United States Nuclear Regulatory Commission - Protecting People and the Environment

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. 

8202040143  
.

                                                            IN 82-12  
                                                            April 21, 1982 
                                                            Page 2 of 3 

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 
.

                                                            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 
.

                                                            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. 
.

                                   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, November 12, 2013