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

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]

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