UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001

December 14, 2006

INFORMATION NOTICE 2006-29: POTENTIAL COMMON CAUSE FAILURE OF

MOTOR-OPERATED VALVES AS A RESULT OF

STEM NUT WEAR

ADDRESSEES

All holders of operating licenses for nuclear power reactors except those who have permanently

ceased operation and have certified that fuel has been permanently removed from the vessel.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert

addressees of potential common cause failure of motor-operated valves (MOVs) at nuclear

power plants as a result of stem nut wear. The agency expects that recipients will review the

information for applicability to their facilities and consider actions, as appropriate, to avoid

similar problems. However, the suggestions contained in this IN are not NRC requirements;

therefore, no specific action or written response is required.

DESCRIPTION OF CIRCUMSTANCES

On March 27, 2006, during the spring refueling outage for the Susquehanna Nuclear Power

Plant, Unit 1, the suppression pool suction valve for the “D” residual heat removal (RHR) pump

failed to close during system functional testing. On April 6, 2006, the suppression pool suction

valve for the “C” RHR pump failed to stroke during system alignment. The licensee identified

the cause of these MOV failures to be excessive wear of the internal threads of the stem nut

that converts the rotational motion of the motor actuator to the lateral motion of the stem to

open and close the valve.

In the MOV Preventive Maintenance (PM) program at Susquehanna, the licensee had relied on

the observation of stem nut thread shavings below the actuator to identify stem nut wear and

the need for stem nut inspection. The licensee had not directly inspected the stem nuts for the

two failed MOVs for 20 years because no stem nut thread shavings had been found below the

actuator. The licensee did not have a periodic overhaul program for safety-related MOVs, nor

procedures to review stem nut thread clearance when conducting diagnostic testing of

safety-related MOVs.

In response to these MOV failures, the licensee evaluated diagnostic traces of rising stem

MOVs at Susquehanna, Units 1 and 2 that were within the scope of Generic Letter 89-10,

“Safety-Related Motor-Operated Valve Testing and Surveillance.” The licensee studied the 

time span required for the rotating stem nut to take-up the clearance between the valve stem 

ML062890437
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threads and the stem nut threads to assist in identifying potential stem nut wear. The licensee

evaluated previous diagnostic traces to determine changes in thread clearance to estimate the

stem nut wear rate and to determine whether sufficient thread would remain through the next

operating cycle. Based on this information, the licensee inspected 31 safety-related MOVs at

Susquehanna and replaced the stem nuts in 18 MOVs with wear exceeding 50% of the original

thread thickness. The licensee plans to conduct additional MOV stem nut inspections in the

future.

The RHR suppression pool suction valves at Susquehanna, Unit 1 are normally open and do

not automatically change position to perform their safety function of allowing emergency core

cooling water to be taken from the suppression pool. These valves also serve a containment

isolation function in that they would be remotely closed if there was a break in the RHR system. 

The RHR suppression pool suction valves are interlocked with the RHR shutdown cooling

suction valves to prevent these valves from being open at the same time and inadvertently

draining the reactor vessel. Therefore, the failure of the RHR suppression pool suction valves

might have impacted the emergency core cooling function of the RHR system, or affected the

primary containment isolation function, depending on valve position at the time of failure. At the

time of the event, primary containment integrity was not required. With the plant shut down for

refueling, the safety significance of having two inoperable RHR suppression pool suction valves

was considered low. Although the RHR pumps associated with the failed valves were not being

relied upon to fulfill emergency injection requirements, valve failures associated with systems

being relied upon for shutdown cooling or emergency core cooling (either shutdown or at

power) might have either prevented or unexpectedly altered system function, thus, complicating

operator actions and necessitating further response.

The licensee submitted Licensee Event Report (LER) 50-387/2006-003-00 on July 26, 2006, in

response to the MOV stem nut failures at Susquehanna (see Agencywide Documents Access

and Management System Accession No. ML062190207). In the LER, the licensee listed the

following as root causes for the event: (1) valve stem visual inspections for evidence of stem

nut thread wear were an ineffective means for monitoring long-term stem nut thread wear;

(2) routine PM activities for periodically inspecting stem nuts did not exist; and (3) procedural

guidance for inspection and acceptance of stem nut thread wear was inadequate. The licensee

reported that immediate corrective actions had been completed to evaluate safety-related

MOVs at Susquehanna using previous diagnostic indications of stem nut thread wear and

inspection of selected stem nuts to assess stem nut condition. Additional long-term corrective

actions planned at Susquehanna include: 

(1) performing additional MOV inspections and replacing stem nuts, when needed;

(2) revising the current 2-year PM activity bases, and to clarify the intent and

limitations of stem inspections in response to the ineffectiveness of visual

inspections to detect long-term stem nut wear;

(3) developing a new PM activity that periodically inspects selected MOV stem nuts;

(4) establishing a methodology and acceptance criteria for measurement of stem nut

wear;
IN 2006-29

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(5) incorporating detailed instructions for performing stem nut inspections into

existing procedures;

(6) preparing procedural direction for monitoring and trending stem nut wear using

MOV diagnostic data; and

(7) modifying stem lubrication PM activities to include valve stroking.

DISCUSSION

In an MOV, the stem nut converts the rotational motion of the drive sleeve in the motor actuator

to the lateral motion of the stem to open and close the valve. Stem nut failure can prevent the

operation of the MOV from either the motor actuator or manual handwheel. Stem nut failure

can also cause valve position for the MOV to be incorrectly displayed in the control room. The

failure of a stem nut for an individual MOV can interfere with the operation of other plant

equipment when its valve position signal is supplied to interlock logic systems. Further, if the

stem nut threads are destroyed, a valve could potentially drift open or closed should the valve

packing be unable to hold the valve stem in position.

The stem nut of an MOV is typically made of bronze material. Over a period of time and use,

the stem nut will undergo wear when rotating to move the steel valve stem to open or close the

valve. Stem nut wear can be influenced by several factors, including the following: (1) stem nut

material; (2) normal operating loads and maximum loads; (3) stem nut rotations during a valve

stroke; (4) number and frequency of valve strokes; (5) stem nut manufacture and threaded

length; (6) stem and stem nut fit; (7) valve stem thread condition; and (8) stem lubricant,

lubrication method and frequency, and environmental conditions for the lubricant.

The unexpected failures and significant degradation of stem nuts in MOVs at Susquehanna

emphasizes the importance of PM activities in identifying MOV stem nut wear in a timely

manner. For example, the absence of significant changes in diagnostic performance for

monitored MOV parameters (such as stem factor, thrust, or torque), or stem nut thread

shavings below the actuator, might not be sufficient to confirm that the stem nut remains in

good condition. Additional PM activities, such as periodic overhaul of safety-related MOVs or

evaluation of MOV diagnostic test data for stem nut thread clearance, might be needed to

identify stem nut wear. Further, it is important for the PM program to ensure that new lubricant

is applied to the stem nut area, such as by stroking the valve, when lubricating safety-related

MOV valve stems.

Excessive stem nut wear represents a potential common cause failure mode that could impact

multiple MOVs at nuclear power plants. It is typically a long-term issue that is addressed as

part of PM programs at nuclear power plants. Industry guidance includes stem nut inspection

as part of MOV technical repair guidelines. Vendors of MOV diagnostic equipment include

stem nut thread take-up clearance as a specific parameter for monitoring stem nut wear.

Appropriate PM activities, such as those to be implemented at Susquehanna, can assist in

identifying significant stem nut wear prior to MOV failure. 
IN 2006-29

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CONTACTS

This information notice requires no specific action or written response. Please direct any

questions about this matter to the technical contacts listed below.

/RA/

Michael J. Case, Director

Division of Policy and Rulemaking

Office of Nuclear Reactor Regulation

Technical contacts: Thomas G. Scarbrough, NRR James M. Trapp, Region I

(301) 415-2794 (610) 337-5186 

E-mail: TGS@nrc.gov E-mail: JMT1@nrc.gov

Alan J. Blamey, Region I Terry A. Beltz, NRR

(570) 542-2134 (301) 415-3049

E-mail: AJB3@nrc.gov E-mail: TAB3@nrc.gov

Note: NRC generic communications may be found on the NRC public Website,

http://www.nrc.gov, under Electronic Reading Room/Document Collections.