ML25008A087

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MOV Thermal Overload Mug Presentation 2025 (Scarbrough) Final
ML25008A087
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Issue date: 01/08/2025
From: Thomas Scarbrough
NRC/NRR/DEX/EMIB
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Motor-Operated Valve Thermal Overload Protection Thomas G. Scarbrough Division of Engineering and External Hazards Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Motor-Operated Valve Users Group Meeting January 2025 1

2 Disclaimer This presentation was prepared by staff of the U.S. Nuclear Regulatory Commission (NRC). It may present information that does not currently represent an agreed upon NRC staff position.

NRC has neither approved nor disapproved the technical content.

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Background===

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  • Motor-operated valves (MOVs) might be required to operate under a wide range of fluid flow, differential pressure, and ambient conditions in the open and close directions.

o Initial opening might require high MOV torque and thrust to unwedge valve disk.

o Differential pressure and fluid flow might require high MOV torque and thrust in the open and closing direction.

o Final closing might require high MOV torque and thrust to wedge the valve disk into the seat to meet leakage limits.

  • Heat buildup can cause motor burnout in a few seconds.
  • Thermal overload (TOL) protection devices can protect the motor by shutting off power to avoid degradation of winding insulation and rotor.

MOV Operating Experience

  • MOV thermal overload protection has been a challenge throughout nuclear power plant operations.
  • NRC Information Notice (IN) 1982-10, Following up Symptomatic Repairs to Assure Resolution of the Problem, reported 16 motors replaced because TOL protection had been bypassed.
  • In 1991, a licensee found that a fire in control room could cause short circuits that can affect MOV operation.
  • NRC issued IN 92-18, Potential for Loss of Remote Shutdown Capability during a Control Room Fire, with examples of the short circuit issue.

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MOV Operating Experience (continued)

  • GL 89-10 included testing of MOVs under design-basis conditions where practicable and verifying MOV switch settings (including thermal overload switches).

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GL 89-10 Experience

  • GL 89-10 programs found many MOVs to be incapable of performing design-basis safety functions because of inadequacies related to design, qualification, testing, switch settings, maintenance, and quality assurance.
  • Many motors suffered burnout during GL 89-10 testing.
  • IN 1992-17 provided examples of GL 89-10 inspection results including importance of thermal overload sizing.
  • GL 96-05 requested licensees to establish long-term MOV programs to verify MOV design-basis capability.

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MOV Experience Lessons Learned 7

  • Significant torque and thrust requirements can occur from high differential pressure and fluid flow.
  • Subsequent to initial conditions, differential pressure and fluid flow might be significantly reduced.
  • MOV shut down by thermal overload protection might be capable of performing its safety function later.
  • Need for MOV to operate immediately during accident or whether safety function can be performed later should be considered in sizing and setting thermal overload protection devices.

10 CFR Part 50, Appendix A Criterion 1, Quality Standards and Records, of Appendix A, General Design Criteria for Nuclear Power Plants, requires components important to safety be designed, fabricated, erected, and tested to quality standards commensurate with the importance of the safety functions to be performed.

Criterion 4, Environmental and Missile Design Bases, requires SSCs important to safety be designed to accommodate the effects of and be compatible with the environmental conditions associated with normal operation, maintenance, testing, and postulated accidents including loss-of-coolant accidents.

Criterion 13, Instrumentation and Control, requires instrumentation be provided to monitor variables and systems over their anticipated ranges for normal operation and for postulated accident conditions and that controls be provided to maintain these variables and systems within prescribed operating ranges.

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10 CFR Part 50, Appendix B Criterion III, Design Control, of Appendix B, Quality Assurance Criteria for Nuclear Power Plants and Fuel Reprocessing Plants, requires measures be established to assure regulatory requirements and design basis are correctly translated into specifications, drawings, procedures, and instructions.

Criterion V, Instructions, Procedures, and Drawings, of Appendix B requires activities affecting quality shall be prescribed by documented instructions, procedures, or drawings, of a type appropriate to the circumstances and shall be accomplished in accordance with these instructions, procedures, or drawings.

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10 CFR Part 50, Appendix B Criterion XI, Test Control, requires a test program be established to ensure systems and components perform satisfactorily and test program includes operational tests during nuclear power plant operation.

Criterion XVI, Corrective Action, requires measures shall be established to assure that conditions adverse to quality, such as failures, malfunctions, deficiencies, deviations, defective material and equipment, and nonconformances are promptly identified and corrected.

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10 CFR 50.55a

  • 10 CFR 50.55a, Codes and standards, requires inservice testing (IST) programs to verify the operational readiness of MOVs to perform their safety functions.
  • ASME has revised the OM Code to require periodic diagnostic testing of MOVs to verify their operational readiness to perform design-basis safety functions.

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RG 1.106 November 1975 12

  • Initial RG 1.106, Thermal Overload Protection for Electric Motors on Motor-Operated Valves, stated that operating experience had shown that indiscriminate application of TOL devices to valve motors could result in needless hindrance to completion of safety functions.
  • RG position specified that TOL protection devices that are normally in force should be bypassed under accident conditions.
  • As an alternative, RG stated that trip setpoint should be established with all uncertainties resolved in favor of completing safety-related actions.

RG 1.106, Revision 1 March 1977 Revision 1 to RG 1.106 included background guidance stating that where TOL devices are bypassed, it is important to ensure that bypassing does not result in jeopardizing completion of the safety function or in degrading other safety systems because of any sustained abnormal motor circuit currents.

Revision 1 to RG 1.106 expands Position 1 to state:

- Provided that completion of safety function is not jeopardized or that other safety systems are not degraded, (a) TOL devices should be continuously bypassed and temporarily placed in force only when valve motors are undergoing periodic or maintenance testing, or (b) TOL devices that are normally in force during plant operation should be bypassed under accident conditions.

Position 2 remained as in the initial RG 1.106.

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RG 1.106 Revision 2, February 2012

  • Revision 2 to RG 1.106 incorporates lessons learned from GL 89-10/GL 96-05 programs to avoid loss of MOV availability as a result of motor thermal overload.
  • Revision 2 is based on three concepts:

- For valves required to function immediately during accident, TOL devices should be bypassed, but in service for testing.

- For valves that operate under normal conditions and automatically actuate, TOL devices should be in service normally, but bypassed under accident conditions.

- For valves that do not have an immediate function, TOL devices should be in service.

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RG 1.106 (Revision 2)

Regulatory Positions To ensure that safety-related MOVs whose motors are equipped with TOL devices integral with motor starter will perform their function, one of three alternatives described in Position 1 as well as Positions 2 and 3, should be implemented:

1. Provided that completion of safety function is not jeopardized or that other safety systems are not degraded:

a) For valves that are required to function immediately to open or close during Design Basis Event (DBE) or Station Blackout (SBO) to perform single safety-related action and that do not change position thereafter, TOL devices should be continuously bypassed, and should be temporarily placed in service only when valve motors are undergoing periodic or maintenance testing.

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RG 1.106 (Revision 2)

Regulatory Positions

1. CONTINUED b) For valves that operate during normal plant conditions and are automatically actuated to perform safety-related function (whether to perform single safety-related action or to modulate during accident condition), TOL devices should normally be in service during plant operation, testing, and maintenance, and should be automatically bypassed under DBE or SBO conditions.

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RG 1.106 (Revision 2)

Regulatory Positions

1. CONTINUED c) For motor-operated valves that do not have an immediate safety function to perform in response to an accident condition, TOL device should be in service at all times (during normal operation, DBE or SBO conditions, and maintenance).

The bypass initiation system circuitry should generally conform to criteria of Sections 4(g), 4(h), 4(i), 4(j), 4(k),

4(l), 5, 5.1, 5.2, 5.3, 5.4, 5.5, and 5.7 of IEEE Std 603-2009, Criteria for Safety Systems for Nuclear Power Generating Stations, and should be periodically tested.

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RG 1.106 (Revision 2)

Regulatory Positions

2. Selection of trip setting of TOL devices should be established with all uncertainties resolved in favor of completing safety-related action.

With respect to uncertainties, consideration should be given to (a) variations in ambient temperature at installed location of TOL devices and valve motors, and (b) inaccuracies in motor heating data and TOL device trip characteristics and matching of these two items.

To ensure continued functional reliability and accuracy of trip point, TOL device should be periodically tested.

Trip setting of TOL device should conform to guidance provided in Annex B of IEEE 741-2007.

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RG 1.106 (Revision 2)

Regulatory Positions

3. Alarm indication should be provided in control room that generally conforms to criteria 5.8 provided in IEEE 603-2009, Criteria for Safety Systems for Nuclear Power Generating Stations, when a safety-related valve motor generates overcurrent (and thus heat) and trips TOL device.

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Additional MOV Thermal Overload Protection Guidance

  • IEEE 741-2007, Standard Criteria for the Protection of Class 1E Power Systems and Equipment in Nuclear Power Generating Stations
  • IEEE 603-2009, Standard Criteria for Safety Systems for Nuclear Power Generating Stations
  • IEEE 1290-2015, Guide for Motor-Operated Valve Motor Application, Protection, Control, and Testing in Nuclear Power-Generating Stations 20

RG 1.106 Application Issue Example

  • Licensee committed to RG 1.106 to remove TOL bypass jumpers during maintenance and testing.
  • Licensee failed to prepare procedural guidance to remove TOL bypass jumpers when performing maintenance testing.
  • Failure to provide those procedure instructions was a performance deficiency.
  • Similar issue later resulted in MOV locked rotor with smoke from motor control center during surveillance testing.

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MOV Thermal Overload Lessons Learned Licensees are required to meet 10 CFR 50.55a(b)(3)(ii) and Part 50, Appendix B, to provide reasonable assurance that MOVs are capable of performing their design-basis safety functions, including lessons learned from GL 89-10 and 96-05 programs.

MOV operating and testing experience revealed need to provide adequate thermal overload protection during maintenance, surveillance testing, and safety functions to avoid motor burnout that would cause the MOV to be unavailable throughout a plant transient, or could initiate a fire that might interfere with availability of plant equipment.

RG 1.106, Revision 2, provides updated guidance for thermal overload protection devices based on lessons learned from MOV testing and experience.

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QUESTIONS?

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Acronyms ASME: American Society of Mechanical Engineers CFR: Code of Federal Regulations GL: Generic Letter IEEE: Institute of Electrical and Electronics Engineers IN: Information Notice IST: Inservice Testing MOV: motor-operated valve NRC: U.S. Nuclear Regulatory Commission RG: Regulatory Guide SSCs: structures, systems, and components TOL: Thermal Overload 24

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