United States Nuclear Regulatory Commission - Protecting People and the Environment

Bulletin 81-01: Surveillance of Mechanical Snubbers

                                                       SSINS No.: 6820 
                                                       Accession No.: 
                                                       8005050075 
                                                       IEB 81-01 

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555 

                              January 27, 1981

IE Bulletin No. 81-01:   SURVEILLANCE OF MECHANICAL SNUBBERS 

Description of Circumstances: 

Several instances of failures of mechanical snubbers supplied by 
International Nuclear Safeguards Corporation (INC) have been identified that
indicate possible deficiencies in these snubbers. A summary of the failures 
that have occurred is provided below: 

1.   On August 9, 1974, the Tennessee Valley Authority submitted event 
     report BFAO-50-260/741W identifying 11 of 14 INC Model MSVA-1A snubbers 
     that were found inoperable on Browns Ferry Nuclear Power Station Unit 2 
     and subsequently identified 5 of 14 inoperable units on Browns Ferry 
     Nuclear Power Unit No 3. All of these units were found to be frozen, 
     and the cause was attributed to a failure to lubricate the parts during 
     assembly. The failed snubbers were replaced with new units produced by 
     the same manufacturer. 

2.   On April 12, 1976, the St. Lucie Plant Unit 1 facility of Florida Power
     and Light Corporation submitted event report No. 50-335-76-9 wherein 
     five INC Model MSVA-1 snubbers were identified as inoperable because 
     they were found to be frozen. The failures were caused by oxidation on 
     the internals and by improper assembly. All INC mechanical snubbers 
     were replaced with units produced by another manufacturer. 

3.   On April 8, 1977, Iowa Electric Light and Power Company submitted event
     report No. 77-23 for the Duane Arnold Energy Center facility that 
     identified 13 INC Model 1MSVA-1 Type AS snubbers to be frozen; the 
     cause of failure was attributed to large amounts of interior oxidation. 
     The units were replaced with those produced by another manufacturer. 

4.   On December 5, 1979, personnel from the Nuclear Regulatory Commission 
     visited Department of Energy (DOE) facilities at Richland, Washington, 
     to obtain information on DOE experience with INC snubbers at the Fast 
     Flux Test Facility (FFTF). The DOE-owned FFTF was equipped with more 
     than 4,000 mechanical pipe restraints (snubbers) supplied by INC. In 
     1978, FFTF examined more than 800 of these mechanical snubbers by 
     removing them from their installation and found that 43, or about 5% of
     those examined, were frozen. The plant was still under construction so 
     the snubbers had seen no service and had been subjected to only normal 
     construction environments for 1 to 2 years. 

Attachment:
Recently issued IE bulletins
.

                                                           IEB 81-01 
                                                           January 27, 1981 
                                                           Page 2 of 6 

     Tests were conducted on three operable snubbers by installing them on 
     a Hanford Engineering and Development Laboratory (HEDL) process line. 
     The three snubbers were subjected to flow-induced low-amplitude 
     vibration (0.003 inches or less). These snubbers were of both the 
     combined carbon steel and stainless steel construction and the all 
     stainless steel construction. Detailed test data are not available to 
     the NRC at this time . However, all three snubbers froze after being 
     subjected to the vibration for periods of 3 to 30 days. 

     The failure modes on all units inspected and tested involved a number 
     of different mechanisms leading to the freezing of the snubbers. 
     Following disassembly of some of the snubbers, inspections showed the 
     failures were caused by improper assembly; overheating of internal 
     components caused by welding (during fabrication); and sensitivity of 
     the design to dirt, corrosion, and inadequate or excessive lubrication.
     DOE concluded that there were generic deficiencies in the design of the
     snubbers of this specific manufacturer for application to the FFTF 
     facility and for pipes subjected to vibration. All INC mechanical 
     snubbers in FFTF have been replaced with snubbers produced by another 
     manufacturer. 

5.   On May 31, 1980, Georgia Power Company reported eight INC snubbers 
     located on instrument and drain lines at Edwin I. Hatch Nuclear Plant 
     Unit 1 were identified as inoperable (LER 321-80-55). The cause of the 
     failures was identified as internal corrosion that caused a frozen 
     condition. In an attempt to free a snubber (750-pound capacity), forces
     up to 1500 pounds were applied in both the "extend" and "retract" 
     directions and the snubber did not move. The inspection of INC snubbers
     was completed at the Hatch facility and, on June 30, 1980, NRC received
     a supplemental report that 45 of the 61 snubbers that had been 
     inspected on Unit 1 had been identified as inoperable and three of the 
     42 snubbers that were inspected on Unit 2 were inoperable. All 
     inoperable snubbers were replaced prior to startup of the affected 
     unit. Some were replaced with mechanical units produced by another 
     manufacturer, some were replaced with later-model INC snubbers, and 
     three were replaced with rigid restraints. Plans are being made to 
     replace all INC snubbers during upcoming refueling outages. Analyses 
     are also being performed on the piping affected by the locked up 
     snubbers. 

In addition to INC snubber failures, failures of mechanical snubbers by 
another manufacturer are identified below: 

1.   On September 7, 1979, Public Service Electric and Gas Company reported 
     the failure of three Model PSA-3 mechanical snubbers manufactured by 
     Pacific Scientific Company that were located on a main feedwater line 
     of Salem Nuclear Generating Station Unit 1 (LER 79-54). These three 
     snubbers could not be rotated around their spherical rod end bearings. 
     The snubbers were removed and inspection revealed that the lead screw 
     and traveling nut assembly, which translates linear to rotational 
     motion, had failed. The snubbers no longer provided seismic shock 
     restraint under this condition. These snubbers are directly upstream of
     the nuclear Class II piping boundary and are included in the stress 
     calculations for the seismic analysis of the nuclear portion of the 
     main feedwater piping. Failure of the snubbers 
.

                                                           IEB 81-01 
                                                           January 27, 1981 
                                                           Page 3 of 6 

     appeared to result from a force many times greater than the design load
     of the snubbers. This force was either an extreme shock load or 
     occurred when the snubber was in the fully retracted condition. The 
     snubbers were replaced with units produced by the same manufacturer. 

2.   On April 10, 1979, Consumers Power Company reported a failure of eight 
     Model PSA-3 Pacific Scientific snubbers at their Big Rock Point Nuclear
     Plant facility (LER 79-017/03L-O). The cause of the failure was 
     improper installation in that a spherical washer was omitted from the 
     transition tube. 

3.   On March 15 and June 11, 1979, Florida Power and Light reported 
     failures of Pacific Scientific Company mechanical snubbers on main 
     steam and feedwater systems at Turkey Point Plant Units 3 and 4 (LER 
     79-006/03L-O and 79-009/03L-O respectively). The cause in both cases 
     was attributed to excessive loading. 

The nature of the above mechanical snubber failures is to prevent the piping
systems, to which they are attached, from moving freely during the normal 
thermal heat up and cool down associated with plant operations. Restraining 
this thermal motion results in higher than normal stresses which, if high 
enough and repeated frequently enough, can lead to a premature fatigue 
failure of the piping system. 

These mechanical snubbers have been installed for a number of years without 
any NRC requirements for periodic surveillance to determine their condition. 
As a result, their current condition is unknown to NRC and therefore NRC is 
requesting a prompt examination of all mechanical snubbers installed to 
date. Because of the high percentage of failures discovered with the INC 
snubbers, the time frame for their examination is the shortest and 
additional operability tests are called for. 

Actions to be Taken by Licensees of Operating Reactors: 

1.   Within 30 days of the issuance date of this bulletin, all normally 
     accessible* INC mechanical snubbers installed on safety-related systems
     or in storage shall be visually examined and tested as follows: 

     a.   Perform a visual examination for damage and, without causing the 
          system to be inoperable except as permitted by the facility 
          technical specifications, verify that the snubbers have freedom of
          movement by performing a manual test over the range of the stroke 
          in both compression and tension. 

     b.   Perform an operability test to confirm that (1) activation 
          (restraining action) occurs in both compression and tension and 
          (2) the drag forces are within the specified range in both 
          compression and tension. The tests shall be performed on all 
          snubbers in storage and on a representative sample (10% of the 
          total of this type of snubber in use in the plant or 35, which 
          ever is less) of the 

  *  "Normally accessible" refers to those areas of the plant that can be 
     entered during reactor operation. 
.

                                                           IEB 81-01 
                                                           January 27, 1981 
                                                           Page 4 of 6 

          normally accessible snubbers that are in service and can be 
          individually removed without causing the system to be inoperable, 
          except as permitted by the facility technical specifications. For 
          each snubber that does not meet the test acceptance criteria, an 
          additional representative sample (as defined above) of this type 
          of snubber shall be tested. For each of these additional snubbers 
          that do not meet the test acceptance criteria, another 
          representative sample of this type of snubber shall be tested. 
          This cycle shall be repeated until no more failures have been 
          found or until all snubbers of this type have been tested. The 
          samples should be made up of snubbers representing the various 
          sizes. 

     c.   Snubbers which have been examined and tested in a manner 
          comparable to Items 1a and 1b above within the last six months may 
          be exempted. 

     d.   If any failures are identified in Items 1a or 1b above, take 
          corrective action and evaluate the effect of the failure on the 
          system operability pursuant to the facility technical 
          specifications for continued operation. 

     e.   If failures are identified in Items 1a and 1b above, and if INC 
          snubbers are known to be located in any inaccessible areas, a 
          plant shutdown shall be performed within 30 days after the 
          discovery of the first inoperable snubber and inspections 
          conducted in accordance with Item 2a and 2b below, unless 
          justification for continued operation has been provided to the 
          NRC. 

2.   Visually examine and test all inaccessible INC mechanical snubbers 
     installed on safety related systems at the next outage of greater than 
     five days duration as follows: 

     a.   Visually examine and manually test all inaccessible snubbers as 
          described in Item 1a above. 

     b.   Perform an operability test on a representative sample of 
          inaccessible snubbers as described in Item 1b above. 

     c.   Snubbers which have been examined and tested in a manner 
          comparable to Items 2a and 2b above within the last six months may 
          be exempted. 

     d.   If any failures are identified in Items 2a or 2b above, take 
          corrective action to evaluate the effect of the failure on system 
          operability pursuant to the facility technical specifications for 
          resuming operation. 

3.   Provide a schedule for an inspection program covering mechanical 
     snubbers produced by other manufactures. As a minimum, this inspection 
     program shall: 

     a.   Include all snubbers installed on safety-related systems; 

     b.   Include the visual examination and manual test described in Item 
          1a if above for all snubbers; 
.

                                                      IEB 81-01, Rev. 1 
                                                      March 4, 1981 
                                                      Page 5 of 6 

     c.   Snubbers which have been examined and tested in a manner 
          comparable to Item 3b above within the last twelve months may be 
          exempted: 

     d.   Require the corrective action and evaluations described in Items 
          1d and 2d above; and 

     e.   Be completed prior to the completion of the next refueling outage.
          Plants which are currently in a refueling outage should perform 
          the visual examination and manual tests of inaccessible mechanical 
          snubbers before resumption of operations unless some other basis 
          for assurance of snubber operability is provided to the NRC. 

4.   Submit a report of the results of the inspections, testing and 
     evaluation requested in Item 1 to NRC within 45 days of the issuance 
     date of this bulletin. Report the results of the inspections, testing 
     and evaluation requested in Item 2 within 30 days after the inspection 
     and testing have been completed. The response to Item 3 shall be 
     submitted within 60 days of the issuance date of this Bulletin. The 
     results of the inspections performed for Item 3 shall be submitted 
     within 60 days after the completion of the inspection. 

     The reports shall contain the following: 

     a.   A description of the visual examinations and tests performed. 

     b.   Number of snubbers examined and tested. Grouping by manufacturer 
          name, model number, and size is acceptable. 

     c.   Number of failures identified; manufacturer name, model number, 
          size, mode of failure, cause of failure, corrective action, 
          snubber location, effect of failure on plant and system safety, 
          and justification for continuing or resuming operation. 

     d.   The above information shall also be provided for the snubbers 
          exempted by Items 1c, 2c, and 3c above. 

Actions to be Taken by the Following Licensees Holding Construction Permits:

Diablo Canyon Nuclear Power Plant Unit 1; San Onofre Nuclear Station Unit 2; 
Watts Bar Nuclear Plant Units 1 and 2; and Virgil C. Summer Nuclear Station 
Unit 1. 

1.   After preoperational and/or hot functional testing and preceding fuel 
     loading, visually examine and test the mechanical snubbers installed on
     safety-related systems as follows: 

     a.   For all snubbers perform a visual examination for damage and 
          verify that the snubbers have freedom of movement by performing a 
          manual test over the range of the stroke in both compression and 
          tension.
.

                                                      IEB 81-01, Rev. 1 
                                                      March 4, 1981 
                                                      Page 6 of 6 

     b.   For INC snubbers, perform an operability test to confirm that (1) 
          activation (restraining action) occurs in both compression and 
          tension and (2) the drag forces are within the specified range in 
          both compression and tension. The tests shall be performed on a 
          representative sample (10% of the total of this type of snubber in
          use in the plant or 35, which ever is less). For each snubber that
          does not meet the test acceptance criteria, an additional 
          representative sample (as defined above) of this type of snubber 
          shall be tested. For each of these additional snubbers that do not
          meet the test acceptance criteria, another representative sample 
          of this type of snubber shall be tested. This cycle shall be 
          repeated until no more failures have been found or until all 
          snubbers of this type have been tested. The samples should be made
          up of snubbers that represent the various sizes. 

     c.   If any failures are identified in Items a or b above, take 
          corrective action prior to fuel loading. 

2.   The schedule for the inspections and tests requested in Item 1 above, 
     shall be submitted within 60 days of the issuance date of this 
     bulletin.*  The results of the inspections, testing, and evaluation 
     requested in Item shall be reported to NRC within 30 days after the 
     inspection and testing have been completed. 

     The reports shall contain the following: 

     a.   A description of the visual examinations and tests performed. 

     b.   Number of snubbers examined and tested. Grouping by manufacturer 
          name, model number, and size is acceptable. 

     c.   Number of failures identified; manufacturer name, model number, 
          size, mode of failure, cause of failure, corrective action, and 
          snubber location. 

Reports, signed under oath or affirmation, under the provisions of Section 
182a of the Atomic Energy Act of 1954, shall be submitted to the Director of
the appropriate NRC Regional Office and a copy shall be forwarded to the 
Director of the NRC Office of Inspection and Enforcement, Washington, D.C. 
20555. 

If you desire additional information regarding this matter, please contact 
the IE Regional Office. 

Approved by GAO B-180225 (S81003) expires December 31, 1981. 
Page Last Reviewed/Updated Tuesday, July 23, 2013