NRC Generic Letter 95-07, Pressure Locking and Thermal Binding of Safety-Related Power-Operated Gate Valves

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Addressees

All holders of operating licenses (except those licenses that have been

amended to possession-only status) 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 previously performed,

(1) evaluations of operational configurations of safety-related, power-

operated (including motor-, air-, and hydraulically 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 the safety functions within the current

licensing bases of the facility.

NRC previously provided guidance on an acceptable approach for addressing

pressure locking and thermal binding of motor-operated valves (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 as necessary 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 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 also includes pressure locking and thermal

binding phenomena in other types of power-operated (i.e., air- and

hydraulically operated) gate valves, as well as MOVs. Additional information

provided in Attachment 1 does not alter the basic approach to addressing

pressure locking and thermal binding given in Supplement 6 to GL 89-10.

Finally, for 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

(1) actions taken by licensees on their own volition to implement the guidance

provided in Supplement 6 (or equivalent industry methods), as well as

(2) actions taken in response to this letter. (In Supplement 6 to GL 89-10,

the staff did not require any licensee response 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 disk 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 disks. Several of the

industry's generic communications have given guidance for identifying

susceptible valves and for 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. In GL 89-10 licensees were asked to

review MOV design bases, verify MOV switch settings both initially and

periodically, test MOVs under design-basis conditions where practicable,

improve evaluations of MOV failures and necessary corrective action, and trend

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 gives 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 functions. Existing surveillance tests or normal

operating cycles might not reveal such failures.

Description of Circumstances

After issuing Volume 9 of NUREG-1275, the NRC staff performed a number of site

visits to discuss pressure locking and thermal binding with licensees (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 previous industry guidance for identification of

susceptible valves and application of preventive and corrective measures. NRC

surveys indicated that in response to the number of generic industry

communications on the subject, 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 motor-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 in light of recent information regarding MOV and system performance,

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 issuing 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

GL 89-10 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 binding of all safety-

related power-operated gate valves, including MOVs, in a consolidated effort

(through evaluation of actions taken in response to 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 (ML9403020090) 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 by 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," alerting licensees to a report by 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.

On June 13, 1995, the Millstone Unit 2 licensee performed an evaluation which

determined that the power-operated relief valve (PORV) block valves were

potentially susceptible to thermal binding. Specifically, it was found that,

if the PORV block valves were closed and a subsequent plant cooldown was

performed, the block valves could experience thermal binding. In GL 90-06,

"Resolution of Generic Issue 70, `Power-Operated Relief Valve and Block Valve

Reliability,' and Generic Issue 94, `Additional Low-Temperature Overpressure

Protection for Light-Water Reactors,' Pursuant to 10 CFR 50.54(f)," the staff

asked licensees to include PORV block valves in their GL 89-10 program.

Therefore, although PORV block valves may not be classified as safety related

at particular plants, licensees will be expected to have evaluated these

valves for potential pressure locking or thermal binding. Similarly,

licensees may need to evaluate for potential pressure locking or thermal

binding other valves outside the scope of this generic letter based on

previous licensing commitments.

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 require a thorough knowledge of components,

systems, and plant operations. Pressure locking occurs in flexible-wedge and

double-disk 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 disks 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 disk causes the disk

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 disk 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

can occur to varying degrees, but does not necessarily render a valve

incapable of operating, although valve damage may occur.

Various plant operating conditions can introduce pressure locking. Pressure

in the valve bonnet 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. Temperature in the valve bonnet 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

decrease 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 is allowed to cool before attempting to open

the valve. Mechanical interference occurs because of different expansion and

contraction characteristics of the valve body and disk materials. Thus,

reopening the valve might be prevented until the valve and disk are reheated.

Solid-wedge gate valves are most susceptible to thermal binding. However,

flexible-wedge gate valves experiencing significant temperature changes or

operating with significant upstream and downstream temperature differences may

thermally bind.

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 90 days of the date of this generic letter, each addressee of this

generic letter is requested to perform and complete the following actions:

1. Perform a screening evaluation of the operational configurations of all safety-related power-operated (i.e., motor-operated, air-operated, and hydraulically 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 individual plant 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 hydraulically operated) gate valves in its plant to identify valves that are susceptible to pressure locking or 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.

Attachment 2 discusses potential resolution options for gate valves found

susceptible to pressure locking or thermal binding.

[Note: If a licensee has previously performed an evaluation of operational

configurations to identify motor-operated gate valves susceptible to pressure

locking or 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 paragraphs 1 and 2 above for MOVs.]

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:

1. 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 or are not susceptible to pressure locking or thermal binding;

2. The results of the susceptibility evaluation and the further analyses referred to in 1 above, including a listing of the susceptible valves identified;.

3. The corrective actions, or other dispositioning, for the valves identified as susceptible to pressure locking or thermal binding, including: (a) equipment or procedural modifications completed and planned (including the completion schedule for such actions); and (b) 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. Plant operation and outage schedules

may also be considered in developing corrective action schedules. 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. In GL 91-18