Information Notice No. 82-12: Surveillance of Hydraulic Snubbers
SSINS No.: 6835
IN 82-12
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
April 21, 1982
Information Notice No. 82-12: SURVEILLANCE OF HYDRAULIC SNUBBERS
Addressees:
All nuclear power reactor facilities holding an operating license or
construction permit.
Purpose:
This information notice is provided as an early notification of a
potentially significant problem pertaining to hydraulic snubbers on safety
related systems. It is expected that recipients will review the information
for applicability to their facilities. No specific action or response is
required at this time .
Description-of Circumstances:
On March 18, 1981, Carolina Power and Light Company (CP&L) reported (LER
81-041/03L) that 21 out of 101 Bergen-Patterson hydraulic snubbers were
considered inoperable after functional testing at the Brunswick 2 facility.
This represented a failure rate of approximately 20 percent. Failures were
identified in systems such as residual heat removal, reactor building closed
cooling water, fuel pool cooling, core spray, reactor core isolation
cooling, and high pressure coolant injection. Of the snubbers that failed to
meet the acceptance criteria, about 30 percent were declared inoperable
because they failed to lock up within the required velocity. As a result of
the high percentage of failures and the modes of failure, the licensee shut
the reactor down on March 4, 1981, and implemented an extended snubber test
program.
CP&L's supplements to the LER, submitted on June 1, 1981, and January 7,
1982, reported that 130 out of a total of 640 snubbers had failed the
functional test. This represented a failure rate slightly in excess of 20
percent. In addition to rebuilding all failed snubbers, another 80 which had
marginally met the acceptance criteria were rebuilt for purposes of
preventive maintenance.
CP&L's evaluation of the test and examination results indicated that a major
cause of failure to pass the functional test was low bleed rate (45 percent
of those rebuilt), Table I lists the reasons snubbers failed the functional
tests.
Those snubbers that were rebuilt were examined by CP&L and the types of
degradation observed were noted. Table II lists the types of degradation
that were observed. The most common types of degradation were worn poppets
(62 percent), spring capture (36 percent), piston/cylinder wear (31
percent), and deteriorated seals (28 percent). Some of the snubbers
exhibited more than one form of degradation accounting for a percentage
total greater than 100 percent.
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IN 82-12
April 21, 1982
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CP&L attempted to relate the observed degradation to the reason the snubber
failed the functional test. Table III lists all of the identified rejection
modes. The most common rejection modes were worn poppets (27 percent) and
spring capture (16 percent). That is, although worn poppets were observed in
62 percent of the snubbers examined, only 27 percent of the test failures
could be positively attributed to that cause.
The three major types of degradation (worn poppets, spring capture, and
piston/cylinder wear) appear to be consequences of service-related
conditions. Pipe vibrations cycle the snubbers to the extent that the
grooves on the poppet heads begin to wear, and the pistons wear on the
cylinder walls. Continued cycling causes the retaining springs to fail or
deform in such a way that the springs can be captured. Of the snubbers
rebuilt, 37 percent were determined by CP&L to be service sensitive. That
is, the operating conditions of the line or snubber were found to have signs
of vibration or water hammer and these conditions could cause the type of
degradation found. Most of the service sensitive snubber failures were
attributed to vibration; a nominal number were caused by water hammer.
CP&L is replacing the poppet valve bodies with a new, more vibration
resistant one developed by Bergen-Patterson. However, until lengthy service
demonstrates the effectiveness of this modification, it would be prudent to
maintain high levels of surveillance and preventive maintenance on hydraulic
snubbers subject to vibration or shock loadings that can reduce their
service life. Additionally, consideration should be given to system design
alternatives which reduce, to the maximum extent possible, or preclude such
vibrations or shock loadings.
In order to help identify the systems with high failure rates, the staff
compared the list of failed snubbers contained in the LER with the list of
safety related hydraulic snubbers contained in Table 3.7.5-1 of the
technical specifications for Brunswick 2. Table IV contains a ranking of the
systems by the failure rates of their attached safety-related hydraulic
snubbers. The difference in the total number of snubbers reported in the LER
(640) and shown in the technical specification table (631) results in a
negligible increase in the average failure rate.
Review of Table IV shows that failures were found in 17 of the 20 systems.
However, the failures were not distributed evenly among these 17 systems.
Rather, the wide range of failure rates in these systems (6 to 83 percent)
indicates that the failures tended to be concentrated in certain systems.
This is further emphasized by the fact that although the nine systems with
failure rates in excess of the overall average account for 68 percent of the
failures (88 out of 130), these systems contain only 39 percent of the
snubbers (244 out of 631) in the plant. These nine systems are ones where
vibrations or shock loadings are likely to be encountered.
A similar problem at Rancho Seco was discussed in Information Notice No.
79-01 based on information reported on December 4, 1978, (LER 78-015/01T)
and January 12 and 15, 1979, (LER 78-017/03L) by Sacramento Municipal
Utility District. Subsequent correspondence from Bergen-Patterson indicated
that its intended modifications to these snubbers would include case
hardening of the
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IN 82-12
April 21, 1982
Page 3 of 3
entire valve body, extending the cavity in which the end of the spring coil
is guided, and reducing the axial travel of the poppet by increasing the
length of the poppet stop.
The number of failures experienced tends to compromise capability of the
affected systems to function properly during operational transients and to
withstand seismic events.
If you have any questions regarding this matter, please contact the Regional
Administrator of the appropriate NRC Regional Office, or this office.
Sincerely,
Edward L. Jordan, Director
Division of Engineering and
Quality Assurance
Office of Inspection and Enforcement
Technical Contact: R. J. Kiessel
301-492-4796
Attachment:
1. Tables I, II, III and IV
2. Recently Issued IE Information Notices
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Attachment
IN 82-12
April 21, 1982
TABLE I
REASON SNUBBER FAILED FUNCTIONAL TEST
Percent of
Reason Total Rebuilt
Low Bleed 45
High Lockup 17
High Bleed 13
No Lockup 13
Other 12
TABLE II
TYPES OF DEGRADATIONS FOUND
Percent of
Reason Total Rebuilt
Worn Poppets 62
Spring Capture 36
Piston/Cylinder Wear 31
Deteriorated Seals 28
Grease in Fluid 7
Side Loading 7
None 8
TABLE III
REJECTION MODE
Percent of
Reason Total Rebuilt
Worn Poppets 27
Spring Capture 16
Deteriorated Seals 3
Piston/Cylinder Wear 3
Side Loading 1
Grease in Fluid 1
Inconclusive Evidence 41
Miscellaneous 8
*Note: These percentages total more than 100 percent because some of the
snubbers exhibited more than one form of degradation.
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TABLE IV
HYDRAULIC SNUBBER FAILURE RATES BY SYSTEM
Failure
System Failures Snubbers Rate(%)
Off Gas 5 6 83
Control Rod Drive 4 7 57
Standby Liquid Control 4 10 40
Steam Relief Discharge 36 93 38
Reactor Core Isolation Cooling 12 33 36
Reactor Feedwater 10 32 31
High Pressure Coolant Injection 11 39 28
Condensate Drains 2 8 25
Nuclear Steam Vent 4 16 25
Service Water 9 45 20
Reactor Circulation 3 21 14
Reactor Building Closed Cooling Water 4 30 13
Residual Heat Removal 19 192 10
Instrument Sensing 1 11 9
Primary Steam 3 34 8
Core Spray 2 24 8
Fuel Pool Cooling 1 16 6
Reactor Water Cleanup 0 1 0
Standby Gas Treatment 0 1 0
Reactor Vessel Instrumentation 0 12 0
Totals 130 631
Overall Average 21
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021