Information Notice 2008-20, Failures of Motor Operated Valve Actuator Motors with Magnesium Alloy Rotors: Difference between revisions
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{{#Wiki_filter:UNITED STATES | {{#Wiki_filter:ML082840609 UNITED STATES | ||
NUCLEAR REGULATORY COMMISSION | NUCLEAR REGULATORY COMMISSION | ||
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OFFICE OF NUCLEAR REACTOR REGULATION | OFFICE OF NUCLEAR REACTOR REGULATION | ||
WASHINGTON, DC 20555-0001 December 8, 2008 NRC INFORMATION NOTICE 2008-20: | WASHINGTON, DC 20555-0001 | ||
December 8, 2008 | |||
NRC INFORMATION NOTICE 2008-20: | |||
FAILURES OF MOTOR OPERATED VALVE | |||
ACTUATOR MOTORS WITH MAGNESIUM | ACTUATOR MOTORS WITH MAGNESIUM | ||
| Line 42: | Line 47: | ||
addressees of recent failures and corrective actions for motor-operated valve (MOV) actuator | addressees of recent failures and corrective actions for motor-operated valve (MOV) actuator | ||
motors due to corrosion of the magnesium alloy rotors. The NRC expects recipients to review | motors due to corrosion of the magnesium alloy rotors. The NRC expects recipients to review | ||
the information for applicability to their facilities and consider actions, as appropriate, to avoid | 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; | similar problems. However, suggestions contained in this IN are not NRC requirements; | ||
therefore, no specific action or written response is required. | therefore, no specific action or written response is required. | ||
| Line 52: | Line 57: | ||
The NRC continues to inform the industry of the potential for MOV actuator motor magnesium | The NRC continues to inform the industry of the potential for MOV actuator motor magnesium | ||
alloy rotors to corrode and subsequently fail under certain conditions. The NRC issued two | alloy rotors to corrode and subsequently fail under certain conditions. The NRC issued two | ||
previous INs to alert the industry of this problem. Technical information in the previous INs is | previous INs to alert the industry of this problem. Technical information in the previous INs is | ||
not repeated in this IN. | not repeated in this IN. | ||
| Line 60: | Line 65: | ||
The following are instances in 2007 where MOVs with actuator motors manufactured with | The following are instances in 2007 where MOVs with actuator motors manufactured with | ||
magnesium alloy rotors did not operate on demand due to motor failure: | magnesium alloy rotors did not operate on demand due to motor failure: | ||
River Bend, Unit 1 | |||
On May 25, 2007, at River Bend, Unit 1, the reactor inlet heater A outboard motor operated | On May 25, 2007, at River Bend, Unit 1, the reactor inlet heater A outboard motor operated | ||
isolation valve failed to operate due to magnesium alloy rotor corrosion that was attributed to a | isolation valve failed to operate due to magnesium alloy rotor corrosion that was attributed to a | ||
steam leak from an adjacent main steam shutoff valve. This main steam shutoff valve, which | steam leak from an adjacent main steam shutoff valve. This main steam shutoff valve, which | ||
also was an MOV with a magnesium alloy rotor, failed to operate approximately four months | also was an MOV with a magnesium alloy rotor, failed to operate approximately four months | ||
later on September 29, 2007. Licensee corrective actions for these MOV failures included | later on September 29, 2007. Licensee corrective actions for these MOV failures included | ||
identifying the MOV actuator motors susceptible to this failure mechanism and performing | identifying the MOV actuator motors susceptible to this failure mechanism and performing | ||
borescope inspections of the susceptible actuator motors. Eight MOV motors at River Bend | borescope inspections of the susceptible actuator motors. Eight MOV motors at River Bend | ||
were replaced because the motor inspections failed the inspection criteria. | were replaced because the motor inspections failed the inspection criteria. Quad Cities, Unit 1 | ||
On September 4, 2007, at Quad Cities, Unit 1, the high-pressure coolant injection (HPCI) | |||
primary containment isolation valve failed to operate. The motor for this valve was installed in | primary containment isolation valve failed to operate. The motor for this valve was installed in | ||
1994. The packing on this valve catastrophically failed in 2002 resulting in high-pressure steam | 1994. The packing on this valve catastrophically failed in 2002 resulting in high-pressure steam | ||
in the immediate area of the motor for several hours. The motor experienced two planned stall | in the immediate area of the motor for several hours. The motor experienced two planned stall | ||
events while placing that valve on its backseat in an attempt to mitigate the packing leak. Motor | events while placing that valve on its backseat in an attempt to mitigate the packing leak. Motor | ||
control center testing had been completed in May 2006 with no issues identified. Quarterly | control center testing had been completed in May 2006 with no issues identified. Quarterly | ||
inservice tests did not identify any degradation. Licensee corrective actions included improving | inservice tests did not identify any degradation. Licensee corrective actions included improving | ||
the procedures, training, and equipment used to perform borescope inspections of susceptible | the procedures, training, and equipment used to perform borescope inspections of susceptible | ||
| Line 96: | Line 102: | ||
actuator motors. | actuator motors. | ||
Dresden, Unit 3 | |||
On September 12, 2007, at Dresden Unit 3, the HPCI primary containment isolation valve failed | On September 12, 2007, at Dresden Unit 3, the HPCI primary containment isolation valve failed | ||
to operate. The motor, installed in 2002, is located in the upper drywell where temperatures are | to operate. The motor, installed in 2002, is located in the upper drywell where temperatures are | ||
approximately 180 ºFahrenheit (F) [82 ºCelsius (C)]. The licensee performed a borescope | approximately 180 ºFahrenheit (F) [82 ºCelsius (C)]. The licensee performed a borescope | ||
inspection of the valve in 2006 with no issues identified. Licensee corrective actions included | inspection of the valve in 2006 with no issues identified. Licensee corrective actions included | ||
improving the procedures, training, and equipment used to perform borescope inspections of | improving the procedures, training, and equipment used to perform borescope inspections of | ||
susceptible actuator motors. The licensee replaced four MOV motors at Dresden, Unit 2, in | susceptible actuator motors. The licensee replaced four MOV motors at Dresden, Unit 2, in | ||
November 2007 because the borescope inspection of the motors failed the inspection criteria. | November 2007 because the borescope inspection of the motors failed the inspection criteria. | ||
| Line 114: | Line 121: | ||
Many safety- and non-safety-related MOVs utilize Limitorque actuators with Reliance motors or | Many safety- and non-safety-related MOVs utilize Limitorque actuators with Reliance motors or | ||
a similarly styled design by a different manufacturer (e.g., Electric Apparatus). Based on torque | a similarly styled design by a different manufacturer (e.g., Electric Apparatus). Based on torque | ||
requirements, aluminum and magnesium alloy rotors are utilized in MOV actuators. Valve | requirements, aluminum and magnesium alloy rotors are utilized in MOV actuators. Valve | ||
actuators with a motor maximum torque of 40 foot-pounds [54 newton-meters] are typically | actuators with a motor maximum torque of 40 foot-pounds [54 newton-meters] are typically | ||
aluminum, and magnesium alloy rotors are used for applications requiring greater than 60 foot- pounds [81 newton-meters] torque. Flowserve Technical Update 06-01, dated December 26, | aluminum, and magnesium alloy rotors are used for applications requiring greater than 60 foot- pounds [81 newton-meters] torque. Flowserve Technical Update 06-01, dated December 26, | ||
2006, states that Limitorque actuator frame sizes 180 and larger are typically constructed with | 2006, states that Limitorque actuator frame sizes 180 and larger are typically constructed with | ||
magnesium alloy rotors. However, one licensee identified that its facility contained Limitorque | magnesium alloy rotors. However, one licensee identified that its facility contained Limitorque | ||
actuator frame sizes 56 and larger that were manufactured with magnesium alloy rotors. | actuator frame sizes 56 and larger that were manufactured with magnesium alloy rotors. | ||
Flowserve Technical Update 06-01 also states that it is Limitorques intention (where possible) | Flowserve Technical Update 06-01 also states that it is Limitorques intention (where possible) | ||
to replace the remaining magnesium alloy rotor designs with an aluminum alloy rotor design in | to replace the remaining magnesium alloy rotor designs with an aluminum alloy rotor design in | ||
| Line 134: | Line 141: | ||
The magnesium alloy rotor bars, end rings, and cooling fins are an integral structure cast | The magnesium alloy rotor bars, end rings, and cooling fins are an integral structure cast | ||
through the iron lamination stack. The rotor assembly is painted to prevent corrosion. The | through the iron lamination stack. The rotor assembly is painted to prevent corrosion. The | ||
magnesium alloy (nominal 90 percent magnesium - 10 percent aluminum) has a linear thermal | magnesium alloy (nominal 90 percent magnesium - 10 percent aluminum) has a linear thermal | ||
expansion coefficient that is double the coefficient for iron. High inrush current heats the motor | expansion coefficient that is double the coefficient for iron. High inrush current heats the motor | ||
internals causing the temperature of the rotor assembly to increase. An MOV motor stall event | internals causing the temperature of the rotor assembly to increase. An MOV motor stall event | ||
is an example of a condition that creates a high current inrush to the motor. Although the rotor | is an example of a condition that creates a high current inrush to the motor. Although the rotor | ||
assembly is painted to limit corrosion, the paint bridging the intersection of the rotor end ring | assembly is painted to limit corrosion, the paint bridging the intersection of the rotor end ring | ||
| Line 148: | Line 155: | ||
and iron lamination would be stressed by differential thermal expansion between the rotor and | and iron lamination would be stressed by differential thermal expansion between the rotor and | ||
iron laminations. Magnesium is an active metal and is anodic to iron. Any gaps, scratches, or | iron laminations. Magnesium is an active metal and is anodic to iron. Any gaps, scratches, or | ||
cracks in the coating will expose any unprotected areas to atmospheric conditions. Bare magnesium surfaces are subject to general and galvanic corrosion in the presence of water | cracks in the coating will expose any unprotected areas to atmospheric conditions. Bare magnesium surfaces are subject to general and galvanic corrosion in the presence of water | ||
(such as through high humidity, steam or condensation). Corrosion can cause a magnesium | (such as through high humidity, steam or condensation). Corrosion can cause a magnesium | ||
alloy rotor assembly to degrade, which can lead to sudden motor failure. | alloy rotor assembly to degrade, which can lead to sudden motor failure. | ||
| Line 158: | Line 165: | ||
The following NRC and industry generic communications address MOV actuator motor failures | The following NRC and industry generic communications address MOV actuator motor failures | ||
due to the corrosion of the magnesium alloy rotor: | due to the corrosion of the magnesium alloy rotor: | ||
General Electric (GE) Service Information Letter (SIL) 425, dated July 17, 1985: This | |||
SIL recommended that licensees determine the plant-specific applications of magnesium | SIL recommended that licensees determine the plant-specific applications of magnesium | ||
| Line 167: | Line 175: | ||
cooling assuming failure of magnesium alloy rotor MOVs a few days after the design | cooling assuming failure of magnesium alloy rotor MOVs a few days after the design | ||
basis event (DBE). Some systems may allow for the operator to position the valve to its | basis event (DBE). Some systems may allow for the operator to position the valve to its | ||
safety position immediately following the DBE instead of waiting for automatic actuation, while other systems may require realignment to a standby mode after initial core cooling. | safety position immediately following the DBE instead of waiting for automatic actuation, while other systems may require realignment to a standby mode after initial core cooling. | ||
| Line 179: | Line 187: | ||
NRC IN 86-02, Failure of Valve Operator Motor during Environmental Qualification | NRC IN 86-02, Failure of Valve Operator Motor during Environmental Qualification | ||
Testing, dated January 6, 1986: This IN described the results of the GE environmental | Testing, dated January 6, 1986: This IN described the results of the GE environmental | ||
qualification laboratory tests on three motors in response to issues at the River Bend | qualification laboratory tests on three motors in response to issues at the River Bend | ||
and Nine Mile Point nuclear power stations. GE tested-to-failure three new motors in | and Nine Mile Point nuclear power stations. GE tested-to-failure three new motors in | ||
varying environmental conditions, with the most limiting failure being a new motor which | varying environmental conditions, with the most limiting failure being a new motor which | ||
| Line 189: | Line 197: | ||
failed after 43 days in a high temperature environment under a maximum temperature of | failed after 43 days in a high temperature environment under a maximum temperature of | ||
223 ºF [106 ºC]. The NRC suggested that licensees review the qualification of these | 223 ºF [106 ºC]. The NRC suggested that licensees review the qualification of these | ||
motors in their DBE applications. | motors in their DBE applications. | ||
NRC IN 2006-26, Failure of Magnesium Rotors in Motor-Operated Valve Actuators, dated November 20, 2006: This IN discussed three MOV actuator failures attributed to | NRC IN 2006-26, Failure of Magnesium Rotors in Motor-Operated Valve Actuators, dated November 20, 2006: This IN discussed three MOV actuator failures attributed to | ||
the corrosion of the magnesium alloy motor rotors that occurred in 2005 and 2006. The | the corrosion of the magnesium alloy motor rotors that occurred in 2005 and 2006. The | ||
IN reaffirms the necessity of adequate inspection and preventive maintenance on MOV | IN reaffirms the necessity of adequate inspection and preventive maintenance on MOV | ||
| Line 204: | Line 212: | ||
==DISCUSSION== | ==DISCUSSION== | ||
Licensees rely on MOVs being operable to satisfy many technical specification requirements. In | Licensees rely on MOVs being operable to satisfy many technical specification requirements. In | ||
addition, MOVs are used in systems credited in accident analyses. Because licensees often | addition, MOVs are used in systems credited in accident analyses. Because licensees often | ||
use MOVs of the same type and manufacturers in redundant trains of several safety systems, MOV actuator failures due to magnesium alloy rotor corrosion raises the possibility of a common | use MOVs of the same type and manufacturers in redundant trains of several safety systems, MOV actuator failures due to magnesium alloy rotor corrosion raises the possibility of a common | ||
mode failure. Many MOVs are within the scope of Title 10 of the Code of Federal Regulations, Part 50, Appendix B, Quality Assurance Criteria for Nuclear Power Plants and Fuel | mode failure. Many MOVs are within the scope of Title 10 of the Code of Federal Regulations, Part 50, Appendix B, Quality Assurance Criteria for Nuclear Power Plants and Fuel | ||
Reprocessing Plants. | Reprocessing Plants. | ||
| Line 218: | Line 226: | ||
reports describing failure of safety-related MOVs to operate due to corrosion of the magnesium | reports describing failure of safety-related MOVs to operate due to corrosion of the magnesium | ||
alloy rotor. This is the third IN that the NRC has issued to alert the industry of this problem. Despite industry awareness of the problem and subsequent corrective measures, failures of | alloy rotor. This is the third IN that the NRC has issued to alert the industry of this problem. Despite industry awareness of the problem and subsequent corrective measures, failures of | ||
MOV actuators attributed to the corrosion of the magnesium alloy rotors continue to occur. | MOV actuators attributed to the corrosion of the magnesium alloy rotors continue to occur. | ||
GENERIC IMPLICATIONS | |||
NRC has initiated discussions with stakeholders to address the generic implications of MOV | NRC has initiated discussions with stakeholders to address the generic implications of MOV | ||
actuator motors susceptible to the corrosion of the magnesium alloy rotor. Specifically, these | actuator motors susceptible to the corrosion of the magnesium alloy rotor. Specifically, these | ||
discussions involve the extent and effectiveness efforts by all licensees to identify and inspect | discussions involve the extent and effectiveness efforts by all licensees to identify and inspect | ||
susceptible safety-related MOV actuator motors, and take corrective actions. An important part | susceptible safety-related MOV actuator motors, and take corrective actions. An important part | ||
of these discussions is licensee evaluations of GE SIL 425. | of these discussions is licensee evaluations of GE SIL 425. | ||
==CONTACT== | ==CONTACT== | ||
This IN requires no specific action or written response. Please direct any questions about this | This IN requires no specific action or written response. Please direct any questions about this | ||
matter to the technical contact listed below or the appropriate Office of Nuclear Reactor | matter to the technical contact listed below or the appropriate Office of Nuclear Reactor | ||
| Line 240: | Line 249: | ||
Regulation (NRR) project manager. | Regulation (NRR) project manager. | ||
Theodore R. Quay for | Theodore R. Quay for | ||
/RA/ | |||
Office of Nuclear Reactor Regulation | Timothy J. McGinty, Director | ||
Daniel H. Dorman, Director | |||
Division of Policy and Rulemaking | |||
Division of Fuel Cycle Safety and Safeguards | |||
Office of Nuclear Reactor Regulation | |||
Office of Nuclear Material Safety and Safeguards | |||
===Technical Contact:=== | ===Technical Contact:=== | ||
Steve Tingen, NRR | |||
301-415-1280 | |||
E-mail: Steve.Tingen@nrc.gov | |||
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections. Despite industry awareness of the problem and subsequent corrective measures, failures of | Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections. Despite industry awareness of the problem and subsequent corrective measures, failures of | ||
MOV actuators attributed to the corrosion of the magnesium alloy rotors continue to occur. | MOV actuators attributed to the corrosion of the magnesium alloy rotors continue to occur. | ||
GENERIC IMPLICATIONS | |||
NRC has initiated discussions with the industry to address the generic implications of MOV | NRC has initiated discussions with the industry to address the generic implications of MOV | ||
actuator motors susceptible to the corrosion of the magnesium alloy rotor. Specifically, these | actuator motors susceptible to the corrosion of the magnesium alloy rotor. Specifically, these | ||
discussions involve the extent and effectiveness efforts by all licensees to identify and inspect | discussions involve the extent and effectiveness efforts by all licensees to identify and inspect | ||
susceptible safety-related MOV actuator motors, and take corrective actions. An important part | susceptible safety-related MOV actuator motors, and take corrective actions. An important part | ||
of these discussions is licensee evaluations of GE SIL 425. | of these discussions is licensee evaluations of GE SIL 425. | ||
==CONTACT== | ==CONTACT== | ||
This IN requires no specific action or written response. Please direct any questions about this | This IN requires no specific action or written response. Please direct any questions about this | ||
matter to the technical contact listed below or the appropriate Office of Nuclear Reactor | matter to the technical contact listed below or the appropriate Office of Nuclear Reactor | ||
| Line 275: | Line 294: | ||
Regulation (NRR) project manager. | Regulation (NRR) project manager. | ||
Theodore R. Quay for | Theodore R. Quay for | ||
/RA/ | |||
Office of Nuclear Reactor Regulation | Timothy J. McGinty, Director | ||
Daniel H. Dorman, Director | |||
Division of Policy and Rulemaking | |||
Division of Fuel Cycle Safety and Safeguards | |||
Office of Nuclear Reactor Regulation | |||
Office of Nuclear Material Safety and Safeguards | |||
===Technical Contact:=== | ===Technical Contact:=== | ||
Steve Tingen, NRR | |||
301-415-1280 | |||
E-mail: Steve.Tingen@nrc.gov | |||
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections. | Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections. | ||
Distribution: IN Reading File | Distribution: IN Reading File | ||
ADAMS Accession Number: ML082840609 | ADAMS Accession Number: ML082840609 | ||
*Concurred via email ** with comments | |||
OFFICE | |||
DCI:CPTB | |||
TECH EDITOR* | |||
BC:DCI:CPTB | |||
D:DCI | |||
BC:DE:EEEB | |||
NAME | |||
STingen | |||
HSpencer | |||
JMcHale | |||
MEvans | |||
GWilson | |||
NAME | DATE | ||
10/27/08 | |||
10/14/08 | |||
10/28/08 | |||
11/06/08 | |||
10/30/08 ** | |||
OFFICE | |||
PGCB:DPR | |||
PGCB:DPR | |||
BC:PGCB:DPR | |||
D:DPR | |||
D:NMSS:FCSS | |||
NAME | |||
DBeaulieu | |||
CHawes | |||
MMurphy | |||
SStuchell for | SStuchell for | ||
DATE | TMcGinty TRQ for | ||
DDorman | |||
DATE | |||
11/21/08 | |||
11/13/08 | |||
12/02/08 | |||
12/8/08 | |||
12/03/08 OFFICIAL RECORD COPY}} | |||
{{Information notice-Nav}} | {{Information notice-Nav}} | ||
Latest revision as of 14:47, 14 January 2025
| ML082840609 | |
| Person / Time | |
|---|---|
| Issue date: | 12/08/2008 |
| From: | Dan Dorman NRC/NMSS/FCSS |
| To: | |
| Beaulieu, David, NRR/DPR/PGCB, 415-3243 | |
| References | |
| IN-08-020 | |
| Download: ML082840609 (5) | |
ML082840609 UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555-0001
December 8, 2008
NRC INFORMATION NOTICE 2008-20:
FAILURES OF MOTOR OPERATED VALVE
ACTUATOR MOTORS WITH MAGNESIUM
ALLOY ROTORS
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, as well as licensees and certificate holders of nuclear fuel cycle
facilities.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform
addressees of recent failures and corrective actions for motor-operated valve (MOV) actuator
motors due to corrosion of the magnesium alloy rotors. The NRC expects recipients to 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
The NRC continues to inform the industry of the potential for MOV actuator motor magnesium
alloy rotors to corrode and subsequently fail under certain conditions. The NRC issued two
previous INs to alert the industry of this problem. Technical information in the previous INs is
not repeated in this IN.
The following are instances in 2007 where MOVs with actuator motors manufactured with
magnesium alloy rotors did not operate on demand due to motor failure:
River Bend, Unit 1
On May 25, 2007, at River Bend, Unit 1, the reactor inlet heater A outboard motor operated
isolation valve failed to operate due to magnesium alloy rotor corrosion that was attributed to a
steam leak from an adjacent main steam shutoff valve. This main steam shutoff valve, which
also was an MOV with a magnesium alloy rotor, failed to operate approximately four months
later on September 29, 2007. Licensee corrective actions for these MOV failures included
identifying the MOV actuator motors susceptible to this failure mechanism and performing
borescope inspections of the susceptible actuator motors. Eight MOV motors at River Bend
were replaced because the motor inspections failed the inspection criteria. Quad Cities, Unit 1
On September 4, 2007, at Quad Cities, Unit 1, the high-pressure coolant injection (HPCI)
primary containment isolation valve failed to operate. The motor for this valve was installed in
1994. The packing on this valve catastrophically failed in 2002 resulting in high-pressure steam
in the immediate area of the motor for several hours. The motor experienced two planned stall
events while placing that valve on its backseat in an attempt to mitigate the packing leak. Motor
control center testing had been completed in May 2006 with no issues identified. Quarterly
inservice tests did not identify any degradation. Licensee corrective actions included improving
the procedures, training, and equipment used to perform borescope inspections of susceptible
actuator motors.
Dresden, Unit 3
On September 12, 2007, at Dresden Unit 3, the HPCI primary containment isolation valve failed
to operate. The motor, installed in 2002, is located in the upper drywell where temperatures are
approximately 180 ºFahrenheit (F) [82 ºCelsius (C)]. The licensee performed a borescope
inspection of the valve in 2006 with no issues identified. Licensee corrective actions included
improving the procedures, training, and equipment used to perform borescope inspections of
susceptible actuator motors. The licensee replaced four MOV motors at Dresden, Unit 2, in
November 2007 because the borescope inspection of the motors failed the inspection criteria.
BACKGROUND
Many safety- and non-safety-related MOVs utilize Limitorque actuators with Reliance motors or
a similarly styled design by a different manufacturer (e.g., Electric Apparatus). Based on torque
requirements, aluminum and magnesium alloy rotors are utilized in MOV actuators. Valve
actuators with a motor maximum torque of 40 foot-pounds [54 newton-meters] are typically
aluminum, and magnesium alloy rotors are used for applications requiring greater than 60 foot- pounds [81 newton-meters] torque. Flowserve Technical Update 06-01, dated December 26,
2006, states that Limitorque actuator frame sizes 180 and larger are typically constructed with
magnesium alloy rotors. However, one licensee identified that its facility contained Limitorque
actuator frame sizes 56 and larger that were manufactured with magnesium alloy rotors.
Flowserve Technical Update 06-01 also states that it is Limitorques intention (where possible)
to replace the remaining magnesium alloy rotor designs with an aluminum alloy rotor design in
order to eliminate the failure mechanism.
The magnesium alloy rotor bars, end rings, and cooling fins are an integral structure cast
through the iron lamination stack. The rotor assembly is painted to prevent corrosion. The
magnesium alloy (nominal 90 percent magnesium - 10 percent aluminum) has a linear thermal
expansion coefficient that is double the coefficient for iron. High inrush current heats the motor
internals causing the temperature of the rotor assembly to increase. An MOV motor stall event
is an example of a condition that creates a high current inrush to the motor. Although the rotor
assembly is painted to limit corrosion, the paint bridging the intersection of the rotor end ring
and iron lamination would be stressed by differential thermal expansion between the rotor and
iron laminations. Magnesium is an active metal and is anodic to iron. Any gaps, scratches, or
cracks in the coating will expose any unprotected areas to atmospheric conditions. Bare magnesium surfaces are subject to general and galvanic corrosion in the presence of water
(such as through high humidity, steam or condensation). Corrosion can cause a magnesium
alloy rotor assembly to degrade, which can lead to sudden motor failure.
The following NRC and industry generic communications address MOV actuator motor failures
due to the corrosion of the magnesium alloy rotor:
General Electric (GE) Service Information Letter (SIL) 425, dated July 17, 1985: This
SIL recommended that licensees determine the plant-specific applications of magnesium
alloy rotors subject to harsh environment, and evaluate the adequacy of long-term
cooling assuming failure of magnesium alloy rotor MOVs a few days after the design
basis event (DBE). Some systems may allow for the operator to position the valve to its
safety position immediately following the DBE instead of waiting for automatic actuation, while other systems may require realignment to a standby mode after initial core cooling.
In addition to operator training, it may be prudent to disconnect some of the valve
actuation logic following safety position actuation to prevent valve realignment without
operator actions.
NRC IN 86-02, Failure of Valve Operator Motor during Environmental Qualification
Testing, dated January 6, 1986: This IN described the results of the GE environmental
qualification laboratory tests on three motors in response to issues at the River Bend
and Nine Mile Point nuclear power stations. GE tested-to-failure three new motors in
varying 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 ºF [106 ºC]. The NRC suggested that licensees review the qualification of these
motors in their DBE applications.
NRC IN 2006-26, Failure of Magnesium Rotors in Motor-Operated Valve Actuators, dated November 20, 2006: This IN discussed three MOV actuator failures attributed to
the corrosion of the magnesium alloy motor rotors that occurred in 2005 and 2006. The
IN reaffirms the necessity of adequate inspection and preventive maintenance on MOV
actuator motors manufactured with magnesium alloy rotors to ensure safe operation of
nuclear power facilities.
DISCUSSION
Licensees rely on MOVs being operable to satisfy many technical specification requirements. In
addition, MOVs are used in systems credited in accident analyses. Because licensees often
use MOVs of the same type and manufacturers in redundant trains of several safety systems, MOV actuator failures due to magnesium alloy rotor corrosion raises the possibility of a common
mode failure. Many MOVs are within the scope of Title 10 of the Code of Federal Regulations, Part 50, Appendix B, Quality Assurance Criteria for Nuclear Power Plants and Fuel
Reprocessing Plants.
The industry has issued several licensee event reports and a number of operational experience
reports describing failure of safety-related MOVs to operate due to corrosion of the magnesium
alloy rotor. This is the third IN that the NRC has issued to alert the industry of this problem. Despite industry awareness of the problem and subsequent corrective measures, failures of
MOV actuators attributed to the corrosion of the magnesium alloy rotors continue to occur.
GENERIC IMPLICATIONS
NRC has initiated discussions with stakeholders to address the generic implications of MOV
actuator motors susceptible to the corrosion of the magnesium alloy rotor. Specifically, these
discussions involve the extent and effectiveness efforts by all licensees to identify and inspect
susceptible safety-related MOV actuator motors, and take corrective actions. An important part
of these discussions is licensee evaluations of GE SIL 425.
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contact listed below or the appropriate Office of Nuclear Reactor
Regulation (NRR) project manager.
Theodore R. Quay for
/RA/
Timothy J. McGinty, Director
Daniel H. Dorman, Director
Division of Policy and Rulemaking
Division of Fuel Cycle Safety and Safeguards
Office of Nuclear Reactor Regulation
Office of Nuclear Material Safety and Safeguards
Technical Contact:
Steve Tingen, NRR
301-415-1280
E-mail: Steve.Tingen@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections. Despite industry awareness of the problem and subsequent corrective measures, failures of
MOV actuators attributed to the corrosion of the magnesium alloy rotors continue to occur.
GENERIC IMPLICATIONS
NRC has initiated discussions with the industry to address the generic implications of MOV
actuator motors susceptible to the corrosion of the magnesium alloy rotor. Specifically, these
discussions involve the extent and effectiveness efforts by all licensees to identify and inspect
susceptible safety-related MOV actuator motors, and take corrective actions. An important part
of these discussions is licensee evaluations of GE SIL 425.
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contact listed below or the appropriate Office of Nuclear Reactor
Regulation (NRR) project manager.
Theodore R. Quay for
/RA/
Timothy J. McGinty, Director
Daniel H. Dorman, Director
Division of Policy and Rulemaking
Division of Fuel Cycle Safety and Safeguards
Office of Nuclear Reactor Regulation
Office of Nuclear Material Safety and Safeguards
Technical Contact:
Steve Tingen, NRR
301-415-1280
E-mail: Steve.Tingen@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.
Distribution: IN Reading File
ADAMS Accession Number: ML082840609
- Concurred via email ** with comments
OFFICE
DCI:CPTB
TECH EDITOR*
BC:DCI:CPTB
D:DCI
BC:DE:EEEB
NAME
STingen
HSpencer
JMcHale
MEvans
GWilson
DATE
10/27/08
10/14/08
10/28/08
11/06/08
10/30/08 **
OFFICE
PGCB:DPR
PGCB:DPR
BC:PGCB:DPR
D:DPR
D:NMSS:FCSS
NAME
DBeaulieu
CHawes
MMurphy
SStuchell for
TMcGinty TRQ for
DDorman
DATE
11/21/08
11/13/08
12/02/08
12/8/08
12/03/08 OFFICIAL RECORD COPY