Resolution of Generic Safety Issue 158: Performance of Safety-Related Power-Operated Valves Under Design Basis Conditions
March 15, 2000
- Background Information
- Recent AOV Performance and Safety Significance
- Industry Initiatives
- Summary of the Issue
- Voluntary Initiatives
- Backfit Discussion
- Federal Register Notification
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.
The U.S. Nuclear Regulatory Commission (NRC) is issuing this regulatory issue summary (RIS) to inform addressees of the closure of Generic Safety Issue (GSI) 158, "Performance of Safety- Related Power-Operated Valves Under Design Basis Conditions," and of the staff's intent to continue to work with industry groups and to monitor addressees' activities to ensure that safety-related power-operated valves (POVs) are capable of performing their specified functions under design basis conditions. No action or written response is requested.
GSI-158, "Performance of Safety-Related Power-Operated Valves Under Design Basis Conditions," was identified by the NRC after reactor operating experience and research results on motor-operated valves (MOVs), solenoid-operated valves (SOVs), air-operated valves (AOVs), and hydraulically operated valves (HOVs) indicated that testing under static conditions was insufficient to demonstrate consistent performance of these valves under design-basis conditions. Operating events involving observed or potential common-cause failures were documented in NUREG-1275, "Operating Experience Feedback Report," Volumes 2 and 6 for air systems and SOVs, respectively, and in AEOD/C603, "Review of Motor-Operated Valve Performance," for MOVs. These issues are also more recently discussed in NUREG/CR-6644, "Generic Issue 158: Performance of Safety-Related Power-Operated Valves Under Operating Conditions." Two related documents, NUREG-1275, Vol.13, "Evaluation of Air-Operated Valves at U.S. Light-Water Reactors," and NUREG/CR-6654, "A Study of Air-Operated Valves in U.S. Nuclear Power Plants," are focused specifically on AOVs.
MOV performance issues were not discussed in GSI-158, as they were documented previously in the resolution of TMI Action Plan Item II.E.6.1, "In Situ Testing of Valves-Test Adequacy Study," which is included in NUREG-0933, "Prioritization of Generic Safety Issues." Resolution of this issue resulted in the issuance of Generic Letter (GL) 89-10, "Safety-Related Motor-Operated Valve Testing and Surveillance," and subsequently GL 96-05, "Periodic Verification of Design Basis Capability of Safety-Related Motor-Operated Valves."
The NRC staff briefed the Advisory Committee on Reactor Safeguards (ACRS) on May 6, 1999, and presented the results of staff and contractor studies of the POV issue. The staff concluded that no new regulations were required to address the issues contained in GSI-158. The current regulations provide an adequate framework to address specific POV issues similar to MOV issues that were resolved through the issuance of GL 89-10 and GL 96-05. The staff also described the voluntary industry initiative being developed to address AOV issues. In its May 14, 1999, letter to the NRC staff, the ACRS stated that the central issue, whether POVs are able to perform their intended functions under design basis dynamic conditions, had not been adequately addressed. The ACRS further stated that unless the NRC staff undertakes a pro-active effort to ensure resolution of this issue, the industry initiative will remain an optional, voluntary program that will not fully resolve the concerns of GSI-158.
Most of the recent staff and industry attention has been focused on AOV performance. The NRC staff stated in a July 2, 1999, letter to the ACRS that it would continue to monitor and work with industry groups developing design basis verification and testing programs for AOVs. The NRC staff noted further that if the actions of the industry did not adequately address the functionality of POVs under design basis dynamic conditions, the NRC staff would take additional regulatory action as appropriate.
The NRC staff previously requested that the industry verify the capability of AOVs with respect to issues involving the plant instrument air supply system. In GL 88-14, "Instrument Air Supply System Problems Affecting Safety-Related Equipment," addressees were requested to verify by test that air-operated safety-related components will perform as expected in accordance with all design-basis events. All addressees were required to respond to the generic letter with confirmation that this verification had been performed. All responses were received by 1993 and the generic letter was subsequently closed.
A recent NRC study of AOVs, documented in NUREG-1275, Vol.13, and NUREG/CR-6654, included a review of AOV operating experience and the results of 7 site visits to 11 U.S. light water reactors conducted in 1997-1998. The seven licensees collectively identified a total of 167 safety-related, high-risk-significant AOVs, ranging from an individual reactor facility high of 36 AOVs to a low of 4 AOVs. In addition, two of those licensees identified a total of 15 AOVs that were non safety-related but high-risk-significant. Most of the licensees visited were planning to verify the design-basis capability of all the referenced AOVs. The licensees' determinations of the high-risk-significant AOVs were based on a variety of methods, including plant-specific probabilistic risk assessment, individual plant examination, and maintenance rule expert panel reviews. Many of the licensees' determinations included evaluations of the risk achievement worth (RAW) and Fussell-Vesely (F-V) risk rankings of the AOVs. Each categorization method was unique.
The major safety concern identified in the NRC AOV study from a risk perspective is the simultaneous common-cause failure of AOVs which disable redundant trains of a system important to safety. The scenario of most concern is that during an accident or transient, AOVs in redundant trains of a safety system fail when subjected to pressure, temperature, and flow conditions different from those seen during normal operation or testing. As discussed in the NRC AOV study, some licensees found that certain AOVs had high RAW and/or F-V risk rankings. Table 6 of NUREG/CR-6654 includes the RAW values for AOVs that were calculated by licensees at three plants. These calculations showed that, in some cases, the RAW could increase by one or two orders of magnitude as a result of common-cause failures. RAW for common-cause AOV failures at those three plants ranged from slightly over 1 to 202. Weaknesses in the design, testing, and maintenance of AOVs could result in common-cause AOV failures which are not addressed in plant safety analyses. The common-cause AOV failures that have been documented in the AOV study did not occur simultaneously with design-basis transients but were identified during operations, maintenance, or testing.
The Joint Owners Group on Air Operated Valves (JOG AOV), which is facilitated by the Nuclear Energy Institute (NEI), presented to the NRC staff in a public meeting on June 3, 1999, the industry's voluntary program to address AOV issues. The JOG AOV program provides guidance to verify valve performance at design conditions and long-term periodic verification of safety-related AOVs categorized as high-risk-significant. For safety-related, low-risk-significant AOVs and AOVs that are not safety-related but are determined to be high-risk-significant, the JOG AOV program also provides guidance for a less-rigorous verification of valve functionality. The activities for safety-related, low-risk-significant AOVs and non safety-related, high-risk-significant AOVs would not necessarily involve verification that the valves would perform under design conditions or require long-term periodic verification. The methodology to determine valve safety significance, as specified in the industry program, may include such risk insight methods as in Regulatory Guide 1.174, "An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis," or programs established to meet the requirements of 10 CFR 50.65, "Requirements for monitoring the effectiveness of maintenance at nuclear power plants," (the maintenance rule) in combination with individual plant examinations and the review performed by a separate expert panel.
This industry program document was completed and distributed to utilities in 1999. The NRC received a copy of the program document in a letter from D. Modeen (NEI ) to E. Imbro (NRC), dated July 19, 1999. NRC comments on the JOG AOV program and its implementation were sent to NEI in a letter from E. Imbro to D. Modeen, dated October 8, 1999. Although the program was noted to have some limitations, the NRC staff recognizes that industry-wide implementation of this program would achieve a uniform level of consistency that would provide increased confidence in the design-basis capabilities of high-risk-significant AOVs in nuclear power plants. As stated above, the NRC will continue to work with industry groups to ensure that safety-related POVs are capable of performing their specified functions under design basis conditions. If POV functionality under design basis conditions is not adequately addressed by the industry, the NRC staff will take additional regulatory action as appropriate.
The NRC has closed GSI-158 on the basis that current regulations provide adequate requirements to ensure verification of the design-basis capability of POVs and that no new regulatory requirements are needed. The NRC staff will continue to work with industry groups on an industry-wide approach to the POV issue and to provide timely, effective, and efficient resolution of the concerns regarding POV performance. The NRC staff will also continue to monitor licensees' activities to ensure that POVs are capable of performing their specified safety-related functions under design-basis conditions.
Although there are no regulatory requirements for licensees to establish an AOV program, licensees are required by 10 CFR 50.65 to monitor the performance of structures, systems, or components (SSCs) in a manner sufficient to provide reasonable assurance that such SSCs (e.g., systems with safety-related and high-safety-significant AOVs) are capable of fulfilling their intended functions. Addressees who implement the JOG AOV program to help ensure the design basis capability of AOVs may wish to consider the NRC comments contained in the NRC's letter to NEI dated October 8, 1999. Addressees who choose to develop plant-specific AOV programs may wish to consider the attributes listed in the attachment to this RIS. These attributes are based on lessons learned from the staff's involvement in the activities related to GL 89-10 and the NRC site visits documented in NUREG/CR-6654.
This RIS requests no action or written response. Therefore, this RIS is not a backfit under 10 CFR 50.109, and the staff did not perform a backfit analysis.
A notice of opportunity for public comment was not published in the Federal Register because this RIS is informational and the NRC staff discussed the closure of GSI-158 in a public meeting with the ACRS.
If you have any questions about this RIS, please contact one of the technical contacts listed below.
|/RA/S. F. Newberry FOR
David B. Matthews, Director
|Technical Contacts:||Joseph Colaccino, NRR
|Harold L. Ornstein, RES
(ADAMS Accession Number ML003686003)
ATTRIBUTES OF A SUCCESSFUL POWER-OPERATED VALVE DESIGN CAPABILITY AND LONG-TERM PERIODIC VERIFICATION PROGRAM
- Include all maintenance rule scope power-operated valves (POVs) in
- Verify POVs in non-safety position are capable of returning to their
safety position if train is assumed operable with valves in their non-safety
- For air-operated valves, verify guidance in GL 88-14, "Instrument Air Supply System Problems Affecting
Safety-Related Equipment," has been successfully implemented, including
periodic monitoring of air quality.
- Evaluate motor-operated valve (MOV) risk-ranking methodologies developed
by the Boiling Water Reactor Owners Group and the Westinghouse Owners
Group for applicability to risk ranking of POVs at the specific plant,
- Focus initial efforts on safety-related, active, high-risk POVs. Information
obtained from these valves and lessons learned may be used to verify
and maintain design basis capability of similar safety-related POVs.
- Verify methods for predicting POV operating requirements using MOV
lessons learned or specific POV dynamic diagnostic testing. Use of the
Electric Power Research Institute (EPRI) MOV Performance Prediction
Method must include all guideline aspects of that methodology and not
only individual EPRI valve test results.
- Justify method for predicting POV actuator output capability by test-based
program established by the vendor, licensee, or industry.
- Address all applicable weak links, including actuator, valve, and
- Ensure quality assurance program coverage.
- Provide sufficient diagnostics when baseline testing to verify capability.
Diagnostics might not be needed if normal plant operation frequently
demonstrates design basis capability.
- Specify when dynamic or static diagnostic periodic testing is needed.
- Ensure post-maintenance testing is adequate to verify capability of
all safety-related POVs and risk-significant functions of non-safety-related
- Ensure POV maintenance procedures are reviewed to incorporate lessons
learned from other valve programs.
- Upgrade training to incorporate lessons learned from other valve programs.
- Apply feedback from plant-specific and industry information, including
test data, to all applicable safety-related POVs.
- Establish quantitative (test data) and qualitative (maintenance and condition reports) trending of POV performance with detailed review following each refueling outage.