Originally embedded in the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, today the Operation and Maintenance of Nuclear Power Plants standard for inservice testing of pumps, valves, and certain dynamic restraints (snubbers) stands on its own. Among other things, the inservice testing program detects and monitors valve safety function readiness and degradation for existing nuclear plants and new builds.
The American Society of Mechanical Engineers (ASME) is one of the oldest standards-developing organizations in the U.S. However, its work extends well beyond the states. It has offices in Beijing, China; Brussels, Belgium; and New Delhi, India. Its stated mission is: “To serve diverse global communities by advancing, disseminating and applying engineering knowledge for improving the quality of life; and communicating the excitement of engineering.”
ASME provides specific operations and maintenance (OM) codes to perform preservice and inservice inspections and testing for many of the components used in existing and new nuclear reactor designs. Between the 1960s and 1980s, many of the inservice testing (IST) requirements were embedded in the ASME Boiler and Pressure Vessel Code, Section XI, which primarily dealt with standardized weld/component inservice inspection (ISI) examinations instead of component functional testing.
The ASME OM Standard Committee was chartered to develop ASME’s Operation and Maintenance of Nuclear Power Plants standard (OM Code), separating the “pump and valve” requirements from Section XI to improve efficiency when developing and refining IST requirements and acceptance criteria. The OM Code requirements apply to:
■ Pumps and valves that are required to perform a specific function in shutting down a reactor to the safe shutdown condition, in maintaining the safe shutdown condition, or in mitigating the consequences of an accident.
■ Pressure relief devices that protect systems or portions of systems that perform one or more of the three functions identified above.
■ Dynamic restraints (snubbers) used in systems that perform one or more of the three functions identified above (Figure 1).
The purpose of IST is to detect and monitor for degradation in components designated as ASME Class 1, 2, or 3 systems and components. Class 1 (Quality Group A) includes systems and components containing reactor coolant and forming the reactor coolant pressure boundary. Items such as reactor safety valves, reactor coolant pressure boundary isolation valves, and power-operated relief block valves fit into the Class 1 category. Class 2 (Quality Group B) items are non-Quality-Group-A systems and components important to safety, such as emergency core cooling, reactor shutdown, and residual heat removal systems. Class 3 (Quality Group C) systems and components are important to safety, but not designated Class 1 or 2. Some examples include diesel generator support systems, and service water and component cooling water pumps.
IST provides reasonable assurance that safety-related components will perform their safety function when activated. The process enables utilities to systematically identify problems and take action to repair or replace safety-related components, if they are degraded. IST is not intended to determine component operability. Rather, Technical Specification (TS) surveillance testing is intended to verify that components and systems are operable and have been since the last TS test. IST is intended to provide confidence that the component will remain operable until the next TS test.
The OM Code is split into three divisions. Division 1, “OM Code: Section IST,” which is the focus of this article, is divided into six subsections. The subsections are:
■ Subsection ISTA, “General Requirements”
■ Subsection ISTB, “Inservice Testing of Pumps in Light-Water Reactor Power Plants—Pre-2000 Plants”
■ Subsection ISTC, “Inservice Testing of Valves in Light-Water Reactor Nuclear Power Plants”
■ Subsection ISTD, “Preservice and Inservice Examination and Testing of Dynamic Restraints (Snubbers) in Light-Water Reactor Nuclear Power Plants”
■ Subsection ISTE, “Risk-Informed Inservice Testing of Components in Light-Water Reactor Nuclear Power Plants”
■ Subsection ISTF, “Inservice Testing of Pumps in Light-Water Reactor Nuclear Power Plants—Post-2000 Plants”
There are five mandatory and 12 non-mandatory appendices. The mandatory appendices are:
■ Appendix I, “Inservice Testing of Pressure Relief Devices in Light-Water Reactor Nuclear Power Plants”
■ Appendix II, “Check Valve Condition Monitoring Program”
■ Appendix III, “Preservice and Inservice Testing of Active Electric Motor Operated Valve Assemblies in Light-Water Reactor Power Plants”
■ Appendix IV, “Reserved for Pneumatic Operated Valve Testing”
■ Appendix V, “Pump Periodic Verification Test Program”
The non-mandatory appendices are:
■ Appendix A, “Preparation of Test Plans”
■ Appendix B, “Dynamic Restraint Examination Checklist Items”
■ Appendix C, “Dynamic Restraint Design and Operating Information”
■ Appendix D, “Comparison of Sampling Plans for Inservice Testing of Dynamic Restraints”
■ Appendix E, “Flowcharts for 10% and 37 Snubber Testing Plans”
■ Appendix F, “Dynamic Restraints (Snubbers) Service Life Monitoring Methods”
■ Appendix G, “Application of Table ISTD-4252-1, Snubber Visual Examination”
■ Appendix H, “Test Parameters and Methods”
■ Appendix J, “Check Valve Testing Following Valve Reassembly”
■ Appendix K, “Sample List of Component Deterministic Considerations”
■ Appendix L, “Acceptance Guidelines”
■ Appendix M, “Design Guidance for Nuclear Power Plant Systems and Component Testing”
Federal Register Rulemaking
The ASME Codes are voluntary consensus standards developed by participants, which include the Nuclear Regulatory Commission (NRC) and licensees. If the NRC believes there is a significant technical or regulatory concern that can be addressed by an approved ASME Code edition or addenda, instead of developing its own standard, the NRC will approve or condition (determine the extent of compliance) the ASME Code edition or addenda “by reference.”
Parts of the standard that are not adopted or conditionally acceptable must be justified. This coincides with the NRC’s policy to maintain nuclear plant safety, while making NRC activities effective and efficient.
The NRC’s conditions are included in Title 10 of the Code of Federal Regulations Part 50.55a (10 CFR 50.55a) “Codes and standards.” Upon incorporation by reference of the ASME Code into 50.55a, the provisions are legally binding NRC requirements (Figure 2).
Effective August 17, 2017, the NRC amended its regulations to incorporate by reference the 2009 Edition, the 2011 Addenda, and the 2012 Edition of Division 1 of the ASME OM Code, with conditions on their use; the 1983 Edition through the 1994 Edition, the 2008 Edition, and the 2009–1a Addenda to the 2008 Edition of ASME NQA-1 Quality Assurance Requirements for Nuclear Facility Applications, with conditions on their use; and OM Code Case OMN-20, “Inservice Test Frequency.”
Code Cases (CCs) clarify the intent of the existing ASME Code, or provide alternative requirements, and are intended to be incorporated into the Code at a later date. A CC can be endorsed by the NRC in Regulatory Guide (RG) 1.192, “Operation and Maintenance Code Case Acceptability, ASME OM Code” with or without conditions (such as relief requests). Alternatively, the NRC may disapprove CCs in RG 1.193, “ASME Code Cases Not Approved for Use.”
In the case of CC OMN-20, it incorporates the use of “grace periods” into the OM Code. The CC allows a ±25% window for up to a two-year test frequency and±6 months for test frequencies greater than 2 years. It is important to note that the CC has not yet been approved in RG 1.192, and the licensee shall not incorporate the grace period into the normal testing schedule.
In July 2006, the ASME Code Committee approved eliminating CC expiration dates. Any published CC that has not been annulled, and that has an expiration date that is after July 2006, is not expired and may continue to be used.
Motor Operated Valves
A motor-operated valve (MOV) is a combination of two separate devices. It has a valve assembly, a mechanical device to optimize a desired fluid control function, such as isolation or throttling, and an actuator, an electrical-mechanical device used to position the valve assembly from a remote location (Figure 3).
The piping systems in a typical nuclear plant include hundreds of MOVs, with many of them being safety-related, that is, the safety of the plant depends on the valve’s ability to operate under severe conditions anticipated during plant design. Safety-related valves are required to prevent catastrophic releases.
One of the major changes to the traditional ISTC testing for MOVs is the use of risk insights and diagnostic testing to extend or eliminate the requirements for OM Code valve position indication verification and stroke time requirements. In many cases, this provides a significant reduction in burden based on the known condition of the MOV and actuator, rather than just a stroke time and exercise test.
Division 1, Mandatory Appendix III, “Preservice and Inservice Testing of Active Electric Motor Operated Valve Assemblies in Light Water Reactor Power Plants,” contains requirements for providing a more comprehensive testing method for MOVs than that provided by Subsection ISTC. It is required that the Mandatory Appendix be followed in its entirety to ensure compliance with the ASME OM Code for testing MOVs. The general and specific requirements for testing and acceptance criteria are provided in the appendix, as well as frequencies and the determination of the testing frequencies, using risk-informed methods. The benefit of implementing Appendix III is that it provides detailed knowledge of the MOV and actuator condition by using diagnostic testing methods, preventive maintenance activities, and continued good performance, to extend the examination/testing intervals to a significant periodicity.
MOV Failures. NRC technical report conference proceedings NUREG/CP-0152, Vol. 9, “Proceedings of the Twelfth NRC/ASME Symposium on Valves, Pumps, and Inservice Testing,” reported in 2014 that “failure of contacts” contributed to the most MOV failures over a five-year period. The contact failures included torque switch, limit switch, motor control center (MCC) contactor relay, and MCC breaker cell contacts. The most frequent failed parts were:
■ Limit switch rotor—10
■ Electrical termination (lug/connector)—8
There are minimal requirements listed in the OM Code regarding testing and examination, and the licensee is provided with some flexibility in the determination of the exam and testing requirements needed to assure MOV operational readiness.
Inservice Testing. IST shall commence when the MOV is required to fulfill its required function and shall be conducted in the as-found condition to assess changes in MOV functional margin, that is, the increment by which an MOV’s available capability exceeds the capability required to operate the MOV under design basis conditions. As-found testing is not required prior to maintenance activities if the MOV is not due for an IST. If maintenance activities are scheduled concurrently with an MOV’s IST, then the IST shall be conducted in the as-found condition, and prior to the maintenance activity.
Static Versus Dynamic. The IST program includes a mix of static and dynamic MOV performance testing, and may be altered when justified by an engineering evaluation of test data. However, dynamic MOV performance testing is not required for certain valve types (such as ball, plug, and diaphragm valves), with acceptable operating experience.
Test Interval. The IST interval, in accordance with Appendix III, includes calculations for determining MOV functional margins that account for potential performance-related degradation. Although maintenance activities can affect IST intervals, it shall be set so the MOV functional margin does not decrease below the acceptance criteria. If insufficient data exist to determine the IST interval according to Appendix III, then MOV IST shall be conducted every two refueling cycles or three years, whichever is longer, until sufficient data exist, from an applicable MOV or MOV group, to justify a longer IST interval. Also, the maximum IST interval shall not exceed 10 years.
Normal Exercising Requirements. All MOVs within the scope of Appendix III shall be full-cycle exercised at least once per refueling cycle (maximum of 24 months). Full-cycle operation as a result of normal plant operations or other code requirements may be considered an exercise of the MOV, if documented. Also, if full-stroke exercising of an MOV is not practical during plant operation or cold shutdown, it shall be performed during the plant’s refueling outage.
Additional or more frequent exercising requirements for MOVs shall be considered in any of the following categories:
■ MOVs with high-risk significance.
■ MOVs with adverse or harsh environmental conditions.
■ MOVs with abnormal characteristics, such as operational, design, or maintenance condition.
Replacement, Repair, or Maintenance. When an MOV or its control system is replaced, repaired, or undergoes maintenance that could affect the valve’s performance, new IST values shall be determined, or the previously established IST values shall be confirmed, before the MOV is returned to service. If the MOV was not removed from service, IST values shall be immediately determined or confirmed. This testing is intended to demonstrate that performance parameters, which could be affected by replacement, repair, or maintenance, are within acceptable limits. Deviations between the previous and new IST values shall be identified and analyzed. Verification that the new values represent acceptable operation shall be documented according to Section III-9000, “Records and Reports.”
Grouping. This reduces the cost and burden associated with testing and examination requirements specified in Subsection ISTC. Grouping MOVs shall be justified by an engineering evaluation, alternative testing techniques, or both. For example, MOVs with identical or similar motor operators and valves, and with similar plant service conditions, may be grouped together based on design basis verification and preservice test results.
Acceptance Criteria. The operational readiness of each MOV within the scope of Appendix III shall be based upon the minimum amount the available actuator output capability must exceed the valve operating requirements. For instance, thrust, torque, or other measured engineering parameters correlated to thrust or torque may be used to establish the acceptance criteria. If the functional margin determined per Appendix III does not meet the acceptance criteria, the MOV shall be declared inoperable.
MOV Code Cases. The applicable MOV CCs are OMN-1, “Alternative Rules for Preservice and Inservice Testing of Active Electric Motor-Operated Valve Assemblies in Light-Water Reactor Power Plants,” and OMN-11, “Risk Informed Testing for Motor Operated Valves.”
The Path Forward
Several activities are underway, or being evaluated for development, including changes to existing ASME OM Codes. Examples include the addition of Mandatory Appendix IV for air-operated valves, similar to the ISTC’s Mandatory Appendix III for MOVs; additional appendices to address other types of power-operated valves; and expansion of the OM Code beyond light-water reactors in order to address new and advanced reactor designs either being built or proposed for design development, such as passive reactors, heavy water reactors, small modular reactors, liquid metal reactors, and high-temperature gas-cooled reactors.
Regarding post-2000 reactors, one of the major differences between Subsection ISTF and Subsection ISTB is that workarounds provided in ISTB are no longer permitted for new reactor builds. For example, the licensee is required to test pumps at full-flow condition in accordance with the pump/system design to satisfy the ASME OM Code criteria. ISTF also eliminated the pump testing allowances for “minimum recirculation flow” testing.
The ASME OM Code must stay ahead of the development and design of these new types of reactors to ensure that the testing methods, requirements, and periodicity of tests/exams are all considered in providing a thorough, cost-effective method for detecting and monitoring OM Code component degradation. ■
—James M. Hylko ([email protected]) specializes in safety, quality, and emergency management issues and is a frequent contributor to POWER, and Ronald C. Lippy ([email protected]) is senior manager engineering programs at True North Consulting in Montrose, Colorado, chairman of the ASME OM Standards Committee, and former chairman of the ASME New Reactors OM Committee Task Group. The opinions and content expressed in this article are those of the authors and do not necessarily reflect those of ASME.