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HomeNRC LibraryDocument CollectionsGeneral CommunicationsInformation Notices1993

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                                 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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*Page Last Reviewed/Updated Thursday, March 29, 2012*

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