Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 84-67
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
August 17, 1984
Information Notice No. 84-67: RECENT SNUBBER INSERVICE TESTING WITH
HIGH FAILURE RATES
Addressees:
All nuclear power facilities holding an operating license (OL) or a
construction permit (CP).
Purpose:
This information notice is provided as a notification of a potentially
significant problem pertaining to recent snubber inservice testing that
resulted in high failure rates. It is expected that recipients will review
the information for applicability to their facilities and consider actions,
if appropriate, to preclude a similar problem occurring at their facilities.
However, suggestions contained in this information notice do not constitute
NRC requirements and, therefore, no specific action or written response is
required.
Description of Circumstance:
The following recent events involving snubber inservice testing with high
failure rates have been reported:
1. On October 17, 1983, the Consumers Power Company reported [Licensee
Event Report (LER) 83-063] the failure of both of the tested steam
generator hydraulic snubbers to lock up during their inservice test at
the Palisades Plant. The cause was attributed to insufficient counter
boring of the valve block end-cap for the hydraulic snubber, which
inhibited the shuttle valve from properly closing the appropriate block
port. Inspection of the remaining 14 hydraulic snubbers revealed the
same defect in 3 additional snubbers (31% over all failure rate). All
defective snubbers were reworked to ensure proper operation. The
hydraulic snubbers were ITT Grinnel Figure 200 (8 in. bore by 5 in.
stroke).
8408140208
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IN 84-67
August 17, 1984
Page 2 of 4
ITT Grinnel indicated in a conversation that they felt the cause of the
problem was the use of a nonstandard spring that gave the snubbers a
higher lockup velocity. It was the longer compressed length of this
spring that prevented proper shuttle valve positioning. ITT Grinnel
also indicated that they were not aware of any other facilities with
this non standard spring.
2. On April 16, 1984, the Virginia Electric and Power Company reported
(LER 84-006) the failure of a number of hydraulic and mechanical
snubbers at Surry Power Station Unit 1 to pass the inservice testing
required by their Technical Specifications. These tests, conducted in
March 1984, were a continuation of tests initially performed in March
1983. All numbers presented below and in the tables are totals of the
1983 and 1984 tests.
Of the 160 hydraulic snubbers tested, failures were found in 48 (30%).
The modes of failure are shown in Table 1. Twenty-one of the hydraulic
snubbers were reset to design conditions and reinstalled. Of the
remaining 27 hydraulic snubbers, 23 had been disassembled as of the
date of the report to determine the cause of failure. The causes of
failure are shown in Table 2. To preclude any generic concerns with the
ITT Grinnel Lynair-type snubbers (67% of the failures were with this
type), the utility plans on replacing as many as possible with ITT
Grinnel Miller-type snubbers.
Of the 66 mechanical snubbers tested, failures were found in 21 (44%).
The modes of failure are shown in Table 3. Five of the mechanical
snubbers were disassembled to determine the cause of failure. Of these,
three had no observable defect and two had broken metallic parts in the
mechanism. All of the mechanical snubbers are Pacific Scientific
snubbers.
3. On April 25, 1984, the Florida Power and Light Company notified the NRC
resident inspector of their intention to shut down Turkey Point Plant
Unit 3 to test the mechanical snubbers in the unit. They indicated that
they were taking this action because of the high number of failures
observed when the mechanical snubbers in Turkey Point Plant Unit 4 were
tested.
Of the approximately 100 mechanical snubbers in each unit, 31 failures
had been observed in Unit 4 and 46 failures were found in Unit 3. This
did not include 17 snubbers in Unit 4 that were found with the
telescoping cylinder not staked at the ball nut thread joint. The modes
of failure are shown in Table 4. All of the mechanical snubbers are
Pacific Scientific snubbers.
4. On June 25, 1984, the Commonwealth Edison Company verbally reported the
failure of the steam generator hydraulic snubbers at Braidwood Station
to pass their qualification tests. All three of the tested snubbers
failed to meet the specified requirements in that the mechanical seals
in the snubbers failed to hold hydraulic fluid at test loads of only 10
to 33 percent of the rated capacity of 2,400 kips. The snubbers were
manufactured by Boeing Company prior to their discontinuing business in
that field. It is currently not known if the same mechanical seal
design is used on other snubber models manufactured by Boeing Company.
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IN 84-67
August 17, 1984
Page 3 of 4
Discussion:
As a part of a multifaceted program to improve overall snubber reliability,
a November 20, 1980 letter from NRC requested all licensees to revise their
Technical Specifications with respect to inservice testing of snubbers that
were previously exempted from testing because of inaccessibility and to
include mechanical snubbers in the testing program. Thus, the first three
failures discussed above resulted from the first inservice testing for the
steam generator hydraulic snubbers at the Palisades Plant and also for the
mechanical snubbers at Surry Power Station Unit 1 and Turkey Point Plant
Units 3 and 4.
Because it was the result of a manufacturing defect, it can be reasonably
assumed that the inoperability of the steam generator hydraulic snubbers at
the Palisades Plant existed since they were installed. However, the length
of inoperability cannot be readily determined for the mechanical snubbers at
Surry Power Station Unit 1 and Turkey Point Plant Units 3 and 4. What is
known, though, is that IE Bulletin 81-01, "Surveillance of Mechanical
Snubbers," required all licensees to ensure that their mechanical snubbers
were not locked up and that:
1. Virginia Electric and Power Company responded on July 30, 1981, saying
that only 1 of the 68 mechanical snubbers at Surry Power Station Unit
1 could not be stroked in accordance with the bulletin.
2. Florida Power, and Light Company responded on July 6, 1981, to say that
only 3 of 64 mechanical snubbers at Turkey Point Plant Unit 3 were
frozen. Their February 19, 1982 response indicated that only 2 of the
60 mechanical snubbers at Turkey Point Plant Unit 4 had been frozen.
From the above it becomes readily apparent that it is important to implement
the revised Technical Specifications for snubbers as soon as practical and
that a higher failure rate than previously expected can be anticipated.
The mechanical snubber test procedures contained two conditions (summarized
below) that some felt accentuated the failure rates observed. However, the
NRC staff does not consider these conditions to be unduly conservative; they
simply represent the conditions under which the snubber would be expected to
function, either in its passive or its active role. The conditions in
question are as follows:
1. The procedure required performing the drag/breakaway test on the
mechanical snubber in the as-found condition before performing the
acceleration tests. This test sequence provides an excellent indication
of the ability of the snubber to perform its passive role of not
impeding thermal growth of the attached piping system after the snubber
has remained in one position for a period of time.
2. The procedure required performing the acceleration test at rated load.
Testing at rated load provides assurance that the full design
capability of the snubber remains available after the snubber has been
in service for a period of time. Because not all snubbers are tested
during each testing period (unless significant failure rates are
detected), testing at rated load should not place an undue strain on
the snubbers that could shorten their expected life.
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IN 84-67
August 17, 1984
Page 4 of 4
No written response to this information notice is required. If you have any
questions about this matter, please contact the Regional Administrator of
the appropriate NRC Regional Office, or this office.
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: R. J. Kiessel, IE
(301) 492-8119
Attachments:
1. Tables 1-4
2. List of Recently Issued IE Information Notices
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Attachment 1
IN 84-67
August 17, 1984
Page 1 of 2
Table 1 Surry Power Station Unit 1
HYDRAULIC SNUBBER FAILURE MODES*
% of Snubbers % of Snubbers
Mode Number Tested that Failed
Low lockup in tension 13 8 27
Low lockup in compression 3 2 6
Low bleed in tension 11 7 23
Low bleed in compression 10 6 21
High lockup in tension 17 11 35
High lockup in compression 10 6 21
High bleed in tension 6 4 13
High bleed in compression 3 2 6
Table 2 Surry Power Station Unit 1
HYDRAULIC SNUBBER FAILURE CAUSES*
% of Snubbers % of Snubbers
Cause Number Tested that Failed
No observable defect 6 4 26
Degraded EP seals 3 2 13
Polyurethane piston seals 7 4 30
Poppet upside down 3 2 13
Debris in fluid 1 1 4
Poppet stuck 1 1 4
Lockup adjustment screw broken1 1 4
Piston/cylinder scoring 2 1 9
Table 3 Surry Power Station Unit 1
MECHANICAL SNUBBER FAILURE MODES
% of Snubbers % of Snubbers
Mode Number Tested that Failed
High drag 14 21 66
Locked up 5 8 24
High acceleration 1 2 5
Locks up in compression 1 2 5
* Failures total more than 100 percent because some snubbers exhibited
more than one failure mode or cause.
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Attachment 2
IN 84-67
August 17, 1984
Page 2 of 2
Table 4 Turkey Point Plant Units 3 and 4
MECHANICAL SNUBBER FAILURE MODES
Unit 3 Unit 4
Mode Failures Failures
Free moving 1 1
Found locked up 1 4
Locked up during test 12 6
Failed drag/breakaway test 16 13
Exceeded 0.02g acceleration limit 1 7
Erratic acceleration response 0 8
Acceleration reduced to zero 0 7
