UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001

July 27, 2006

INFORMATION NOTICE 2006-15: VIBRATION-INDUCED DEGRADATION AND FAILURE OF

SAFETY-RELATED VALVES

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 vibration-induced degradation and failure of valves supplied by Fisher Controls

and other manufacturers. 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 CIRCUM STANCES

During a plant startup in October 2003, Vogtle Electric Generating Plant, Unit 1, experienced a

failure of an auxiliary feedwater (AFW) discharge control valve. The valve was a 4-inch, model

SS-120, manufactured by Fisher Controls. Although the valve indicated full open, operators

noted that AFW loop flow was reduced and did not change during valve throttling. The valve

degradation was attributed to the flow-induced metal fatigue failure of a cotter pin designed to

secure the pilot plug assembly retaining nut to the valve stem. Consequently, the retaining nut

backed off completely, releasing the pilot plug spacer and a washer from the pilot plug, allowing

them to be transported downstream and block flow through a restricting orifice. A similar failure

of an AFW discharge control valve occurred in 1989 at Vogtle.

The valve vendor issued an advisory regarding this failure mechanism in 1988. In this advisory,

the vendor stated that failures of Fisher Controls type AP, EP, EWP, and SS-120 valves may

occur and recommended that all valves affected by the advisory be disassembled to inspect the

main-plug/pilot-plug restraining nut assembly. The assembly is held together by a large nut

which is restrained from turning by either a star lockwasher with bend-up tabs, or a single cotter

pin design. The vendor stated that the hex nut may unscrew because of improper installation of

either type locking mechanism. Specifically, reuse of the star lockwasher has resulted in fatigue

and subsequent breaking of the tabs, and the cotter pin design has failed from improper

replacement or reuse which has allowed the pin to vibrate and fail through fatigue.

M L061790443
IN 2006-15

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In response to this failure event, the licensee performed visual inspections of 15 similar valves 

and identified that all of the motor-driven AFW discharge control valves were missing cotter pins. 

The cotter pins associated with the turbine-driven AFW discharge control valves appeared

unaffected, probably because of the much lower operational flow time. The licensee repaired

the valves by staking the threads on the valve stem against the retaining nut, instead of securing

the nuts with cotter pins.

DISCUSSION

Over the years, nuclear power plants have experienced vibration-induced degradation of plant

equipment during operation at the original licensed power and under power uprate conditions. 

The NRC has issued several INs on this subject, including NRC IN 83-70, “Vibration-Induced

Valve Failures,” dated October 25, 1983 (http://www.nrc.gov/reading-rm/doc-collections/

gen-comm/info-notices/1983/in83070.html), to alert nuclear power plant licensees of instances

of valve failures and system inoperability that occurred as a result of normal operational

vibration.

In January 2004, NRC IN 2002-26, Supplement 2, “Additional Flow-Induced Vibration Failures

After a Recent Power Uprate,” (Agencywide Documents Access and Management System

(ADAMS) Accession No. ML040080392) was issuedwhich described that increased steam and

feedwater flow can increase the vibration of plant equipment, including valves and valve

actuators. The higher vibration levels can impact the appropriate inspection intervals for some

plant components.

San Onofre Nuclear Generating Station, Units 1 and 2, experienced degradation of butterfly

valves in 2003, as discussed in NRC IN 2005-23, “Vibration-Induced Degradation of Butterfly

Valves,” dated August 1, 2005 (ADAMS Accession No. ML051740299). The failures resulted

from lost taper pins used to connect the valve disc to the valve stem. These valves were

manufactured by Fisher Controls. Problems have been attributed to failed taper pin connectors

in butterfly valves supplied by other manufacturers. In 1989, Turkey Point Nuclear Plant, Unit 4,

lost taper pins in a 36-inch intake cooling water isolation valve manufactured by the Henry Pratt

Company. In 2003, Davis-Besse Nuclear Power Station, Unit 1, lost taper pins in a 10-inch

decay heat removal cooler valve with the brand name Valtek marketed by the Flowserve

Corporation.

In June 2005, the licensee at Hope Creek Generating Station shut down the unit and entered into

its emergency plan because it exceeded limits for unidentified leakage inside primary

containment. This event was discussed in “Hope Creek Nuclear Generating Station - NRC

Integrated Inspection Report 05000354/2005005 and Exercise of Enforcement Discretion,” dated

January 26, 2006 (ADAMS Accession No. ML060270171). The licensee identified an

approximately 285-degree circumferential crack in the position-indicating tube for the “A” residual

heat removal shutdown cooling return testable check valve. This through-wall leak was caused

by vibration of the attraction sleeve (located at the end of the actuator rod), in the presence of the

switch magnetic force, resulting in the attraction sleeve fretting and wearing through the position-
IN 2006-15

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indicating tube. Licensee corrective actions included modifying both the “A” and “B” train check

valves by removing the position indicator tubes. Six additional check valves that use the same

position indicator tube underwent ultrasonic testing, which revealed no similar wear indications.

In summary, operating experience associated with vibration-induced valve degradation shows

that certain valve sub-components (such as yoke-to-bonnet hold-down studs and nuts, stem-todisc connectors, valve stem clamp setscrews) may be more susceptible to failure. Changes to

system flow characteristics and vibrational harmonics may serve as indicators that further

evaluation of these effects on system components is needed. Initiatives to preclude valve

failures may include identifying components that could be subjected to vibration-induced stress

and wear, fully understanding the long-term effects that vibration-induced stress may have on

these components (including sub-components that may be prone to early failure), and

thoroughly evaluating and inspecting components on a schedule consistent with the overall risk

significance associated with a failure.

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/

Ho K. Nieh, Acting Director

Division of Policy and Rulemaking

Office of Nuclear Reactor Regulation

Technical contacts: Steven M. Unikewicz, NRR John Zeiler, Region II

(301) 415-3819 (803) 345-5683

E-mail: SMU@nrc.gov E-mail: JXZ@nrc.gov

Terry A. Beltz, NRR

(301) 415-3049

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.