Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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
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IEB 81-01
January 27, 1981
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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
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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.
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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;
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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.
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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.
