valves;
    * RHR inboard suction isolation valves;
    * HPCI steam admission valves;
    * Power-operated relief valve (PORV) block valves;
    * Reactor coolant system letdown isolation valves;
    * RHR suppression pool suction valves;
    * Containment isolation valves (sample line, letdown exchanger 
inlet header);
    * Condensate discharge valves; and
    * Reactor feedwater pump discharge valves.
    2. Perform a further analysis of the safety-related power-operated 
gate valves identified (in 1 above) as susceptible to either pressure 
locking or thermal binding to ensure all such valves can be opened to 
perform their safety function under all modes of plant operation, 
including test configuration.
    If a safety-related power-operated gate valve is found to be 
susceptible to pressure locking or thermal binding and the addressee 
relies on the capability of the actuator to overcome pressure locking 
or thermal binding, consideration of the uncertainties surrounding the 
prediction of the required thrust to overcome these phenomena should be 
included in the evaluation. Credit for bonnet pressure decay within the 
valve response time may not be acceptable unless operation of the 
actuator under those conditions will not degrade actuator capability.
    Attachment 2 to this generic letter describes potential resolution 
options that may be used by licensees for power-operated gate valves 
found susceptible to pressure locking to thermal binding. Several 
preventive and corrective measures for pressure locking and thermal 
binding are also discussed in NUREG-1275, Volume 9, though each method 
has limitations with respect to applicability, safety, effectiveness, 
and cost.
    The NRC regulations require an analysis under 10 CFR 50.59 for any 
valve modifications and the establishment of adequate post-modification 
and inservice testing of any valves installed as part of the 
modification. For example, addressees may need to evaluate the effects 
of drilling the hole in the disk if this option is used to resolve a 
pressure locking concern. One consideration is the fact that, with a 
hole in one disk and the other disk flexible allowing fluid to enter 
the valve bonnet, the valve will be leaktight with respect to pipe flow 
in only one direction.
    As required through appendix B to 10 CFR part 50, the addressee may 
need to establish training for plant personnel to perform any necessary 
actions and incorporate specific procedural precautions/revisions into 
the existing plant operating procedures. For example, plant personnel 
might periodically stroke certain valves to reduce the potential for 
thermal binding.

Attachment 2--Description of Potential Resolution Options for Gate 
Valves Found Susceptible to Pressure Locking or Thermal Binding

1. Analysis Only To Justify Adequate Capability to Overcome the Thrust 
Requirements of Pressure Locking or Thermal Binding

    The staff considers the prediction of the thrust required to 
overcome pressure locking or thermal binding to be very difficult. An 
addressee may be able to justify adequate actuator capability in 
response to pressure locking for small valves. The staff does not 
consider this alternative appropriate to resolve concerns regarding 
thermal binding.

2. Testing Only To Justify Adequate Capability to Overcome the Thrust 
Requirements of Pressure Locking or Thermal Binding

    An addressee may be able to demonstrate through an in-situ or 
prototype test that the actuator has adequate capability to overcome 
pressure locking for a particular valve. The staff considers this 
alternative difficult to justify for thermal binding concerns because 
of the uncertainty in modeling actual plant and valve conditions.

3. A Combination of Testing and Analysis To Justify Adequate Capability 
to Overcome the Thrust Requirements of Pressure Locking or Thermal 
Binding

    An addressee may be able to demonstrate adequate capability of the 
actuator to overcome pressure locking based on test information from 
the particular valve or similar valves from other sources together with 
an analysis to demonstrate applicability. As with Alternative 2, the 
staff considers this alternative difficult to justify for thermal 
binding concerns.

4. Equipment Modifications To Prevent Pressure Locking or Thermal 
Binding

    The staff considers this to be the least difficult alternative to 
justify and address pressure locking of susceptible gate 
valves. [[Page 15804]] 
    Examples of possible modifications to prevent pressure locking are 
provided in NUREG-1275, Volume 9. Modifications to prevent thermal 
binding are also possible, such as replacing a wedge gate valve with a 
parallel-disc gate valve.

5. Procedure Modifications To Prevent Pressure Locking or Thermal 
Binding

    The staff considers procedure modification to be a strong 
alternative for preventing thermal binding of gate valves. However, 
procedure modifications are less likely to be a justifiable alternative 
to prevent pressure locking of gate valves.

    Dated at Rockville, MD, this 20th day of March, 1995.

    For the Nuclear Regulatory Commission.
Brian K. Grimes,
Director, Division of Project Support, Office of Nuclear Reactor 
Regulation.
[FR Doc. 95-7431 Filed 3-24-95; 8:45 am]
BILLING CODE 7590-01-M