[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]
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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.
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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
