UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, DC 20555-0001

November 20, 2006

NRC INFORMATION NOTICE 2006-26: FAILURE OF MAGNESIUM ROTORS IN

MOTOR-OPERATED VALVE ACTUATORS

ADDRESSEES

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

permanently ceased operations and have certified that fuel has been permanently removed

from the reactor vessel.

PURPOSE

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

addressees of recent failures of motor-operated valve (MOV) actuators that were attributed to

the oxidation and corrosion of the magnesium motor rotor fan blades and shorting ring resulting

from exposure to high humidity and temperatures. This IN serves to reaffirm the necessity of

adequate inspection and/or preventive maintenance on MOV actuators manufactured with

magnesium rotors to ensure the safe operation of nuclear power facilities. It is expected that

recipients will review the information for applicability to their facilities and consider actions, as

appropriate, to avoid similar problems. However, suggestions contained in this IN are not NRC

requirements; therefore, no specific action or written response is required.

DESCRIPTION OF CIRCUMSTANCES

A recent NRC staff review of MOV actuator failures at certain plants identified the following

examples:

1. Failure of a Main Feedwater Isolation Block Valve to operate automatically (Crystal

River 3; October 28, 2005; Licensee Event Report (LER) 50-302/2005-004-00). The

licensee attributed this failure to the corrosion and oxidation of the magnesium fan

blades and shorting ring of the motor rotor as a result of exposure to high humidity and

temperatures.

2. Failure of the Residual Heat Removal (RHR) Cold Leg Injection Valve to open when

placing RHR in operation for cooldown (Turkey Point 3; March 6, 2006;

LER 50-250/2006-003-00). The licensee attributed this failure to the corrosion and

oxidation of the magnesium fan blades and shorting ring of the motor rotor as a result of

exposure to high humidity and temperatures.

ML062070124
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3. Failure of a Recirculation Pump Suction Valve to operate (Browns Ferry 3;

January 15, 2006; documented in the licensee's corrective action program). The

licensee attributed this failure to the corrosion of the motor rotor fan blades and shorting

ring.

BACKGROUND

Many safety- and non-safety-related MOVs utilize Limitorque actuators with Reliance motors or

a similarly styled design by a different manufacturer. Based on torque requirements, aluminum

and magnesium alloy cast-squirrel-cage rotors are utilized in MOV actuators. Valve actuators

with a motor maximum torque of 40 foot-pounds force (54 Newton-meters) are typically

aluminum, and magnesium actuators are used for applications requiring greater than

60 foot-pounds force (81 Newton-Meters).

The typical magnesium rotor is made of stacked, steel punched core plates with AM100A

magnesium alloy (approximately 90% magnesium, 10% aluminum, 0.1% manganese)

components—the conductor bars, end rings, and cooling fan blades—cast to complete the

rotor. While magnesium provides higher torque through its higher resistivity, this relatively

brittle cast alloy is susceptible to shrinkage cracking and gas porosity. Specifically, magnesium

rotors are susceptible to three main failure mechanisms: galvanic corrosion, general corrosion,

and thermally induced stress. 

The first failure mechanism is galvanic corrosion. Following manufacture, the electrical

potential difference between the magnesium and the steel core is 1.9 volts creating the

conditions for galvanic corrosion, with the most vulnerable area being the interface between the

steel core and the magnesium end ring. Most manufacturers alleviate this by protecting the

magnesium end rings with a paint and/or lacquer coating. Though the rotor might be initially

protected, even the smallest scratch or chip in this exterior coating will cause localized,

accelerated corrosion in the form of magnesium hydroxide (MgOH) powder. The formation of

MgOH powder leads to rotor cracks that add to the existing problems of shrinkage cracking,

gas porosity, and MgOH volume difference. Motor overheating events (typically due to locked

rotor conditions) accelerate this coating degradation. A propagating crack at the interface

between the stacked core and the end ring causes a high resistance connection with the end

ring, which in turn causes a high current density (due to current redistribution) on the opposite

side of the rotor. This increased current density increases the temperature on that side of the

rotor resulting in thermal stress. At the steel-magnesium interface, the higher temperature may

melt the magnesium into small beads. These thermally-stressed rotor areas and the melted

magnesium beads then provide new opportunities for coating degradation and cracking

resulting in new areas of high resistance between the stacked core and end ring and new areas

of the rotor with a higher current density. This cycle of events can then repeat around the rotor.

The second major failure mechanism affecting magnesium rotors is general corrosion. Most

actuator motors for safety-related MOVs that are located in potentially harsh environments have

T-drain pipe plugs to allow moisture to escape. These same plugs allow moisture to enter and

condense inside the motor. This moisture leads to the formation of MgOH and magnesium

ox 2ide (MgO ). The white MgOH powder can form a light haze on the inside of the motor without

2

impacting its operation. However, MgOH and MgO can form beads between core plates (from
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the magnesium conductor bars) and at the interface between the stacked core and the end ring

causing high resistance points and the high current density phenomena stated above and even

further cracking. The rate of general corrosion increases in a higher humidity operating

environment.

The third major failure mechanism affecting magnesium rotors is thermally induced stress

which reveals itself in different ways. First, because galvanic corrosion is thermally catalyzed,

the corrosion rate increases with temperature, with a significant increase in the corrosion rate

occurring at temperatures above approximately 93 EC (200 EF). The rate of galvanic corrosion

increases when the motor is located in a higher temperature environment, as well as during

general motor high-current conditions and/or within the high current density regions mentioned

earlier. Secondly, magnesium has twice the thermal expansion coefficient of steel. This

produces uneven axial and radial forces across the rotor causing further cracks in the

magnesium and its paint and/or lacquer coating. Finally, many rotors experience significantly

higher temperatures because their thermal overloads are set higher than the recommended

10 to 15 seconds for locked rotor current conditions (in order to ensure safety-related function

as given in NRC Regulatory Guide 1.106, Revision 1, “Thermal Overload Protection for Electric

Motors on Motor-Operated Valves”). For example, some rotors reach 700 EF (371 EC) in

15 seconds, and temperatures of 700 EF to 850 EF (371 EC to 454 EC) cause a significant loss

of magnesium yield strength.

Various laboratory tests have been conducted to better understand magnesium rotors. General

Electric (GE) tested-to-failure 3 motors in varying aged and environmental conditions, with the

most limiting failure being a new motor which failed after 43 days in a high temperature

environment under a maximum temperature of 223 EF (106 EC). The Institute of Electrical and

Electronics Engineers (IEEE) inspected 14 magnesium rotors and discovered 5 showing

varying levels of degradation. Finally, IEEE reviewed plant motor failure rates and found

magnesium rotors failing at three times the rate of aluminum rotors.

The following documents address similar MOV failures with related technical details:

! NRC Information Notice 86-02, “Failure of Valve Operator Motor during Environmental

Qualification Testing,” January 6, 1986: this IN reported on the results of the previously

discussed GE laboratory test on three motors in response to issues at the River Bend

and Nine Mile Point 2 nuclear power stations. In addition to the technical details stated

earlier, the NRC within this IN suggested that licensees review the qualification of these

motors in their Design Basis Event applications.

! NUREG/CR-5404, ORNL-6566/V1, “Auxiliary Feed Water Aging Study,” July 1993: 

while this report is extensive and covers many wide-ranging aspects, Section 4.5

(Alternate Methods of Valve Actuator Motor Testing) reviews two methods for the

preventive maintenance of magnesium rotors.

! NUREG/CR-6205, ORNL-6796, “Valve Actuator Motor Degradation,” December 1994: 

this NUREG provides a detailed review of the technical phenomena citing all of the

failure mechanisms with insights from the GE test and the IEEE report.
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! IEEE Transactions on Energy Conversion, Vol. 3, No. 1, “An Investigation of Magnesium

Rotors in Motor Operated Valve Actuators,” March 1988: this IEEE report provides a

detailed, technical analysis of the failure mechanisms and material impact of

magnesium rotors. This analysis includes the review of various laboratory tests and

licensee database reviews. This report includes a detailed inspection procedure for user

guidance.

The IEEE report, NUREG/CR-5404, Crystal River LER 50-302/2005-004-00, Turkey Point LER

50-250/2006-003-00, and the operating experience from Browns Ferry provide some specific

methods for preventive maintenance:

1. The IEEE report and the LER’s from Crystal River and Turkey Point provide detailed

inspection procedures with acceptance criteria. They specifically discussed boroscopic

inspections of MOV actuators through the T-drain pipe as a preventive maintenance

method.

2. The Crystal River LER also provides detail on performing electrical Polarization Index

inspections from measurements of the motor winding insulation resistance. 

3. NUREG/CR-5404 reviews motor current signature analysis as a method for revealing

broken or distorted rotor bars.

4. The IEEE report reviews ideal thermal overload setpoints in order to avoid the thermally

induced stresses discussed earlier but also proposes graduated inspection criteria if

these setpoints are not met.

5. Operating experience from Browns Ferry describes their consideration of duty cycle

limitations to ensure the motors are not actuated without a proper cooldown interval in

order to avoid or not exacerbate thermally induced stresses.

DISCUSSION

Recent failures of MOV actuators as a result of galvanic corrosion, general corrosion, and/or

thermally induced stress highlight the particular vulnerabilities of motor actuators with

magnesium rotors, particularly when the motor is located in a high humidity and/or high

temperature environment. These MOV failures illustrate the necessity of adequate inspection

and/or preventive maintenance on actuators manufactured with magnesium rotors.
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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 or the appropriate Office of

Nuclear Reactor Regulation project manager.

/RA by Theodore Quay for/

Michael J. Case, Director

Division of Policy and Rulemaking

Office of Nuclear Reactor Regulation

Technical Contacts: James Polickoski, RII Scott Stewart, RII

803-345-5683 305-245-7669

E-mail: jtp@nrc.gov E-mail: jss1@nrc.gov

Tom Morrissey, RII Robert Monk, RII

352-795-7677 256-729-6196

E-mail: txm1@nrc.gov E-mail: rlm2@nrc.gov

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

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