United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 84-67: Recent Snubber Inservice Testing with High Failure Rates

                                                           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 
.

                                                         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. 
.

                                                         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. 
.

                                                         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 
.

                                                         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. 

.

                                                           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  

Page Last Reviewed/Updated Tuesday, November 12, 2013