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UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D. C. 20555-0001
December 15, 2000
| NRC INFORMATION NOTICE 2000-21: IN 2000-21 | DETACHED CHECK VALVE DISC NOT DETECTED BY USE OF ACOUSTIC AND MAGNETIC NONINTRUSIVE TEST TECHNIQUES |
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.
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to the potential that nonintrusive testing (NIT) of check valves may not provide accurate results if the NIT method was not qualified and if baseline performance was not established when the valve was known to be operating acceptably. It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid problems. However, suggestions contained in this information notice are not NRC requirements; therefore, no specific action or written response is required.
During an inservice test of high-pressure safety injection pump P-66A at the Palisades Plant on June 21, 2000, the pump failed to achieve its hydraulic reference values. The probable cause of this condition was that piston check valve CK-ES3340 in the P-66A recirculation line was stuck in mid-position. A simplified system diagram of the Palisades safety injection system is included as Attachment 2.
The Palisades inservice testing (IST) program designated check valve CK-ES3340 as having safety functions in both the open and closed positions. Upon discovery of this valve stuck in mid-position, the licensee designated swing check valve CK-ES3332 to provide the safety function to close, previously performed by CK-ES3340, in addition to its originally-designated safety function to open. The open safety function of CK-ES 3332 had been tested previously under the IST program by verifying that the valve passed the maximum accident condition flow. NIT had been performed in 1997, and the licensee concluded then that the acoustic monitoring data provided indications of valve closure. The valve had never been disassembled for internal inspection.
On June 21, 2000, Palisades performed NIT, using acoustic and magnetic monitoring techniques, to verify the closure capability of check valve CK-ES3332 as a substitute for the closure function performed by CK-ES3340. The licensee concluded that the acoustic test data provided closure indications of the check valve disc impacting its seat and also provided open indications of the disc impacting the backstop. The analysis of magnetic test data also indicated that the valve disc had moved to the open position. However, because of changes that were made to the Palisades NIT data collection software between 1997 and 2000, the licensee could not trend the data between these tests. No additional testing or examination was performed on June 21, 2000.
On September 5, 2000, the licensee radiographed valve CK-ES3332 to obtain more actual data. Radiography revealed that the valve disc and connected swing arm had separated from the hinge pin and were resting on the bottom of the valve. Opening the check valve to perform an internal inspection confirmed the radiography and revealed that the swing arm and disc were intact. There was no indication of service wear on the hinge pin. A subsequent licensee event report (LER, Reference 1), attributed this condition to improper assembly of the valve during the plant's construction. In addition, the LER stated that "the actual condition of CK-ES3332 was not ascertained in June 2000, during acoustic testing, because the results obtained from the acoustic testing corresponded with generically expected open and closed indications."
In accordance with the Code (References 2, 3 and 4), a check valve that is required by 10 CFR 50.55a to be included in the IST program must be exercised to the positions in which it performs its safety functions. Attachment 1 to NRC Generic Letter 89-04 (Reference 5), details 11 separate NRC Staff Positions on potential generic deficiencies related to IST programs and procedures. Position 1, "Full Flow Testing of Check Valves", states that a check valve's full stroke to the open position may be verified by passing the maximum required accident-condition flow through the valve. Position 1 also provides guidance on qualifying other techniques (e.g., establishing a baseline when the valve is known to be in good working order and specifying adequate acceptance criteria). For backflow testing of check valves, Position 3 states that check valve closure may be verified by visual observation, by an electrical signal initiated by a position-indicating device, by observation of appropriate pressure indication in the system, by leak testing, or by other positive means.
Section 4.1.2 of NUREG-1482 (Reference 6) provides guidance on exercising check valves with flow and on using NIT. It states that nonintrusive techniques may be used to verify the capability of check valves to open, close, and fully stroke in accordance with quality assurance program requirements. These techniques are considered "other positive means" in accordance with requirements for testing the check valve obturator movement (References 7, 8, and 9).
Information on qualification of NIT is given in the summary of NRC public workshops on the revision of Inspection Procedure 73756 (Reference 10). In response to a question about expectations for qualification of an NIT method, the NRC stated that a "qualified" NIT method is a technique that has been successfully and reliably demonstrated for the examination method and for the specific valve application.
In the late 1980's and early 1990's, the Nuclear Industry Check Valve Group conducted an experimental research and testing program to evaluate the available NIT technologies to determine their acceptability and reliability for use in check valve testing (Reference 11). The group obtained baseline information on check valves and concluded that, with baseline information available, the NIT methods investigated could detect a missing disc. Their report also indicates that having a complete operational history of the valve will improve accuracy of the data evaluation.
In accordance with the guidance in Reference 6, the NRC considers NIT acceptable for inservice testing of check valves provided that the method used is qualified. Qualification includes establishing a performance baseline when the check valve is in good operating condition. A check valve's performance can then be assessed against this baseline. In order to meet the check valve obturator testing requirements, the NIT technique must be repeatable. Both the NRC and industry have provided guidance on the use of NIT, as referenced above.
The only means to determine if an NIT technique will provide accurate and repeatable results for a specific check valve is to qualify the technique prior to its use. The qualification process may reveal that certain NIT techniques give inconclusive results for a particular application. For example, other plant noise sources may affect the sound pattern of the disc striking the valve backstop or seat, which may affect the results of the acoustic monitoring test. Such issues are typically addressed in the NIT qualification process and are documented for future reference.
In the case of Palisades, check valve CK-ES3332 had not been verified to be in good operating condition prior to the use of NIT. When NIT was performed on June 21, 2000, the technique had not been properly qualified. The individuals who examined the results of the test concluded incorrectly, based on their experience, that the valve was moving properly to its open and closed positions. Had the NIT method been qualified for valve CK-ES3332, the initial examination should have identified the valve's improper assembly.
If NIT techniques used to verify the opening or closing capability of safety-related check valves are not properly qualified and a baseline established for each individual valve when the valve is known to be operating acceptably, potentially inadequate valve performance may be undetectable in the analysis of NIT results.
This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact the technical contact listed below or the appropriate Office of Nuclear Reactor Regulation (NRC) project manager.
| /RA/
Ledyard B. Marsh, Chief |
| Technical contact: | J. Colaccino, NRR 301-415-2753 E-mail: jxc1@nrc.gov |
| Attachments: | 1. List of References 2. Palisades Safety Injection System Simplified Diagram 3. List of Recently Issued Information Notices |
