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Information Notice No. 93-42: Failure of Anti-Rotation Keys in Motor-Operated Valves Manufactured by Velan
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 June 9, 1993 NRC INFORMATION NOTICE 93-42: FAILURE OF ANTI-ROTATION KEYS IN MOTOR-OPERATED VALVES MANUFACTURED BY VELAN Addressees All holders of operating licenses or construction permits for nuclear power reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to potential problems resulting from the failure of anti-rotation keys in motor-operated globe valves manufactured by Velan Valve Corporation. It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice do not constitute NRC requirements; therefore, no specific action or written response is required. Description of Circumstances On March 28, 1993, Wisconsin Public Service Corporation, the licensee for Kewaunee Nuclear Power Plant, reported that during motor-operated valve (MOV) dynamic testing on March 27, 1993, one of two redundant safety injection recirculation valves to the refueling water storage tank failed to close completely. Investigation revealed that the anti-rotation device, an L-shaped key between the valve stem and the yoke bushing (Attachment 1), had broken. The shorter part of the broken L-shaped key had apparently worked its way toward the valve stem and had jammed the valve stem, preventing closing. The valve that failed was a 2-inch bonnetless globe valve manufactured by Velan Valve Corp. The licensee inspected all nine similar motor-operated valves and found that six had broken anti-rotation keys. The inspections were visual and did not require disassembly of the motor-operated valve. Failures usually can be observed on normally closed valves, since the broken long leg of the L-shaped key slides down partially and becomes visible below the yoke bushing. According to the licensee, the keys failed in a brittle manner. The fractures began on either side of the key at the sharp radius of the inside corner of the L-shaped key and propagated into the body of the key. Machining of the sharp inside corner of the L-shaped key may have contributed to the failure of the anti-rotation keys by producing high stresses in the corner of the keys. It is likely that the rotational action of the valve stem impacted on both sides of the key, exceeding the impact strength of the material at the sharp radius of the key. 9306030147. IN 93-42 June 9, 1993 Page 2 of 3 In 1990, at the Maine Yankee Atomic Power Station, anti-rotation keys in 4-inch MOVs manufactured by Velan failed. Maine Yankee Atomic Power Company, the licensee, discovered that spare keys supplied by Velan for several of the valves were not of the proper hardness; they were Rockwell C-20 instead of the harder Rockwell C-59 as originally specified. Valves that may have had the wrong anti-rotation keys installed were disassembled, and the licensee replaced the Rockwell C-20 keys with keys made of the correct material. The non-conforming keys that were removed showed signs of significant wear. During post-maintenance testing, several of the new (hardened) keys broke at the corner of the "L" shape. This happened because the keyway in the valve adaptor plate was too tight considering the service-related wear that had occurred to the rest of the keyway. Maine Yankee engineers specified that new larger keyways should be cut in the adaptor plate to prevent the key from breaking when the valve is actuated. The licensee repaired all the affected valves and then tested the valves to ensure that they operated satisfactorily. Before testing MOVs at Prairie Island this past winter, an inspection of Velan globe valves also discovered anti-rotation key failures (2 broken keys found in 16 valves) in similar systems. Because of the location of the break on the anti-rotation key, and the length of the key and key slot, Northern States Power Company, the licensee, believed that the broken anti-rotation keys would still perform their intended function and that the broken piece (i.e., the short leg of the L-shaped key) would not interact with the valve stem so as to impede valve operation. This is at variance with the Kewaunee event discussed above. The licensee added a step to their generic actuator replacement procedure to inspect, clean, debur, and lubricate anti-rotation devices. Discussion On April 5, 1993, Velan issued the attached (Attachment 2) Service Bulletin #SB-106. This bulletin recommends that the anti-rotation keys be inspected for possible wear which could result in failure and that consideration be given to replacing the keys at the earliest convenient time. The replacement keys, which have been supplied by Velan for the past 2.5 years, are made of material meeting American Iron and Steel Institute (AISI) 4140 alloy steel, as opposed to 440C stainless steel (440C SS). AISI 4140 is significantly tougher than 440C SS. According to Velan, the material was changed to 4140 because 4140 is easier to procure and to heat treat properly. Velan plans to send this bulletin to all U.S. nuclear utilities. On the basis of conversations between the NRC staff and Velan subsequent to the issuance of the bulletin, and because of the hardness of anti-rotation keys made out of 440C SS, Velan suggests that both the keys and the keyways be inspected for damage and deterioration. The experience cited above indicates that the use of brittle material in anti-rotation keys can lead to failures, although valve operation may not always be impaired.. IN 93-42 June 9, 1993 Page 3 of 3 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 (NRR) project manager. ORIGINAL SIGNED BY Brian K. Grimes, Director Division of Operating Reactor Support Office of Nuclear Reactor Regulation Technical contact: David C. Fischer, NRR (301) 504-2728 Attachments: 1. Figure 1, Velan Valve and Anti-Rotation Key 2. Velan Service Bulletin #SB-106 3. List of Recently Issued NRC Information Notices .
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