NUREG/CR-4279

PN L-5479

Vol. 1

Aging and Service Wear of

Hydraulic and Mechanical Snubbers

Used on Safety-Related Piping and

Components of Nuclear Power Plants

Phase I Study

Prepared by S. H. Bush, P. G. Heasler, R. E. Dodge

Pacific Northwest Laboratory Operated by Battelle Memorial Institute

Prepared for

U.S. Nuclear Regulatory Commission
I

4 1

NOTICE

JThis report was prepared as an account of work-sponsored by an agency of the United States

Government. Neither the United States Government nor any agency thereof, orany of their

employees, makes any warranty, expressed or implied, or assumes any legal liability of responsibility for any third party's use, or the results of such use, of any information, apparatus.

product or process disclosed in this report, or represents that its use by such third party would

not infringe privately owned rights.

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I - -

,- - ' - NOTICE ;

- < Availability of Reference Materials Cited in NRC Publications

Most documents cited in NRC publications will be available from one of the following sources:

1 The NRC Public Document Room, 1717 H Street, N.W.

Washington, DC 20555

2. The Superintendent of Documents, U.S. Government Printing Olfice, Post Ollice Box 37082,

Washington, DC 20013-7082 -

3. The National Technical Information Service, Springfield, VA 22161 , >

Although the listing that follows represents the majority of documents cited in NRC publications,

it is not intended to be exhaustive.

Referenced documents available for inspection and copying for a fee from the NRC Public Docu

mient Room include NRC correspondence and internal NRC memoranda; NRC Office of Inspection

and Enforcement bulletins, circulars, information notices, inspection andl investigation notices;

Licensee Event Reports; vendor reports and correspondence; Commission papers; and applicant and

licensee documents and correspondence. -

The following documents in the NUREG series are available for purchase from the GPO Sales

Program: formal NRC staff and contractor-reports, NRC-sponsored conference proceedings, and

NRC booklets and brochures. Also available are Regulatory Guides, NRC regulations in the Code of

Federal Regulitions, and Nuclear Regulatory Commission Issuances.

Documents'-available from the National Technical Information Service include NUREG series

reports and technical reports prepared by other federal agencies and reports prepared by the Atomic

Energy Commission, forerunner agency to the Nuclear Regulatory Commission.. - - :

Documents available from public and special technical libraries include all open literature items,

such as books, journal and periodical articles, and transactions. Federal Register notices, federal and

state legislation, and congressional reports can usually be obtained from these libraries;: -

Documents such as theses, dissertations, foreign reports and translations, and non-NRC conference

proceedings are available for purchase from the organization sponsoring the publication cited.

Single copies ol NRC draft reports are available free, to the extent of supply, upon written request

to: the Division of Technical Information and Document Control' U.S. Nuclear Requlatory Cor

mission. Washington. DC 20555. : . - -

Copies of industry codes and standards used in a substantive manner in the NRC regulatory process

are maintained at the NRC Library, 7920 Norfolk Avenue, Bethesda, Maryland. and'are available

there for reference use by the public. Codes and standards are usually copyrighted'and may be

purchased from the originating organization or, if they are American National Standards, from the

American National Standards Institute, 1430 Broadway, New York, NY 10018.

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NUREG/CR-4279

PN L-5479

Vol. 1

RM, RV

Aging and Service Wear of

Hydraulic and Mechanical Snubbers

Used on Safety-Related Piping and

Components of Nuclear Power Plants

Phase I Study

Manuscript Completed: January 1986

Date Published: February 1986

Prepared by S. H. Bush, P. G. Heasler, R. E. Dodge

Pacific Northwest Laboratory

Richland, WA 99352

Prepared for

Division of Engineering Technology Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, D.C. 20555

NRC FIN B2865

-
FOREWORD

This report is based on the snubber operating experience for the past

several years, primarily licensee event reports (LERs). Aging mechanisms that

influence snubber failures are assessed. The adequacy of current testing and

examination procedures is discussed, and suggestions are given for changes when

the procedures are considered to be inadequate. A conclusion from this report

is that many snubbers installed in nuclear power plants may be unnecessary and

could be removed. Work outside the scope of this report has confirmed that the

removal of many snubbers can be justified. An approved approach to evaluate

snubber removal has been incorporated into ASME III and approved by the U.S.

Nuclear Regulatory Commission (NRC) on a case-by-case basis.

This report constitutes an NRC special study of snubber experience from

the standpoint of reducing the number of snubbers currently in use. The study

includes a preliminary assessment of snubber aging characteristics and mechanisms. However, a treatment of the snubber as an assembly and systematic

consideration of each component or subcomponent in terms of materials of construction, stresses, failures due to aging and service wear, and measurable

functional indicators was not undertaken. Such work will be included in

Phase II of this investigation.

This review was conducted under the NRC's Nuclear Plant Aging Research

Program; it is a transition report that may be modified in the future. In particular, the positions relative to examination and testing and the suggested

changes in these requirements may be modified. Obvious interfaces exist with

the ANSI/ASME OM4 Committee on Snubbers and ASME XI, Section IWF. Comments and

suggestions that could impact the Phase II study are welcomed. There is a

recognized need to develop a viable interface among the NRC, the relevant codes

and standards, and the nuclear industry with regard to the scope of the

Phase II study. Currently, the scope of the Phase II study consists of: 1) a

comprehensive aging assessment of hydraulic and mechanical snubbers; 2) a

review and verification of inspection, surveillance, and monitoring methods;

and 3) establishment of application guidelines within appropriate codes, standards, and regulations.

The information presented in this report was obtained from many sources

over a period of several years. Reference notations are given where possible.

A bibliography is also provided.

iii
ABSTRACT

This report presents an overview of hydraulic and mechanical snubbers used

on nuclear piping systems and components, based on information from the literature and other sources. The functions and functional requirements of snubbers

are discussed. The real versus perceived need for snubbers is reviewed, based

primarily on studies conducted by a Pressure Vessel Research Committee. Tests

conducted to qualify snubbers, to accept them on a case-by-case basis, and to

establish their fitness for continued operation are reviewed.

This report had two primary purposes. The first was to assessxthe effects

of various aging mechanisms on snubber operation. The second was to determine

the efficacy of existing tests in determining the effects of aging and degradation mechanisms. These tests include breakaway force, drag force, velocity/

acceleration range for activation in tension or compression, release rates

within specified tension/compression limits, and restricted thermal movement.

The snubber operating experience was reviewed using licensee event reports and

other historical data for a period of more than 10 years. Data were statistically analyzed using arbitrary snubber populations. Value-impact was considered in terms of exposure to a radioactive environment for examination/

testing and the influence of lost snubber function and subsequent testing

program expansion on the costs and operation of a nuclear power plant. The

implications of the observed trends were assessed; recommendations include

modifying or improving examination and testing procedures to enhance snubber

reliability. Optimization of snubber populations by selective removal of

unnecessary snubbers was also considered.

v
CONTENTS

FOREWORD .; iii

ABSTRACT ..;,.; .......v

1.0 INTRODUCTION * * ; :0 1.1

20O BACKGROUND' INFORMATION ...... 2.1

- 2.1 WHAT IS'A'SNUBBER? ' 2.1

2.2-,TYPES OF SNUBBERS * '- 2.3

* 2.3 FUNCTIONAL REQUIREMENTS ....... ... 2.5

2.4 MATERIALS'AND CONSTRUCTION ........................... 0 -'- 2.9

, ... . .¶

2.5 'SNUBBER LOCATIONS ........ 0.0.0.0.000.*...0.... .; 0 00.. '0 .;; 2.12

3.0 TECHNICAL-SPECIFICATION REQUIREMENTS'INCLUDING REVIEWOF

RELEVANT STANDARDS AND GUIDES .......****...*.*.*....*.***.*... 3.1

3.1 SNUBBER DESIGN CRITERIA ... ...................00 0000*0.. ** 3.2

3.2 QUALIFICATION VERSUS ACCEPTANCE TESTING ..................... *. 3.6

3.3 TECHNICAL SPECIFICATIONS .. 00...................... 3.8

3.4 CURRENT PRESERVICE AND IN-SERVICE TESTING REQUIREMENTS

IN RELEVANT-CODE/REGULATORY'DOCUMENTS ............... 3.'...O 3.10

3.5 EXISTING AND'POTENTIAL'TESTING PROBLEMS--';.; .............. 3.11

3.6 SUGGESTED CHANGES IN'SNUBBER TESTING AND'EXAMINATION,' '

' ' PROCEDURES.;- ............. , 3.13

4.0 OPERATIONAL STRESSES '' 4.1

4.1 MECHANICAL ................ 4.1^

4.2' THERMAL .......... 4.1 , ; 

4.3-,CHEMICAL/ENVIRONMENTAL ............. ' 4.1 . ';

vii,

_l
4.4 RADIATION ........................... ...... ............... .... . 4.2

4.5 SYNERGISTIC EFFECTS .............. ................. ... 4.2

5.0 REVIEW OF LICENSEE EVENT REPORTS FROM 1970 TO 1983 .............. 5.1.

5.1 PIPING SYSTEMS ................................ *........... . 5.1

5.2 SNUBBER FAILURES ........... 1..... .. ** ... *.......... .. 5.1

5.3 FAILURE MECHANISMS .......... ... ... ........... ... ..... ...... 5.2

5.4 STATISTICAL ANALYSES OF FAILURE DATA ........................ 5.10

6.0. PERSONNEL EXPOSURE TRENDS ....... ...... .......................... 6.1

7.0, COSTS AND VALUE-IMPACT ..... ..................................... 7.1

7.1 PRIOR VALUE-IMPACT STUDIES ................................. 7.1

7.2 VALUE-IMPACT ANALYSIS ....................... s*............. 7.4

7.2.1 Snubber Unit. Costs .................................... 7.4

7.2.2 Reducing the Number of Snubbers ...................... 7.12

* 7.2.3 Increasing Snubber Reliability 7.14

8.0 IMPLICATIONS OF AGING AND DEGRADATION OF SNUBBERS ............... 8.1

9.0- PHASE II STUDY .................................................. 9.1

10.0. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS ....................... 10.1

11.0. BIBLIOGRAPHY ............................. 1.

APPENDIX A - CONTENTS OF A FUNCTIONAL SPECIFICATION FOR SNUBBERS

USED IN SYSTEMS IMPORTANT TO SAFETY ..... ................. A.1

APPENDIX B - QUALIFICATION OF FUNCTIONAL PARAMETERS FORSNUBBERS

USED'IN'SYSTEMS IMPORTANT TO SAFETY ..... ................. B.1

APPENDIX C - ACCEPTANCE TESTING CRITERIA FOR THE PRODUCTION OF

SNUBBERS USED IN SYSTEMS IMPORTANT TO SAFETY ............. C.1

APPENDIX D - WESTINGHOUSE STANDARDIZED TECHNICAL SPECIFICATIONS ....... D.1

viii
FIGURES

1.1 Snubber Reduction Procedure ......... ......... ........................ 1.4

2.1 Hydraulic Snubber Fully Extended Piston Rod ..................... 2.4

2.2 Reservoir of Hydraulic Snubber in Extended, Overextended,

and Retracted Positions ......... ......... .... * ** ........ .. 2.4

2.3 Acceleration-Controlled Mechanical Snubber - High Pitch

Lead Screw ........... ............................................ 2.6

2.4 Acceleration-Controlled Mechanical Snubber - Recirculating

Ball Assembly ...... *.... ....... 2.7

2.5 Velocity-Controlled Mechanical Snubbers ......................... 2.8

7.1 Sample Letter Sent to 62 Companies Requesting Snubber

Information .. ......... .......................................... 7.6

10.1 Nuclear Plant Aging Research Program Strategy ................... 10.2

ix
TABLES -

2.1 Subcomponents in Hydraulic Snubbers ............................. 2.10

2.2 Hydraulic Snubber Seals ......... ................................ 2.11

2.3 Materials Used for-Seals in Snubbers ............................ 2.11

2.4 Measurements to Characterize'Seal Properties .................... 2.12

3.1 Significant functional Parameters Pertinent to Snubbers ......... 3.12

3.2 Significant.Functional ParametersPertinent to Snubbers Cited

in Draft Regulatory Guide . ................... .. .... ............ . 3.13

3.3 Common Causes forDegradation in Snubber Operating

Characteristics ......................................................... 3.16

3.4 Types of Test Conditions ....... ............................. 3.19

5.1 Snubber Failures from 1973 through 1983 ......................... 5.2

5.2 Snubber Incidents by Plant and Year from LER Reviews ............ 5.3

5.3 Failure Causes and Mechanisms for Hydraulic Snubbers ............ 5.5

5.4 Failure Causes and Mechanisms for Predominately Mechanical

Snubbers ..................................................... 5.6

5.5 Failure Causes and Mechanisms forHydraulic and Mechanical

Snubbers ................................. 5.7

5.6 Failure Modes and Causes for Surry Power Station Unit 1

Snubbers ........ **...**.... *.................................. 5.11

5.7 Comparison of the Number of Incidents by Year for Three

Data Sets ....................................................... 5.12

5.8 Comparison of Snubber. Incidents by Plant for Three Data Sets .... 5.13

5.9 Failure Modes for Data Set A .................................... 5.15

5.10 Ultimate Failure Causes for Data:Set A, .......................... 5.15

5.11 Snubber Incidents byPlant and Year for Data Set A ............... ., 5.16

xi
5.12 Snubber Incidents by Plant and Year for Data Set B .............. 5.18

5.13 Snubber Failures by Year and Snubber Type for Data Set A ........ 5.19

5.14 Snubber Failures by Plant and Snubber Type for Data Set A ....... 5.20

5.15 Proportion of Failed Snubbers in Operating Nuclear Power

Plants for Data Set A ............................ ... 5.22

5.16 Snubber Failure Data by Plant for Data Set A .................... 5.22

5.17 Snubber Failure Data by Failure Mode for Data Set A .............. 5.23

5.18 Snubber Failure Data byFailure Cause for Data Set A ............ 5.23

5.19 Number and Proportion of Snubber Failures by Manufacturer

for Data Set A .................................................. 5.24

5.20 Snubber Failure Data by Year for Data Set A, Assuming a

Snubber Population ................ ......... ............. .......... 5.25

5.21 Snubber Failure Data by Plant for Data Set A, Assuming a

Snubber Population ............. ............................................. 5.26

5.22 Snubber Failure Data by Failure Mode for Data Set A,

Assuming a Snubber Population ................................... 5.28

5.23 Snubber Failure Data by Failure Cause for Data Set A,

Assuming a Snubber Population ................................... 5.29

6.1 Personnel Exposure Data ........ ................................. 6.1

6.2 Relevant Snubber Data in NUREG/CR-2136 ........................... 6.2

6.3 Exposure Values Cited by Country for BWRs in

Past 2 to 3 Years ................................................ 6.4

7.1 Value-Impact Data for Snubbers .................................. 7.3

7.2 Snubber Manufacturers Responding to Request for Information ...... 7.7

7.3 Manufacturer Data on- Snubbers ................. ........ .......... 7.8

7.4 Estimated Size Distribution of Snubbers ......................... 7.9

7.5- Summary of Snubber Costs Assuming a 40-Year Plant Life and

Discount Rates of 2%, 5%, and 10% ...... ........... . ......... 7.11

xii
7.6 Estimated Cost Savings Resulting from Reducing the Number

of Snubbers in an Existing Plant ................................ 7.13

7.7 Average Total Sample Size for Specified Mean Failure Rates ...... 7.14

7.8 Summary of Net Present Value of Snubber Costs for Various

Mean Failure Rates ..................................................... 7.14

xiii
1.0 INTRODUCTION

The first light-water reactors (LWRs) used few, if any, mechanical or'

hydraulic snubbers or pipe whip restraints. Most LWR piping designs were similar to fossil plants, which have'flexible piping systems.'' During the 1970s, an

increasing concern was expressed by'U.S. Atomic Energy Commission and U.S.

Nuclear Regulatory Commission'(NRC) regulatory personnel concerning'the

behavior of piping under severe seismic loads. Essentially concurrently, the

large 'pipe break, or'iginally proposed as a mechanism 'for sizing containments'

through back calc'ulation'of pressure loads, emerged as a design-basis accident.

'Appropriate measures' were required to'prevent" pipe whip. A logical extension

'of the'seismic and pipe break criteria was to consider that they occur

essentially'simul'tan'6ously. '

The piping failure mode proposed for severe seismic loads was plastic collapse. Although this failure mode had not been confirmed experimentally and

analytic validation was based on very conservative assumptions, it became

accepted that plastic collaps' would be the cohtrolIing pipe break mechanism.

A natural consequence'of this' failure'mode was a movement'toward progressively

'stiffer'piping systems'to "prevent" plastic collapse.

Several assumptions made during 'seismic modelinfg further increased the

-total number of supports on a piping.system. Examples include the manner of

bounding the seismic spectrum; assumptions .on'combi'ning'loads; the handling of

spectra broadening; and, in particular, the use of conservative values for

seismic damping. >The-se' assumption's led to more and more supports and snubbers

being-added t6 piping systems. Nuclear plants in the near-term operating

license stage may have more'than 1000 snubbers; plants licensed during the

1970s may fia' an 'average of about 500 snubbers.

;;As more snubbers were-'used, several operating problems arose;"for ekample,

degradation and leaking of seals 'on hydraulic 'snubbers 'and functional'failures

of both hydraulic and'mechanical snubbers'. These' poble'ms led to increased ' -

qualification and testing requirements. Thus, the original cost of a snubber

represents onlya''small'fraction of the'overall'c'ost''of qualification, installation,''mainten'ance,'and :testing. t'In addition, 'maintenance and testing' result

in substaitail'orndi atjoriexposure 'in older plants. ' ' ' I ' -- ' , ; i' X se .) ,-. Jt~t' 

Another problem that was not recognized initially was the limitation on

in-service inspection (ISI) resulting from the large numbers of snubbers and

supports that prevent access to many welds in piping systems. A further

problem was the concern that stiff piping systems may be inherently more

susceptible 1to overloading and possible. failurie than flexible systems. Obvious

examples 'include imp',rope'r snubberilnstalla'tion and premature activation, which

can result in severe'loads-during'h'heatup'~rid co'0ldown.-'; The possibility that
- I

ratcheting may be a more probable failure mode than plastic collapse is another

factor to be considered. In fact, it has been reported that a stiff piping

system failed during an earthquake while a flexible system in the same plant

did not.

As a result of the preceding concerns, Technical and Steering Committees

on Piping were organized under the Pressure Vessel Research Committee (PVRC)

with active industry and NRC participation. In the past.two years, these

groups have developed a more relaxed interim position on seismic damping,.a

modified and less conservative, position on spectra broadening, and a document

on industry practice 'related'to design approaches leading to fewer snubbers.

These.positions have been accepted. by the NRC on a case-by-case basis, and portions have been incorporated into Appendix N of ASME III (the reactor construction code). A task group on seismic design under an NRC Piping Review

Committee has recommended that the case-by-case status be converted to generic

positions. The NRC Executive Director for Operations has issued a directive to

develop, such generic positions as cited in NUREG-1061.

The implications of the preceding changes on a nuclear plant containing a

large number of snubbers (for example, 500 to 1000): are dramatic. If a new

design' analysis is conducted, the number of supports could be reduced by 25% to

50%. These numbers have been confirmed by sensitivity studies conducted at

Duke Power Co. and Lawrence Livermore National Laboratory (LLNL). Furthermore,

if the design suggestions presented in the industry practice document are utilized, the number of remaining snubbers could be further reduced.

A recent document reports the results of the sensitivity tests using the

PVRC-proposed damping values and spectra broadening.(a) Three piping systems

were considered: auxiliary feedwater system (in part), reactor heat removal/

safety injection system, and reactor coolant systems. The auxiliary feedwater

system at the Zion 1 nuclear plant was modeled in depth. The original auxiliary

feedwater system contained 25 vertical and horizontal supports or restraints

and.,two snubbers. Using the PVRC damping values would permit removal of both

snubbers and seven of the horizontal restraints, based on the analysis.

While snubbers can provide a valuable function wh-ere space is very

restricted, they often have been used instead of conducting a more sophisticated analysis. Somewhat belatedly, the implications of excessive snubbers in

terms of costs of original purchase, qualification, installation, maintenance,

(a)' Chuang,-T.,Y., et al. 1984. Impact of Changes in Damping and Spectrum'

Peak Broadening on the Seismic Response&of Piping Systems. NUREG/

CR-3526, Lawrence Livermore National Laboratory, Livermore, California.

1.2
and testing (which requires removal-and-reinstallation).are being recognized.

A further factor is the cumulative exposure incurred during the maintenance and

testing stages.

At a recent conference, several papers were presented pertaining to the

removal of snubbers using approaches such as that of,,T. -Y. Chuang et al. One

paper (Jimenez and Requena)(a) discussed a snubber optimization program based

on analyzing stress and damage with a computer code. The reanalysis permitted

removal of 95% of the snubbers and 25% of the supports from an intermediate

energy line and 85% snubbers and 17% supports from a high energy line.

A Babcock and Wilcox study(b) used a multiple response spectra approach

with 0.5% and 5% (PVRC) damping on a high-pressure spray line. The analytic

approach reduced stresses by factors of 2 to 3 compared with an enveloped

spectra. By evaluating primary and secondary stresses, the number of snubbers

on the system was reduced from 15 to 3 with 3 snubbers replaced by link-bars.

An Impell study(c) made several suggestions, including a change in Equation 9 of ASME III. Figure 1.1, which was taken from this study,(C) illustrates the snubber reduction procedure that has many aspects in common with

similar snubber reductions. Their best case--which combines PVRC damping,

multiple level response spectra, peak shifting, direct'generation of response

spectra, and fracture mechanics to justify' leak-before-break--reduced "snubbers

on steam generator blowdown line to 1."

The PVRC actions cited in previous paragraphs'have not been described in

depth. The interested reader is referred to WRC Bulletin 300, which discusses

positions concerning damping, spectral broadening,-and,industry practices.

This information provides the basis for comments presented in later sections

concerning reducing the number of snubbers.

(a) Jimenez, P., and A.'Requena. 1985." "Snubber Optimization Program."

Presented'at ASME Pressure Vessel and Piping'Conference, June 24-27, 1985,

-New Orleans, Louisiana.i

(b) Tuttle, E. B., and A. D.-'DuBose. _1985. "Snubber Optimization Using

Multiple Response Spectra and Increased Damping Techniques." 'Presented at

ASME Pressure Vessel and Piping Conference, June 24-27, 1985,' New Orleans,

Louisiana.

(c) Eidinger, J., and G. Hau. 1985. "Snubber Reduction-by Reduction of

Unnecessary:Seismic Margins." -Presented at-ASME Pressure Vessel and

Piping Conference, June 24-27,'1985,'New Orleans, Louisiana.

1.3
Step 1

Analysis Techniques

* Direct Generation of Response

Spectra

. Primary-Secondary Interaction

* PVRC Damping

* Spectrum Peak Shifting

* Multiple Level Response

Spectra Analysis

* Break Postulation

* Thermal Parameters

* Support Optimization

* Accurate Mode and Level

Co'mbinations

For Remaining

Snubbers

Step 2

Snubber Testing

Acceptance Criteria

. Drag Force

* Acceleration Threshold

For Failed

Snubbers

Step 3

Pipe Functionality

Stress Limits [ . Strain Limits.

Snubber Reduction

More Snubbers

-01.- Pass the Test

Justification for

Interim Operation

i

FIGURE 1.1. Snubber Reduction Procedure

While the purpose of this Pacific Northwest Laboratory (PNL)(a) review was

to consider the implications of aging and service wear of snubbers, an essential lead-in is to establish the need for the large number of snubbers now in

use, together with the implications of errors in design and installation. In

this report, available data on snubber behavior and operating experience are

reviewed, together with the current and proposed status of surveillance and

testing. Failure modes and mechanisms are also reviewed to assess the overall

contribution of aging in contrast to other failure modes.

(a) Operated for the U.S.Department of Energy.(DOE) by Battelle Memorial

Institute under Contract DE-AC06-76RL0 1830.

1.4
-. 2.0 rc:BACKGROUN4D-INFORMATION-; ;.I- * 1 r; -

2.1 WHAT IS A SNUBBER? . .-- , .

If one queries an engineering-based computer. information-system, the word

"snubber" somewhat surprisingly appears only when listed in titles or key

words. ' It is not-used as an-entry term in.the conventional-thesauruses used to

search the literature. ... ,,,.

- Snubbers of the type;.discussed-in this-report.have two.functions:. .1).they.

should.move freely.at low, accelerations and,2) they should l'ck'up at higher .

accelerations. The valid use of snubbers.is in locations of limited clearance'.

and possible high thermal expansion. Unfortunately, designers have often

specified snubbers instead of validatinga method that could permit the use of

conventional-supports. ,, , . ,

-Snubbers are available in awide range of sizes from~very small units on

smaller lines to units weighing.thousands ,of, pounds used on steam.generators.. I

Two types of-snubbers are in use. The hydraulic type depends on the flow of a

fluid through a yalve.or orifice until the valve closes or.the flow is choked

in the orifice. The mechanical type is a device where movement of a piston A

leads to rotation of a screw or a roller in a screw. Again, rapid movement

results in activation.

While essentially all snubbers.on LWRsystems~fall.into the.above categories,;there are other,.snubber,.deviceswith no moving parts. Examples include

massive blocksof an elastom'eric material.used to handle structural movement.

Passive devices have been developed containing no moving parts that absorb high

levels of..energy if there is substantial piping displacement; however, these

devices are not used commercially. ," . ., ., ., .,,

Recent emphasis on maximizing the reliability of piping systems,' the escalating require'ments for snubb'er-ISI-, and the field problems that some snubbers

have recently exhibited have increased the importance of minimizing the number

of dynamic'load restraints.' Ther eare. definite economic and reliabilitygains

associated with limiting the use lof these devices.'The' first and most obvious

is the' elimin'ation ofithe cost" -fthe harware ahid its installation,'particularly if the 'dyinamic load8restrainttis"a s'nubbbr. , Be'cause the' snubber is a

mechanical or hydraulic device', initallation procedures mu'st cons'ider the'

impact on operability. Manipulation or handling of the device is more difficult than for' a stahdardIrigid-ty'esupport'.' That is, the 'insitallationn',nmust

allow for travel of the pisst-onr, Ympaking it'necessary io'-control en -to-en&

dimensions. Hydraulic 'nubb6er'smust b'*inst lled with'the reservoir' in', the

proper position; 'and -piston rods must'be fre'of paint, nicks, and weld -

spatter. An 'additional and"significant-problem ass'ociate'd with snubbers is the

2.1
requirement for periodic inspection (usually-visual). Present regulations for

testing hydraulic snubbers require removing the device from its installed location, transporting it to a test fixture elsewhere in the plant, running the

test, and (if acceptable) reinstalling it. The potential for damage to occur

as a result of this additional handling is substantial if the snubber is

removed." In situ testing is also an alternative under the regulations.

The functional test is only a small part of the overall cost. The greater:

economic impact is associated with the removal and subsequent reinstallation'of

the snubber, the radiation exposure of personnel, and the very real possibility

of extended plant outages as statistical test sample sizes are increased due to

test failures. 'It is, therefore, critical that the analyst recognize these

factors and make-every effort to specify as few dynamic load restraints that

will satisfy pipe stress and equipment load criteria.,

Qualitatively, reliability as related to snubbers is simple: a system

without these devices is more reliable than a system with them. Anytime a

mechanical or hydraulic device must be counted on to function, reliability'is

reduced. Nonetheless, it must be recognized that some snubbers are needed so

that piping systems in confined spaces can be designed to be flexible enough to

absorb thermal expansion loads and, at the same time, be rigid enough to withstand the dynamic loads imposed on them.

In summary:

* The snubber is a device that relies on mechanical or hydraulic mecha--'

nisms to function.' It'is expensive to purchase, critical to install,-

-and requires inspection and testing for'the life of the plant.

* Snubbers accommodate the dynamic loads':imposed on the piping system

while allowing for free thermal motion during normal operation.

Without such devices, it would be difficult, if not impossible, to

design for' the myriad of both real and postulated loading events.

* Due to schedule pressures, the initial reaction is to develop a system geometry that provides thermal flexibility and then to use

snubbers to accommodate the dynamic load events. However, additional

time and effort on-the part.ofthe analyst could provide the same

system protection while limiting the use of snubbers.

,,X( f,., 1 , .4 , ................... .. ........

* Several organizations have reported that a typical 1100-MWe capacity

b"oiling-water reactor (BNR) can have'9,000-to 10,000 supports on

seismic Category-I piping (as many as'800 spring hangers and 1500

snubbers).' AnA1100-MWe pressurized-water reactor- (PWR) could have

7,000 to 10,000 'supports (200 'spring hangers and 950 snubbers).

2.2
* Engineering, fabrication, construction','and hardware costs will be

lower with fewer supports; however, when-the cost~of items such as

analysis, computer time, and reconciliation are considered, the cost

difference in the two approaches mayinot be significant. The difference is heavily influenced by two fact6rs: 1) the installed cost per

support and 2) the total life (40 years) cost-per-support. The

second factor refers to inspection and maintenance:;costs associated

awith snubbers--typically $5,000 td $10,000 per snubber on small-bore

'piping. ' ' - > I

* The spring sway brace (a-standard .component..support).should be-considered for controllin'g -vibration-if high amplitudes are observed or

expected. Snubbers are not 'reco'mmendekd'for'controlling vibration.

* In the analysis process, supports'and-restraints-are requiret

d to protect the system against various typees of loading (fr6m as simpl'e' as-''

the weight of the piping toza's complex'as*th'e dynamic loads asso-;

ciated with water/steam hammer).'2 The'vselectin 5f'the typ'e'_'and location of -supports and restraints controls a number of items other'than'

the -acceptability of the piping. For example, the use of snubbers

has-*an impact on-the inspection and;examination requirements for the'- '

life of the plant. ' .

2.2 TYPES OF SNUBBERS

The two basic -typesiof snubbers are~hydraulic and mechanical.- The hydraulic snubber consists of a double-acting cylinder, a flow cortrol-device, and, in

most cases, a hydraulic fluid reservoir (Figure 2.1). The reservoir of a

hydraulic snubber in the extended,' overextended, and retracted positions is

shown in Figure 2.2. An alternative to the external reservoir-type snubber is

one using an orifice with high-fluid resistance within the piston where-fluid

does not pass'through external'hydrailic controls -or reservoirs. This'design is

simpler.and more compactly 'constructed than the design with-external reservoirs.

In the inactive mode, free the'rmal expansioni'occurs by. the passage-of'-'''--

hydraulic fluid from one side of the piston to the other. In-the activated

mode, flow of fluid is restricted, thus limiting piston motion. Release rate

velocities after activation of'hyd duliicfsniibbers ire controlled by bypass circuits or through inherent leakage'inithe' flow control-device. An external

reservoir serves a variety of purposes such as accommodating the thermal expansion of the fluid and volumetric changes'dudring'snubber motion and providing

reserve fluid.

When applied to the snubber piston,,any load from the pipe or other component will cause fluid to flow through thecontrol device.- These-pistons may
I . I I J

a;

Reservoir Piston

Packing

Overfill Protection Vent

; (Weep Hole),

Reservoir Indicator Rod

Piston /

.'r 1 : '.! /.. ,'.

RET =Minimum Rod Position

| EXT = Maximum Rod Position

xMaximum

LD Rod

"A" Position

Di Maximum Stroke

FIGURE 2.1. .. .

Hydraulic Snubber Fully Extended Piston Rod

 I

- : .- r.f- j.

A -

I - . .i I;., . I . I . 1 Reserve Displacement

Abnormal *

Fully Extended'

: n4 ,; , .

Normal

Fully Extended

Normal

Fully Retracted

,. I> -_ . ! _ I, . . I

FIGURE .2.. Reservoir of Hydraulic Snubber in Extended,

-Overextended, and Retracted Positions

2.4
be either single-ended or double-ended. While the single-ended design is more

compact, it requires more careful bearing and seal design; and these components

have often been a source of problems. Also, the relative behavior in tension

may differ from that under compression. The/obvious-advantages'of the doubleended design include more stable bearing' and seal loads. A disadvantage is

that the overall assembly will be longer. ;

Hydraulic snubbers are velocity controlled. Acceleration control is more

common in mechanical snubbers. Figures 2.3 and 2.4 illustrate the more usual

mechanical snubber. Mechanical snubbers'~usually consist of a motion-sensing

device, an activating rod, and a braking mechanism. Braking devices may be

included to limit mIotions of the'movable rod end.: Such-snubbers are activated

by 1) a progressive increase in load'(force) withithe increase of rod motions

through a gear mechanical escapement or 2)<a braking action applied on a rotating mass threaded to the active rod. 1Alternative approaches employ friction,

wedging, or spring action for activation.'

The snubber illustrated in Figure 2.'3.,utilizes-a brake on a rotating mass

and has a motion sensor that responds to 6changes in the rate of angular rotation of the mass. Application of a dynamic.fbrce or displacement activates an

internal brake or clutch and "locks" the ,snubber., Figure 2.4 illustrates a

snubber that uses a recirculating ball assembly to convert the linear motion of

the piston rod to rotation in the braking mechanism.

An alternative to the acceleration-controlled mechanical snubber is one

that is velocity controlled (Figure 2.5).' 'Displacement occurs at a specified

constant velocity and activation occurs`'at'higher values. An oscillating verge

(specially shaped cam) with connecting gearing limits the velocity of the gear

rack, thus controlling the rate of linear. displacement of piping or components.

During normal thermal transients, the ;rack is fr'ee to displace with relatively

small resistance; however, a dynamiciload results in oscillation of-the verge,

which limits movement of the gear track to a velocity proportional to the

applied load. On-line experience with this specific type of snubber,appears to

be limited.

2.3 FUNCTIONAL REQUIREMENTS

Snubbers, whether hydraulic or mechanical, have two principal purposes:

1) they should move freely to accom'mo'date thermal movement of piping and

2) they should lock up if piping-orcomponentsaire-'subjected to rapid movement

and high amplitudes caused by a seismic event or dynamic loads such as water

hammer. The handling of dynamic'loads at accelerations and magnitudes greater

2.5
Rod and Bearing Spherical Bearing

Guide Plate.

Torsion Rods

Screw Shaft

Shaft Nutg Pivot and Ball

-- Outer Tube

Inner Tube

Retaining Ring

Capstan Spring. -Clutch Spring

FIGURE 2.3. Acceleration-Controlled Mechanical Snubber - High Pitch Lead Screw

2.6
Telescoping CylinderRecirculating Ball,

Assembly

.B . a r

Ball Bearing __ .

End Plug

Assembly

osition Indicator Tube

- Support Cylinder

Precision Lead Screw

Spring

Torque Drum.

.)

Inertia Mass

Dust Cover

FIGURE 2.4.

. ~~~~ 1 . .. ~~. 

Acceleration-Controlled Mechanical Snubber --Recirculating Ball

Assembly

.. . . .,:! . . ,; .. , . %. I,

2.7
End

Connector

End

Connec

FIGURE 2.5. Velocity-Controlled Mechanical Snubbers

2.8
than those typically generated by 'seismic-events is desired but may not be met

because of damage to the snubbers, particularly the mechanical type. There

have been s-everal instances where severe-snubber damage-and lockup occurred on

a system exposed to a water hammer.

These functional requirements are based on' premise that may not be

valid; namely, that piping will fail in a collapsing mode when exposed to dynamic seismic loads. Actual experience in piping systems'under seismic loads has

not confirmed such a failure mechanism. In fact, there have been very few

piping failures during seismic events, and these failures have usually occurred

because the piping was tied to a structure that failed. The general failure

mechanism has been challenged and there may be a movement to return to a degree

to the flexible systems typical of fossil fuel plants and early nuclear power

plants. An extreme case might be the reanalysis of piping systems followed by

the removal of.a~substantial number of snubbers. ,

2.4 MATERIALS AND CONSTRUCTION

Although hydraulic and mechanical snubbers are relatively complex mechanisms, the materials used are not exotic. The critical component in the

hydraulic snubber is the plastic seal. The current material-is a polyethylene

that has proved satisfactory. -

Tables 2.1 through 2.4 present information on the various components of

hydraulic snubbers. Various control mechanisms, reservoirs,, cylinders, and

seal materials are used (Table 2.1). Tables 2.2 and 2.3 illustrate where seal

materials are used and their associated problems and limitations. Table 2.4

lists measurements that are-used to characterize seal properties.' Seal materials are subject to the following problems and should be selected carefully:

* extrusion (minimize with backup rings)

* nibbling,-wear, embrittlement, or roll -

* installation error

i r -. :i f -- ' .- :.

* poor manufactured finish or improper groove depth (manufacturing'

error) - --. a ;

- design/application problems: fluid incompatibility;'temperature

incompatibility; radiation incompatibility; steam/moisture incompatibility; vibration.

I 2. '9
TABLE 2.1. Subcomponents in Hydraulic Snubbers

Item

Control Valve:

Activation/bleed

Bleed orifice in poppet

Fixed orifice

Temperature compensating

Reservoir:

Nonpressurized (vented)

Pressurized

Remote

None

Equi pment Piping

a,b,c,d,e,f

i

b

a,c

i

a,b,g,h

c

b,h

a,c,g

Cylinders:

Single-ended

Double-ended

Tie rod

Screwed end

piston rod

piston rod

a,h,c,f

d,e,i

a,b,h

a,c

Seal Locations:

Piston rod

Rod wiper

Cylinder end

Piston

Valves

Reservoir

Fill plugs

Seal s:

Viton

Special

Polypak (EP)

Tee (EP)

EP

Tefzel

Metal I ic

d,e

i

a

b,f

c

c

a

9

b,h

C

c

(a) Bergen-Paterson Pipe Support Corp.

(b) ITT-Grinnell Corp.

(c) Paul-Munroe Energy Products

(d) Milwaukee

(e) Anchor-Holth

(f) Rexnord.

(g) EP Systems

(h) Basic Engineering

(i) Taylor Devices

2.10
TABLE 2.2. Hydraulic Snubber Seals

Item/Factor

Loading: Dynamic

Static

Piston

X

Rod

X

-Reservoir

Plunger

X

Location

Rod -Cylinder Plugs/

Wiper End Valves

x -

Threaded

Seals . . I I . .

X X X

Seal Packing

Material/Type:

Polypak (EP)(a)

Miller

Vee (Chevron)

O-ring

Tee

U-cup

Gasket

Tefzel

X X.

X.

X

X

X

X

X

X

X

X

X X

(a) Ethylene propylene.

- I - TABLE 2.3

M e a f r , e !,

Materials.Used for:Seals in,Snubbers

Material

Elastomers:

Polyurethane

Nitrile-rubber

(Buna-N)

Viton (fluoro- elastomer)

Advantages Disadvantages

Poor silicone fluid

compatibility.

Poor elevated temperature

properties.

Good elevated temperature- Poor steam resistances.

properties. Resists'aging.-'Moderate radiation

resistance.

Ethylene propylene

(EP, EPDM)

Good radiation resistance.

Good fluid compatibility.

Poor resistance to petroleum

solvents. Moderate temperature aging resistance.

Thermoplastics:

Tefzel

Inelastic; springs required.

Metallics: O-rings Limited springback.

2.11
TABLE 2.4. Measurements to Characterize Seal Properties

Property Measurement

Hardness Extrusion resistance (typically 70 to 90 Durometer)

Tensile strength(a) Embrittlement

Elongation(a) Typically 300% elongation at 2000 psi

Modulus: tensile

compression Modulus of elasticity

Compression set Low pressure sealing force; constant stress

(400 psi at 1500F for 30 min); constant strain

environment for environmental effects

(a) At room temperature.

2.5 SNUBBER LOCATIONS

Snubber locations vary substantially with the age of the plant. Typically

snubbers are used on large components such as steam generators, valves, pumps,

and pressurizers and on safety-related piping systems. Unrealistic design

approaches have resulted-in snubbers being used to control vibration, which is

a poor practice. Snubbers have been located at artificial benchmarks resulting

from computer analysis and boundary limits set by the computer. Modifications

in seismic spectra criteria have greatly increased the numbers of snubbers to:

where they may be located 10 to 20 ft apart on piping systems. This practice

is an outgrowth of the concept that a stiff piping system will not fail where a

flexible system will. Many snubbers are virtually inaccessible; they may be

high above floor level or within a maze of piping. Snubbers are distributed

throughout a plant on Class 1, 2, or 3 piping systems. There are no definitive

ground rules that permit the prediction of where snubbers will be in a specific

plant.

2.12
3.0 TECHNICAL SPECIFICATION REQUIREMENTS INCLUDING REVIEW OF

RELEVANT STANDARDS AND GUIDES

Snubber'testing can be divided into three categories:-- 1) qualification.,

testing, 2) acceptance testing, and 3) preservice and in-service testing.

These three categories have been dealt with to a greater or lesser degree in a

spectrum of documents.

Qualification testing is performed on a sample'of snubber units''odfeach

size, type, and manufacturer to demonstrate compliance with design requirements. The draft Regulatory Guide Qualification and Acceptance Tests for-Snubbers Used in Systems Important to Safety deals primarily with qualification

testing, acceptance testing,"and functional specification.(a) The contents of

this guide will be discussed more extensively later in this section; and

Appendices A, B, and C of' the draft guide are attached to this report.

Acceptance testing is briefly discussed in ASME pVp-45.(b) However, no

other source with the exception of the draft Regulatory Guide covers acceptance

testing. Preseryice andjin-service testing has been addressed in ASME XI and

ANSI/ASME 0M4. c Plant-specific or standardized technical specifications also

deal explicitly with this area.

The design criteria for snubbers were reviewed to determine what they need

to provide and to use these criteria as benchmarks to measure the adequacy of

qualification, acceptance, or in-service testing. Obvious sources of information were ASME III, Section NF, and the NRC Standard Review Plan,

Section 3.9.3.(d)

(a) U.S. Nuclea'r Regulatory Commission. 'February 1981.' Qualification and

Acceptance Tests for Snubbers Us'ed in Systems Important to-Safety. Draft

Regulatory Guide SC 708-4, Rev. 1,' Washington, -D.C.-

(b) American Society of Mechanical Engineers.' -1980. Criteria for Nuclear

Safety Related Piping and Component Support Snubbers. PVP-45, presented

at the Pressure.Vessel.:and Piping Conference,;ASME Century.2 - Emerging.

Technology Conferences, San Francisco, California.

(c) American Society of Mechanical Engineers. 1982. Examination and

Performance Testing of Nuclear Power Plant Dynamic Restraints (Snubbers).

ANSI/ASME OM4, New York.

(d) U.S. Nuclear Regulatory Commission. July 1981. USNRC Standard Review

Plan for 3.9.3, ASME. Code Class 1, 2, and 3 Components, Component

Supports, and Core Support Structures. NUREG-0800, Washington, D.C.

3.1
3.1 SNUBBER DESIGN CRITERIA

Section NF-3000 of ASME III covers design rules for supports. Specifically, NF-3411.3 covers concurrently acting loads stipulated in the design

specifications. For example:

* transmitted loads including dynamic loadings

* structural interaction of intervening element with the component and

component supports

* support load path material requirements

* temperature effects or other environmental effects

* design, fabrication, examination, testing, and installation

requi rements

- documentation requirements

* connecting requirements of intervening element to building structure.

Section NF-3411.3 (c) gives explicit instructions for snubbers; namely,

that they must meet:

(1) the following occasional loads

(a) seismic inertia

(b) seismic anchor displacement

(c) hydraulic transient loads resulting from but not limited to

water hammer, steam hammer, pump startup, pump shutdown, safety

and safety relief valve discharges as specified in the design

operations

(2) thermal expansion and thermal anchor displacement - The snubber shall

not resist the effects of thermal growth of the component, the piping

system, and the anchorage to the degree where it imposes a significant load or stress on the piping or component.

Section NF-3412.4 deals explicitly with snubbers. In addition to the

design loadings cited previously, the design specification should contain as a

minimum:

3.2
(1) the required force, time, and'displacement relationships

(2) the environmental conditions'that the snubb'ers- will be expo'sed to

such as temperature, irradiation, corrosive atmosphere, moisture', and

airborne particles -''

(3) consideration of material characteristics,' such as'compatibility,

stability, fire resistance, wear, and aging

(4) tests that are required-prioi to'installation; design'of functional

members such 'as interconnections, tubin'g and fittings, reservoirs,

and flow distributors 'shall con'sider''the-effect'of internal pressure,

thermal expansion, and vibration loading.''

Section 3.9.3 of the NRC Standard Review'Plan cites'the criteria applied

by the NRC for snubbers. SubsectionHII.3b'1-7 deals'specifically with snubbers; that section is quoted below:

(b) Where snubbers are utilized as supports for safety-related

systems and components, acceptable criteria for snubber

operability assurance should contain the following

elements:

(1) Structural Analysis''and 'Systems 'EvaluationSystems and components which utilize'snubbers as shock and

vibration arrestors must be analyzed to'a'scertain th'e interaction of such' devices with'the systems0and components''to which

'they'are attached.'- Snubbers may be'used as shock and'vibration

arrestors and in:some instances' as'X'dual' purpose snubbers.' When

used as' a'vibration-arrestor or dual purpose snubbers, fatigue

:strength must be considered. :Important' factors' in the fatigue

evaluation include:' (i) unsupported system 'component movement

or amplitude, (ii) force-imparted to'snubber and corresponding

reaction on system or component due to restricting motion

(damped amplitude), (iii) vibration frequency or numbe'r of-lead

cycles, and (iv) verification of system or component and snubber

fatigue -strength.'- '1; -- -

Snubbers used -as shock rarrestors do -not 'require fatigue

-evaluation if it 'can be "demonstrated that!(i) the 'umbe'r of load

cycles which -the isnubberj-will ,experi'ence 'during normal plant

,operating conditions is small '(<2500)"or (ii) motion during

normal plant'operating conditions does 'not exceed snubber -dead

'band. ' , I ' ' - . ; . ;. -

3.3
Snubbers utilized in systems or components which 'may

experience high thermal growth rates either during normal

operating conditions or as a result of anticipated transients

should be checked.to assure that such thermal growth rates do

not exceed the snubber lock-up velocity.

(2) Characterization of Mechanical Properties

A most important aspect of the structural analysis is realistic characterization of snubber mechanical properties (i.e.,

spring rates):-in the analytical model. Since the "effective".

stiffness of a snubber:jis generally greater than that for the

snubber support assembly (i.e.,rthe snubber plus clamp, transi-.

tion tube extension, back-up support structure, etc.) the

snubber response characteristics.may. be "washed out" by the

added.flexibility in.the support structure. The combined effective stiffness of the snubber and support assembly must, therefore, be considered in evaluating the structural response of the

system or component.

Snubber spring rate should be determined independent of

clearance/lost motion, activation level, or release rate. The

stiffness should be based on structural and hydraulic compliance

only, and should consider the effects of temperature.

..The snubber end fitting clearance and lost motion must be

minimized and should be considered when calculating snubber

reaction. loads: and stress: whichare. based ona. linear analysis

of. the system or component. Thisjis especially important in

*multiple snubber applications where mismatch of end fitting

clearance has.a greater effect on.,the, load sharing of these

snubbers than doesthe mismatch of activation level or release

rate. Equal load sharing of multiple snubber supports should

not be assumed if mismatches .in end fitting clearance exist.

(3) Design Specifications

The required structural and mechanical performance of snubbers is determined from the user's system analysis described in

(1) and (2). The:,snubber Design Specification is the instrument

provided by. the purchaser to.the.supplier to assure-that the

requirements are met., The Design Specification-should contain

(i). the general functional requirements, (ii) operating environment,:.(iii) applicable codes and standards, (iv) materials of

construction and standards for hydraulic fluids and lubricants,

3.4
(v) environmental,.structural , and'performance design.ver.ifica-

*tion tests, (vi).production unit functional -verification tests

and certification;-(vii!) packaging,- shipping,5.handling, and

storage requirements, and (viii) description of provisions for

-attachments and installation.;;

'In addition;,the snubber manufacturer should be requested

to submit his quality-assurance and assembly quality control

procedures for review-and acceptance by the purchaser..

(4) Installation and Operability-Verification - ."-

.. Assurance that all snubbers are properly-installed prior to

preoperational.piping vibration and plant-startup.tests should -

be provided. Visual observation of piping'systems and measure-,

ment.-of thermal :movements during.plant ,start-up~tests could

verify that snubbers.are operable (not lockedtup). Provisions

for such ,examinations'and measurements :shouldbe discussed in-;:'.

the~piping preoperational vibration and plant.startup test pro--

grams-as described in:SRP.Section'3.9.2.-

(5) Use of Additional Snubbers

.' Snubbers could in-some'instances be.-installed'during or

after plant construction-which'may notihave'been~'inclu'ded ins-the;;

designranalysis.c This;-could:occur as.a result of unanticipated

piping vibration as discussed'in SRP-Section.3.9.2 or-inter-fer-..-'

ence problems during construction. -The:effects of such inst'al---_'.

lation should be fully evaluated and documented to demonstrate

that normal plant operations and safety'lare not diminished.. c

(6) lInspectidn land'Testing'

In'serVice inspection ard teting aret'crittical elementsof

operability tassurancerprograms-for' mchanical c'omrponents. The

applicant should provide a discussion'oft'accesssibility p'rovisions for maintenance, in-service inspection and testing, and

-possible'repair or replacement-of'snubb'ers"'consiste'nt withbthet

requirements of the NRC.Standard Technical Specification s.'

(7) Classification and Identification

All safety-related components which utilize snubbers in

their support systems should be identified..and-.tabulated in the;,

-FSAR. The tabulation.sho6ld include' he:.following information:.

3.5
(i) identification of the systems and components in those systems which utilize snubbers, (ii) the number of snubbers utilized in each system and on components in that system, (iii) the

type(s) of snubber' (hydraulic or mechanical) and the corresponding supplier identified, (iv) specify whether the snubber was

constructed to the rules of ASME Code Section III, Subsection

NF, (v) state whether the snubber is used as a shock, vibration,

or dual purpose snubber, and (vi) for snubbers identified as

either dual purpose or vibration arrestor type, indicate if both

snubber and component were evaluated for fatigue strength.

In addition, Subsection III.3 touches on operability, namely:

The reviewer should be assured that the applicant's PSAR contains discussions and commitments to develop and utilize a snubber

operabilityfassurance program containing the'elements specified in

paragraphs (1) through (6) of subsection II.3.b of-the SRP section.

A commitment to provide in the FSAR the information specified in

paragraph 71 of subsection II.3.b of this SRP section is sufficient

for the CP review stage. During the Operating License review the

FSAR should contain summaries in sufficient detail to verify the PSAR

commitments.

The preceding quoted material states rather explicit criteria to be met;

however, snubbers-often do not-meet the cited criteria. For example, many

mechanical snubbers fail to handle vibrations-or severe dynamic loads such as

water hammer. (Tables 5.4 and 5.5 in Section 5-present data on dynamic

loads.- Extensive work related to the Fast Flux Test Facility has established

mechanical snubber degradation because of vibrations.)

3.2 QUALIFICATION VERSUS ACCEPTANCE TESTING

The draft Regulatory Guide and ASME PVP-45 deal with qualification testing, and the same requirements can be applied to acceptance testing. In addition, DOE internal standardNE E7-9T(a) leans heavily on the draft Regulatory

Guide. The draft. Regulatory Guide recommends that six functional parameters

used in the design of systems .be measured:

* activation level ->The axial velocity or acceleration that causes the

snubber to convert.to the restraint mode.

(a) U.S. Departmentfof Energy. September 1984. Mechanical and Hydraulic

Snubbers for'Nuclear Applications. NE E7-9T, Washington, D.C.

3.6
- breakaway force -'The minimum'applied'force required tobbegin exten-:

sion or retraction'of the snubber>!,'

* dead band - The free (or nearly free) axial movement of the snubber,

between the two activations of opposite directions. This is considered to include theieffects of clearance'.at the snubber and connections as well as effects internal to the snubber' design. (

* drag force - The force required-to.maintain snubber movement at a

specified constant velocity prior to 'activation.."

* release rate.- The axial velocityi or acceleration of snubber..movement

under. a specific load after activation. ,J.,.; , . _

* spring rate -A linear approximation of the force-Zdisplacement

relationship.

The regulatory position in the draft Regulatory Guide recommends-the

following: s c at i confor t Appendix A o t af

*a functional specification in conformance to-Appendix:.A of the draft . i; . C

KegUIdLUVY uuiue - .  I r 11.

* construction to ,ASME III, Section. NF.

* establishment of the compatibility of material not covered in:

Section NF (for example, seals, lubricant)

-designs should consider the recommendations of Appendix B of the<:

draft Regulatory Guide I

* All snubbers should be accepted from the production line only after

meeting Appendix C requirements.

,1.~! I J I e * * . i-

.1 , i i (-, - I . J l: 

!  I I I Id C i - : -f- 11

I I I I

3;7
Meeting the requirements of.Appendices A,.B, and C also assures that both qualification and acceptance requirements will be met. Because of their importance, Appendices A, B, and C of the draft Regulatory Guide are attached to

this report. ,;-

Qualification testing as cited i9 the draft Regulatory Guide is discussed

in a number of papers in ASME.,PVP-42. a) A variety of qualification tests are

discussed; some are comparable to the draft Regulatory Guide.

ASME PVP-45 covers much of same material as ASMEIII, Section NF, and the

draft Regulatory Guide. The design criteria are cited, and several possible or

suggested criteria dealing with snubber installation and testing are discussed.

The document covers IS1as well.' Appendix A of ASME PVP-45 covers the parameters to be considered in a snubber design specification; these parameters are

discussed in substantially greater detail than in ASME III, Section NF.

3.3 TECHNICAL SPECIFICATIONS

The time period of this report is a time of transition for technical -

specifications. There is a conscious effort by the NRC to achieve uniformity

by removing' the diff rences that exist' from plant, to plant. For example,

Generic Letter 84-13?b) provides a suggested format for 3/4.7.9 snubbers.

Newer technical specifications (both plant-specific and standardized) follow

the suggested format closely; Palo Verde and Westinghouse's standardized

technical specifications are cited in the references. The Westinghouse

specifications are shown in-Appendix D.

Section 4.7.9(b) provides the following criteria for modifying the inspection period as a function of the number of inoperable snubbers detected during

the usual inspections:

(a) American Society of Mechanical Engineers. 1980. Component Support

Snubbers - Design, Application and Testing. PVP-42, presented at

the Pressure Vessel and Piping Conference, ASME Century 2 - Emerging

Technology Conference, San Francisco,' California.

(b) U.S. Nuclear Regulatory Commission. May 3, 1984. Technical

Specifications for Snubbers. Generic Letter 84-13.

3.8
No. of Inoperable Snubbers of Each Type .Subsequent Usual

(on any system) per Inspection:Period Inspection.Period(a)

0 18 months +25%

1 12 months ±25%

2 . -6 months ±25%

3, 4 <.124 days+±25%

5, 6, 7 62 days-±25%

8 or-more 31 days ±25%

(a) The inspection intervalifor each.type of snubber.(on a -given system) shall not!be lengthened more than one step

at a time.unless a generic problem has been identified

and.corrected; in that event,.the inspection interval

may be lengthened one stepithe first time and two steps

thereafter if no inoperable snubbers of that type are

found (on that system).

The technical specification establishes visualinspection criteria

requirements after potentially damaging transients..and explicit requirements

for functional tests.- These inspections-are conduicted-at 18-month intervals

(see Appendix D for sample size options). Both failure-and acceptance criteria

are included.

The technical specifications in Appendix D under "Functional Test Acceptance Criteria" and "Snubber Service'Life' Proga'm"are similar to the next section on in-service testing. The first cites activation, bleed, or release

rates and force to initiate ormmaintain motiont'(ft'.mechanical snubbers), all

within a given range under tension anid compression. `For snubbers that are not

required to displace under continuous load, the ability to withstand load without displacement must be established. * ;

With regard to snubber seals, their service life is to be monitored to

ensure that they do not exceed the permissible limits between surveillance

inspections. They-should be replaced if'thelimits will be exceeded. The same

is true for springs, etc"

In October 1984, Region II cited inconsistencies in snubber technical

specifications from plant to~plant andirecommended changes. Generally, the

recommendations would-upgrade requirements on older plants.- .Perhaps the most

significant change was the recognition of the effect ofvisually testing plants

with many snubbers (1000 to 2000) versus plants with few (50 to 100) snubbers.

The probability of detecting one or-more inoperable snubbers in a 10% sample of

many versus few snubbers is'obvious,(for.example, in.the-case of loss of

hydraulic fluid). -A shift frlo6m, absolute"_numbeirs to a percentage of the total

-3.9
was recommended. Another option was to retain the numbers as an incentive for

utilities to reduce the number of installed snubbers.

3.4 CURRENT PRESERVICE AND IN-SERVICE TESTING REQUIREMENTS IN RELEVANT

CODE/REGULATORY DOCUMENTS

The "official" testing document is AStHE XI. In addition, there are ASME

01 documents. Section IWF of ASME XI covers supports in general, including

snubbers. Pertinent information is in IWF-2430 (c) and (d), which require

that hydraulic snubber fluid reservoirs be refilled if they are outof-specification. Section (d) requires readjustment of hot settings if outof-specification. Table IWF-2500-1, Section'F 3.50, cites visual VT-4 for

snubbers as well as examination requirements, acceptance standards, extent of

examination, and frequency of examination.

Section IWF-5000 relates directly to the testing of snubbers. Currently,

only snubbers less than 50 kips in size are covered in IWF-5400. Sections IWF5400 and IWF-5500 are repeated below:

IWF-5400 In-Service Tests for Snubbers Less Than 50 kips

(a) In-service tests shall be performed either during normal

system operation or plant outages.

(b) A representative sample(a) of 10% of the total number of

nonexempt (IWF-1230) snubbers whose load rating is less than

50 kips shall be tested each inspection period. Each representati've sample shall consist of previously untested snubbers.

After all nonexempt snubbers in the plant have been tested, the

tests shall be repeated taking the same snubbers (or their

replacements) in the same sequence as in the original tests.

These tests shall verify that:

(1) during low velocity displacements, the specified

maximum drag or free movement force will initiate motion of

the snubber rod in both tension and compression;

(2) activation (restraining action) is achieved within the

specified range of velocity or acceleration in both tension

and compression;

(a) A representative sample shall include snubbers from various

locations, taking into consideration service and

environment.

3.10
(3) snubber bleed, or release rate, where required, is

within the specified :range in compression or tension. For

units specifically required not to displace under

continuous load, the ability of the snubber to withstand

load without displacement shall be demonstrated.

(c) Snubbers that fail the in-service tests of (b) above shall

be repaired in accordance with'IWF-4000 and retested.' An additional sample of 10% of the total number of snubbers shall also

be tested at that time. Additional sample testing shall be continued until all units within the samples have met the requirements of (b) above.

(d) Components whose supports fail the test requirement of

(b) (1) above shall be evaluated to ensure that the supported

component has'not been impaired.

(e) Inspection and test results shall be recorded for each

snubber.

IWF-5500 SuccessiveTesting

Any snubber that fails an in-service test of I1WF-5400 shall be

retested during the next test period.

ANSI/ASME 0M4 expands substantially on the ASME XI requirements in terms

of degree of detail for both visual examination and testing. Similar items in

ASME XI and ANSI/ASME 0M4 are compared in Table 3.1. Although the draft

Regulatory Guide on snubbers is a qualification/acceptance document, it covers

various testing criteria relevant to ISI. These criteria are summarized in

Table 3.2.

3.5 EXISTING AND POTENTIAL TESTING PROBLEMS

There is evidence that so-called inoperable or out-of-specification snubbers, based on bench testing, are often operable. Studies at Peach Bottom with

hydraulic snubbers indicated failure to meet'activation and bleed velocity set

point1.ta) However, these failures were due to the inability of the test operator to accurately determine the maximum values during bench testing because of

,, . . .. 

(a) Kohler, E. G.,;J.-J.-SmerkeIII',.and H. F. Dobson. 1983. "Peach.Bottom

Atomic Power Station Hydraulic Snubber Failures." ANS Trans. 45:565.

3.11
TABLE 3.1. Significant Functional Parapeiers Pertinent to Snubbers

(ANSI/ASME 0M4 and ASME XI)Ta

Inspection

ANSI/ASME OM4

Preservice In-Service

Breakaway force X X

Drag force

Velocity/acceleration range for

activation in tension or

compression

Release rate within specified

tension or compression

X X

ASME XI

X

X

X

X X

X X

Visual X.

Can carry load X

Does not restrict thermal movement

Fluid level/hydraulic

X

X

Visual examination sample size

Operability

100% initial,

reduced to no

lower than 10%

18 months >25%

X

Frequency

Sample 10% or 35 (whichever is less)

X

Failed test 50% of original

sample added

(a) A revision of ANSI/ASME 0M4 is currently being drafted.

rapid changes in the readings. Substitution of a peak.holding velocity indicator that retained the maximum reading led to decreased rejection rates.

Studies at Pacific Scientific (h c) confirm that there are substantial

differences in test equipment and test procedures, both of which can indicate

(b) Pacific Scientific.

(c) Pacific Scientific.

PSA-3 Shock Arrestor

California.

August 1984. Data Report 1700, Anaheim, California.

1984. A Simulated 40-Year Service Life Test for a

with 2% to 3% Friction.. Test Report 871, Anaheim,

3.12
TABLE 3.2. Significant Functional Parameters Pertinent to Snubbers Cited

in Draft Regulatory Guide ,, -

Parameter

Breakaway force and drag force for range

of piston velocities and strokes,_,

Dead band for range of working loads and

piston locations

Activation level, or'for-snubber design

without distinct activation level,

limits for force motion behavior

Release rate for range of working loads,

Value

Limits specified

Limits, specified

Test'both directions ,

5%, 10%, 25%, 50%, 100%

rated and emergency loads

Spring rates for range

and piston' locations - I I : .!,

of working loads , !: I .;

By dynamic cyclic loading

' equal'to rated load,'1/4,

- 1/2, 3/4;stroke locations 1, ' ; .'. , .f! : , I- ~~~ r~~ o -, n i, I.j I. ,

Testing frequency of 3 to

33 Hz for at least 10 s,

,, . - . -: : , ...o ,

Above room or design temperature,

whichever is lower -

.. .,,

that snubbers are out of'specification when they are'not.. The r'everse is

acceptance of out'-of-'specificati'on snubbers. Specific concerns include

accurate measurement of drag force~and acceleration'level, 'both'of which are

difficult to determine.; Pacific Scientific has suggested specific procedures. - . s . I1 ! @ ' . ., .'tii.'- 

3.6 SUGGESTED CHANGES IN SNUBBER TESTING AND EXAMINATION PROCEDURES . ,~~~~~~~~~~~~~~~~~

The following criteria or,'modificationstto criteria were culled from'

several sources. It is assum;ed that':

* The snubbers meet ASME III nondestructive examination (NDE)

re'quirem~ent s.* ;- -?1 ,'it r;t.,^ ;!'a

* A functional specification'was developed in c`onformance with Appendix A of the draft Regulatory Guide..

* The snubbers have'been qualified to Appendix B,of the draft Regulatory Guide, including environmentai qualification.

*All snubbersare acc pte from-the'production line only after meeting

the requirements -of Appendix C o`fthe, raft RegulatoryGuide.

; .,. , ¢T Iii, . ; . . - . .

3.13
I

Tests to establish compliance with the draft Regulatory Guide measured the

following functional parameters:

* activation level, outside limits (tension; compression; velocity;

acceleration)

* breakaway force, outside limits (tension; compression)

* drag force, outside limits (tension; compression)

* dead band, exceeds limits (tension; compression)

* release rate, outside limits (tension; compression)

* spring rate, outside limits (tension; compression).

The acceptance/rejection limits for each of these parameters will be a

function of type and size of snubber. These limits help establish the design

modeling assumptions since they are specific; only general comments are given

below:

* Linear modeling is the usual approach in dynamic system analysis.

Therefore, parameters such as activation level and dead band, which

contribute to nonlinear behavior, should be minimized.

* The activation level should be greater than the maximum thermal

growth rate combined with minimum breakaway and drag forces so that

there is minimal resistance to normal thermal movement. The

undesirable condition would be a substantial force so that thermal

growth builds up forces within the system until-the breakaway level

is reached. If the forces are high and breakaway is abrupt, the

snubber could be activated and cause undesired resistance.

* Some parameters are sensitive to the specific snubber application.

For example, the release rate determines snubber motion after activation occurs. The value would he substantially different for a

relief valve snubber than for a seismic snubber.

* Dead band is sensitive to factors such as installation tolerance, air

in hydraulic fluid, and manufacturing tolerance; therefore, it is

better to determine dead band through testing.

* Breakaway force is sensitive to both vibration and extended periods

of inactivity; inactivity may increase breakaway force levels substantially. Both conditions may exist and should be considered.

* The usual tendency is to minimize the dead band level to minimize

impact loads-in the snubbers and attached components. This tendency

needs to be counterbalanced against the increased tolerance to higher

vibration levels at higher dead band levels.

3.14
* The spring rate or load displacement~'is an indication-of the stiffness in the snubber; however, stiffness is controlled by the associated hardware attached to the structure, the snubber, and the

.-component. Therefore, the -'spring rate of-the snubber is only a part

of the picture and evaluations based on the cited values may not be

valid.,

* The various measured parameters are quite sensitive to the test and

the test procedures. An acceptable snubber may be rejected or an

unacceptable snubber may be accepted due to variability in test '

equipment and procedures. This factor is not recognized in the

various codes and standards. A definitive set of criteria should be

developed to control this variable.

* In some snubbers there is no distinct activation level and release

rate; thus, it is necessary to define the force-motion relationship

,over the'appropriaterange.

* Snubbers'should be tested by free'(without-activation)Jexercisingfor

not less than five full-stroke cycles to establish that they are in

working order.,

* Experimental evidence confirms that the activation level of.

mechanical snubbers is insensitive to load over a wide range thus,

/.it is not'necessary to conduct a full range of load tests. a'

/ *Cyclic loads'as such have a`minimal'effect on lost motion aand'acceleration based on tests to 40,000-cycles; however, the drag force is

changed substantially. For example, the drag force is reduced by.

nearly 50% under room temperature cyclic loads without simulation of

environmental effects. b)

The concern:with regard to aging of.isnubbers relates to the optimum tests

for detecting various degradation-modes. Table 3.3 is'taken from the DOE

Nuclear Standard NEE7-9T and illustrates soI of 'these 'interrelationships. ; , . .-... ,, ; / .' -, , n a:e ',-,'

The parameters in Table 3.-3 such as corrosion and viscosity are strongly

influenced by temperature and irradiation. Thus, times to failure may differ

markedly from one portion ofAthe plant to another. This is of concern because

(a) Pacific Scientific.

(b) Pacific Scientific.

PSA-3 Shock Arrestor

California.

August 1984. Data Report 1700, Anaheim, California.

1984. A Simulated 40-Year Service Life Test for a

with 2% to 3% Friction. Test Report 871, Anaheim,

3.15
TABLE 3.3. Common Causes for Degradation in Snubber Operating

Characteristics

Measured Parameterva)

Stiffness Activation Release Dead Friction

Cause (spring rate) Force Rate Band (drag/breakaway)

Wear - - - M M

Corrosion - - - - H,M

Viscosity H - - -

Temperature H H H

Entrapped air H H H H

Contamination - - - - H,M

(a) M - mechanical; H - hydraulic.

an inherent assumption in testing a small sample is the homogeneity of thepopulation. Snubbers taken from one region of a reactor (for example, a cooler

region) could display a markedly different failure history than ones removed

from another region (for example, a BWR dry well). This issue could warrant

consideration in selecting the sample to be tested.

It is apparent that acceptance and qualification of snubbers is based on

some or all of the following parameters: activation, breakaway, dead band,

drag, release rate, and'spring rate. Therefore, any consideration of preservice and in-service-testing should consider these same parameters and determine

if they will detect the various aging/degradation mechanisms to which snubbers

are subjected. As can be seen from the preceding table, measurement of these

parameters can detect wear, corrosion, contamination, and changes in hydraulic

fluid caused by temperature, changes in viscosity, or entrapped air. Loss of

fluid in hydraulic snubbers is obvious, and mechanical snubber lockup can be

detected by several tests.

In hydraulic snubbers:

* The hydraulic fluidsshould have adequate lubricity to minimize galling, be compatible with other materials (e.g., seals), be stable

under operating. conditions, provide corrosion protection, resist

fire, resist radiation damage, and be-capable of cleansing by filtra-

-tion. The effects of entrained or dissolved gases should also be

considered.'

* Seal materials-should be selected considering-the effects of radia-'

tion, humidity, temperature, possible incompatibility, aging, and

resistance to abrasion.

3.16
* Snubber spring materials should be selected on the basis of elevated

temperature stability, corrosion resistance, friction and galling

effects, and minimization of structural defects (also mechanical

snubbers)..

* Leakage rates should be determined throughout life.

Appendix B of the draft Regulatory Guide suggests testing over a'frequerncy

range'of'3 to'33 .Hz at 5-Hz steps from the 1/2 stroke position. This testing

could be appropriate during in-service testing under-some circumstances.

ANSI/ASME OM4 is more explicit concerning visual examinations (both preservice and in-service) and should serve as a model.^ This also applies to

validation of thermal movement and swing clearances.

If multiple small snubbers are used in lieu of a larger snubber, the DOE

standard suggests specific values for load sharing, which is a strong function

of mismatch, lost motion, and end fitting clearances as well as a lesser function of mismatch of activation level and release rate. The following values

are suggested:

* Differential clearance/lost motion between any two snubbers should be

less than or equal to 0.02 in.

* Differential in activation levels between any two snubbers should be

less than or equal to 10 in./min., or 0.005 g, or 50% of smallest

activation level.

* Differential in release rates between any two snubbers should be less

than or equal to 10 in./min, or 0.005 g, or 100% of snubber release

rate. Maximum rate should be greater than or equal to 10 in./min or

0.001 g.

* Load sharing-peak load should be 1.2 times average load.

Types of test conditions versus specific tests or examinations are compared in Table 3.4. The information in the table was taken from ASME PVP-45

and gives some idea of the scope of tests and examinations as well as where

they are applied. Basically, the information agrees with the draft Regulatory

Guide, ANSI/ASME 0M4, and ASME XI.

.. . .

. .-. .

3.17
Recent studies(a b) su~gjst approaches to relaxing current testing criteria. A paper from MPe11 a suggests both a generic improvement of acceptance limits (Phase 1) and detailed computer evaluations (Phase 2). They

expect that the drag/breakaway limits and Acceleration limit could be increased

substantially. A Babcock and Wilcox papertb) deals specifically with acceleration limits and dead-band displacement. They used sample problems with a multiple response spectra approach to indicate that acceleration limits could

increase from 0.02 to 0.26 g without exceeding design stresses. Dead-band displacement proved to be snubber specific. In some instances, only limited

increases from the usual 0.030 to 0.040 in. were possible. In others, a threeto four-fold increase was possible.

(a) Eidinger, J., and G. Hau. 1985. "Snubber Reduction by Reduction of

Unnecessary Seismic Margins." Presented at ASME Pressure Vessel and

Piping Conference, June 24-27, 1985, New Orleans, Louisiana.

(b) Gurdal, R. J. 1985. "Snubber Lockup Accelerations and Acceptable DeadBand Displacements by Extension of the Response Spectra Method."

Presented at ASME Pressure Vessel and Piping Conference, June 24-27, 1985,

New Orleans, Louisiana.

3.18
TABLE 3.4. Types of Test Conditions(a)

0;

Specific Testbr

Examination

Drag force

Breakaway (RT, Top, friction)

Activation level (RT, TOP

(v or a)

Release rate (RT, Top)

Dynamic characterlstics (1/4,

1/2, 3/4 stroke faulted one- cycle dynamic

Abnormal envlronment~translents (repeat others at RT) ; - I - - . ." . ,--)

Static test:(maxlmum rated

load)

Dead band:(RTTp .

Margin to failure at-faulted

Low amplitude agial

vibration (5xlO cycle)

Spring rate (if applicable)

Fluid pressure Integrity

Full stroke

Freedom of motion

VT for Installation defects

Cbrrect.locationCorrect IIne of actIon and

swing clearancex

x

x

x

x

x

x

x

x x

x

x

x

Hot Functional

and Startup

Qualification Acceptance Testing In-Service

Testing Testing Installation (preservice) Testing

x

x x

x

x

x

x

x

x

x

x x

x

x

x
TABLE 3.4. (contd)

Specific Testbr

Examination

Hydraulic chambers without

leaks, cracks

Qualification Acceptance

Testing Testing

X

Hot Functional

and Startup

Testing

Installation (preservice)

X

In-Service

Testing

X

Hydraulic snubbers correctly

oriented and seals Intact

Sliding-surface free from

burrs, weld spatter, paint,

etc.

Connecting pins present and

locked

Structural and piping con- nections correctly oriented

to handle lateral movement

Cold position wilihandle

maximum thermal travel

Shipping Installation

locking features removed

x

x

x

x

x

x

N,

Visual examination at refueling

Hydraulics - vents OK;

fluid level.OK

(a) Sources: ANSI/ASME OM4; ASME PVP-45.

(b) RT - room temperature; Top - operating temperature; V - velocity; a - acceleration.

X

x
4.0 OPERATIONAL STRESSES

Mechanical loads are the primary operational stressors that contribute to

the failure of mechanical snubbers. To a lesser extent, mechanical-loads may

also impact the life of hydraulic snubbers. The primary operational stressors

that impact hydraulic snubbers are thermal, chemical/environmental, and radiation effects (separately or in combination). These stressors may also degrade

mechanical snubbers. ',>

4.1 MECHANICAL

Snubbers are installed to handle a spectrum of dynamic mechanical loads on

piping, steam generators, pressurizers, pumps, valves, etc.. The major design

load is usually seismic; however, snubbers'are also used for loads'such as

valve opening or closure. These loads are anticipated within the designs

envelope; of greater significance are mechanical loads outside the design

envelope such as water or steam hammer and water slugging. Such loads have

caused mechanical snubbers to lock up, which can severely load piping during

thermal movement. Essentially all snubbers on a given piping system have been

locked up by water hammer loads.

Other mechanical loads not normally considered may render snubbers inactive. For example, there have been several cases where snubbers have been used

as a ladder. Other mechanical loads include twisting or bending during installation or incorrect installation so that a bending moment occurs.

4.2 THERMAL

Temperature changes can have two effects on snubbers. If incorrectly

installed, piping movement may render them inoperable because of bending. A

more general effect is degradation. Elevated temperatures age and degrade the

plastic seals in hydraulic snubbers, change fluid viscosity, and may lead to

high corrosion rates or to distortion of components in mechanical snubbers.

Continued snubber operation in BWR dry wells is doubtful if temperatures are

high enough to lead to premature aging.

4.3 CHEMICAL/ENVIRONMENTAL

Obvious long-term aging/degradation mechanisms related to chemical and

environmental effects include a combination of high humidity and temperature.

This combination can lead to oxidation and possible loss of function in

mechanical snubbers and in various components of hydraulic snubbers. The

4.1
preceding conditions exist; if other chemical contaminants are inadvertently

introduced, the degradation could be accelerated.

4.4 RADIATION

Radiation shortens the life of the seal materials used in hydraulic snubbers. However, radiation is only one parameter contributing to seal aging;

temperature will have the same effect over an extended period. Radiation can

influence viscosity and may change operating characteristics of hydraulic

snubbers.

4.5 SYNERGISTIC EFFECTS

Thermal, chemical/environmental, and radiation effects tend to interact.

It is possible that the life of some components, particularly seals, could be

dramatically reduced. These effects have not been quantified.

4.2
5.0 REVIEW OF LICENSEE EVENT REPORTS FROM 1970 TO 1983

.The licensee event report (LER) literature pertinent to snubbers has been

quite voluminous over the past 10 to 15 years. Earlier reactors.used hydraulic

snubbers almost exclusively. Surprisingly, the number of snubbers used in some

plants in the late 1960s and early 1970s was quite high. For example, Indian

Point 2 used 540 snubbers, of which 356 were inside containment (inaccessible);

and Fort Calhoun is cited as having 324 snubbers outside containment (accessible). Apparently, some early plants have continued to add snubbers or to

switch from one type to another, as indicated by comments in.the LERs.

This review makes an effort to statistically analyze'the trends. Some

obvious perturbations will be cited to permit a relatively broad overview of

snubber experience. Because of the ambiguity in citing failures, the cited

numbers-should be considered to represent approximate and not absolute'values.

Three areas will be discussed: 1) where snubbers are used; 2) numbers of

"failures" per year; and 3) various.failure mechanisms and-the annual trends

observed within these mechanisms.

5.1 PIPING SYSTEMS . .,

The LERs vary widely with respect.,to citing piping systems where snubbers

are used; however, virtually all safety-related piping systems have snubbers.

Systems-cited again and again in the LERs include the main steam, feedwater,

high pressure core injection, low pressure.core injection, reactor heat

removal, decay heat, chemical volume control, core spray,,and containment

spray; many other systems are noted. There does.not appear to be an obvious.

trend insofar as including'or excluding systems.

5.2 SNUBBER FAILURES

Hydraulic snubbers are the.major type'being used (see Table 5.1), which

accounts for the preponderance of failures in such units. The "not defined"'

category in Table 5.1 covers cases where the LERs were not explicit because the

type of snubber'was not a factor (for example, a water hammer incident where

supports were damaged). In many such cases,.rthe ,utility did not cite the type

of snubber. .... .

The 1973-1974 figures are not typical because severe seal degradation

occurred and repetitive inspections were required over short.time intervals (IE

Bulletin 73-3). In essentially all'of the older units, the polyurethane seals

were replaced with ethylene propylene seals.

5.1
TABLE 5.1. Snubber Failures from 1973 through 1983

Type 1983 1982 1981 1980 1979 1978 1977 1976 1975 1974 1973

Hydraulic 223 135 >255 130 >374 197 510 152 49 177 7

Mechanical 86 -24 71' 46 11 -- 21 17 -- -- --

Not defined 6 18 -30 16 35 21 15 -- 4 7 --

-Snubber failures by year andoperatin'g reactor are presented in Table 5.2.

The figures represent th'e number'of failed snubbers cited in one or more LER.

The Crystal Rive'r 3 value of 110 represents a one-time case. It was not

possible to discern any obvious trends from these data.

5.3 FAILIURE MECHANISMS

Tables 5.3, 5.4, and 5.5 detail snubber failure mechanisms. These data

will almost certainly correlate to some degree with the statistical analyses.'-

There is an attempt to mirror annual trends by classes of snubbers and failure

mechanisms.

Table 5.3 covers failure mechanisms for hydraulic snubbers. The seal

failures due to selection of an improper material were cited previously. Loss

of fluid remains a problem with very little improvement observed from year to

year. The seal'degradation/aging problem is less severe'; however, leaking

fittings, damaged seals, etc., are still problems. A simple problem such'as

keeping the oil 'reservoir upright' has not been solved in 10 years, although

this problem is less significant with pressurized reservoirs. The number of

mechanisms leading to loss of fluid are not all-inclusive, but they are

indicative of the causes.

Major attention has been given to keeping hydraulic snubbers within specification hands for activation, bleed rate, etc. These factors are influenced

by loss of fluid to a major degree. In addition, several other factors also

play a role, including fluid viscosity and valve settings. An attempt has' been

made to mirror the reporting of snubbers that are outside the specification

band but still generally operable. -As noted, the numbers are large.

A problem almost generic in nature that does not surface in Table 5.'3 is''

contaminated fluid, which can cause seal degradation, severe leakage, valve'

plugging, etc. Many units were not adequately cleaned prior to leaving the

factory, and the problem of'contaminated fluid has continued for many years.

The authors are aware of such incidents that have not been reported in LERs.

5.2
TABLE 5.2. Snubber Incidents'by-Plant and Year from LER Reviews

'PIant 1983

Arkansas l

Arkansas 2

Beaver Valley 1

Big Rock Point 1

Browns Ferry I

Browns Ferry 2

Browns Ferry 3

Brunswick 1

Brunswick 2

Calvert Cliffs 1

Calvert Cliffs 2

Cook 1

Cook 2

Cooper Station

Crystal River 3

Davis-Besse 1

Diablo Canyon 1

Dresden 1 -

Dresden 2

Dresden 3 --

Duane Arnold

Farley 1

Farley 2 -

Fitzpatrick

Fort Calhoun 1

Fort St. Vraln

Ginna

Haddam Neck

Hatch 1 -

Hatch 2

_ _

I

6

110

45

8

* 16

1

7I 

* 1

1982

.3

4

.3

__I

1981

1

27

132/640

1

3

-3

12

'5

-2

1980

7.

7

5

2

2

13

12

1*-

'. 'I

1-

26

48/61

1 -

I

1979

23/2218a)

8

1978

:3

1

.1977

- 1

r__

13 13

44

1

2

I1-

-- I

10

8

-1976

18

13

2

6

3

* 9

8

5

8

-2

44

158/230

16

31

2

9

55

I

15-

1974 or

1975 Earlier

: , -- . _4 

4_

- i

8-

13

17

1

2 __

'. 1

2 1

2 2

5

51 16

2 4

21 - 135'

2

8

4 (b)

1

;- 2

. 4

__ __1

--. 1

3 --!

2 1. 8

2.- -12 .

__ 1

-- .4 .

12 - --- -

_ 1

2 ---

__ 40

__ I _ ,

-9

. 2

-7

-1.

Indian Point

Indian Point

Indian Point

Kewaunee

La Crosse

2

3

3 2

3 3

_- __

37 40/115

3

I.

, __

-3

LaSalle I

Maine Yankee

McGuire 1

McGuire 2

Millstone 1

Millstone 2

Monticello

Nlne Mlle Poil

North-Anna I

North Anna 2

Oconee 1 .I.-

Oconee 2'

Oconee 3

Oyster Creek 1

Palisades''

5

- 7

* 2'2

It' - 2

t-I. __

! , _

- - t 2 I

I *.. 3

16

4

2

4

__

_- _

1-

., 10

'6

.i1H'K2~. -. .. ,2.

2

, __' .. 5_i .

-: 1 ~ . -,

.5

I , _ _

5

3

__4

__

l

_.w

2

I .i.

2

'. -- _

__

4

.. ,

1 '

117

_ 3

3'

8

I 1I . 1 . . -- ::

14 a -- .-,,7 .- 5 s B-' .. 5 . ' I -'

5.3
TABLE 5.2. (contd)

P lant*

Peach Bottom 2

Peach Bottom 3

Pi IgrIm I

Point Beach I

Point Beach 2

Prairie Island I

Prairie Island 2

Quad-CIties 1

Quad-Cities 2

Rancho Seco 1

Robinson 2

Salem-l

Salem 2

San Onofre 1

San Onofre 2

San Onofre 3

Sequoyah 1

Sequoyah 2

St. Lucie 1

St. Lucie 2

Summer 1

Surry I

Surry 2

Susquehanna 1

Three Mile Island I

Trojan

Turkey Point 3

Turkey Point 4

Vermont Yankee 1

Yankee-Rowe I

1983

2

3

2

10

6

3

5

30

28

__

1982

2

2

3

16

4

5

5

1981

4

1

_2

2

2

4

1980

6

2

8

2

1979

6

2

3

5

2

3

l

1978

5

4

2

2

14

1977

6

16

1

1

6

36/81

1976

26

10

4

5

12

1975

7

2

4

1974 or

Earlier

1

41

14

4.

__

3

9

3

3

12 6

I 1

3 2

3

__

I

4

3

22

19

2

2

12

36

2

20

4

l

45/259

31

2

3

4 5

__

8

Zion 1

Zion 2

-- 1 II 1

3 -- -_ __

__ 5 8

3 22 11

(a) In some Instances, fa Ilures were cited as well as the number of snubbers tested.

(b) One LER said most snubbers were out-of-specificatlon.

It is apparent from

had many problems. Many

ever, it is too early to

the number of incidents that hydraulic snubbers have

utilities are changing to mechanical snubbers; howtell if they are simply switching problems.

Far less failure data are available on mechanical snubbers (Table 5.4).

The-corrosion/oxidation category in Table 5.5 primarily applies to mechanical

snubbers. One type of mechanical snubber has been particularly susceptible to

failure by corrosion. A major problem with mechanical snubbers is that they

fail in an unsafe fashion; they lock-up and represent a rigid constraint that

could lead to failure of the mounting or pipe (IE Bulletin 81-01). While

hydraulic snubbers are somewhat susceptible to dynamic loads such as water

5.4
TABLE 5.3. Failure Causes and Mechanisms for Hydraulic Snubbers

Failure Mechanism

Design Weakness:

Leakage/loss of-fluid (cause often

not given)

Cracked/falled/damaged O-ring/bushing/

seals/gaskets

Fitting leak

Seal degradation/aging (wrong material)

Scoring of piston/body

Worn out 0-rings/bushings

Seal failure/particulates remaining after

fabrication

0-rings or seals Installed incorrectly

Broken/cracked fluid reservoir

Loose packing nut/screw

Sight glass leak

Heat damage to fluid reservoir

Scored bushing/worn threads on piston shaft

Rod end cylinder seal

Piston rod seal worn

Scored/broken poppets, poppet springs

Valve leakage

Failure to lock up or outside specifications

Faulty closure shuttle valve

Poppet springs out of place

Scored poppet seats

Outside of specifications

Did not meet activation or bleed rates

(high or low)

Length of stroke

1983 1982 1981 1980 1979 1978 1977. 1976 1975

-- .-- 9 15 .8 25 27 . 26 13

.-- . 3 14 7 4 15 7 26 . 5

2 2

6 11

__ 1

__ 2

4

1 5

2

6 5 6

4 -- 16

__ -_ 2

3 3 21

-- 1 4

1 1

I 13

II

38

12

3

11

.1

24

17

1

3

3

4

3

__ -- 2

2 . 1 2

__ _- 2

__ __ I

__ -- 2

__ -- 2

__ -- 3

l

I

1

3

__ 1

. __

2 I 4 __ 3

115 59 170 '19 8

-- -- 11 I ~~~~ -

-- -- -- ~3

--- I -- 5

-..m

Outside Design Envelope:

Reservoir upside down/Improperly Installed

Reservoir tube cut

_ -- 25

__ -- 2

6 7 3 7 5 -

5.5
TABLE 5.3. (contd)

failure Mechanism

Outside Design Envelope: (contd)

Foreign material plugging bleed screw

activation valve

1... 1982 1981 1980 1979 1978 1977 1976 1975

-- -- -- -- -- 1 -- -- --

Maintenance error

Out of adjustment

Did not meet acceleration test

Improperly lubricated with grease fluid,

contaminated

Water

Solid debris 2 -- I 1 -- -- -I -

Unknown:

Cause not stated 3 1 -- __ __

TABLE 5.4. Failure Causes and Mechanisms

Mechanical Snubbers

for Predominately

Fal lure Mechanism

Design Weakness:

Microcracking of capstan springs(a)--

generic frozen and/or broken components

Scored components

Corrosion/oxidation

Outside specifications

Did not meet drag force criteria

Failed stroke test

1983 1982 1981 1980 1979 1978 1977 ...

-- 40 -- 45 2 2 15

46 45

Outside Design Envelope:

One-time dynamic load

Water hammer

Steam hammer

Unknown:

2

3

7

2

I

2 2

No reason given 5 -- -- -- 1

(a) Microcracking cited; no specific numbers.

5.6
TABLE 5.5. :Failure Causes and!Mechanisms for Hydraulic and

8 . Mechanical Scrubbers . E. .i.. . I .- ;97 6 . 1

Failure Mechanism 1985 1982 1981 1980 1979 1978 1977 1 976 1975

Design Weakness:_

Loose set screws

Oxidation/corroslon (prlma r mechanism)

Design/installation errors

Worn/broken components

Extruded bearings

Loose end clamps, etc.

Excessive wear- --

Outside Design Envelope:

Improper installation

Out of alignment/inadequate clearance/

rotated/missing anchors/bolts

Loose clamp

One end disconnected

Rod end bushing not staked

Bushing missing or disengaged

Bracket pin missing

Missing load pin

Maintenance error :

Component fa Ilure

Bent/damaged/broken components

installation/maintenance error '-

Mechanical'binding, no detail

- .2

- f

7 r

.. -C

r. I

"4'- 2.1.

I,

-- -- I

*.

- 1

1 -- --

I. -- --

C'

2 7 ''1'

1 -- _

8 '>, .1?7 --

__ -- 3

I'd1-

. 9 '

2

3

'_-

3

-1

' I I

2,

.1':

* .- -1 - - -

-- 1 2 A 1- *-2

5 7

:

A,

2 1- -- - -~2-

1 - - - - -

2

I

__

6 -

1 , r

12. . 1 --

. I

Excessive lateral loads (thermal) -- --

Probably used as a ladder -- --

Maintenance-mishandling,:. .- , --, I; i ,--

Vibration I '' '' ;- ' **''-. ; '''''

2 1 1 - - - -

Missing pins,, ..

Water hammer (see mechanical ) .

1.1i , ~ .. 3.--I

1 2 .Kii.

r. I- . --

71

__

_

Support damage -' : .. . "...

* - ,

. -, ; j  - .. , , -

. . z .. ! I :  I " ;

5 * . I. .3

5.7
hammer, mechanical snubbers are much more susceptible. They suffer severe

internal damage, remain in a locked position, and may damage the piping system.

Since water hammer is more probable than a seismic event, it will remain a

problem.

A study supported by the Electric Power Research Institute (EPRI) on

mechanical snubbers removed from Three Mile Island Unit 2 (TMI-2) supports the

failure mechanisms cited in the LERs. a) Some snubbers locked up. Examination

of the disassembled snubbers revealed substantial damage to the internals in

the form of bowed or twisted torsion rods and screw shafts. The postulated

damage scenarios included water hammer or high vibrations during an accident

from causes such as cavitation of the primary pump or blow off of the pressureoperated relief valve.

- Loss of function of mechanical snubbers at other reactors has been attributed to both water hammer and high vibration; thus, the scenarios are convincing. An undetected water hammer has led to snubber lockup, and subsequent

heatup and cooldown tore the snubber mounts out of the wall by thermal

expansion. Vibration may act similarly.

ASME PVP-55(b) includes a series of papers on factors leading to a loss of

function. Some obvious examples include low level vibrations, lateral loads,

and environmental effects, particularly salt air.

An NRC report(c) discusses mechanical snubber failures. The impetus for

the report was the lockup of five safety-related mechanical snubbers in 1983.

While these conditions appear to indicate severe loads such as water hammer, no

such incidents were known to occur.

The search was extended to other reactors. For example, seven mechanical

snubbers were found to be inoperable at another plant; the probable failure

cause was mishandling and/or abuse during construction or installation. Failure incidents at some other facilities were believed to be due to pipe vibration with thermal effects possibly contributing to slow deterioration. At

another plant, lockup was attributed to maintenance misuse (for example,

(a) Wadsworth, F. L., et al. 1983. Examination of Mechanical Snubber from

TMI-2. EPRI-NP-2966, Electric Power Research Institute, Palo Alto,

California.

(b) American Society of Mechanical Engineers. 1981. Snubber Design

Application and Minimization Methods. PVP-55, New York.

(c) Hsu, C. March 23, 1984. Mechanical Snubber Failure. AEOD/E406, Office

of the Analysis and Evaluation of Operational Data, U. S. Nuclear

Regulatory Commission, Washington, D.C.

5.8
stepping on the snubbers or using themas rigging points). At another plant,

several snubbers failed to meet the functional stroke test.

These incidents appear to indicate that several mechanisms can lead to

mechanical snubbers locking up or being out of specification. The problem

could be more widespread than recognized. The responses to IE Bulletin 81-01

have been assembled and reviewed (AEOD/E406), which may lead'to further

clarification.

Failures not unique to a class of snubbers are listed in Table'5.5.

Installation and design errors often leadjto failure of. snubber supports

because thermal expansion is not accounted for; supports have literally been

torn apart under such conditions. A recurring problem that is often linked to

vibration is the loss of function because one end is disconnected. Vibration

may lead to total loss of function, unscrew the pistons, extrude the bearing

and shake out the retaining pins. A source of vibration relatively near a

snubber can result in loss of function. Loss of function can also occur

because snubber parts are bent or broken when used as a ladder or due to

lateral loads from thermal expansion.,

Table 5.5 lists several failure mechanisms. In turn, these mechanisms can

be grouped into modes related to underlying causes of failure such as errors in

design, installation, or operation. ,It is possible to correlate failure causes

into the specific categories of design, installation, and operation.

Recent IE Information Notices--IN 84-67(a) or IN 84-73(b)--as well as

AEOD/E423 c) cite functional failures of both hydraulic and mechanical snubbers. IE IN 84-67 mentions the Palisades plant where 5 of 14 large hydraulic.

snubbers were tested on the steam generators and failed to lock up. The cause

was attributed-to insufficient counterboring of valve block endccaps, which

inhibited the shuttle valve from properly closing the port blocks. The root

cause was a nonstandard spring that gave high activation velocities and prevented proper shuttle valve positioning. Subsequent testing confirmed that all

14 snubbers failed to lock up.- AEOD/E423;expands on this case, citing other

plants and other manufacturers whose large hydraulic snubbers that are

(a) IE IN 84-67. August 17, 1984. Recent Snubber Inservice Testing with High

Failure Rates. U.S. Nuclear Regulatory-Commission; Washington, D.C.

(b) IE IN 84-73. September 14, 1984. Downrating of Self-Aligning Ball

Bushings Used in Snubbers. AU.S. Nuclear Regulatory Commission,

-Washington, D.C.' " '

(c) Brown, E. J., to K. V. Sefrit, memo. September 20, 1984. Failure of

Large Hydraulic Snubbers to 'Lockup. AEOD/E423,'Office of the Analysis and

Evaluation of Operational Data,-U.S. Nuclear Regulatory Commission,

Washington, D. C.

5.9
typically used on steam generators had malfunctioned. Causes included:

inadequate design, inadequate control of design changes, inadequate control of

manufacturing procedures to implement design changes, inadequate functional

specifications, lack'of requirements for functional qualification testing,

derating of subcomponents after equipment was placed in service, component

aging during service or shelf life, questionahle qualification testing, and

exemption of large'snubbers from surveillance testing requirements in the

technical specifications. The preceding list covers snubbers that probably

never functioned correctly.as well as snubbers that suffered aging.

IE IN 84-67 also discussed hydraulic-and mechanical snubber failures at

Surry Power Station Unit 1. Hydraulic snubber failure modes and causes and

mechanical failures modes are shown in Table 5.6. Several failure mechanisms

are related to aging/degradation. More significantly, a substantial number-of

both hydraulic and mechanical snubbers failed to lock up; 30% of the hydraulic

snubbers and 32% of the mechanical snubbers failed activation tests. Some

snubbers failed in more than one mode.

Turkey Point's mechanical snubbers exhibited similar behavior; about 40%

of those tested failed to meet specifications. In addition, 17 snubbers were

not staked at the ball-nut thread joint. At Braidwood, the steam generator

hydraulic snubbers failed to pass the qualification test. These units would

not hold hydraulic fluid at loads of 10% to 33% of stated capacity because the

mechanical seals failed.

IE IN 84-67 cites arguments that drag/breakaway tests and acceleration

tests at rated load may actually accentuate failure. The NRC does not agree

with this supposition.

IE IN 84-73 describes another problem pertinent to both hydraulic and

mechanical snubbers;. Self-aligning ball bushings are used in both types of

snubbers. At one plant, one such bushing failed at a static load below the

rated load and rendered the snubber inoperable. At least two manufacturers had

downrated such bushings, which originally had been used in the aircraft industry, on the basis that snubber specifications exceeded the ideal conditions of

the aircraft industry.

5.4 STATISTICAL ANALYSES OF FAILURE DATA

Three sets of data were compiled from LER abstracts stored in the RECON

Information Retrieval system maintained by the DOE Technical Information Center

at Oak Ridge. The Oak Ridge data base contained approximately 650 LER

abstracts relevant to snubber failures from 1973 to 1983. Each of the three

sets of data was compiled by different researchers using essentially the same

LER data base.

5.10
' TABLE 5.6. Failure Modes aod Causes'for Surry Power Station

Unit l'Snubbersta) : D I ,IU.B.R.FA;LUR. . D .I

a) HYDRAULIC SNUBBER7 FAILURE MODES(a.) .- ,

Mode

Low activation in tension

Low activation in compression

Low bleed in tension

Low bleed in compression

High activation in tension

High activation in compression

High bleed in tension

High bleed in compression

No.. of

Failed Snubbers

. .- 13 r I

3

I 11

-. 10 .

;17

10

6

3

% of Snubbers

Tested ''

.. ~ 8 ; :'

2

7,,7

*6 . -i 6 -

6

4

,2..

-.% of.Snu rs

Fai ledI '

. 2 -

6

* 23

21

. .35

I '' 21

13

6

b) HYDRAULIC

Mode

No observable defect

Degraded EP seals

Polyurethane piston seals

Poppet upside down

Debris in fluid

Poppet stuck

Activation adjustment screw broken

Piston/cylinder scoring

.SNUBBER FAILURE CAUSES

No. of, --%-of Snubbers;

iled Snubbers Tested

6 4

32

7 4

3 2

: 1 - *N,, 1

1 1

% of:Snubbers :

Failed

-* 26

: -13

30', ,

13

4 - . 4

4

9 i

3 

2 13

. 

'. 

zj 

.' " : ..  .  . I -II ; .-

.

' 

' . , - 4

I 

! 

1 

- .16- I ' ' - ' 4

I 

1 

. ;.2 ..- 1

c) MECHANICAL SNUBBER FAILURE MODES(c)

Mode ; .

High drag

Locked up

No. of i % of Snubbers

Failed Snubbers' Tested,'

14 21

. % of Snubbers

Failed

66

5 8 24 _ .

High acceleration 1 2 5

Locked up in compression - 1 - i' 2. 5-. .

(a) A total of'160 hydraulic snubbers were tested; 48 snubbers-(or 30%)

failed.

(b) Failures total more than 100% because some snubbers exhibited.:,

more than one failure mode or cause.

(c) A total of,66 mechanical snubbers were tested; 21 snubbers (or 32%)

failed. :.

5.11'
These three compilations are compared in Table 5.7. This table illustrates some of the difficulties in using LER information. A more detailed

plant-by-plant comparison of the three compilations is presented in Table 5.8.

These tables indicate an important source of errors in snubber failure calculations. Using present LER information, it is frequently not possible to

unambiguously categorize snubber failures according to a consistent scheme.

Consequently, the number of failures that are assigned to a certain category

might differ markedly from one study to another.

Data Set A is the result of an exhaustive computer search of the LER

abstracts; Data Sets B and C result from manual reviews of the data.

Therefore, in most of the following tabulations of LER failures, Data Set A

will be used.

Tables 5.9 and 5.10 list failure modes and causes extracted from the LERs;

not too surprisingly, the operational modes tend to control in total numbers.

This relates back to Tables 5.3, 5.4, and 5.5, where the large spectrum of

aging/degradation mechanisms are cited. It is obvious that correcting a single

problem will not solve the entire problem.

Tables 5.11 and 5.12 permit an analysis of failures by reactor by year

from 1973 through 1984. A summary of the failures by snubber type is shown in

Table 5.13. The higher levels of hydraulic snubber failures relate to the

TABLE 5.7. Comparison of the Number of Incidents by

Year for Three Data Sets

Snubber

Type _ X7T

Data Set A:

No. of Snubber Failures

1974 1975 1977 1978 1979 1'980 1981 1982 1983

Unknown

Hydraulic

Mechanical

__ 25

272 472 __ 14

30

54

1 12

161 385

17 21

7 75 5 12 23 67

248 308 114 330 134 66 -- 11 46 83 20 49

Data Set B:

Unknown

Hydraulic

Mechanical

-- 18 193

267 324 82 -- 11 2

__ 7 3

169 441 133

5 13 __

306 82

57 45

35 25 54

341 133 89

9 18 26

Data Set C

(through 1984):

Unknown

Hydraulic

Mechanical

7

7 4

177 49 -- 15 21 35 16 30 18 6

152 510 197 374 130 255 135 223

17 21 -- 11 46 71 24 86

5.12
TABLE 5.8. Comparison of Snubber Incidents by Plant for Three Data Sets

Power Plant

Arkansas 1

Arkansas 2

Beaver Valley 1

Big Rock Point 1

Browns Ferry 1

Data Set A

24

24

24

10

62

--Data Set B

23

24

24

10

11

Data Set C

22

38

38

10

55

Browns Ferry 2

Browns Ferry 3

Brunswick 1

Brunswick 2

Calvert Cliffs 1

Calvert Cliffs 2

Cook 1

Cook 2

Cooper 1

Cooper 2

Crystal River 3

Davis-Besse 1

Diablo Canyon 1

Dresden 1

Dresden 2

Dresden 3

Duane Arnold

Farley 1

Farley 2

FFTF

FitzPatrick

Fort Calhoun 1

Fort St. Vrain

Ginna

Haddam Neck

Hatch 1

Hatch 2

Humboldt Bay

Indian Point 1

Indian Point 2

Indian Point 3

Kewaunee

La Cross

LaSalle 1

Maine Yankee

31

6

44

221

9

20

32

4

43

107

19

8

4

26

46

47

137

9

73

77

147

6

12

117

8

4

1

197

38

2

25

8

28

12

46

188

19

I

18

4

38

2

90

5

45

3

82

47

72

136

3

43

76

67

136

6

126

5

26

39

2

8

7

19

11

58

221

8

20

27

4

40

141

19

67

7

25

30

87

137

3

126

49

144

6

10

118

11

40

90

8

3

. 9

7

5.13
TABLE 5.8. (contd)

Power Plant

McGiire 1

McGuire 2

Millstone 1

Millstone 2

Monticello

Data Set A

3

101

84

55

Data Set B

2

79

24

57

Data Set C

7

17

98

2

Nine Mile Point 1

North Anna 1

North Anna 2

Oconee 1

Oconee 2

Oconee 3

Oyster Creek 1

Palisades

Peach Bottom 2

Peach Bottom 3

25

14 '

3

8

19

3

142

26

41

177

26

4

2

7

8

4

10

8

7

11

3

7

9

3

160

213

50

177

3

44

167

53

75

Pilgrim 1

Point Beach I

Point Beach 2

Pairie Island

Pairie Island

Quad-Cities 1

Quad-Cities 2

Rancho Seco 1

Robinson 2

Salem 1

7

23

2

28

9

5

1

2

Salem 2..

San Onofre

San Onofre

San Onofre

Sequoyah 1

1

2

3

Sequoyah 2

St. Lucie 1

St. Lucie 2

Summer 1

Surry 1

Surry 2

Three Mile Island 1

Three Mile Island 2

Trojan

Turkey Point 3

Turkey Point 4

Vermont Yankee 1

Yankee-Rowe 1

Zion 1

Zion 2

13

35

85

7

39

15

20

5

3

3

3

23

5

78

97

86

2

1

34

34

40

23

39

10

19

7

1

3

13

34

95

7

33

6

20

88

8

33

3

23

4

50

3

18

5

84

10

21

5

3

3

65

122

2

6

90

63

1

I

24

12

8

26

1

23

39

9

24

39

5.14
TABLE,5.9. Failure-Modes for Data-Set A (1973-1984)

Failure ModeUnknown - -

Locked up

Out of tolerance

A - , .;Number of

Failed Snubbers--

- 49 --

255

e 923

Would not activate 1835

I

TABLE 5.10. Ultimate Failure Causes for Data Set A (1973-1983)

Ultimate Failure Cause

Unknown -

-Design

Installation

Operation

Number of

Failed Snubbers

- 25

886

257

1894

numbers in a given plant. It is. a fair assumption that the number of mechanical snubber failures will increase in the future because of the large numbers

being used. -'The'data from Table 5.13 are expanded in Table 5.14, showing

plant-specific failures.

Table 5.15 gives failure proportions.by year, calculated using the ratio

of the number of failures reported to the total number of snubbers'in'spected.

Only a fraction of the-LERs (^8%) reported the number of snubbers that!were

inspected; thus, the proportions calculated in Tahle 5.15 may be biased.

Similar.dataiare given in Table'.5.16 by plant. The-data-are summarized in

Tables 5.17 and 5.18 according to very broad failure mode and failure cause

categories. -In Table 5.19, theamean failure rates-by manufacturer andifailure

cause are compared as related to design, installation, and operation.,

Failure rates have been calculated and categorized in Tables 5.20-through

5.23. The failure rate is-calculated using the ratio:

No. of reportedjfailures--

'Total no. of snubbers in operation

5.15
TABLE 5.11. Snubber Incidents by Plant and Year for Data Set A

Plant 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983

Arkansas 1

Arkansas 2

Beaver Valley 1

Big Rock Point I

Browns Ferry 1

Browns Ferry 2

Browns Ferry 3

Brunswick 1

Brunswick 2

Calvert Cliffs 1

Calvert Cliffs 2

Cook 1

Cook 2

Cooper Station

Cooper 2

Crystal River 3

Davis-Besse 1

Diablo Canyon 1

Dresden 1

Dresden 2

Dresden 3

Duane Arnold

Farley 1

Farley 2

FItzPatrick

Fort Calhoun 1

Fort St. Vraln

Ginna

Haddam Neck

Hatch 1

Hatch 2

Humboldt Bay

Indian Point 1

Indian Point 2

Indian Point 3

Kewaunee

LaCrosse

LaSalle 1

Maine Yankee

McGuire I

McGuire 2

Millstone 1

Millstone 2

Monticello

Nine Mile Point 1

North Anna 1

North Anna 2

Oconee 1

Oconee 2

Oconee 3

-- -- 18 1 5 -- -- -- -- --

-- -- -- -- -- 1 23

-- -- -- -- -- -- 8 -- -- 1 1

6 55 -- -- -- -- -- -- 1 -- --

-- 22 4 -- 2 -- 1 -- -- 2 --

-- -- -- -- 6 -- -- __ _- -- -

-- -- -- -- 3 7 28 3 3 -- --

-- -- -- 20 7 10 44 7 132 1 --

-- -- -- -- -- -- -- 5 1 1 2

-- -- -- 8 8 1 1 2 -- -- --

-- -- -- 13 2 10 2 -- 3 -- 2 __ __ __ __ -- 1 -- -- 3 -- --

-- 6 -- 19 1 1 -- 2 3 4 6

-- -- -- -- 1 -- 2 13 87 1 3

-- -- -- -- 5 -- 1 12 -- -- 3 -- -- -- -- 8 -- -- __ __ -

__ __ __ 3 -- 1 -- -- -- -- --

-- 12 -- 2 2 -- -- -- 4 -- 6

11 -- -- 2 -- 8 -- 1 2 1 1

-- 1 1 -- 34 1 2 -- -- 2 6

-- -- -- -- -- -- 135 1 1 -- --

1 2 6

-- -- 3 -- 57 4 2 3 -- 2 2

30 10 -- 10 16 -- 11 -- -- -- --

-- -- -- -- 31 9 7 23 16 51 10

2 2 1 - - - 1

3 8 1 -- -- -- - --

__ __ __ __ 55 5 -- 51 4 1 1

3 1 2 2 --

4 - - _- - __ __ __ __ __ __

- 1~ -- -- __ __ __ __-- -- -

-- 149 -- 1 3 37 2 2 3 --

_- __ __ __ -- -- 35 3

__ __ __ __ -- 2 -- -- -- -- --

__ __ __ __ __ _- _- -- -- 4 21

__ __ __ 4 _- -- -- -- -- 2 2 __ __ __ __ __ _- - - - -- 3

76 8 5 8 2 -- -- -- -- 2 --

-- -- -- 1 2 -- 1 3 75 1 1

42 10 3

15 -- -- -- 4 1 1 -- 1 -- 3

_- __ __ -- -- 5 3 3 3

2 1 -- -- -- -- -- -- 5 - 1 2 __ __ __ __ __ 5 __ 4 __ --

-- -- -- -- -- -- -- 1 1 1 -

5.16
TABLE 5.11. (contd)

Plant : 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983

Oyster Creek 1 73 34 -- -- -- -- 7 3 13 10 2

Palisades -- 1 -- -- 1 3 3 3 8 6' 1

Peach Bottom 2 -- 1 -- 24 6 4 -- -- 4 2 --

Peach Bottom 3 -- 148 2 .9 14 -- -- -- 1 3 --

Pligrim I 1 14 -- -- 6 5 -- --

Point BeachI I- -- -- - :1 1 -- I -- 1. --

Point Beach 2 -- -- -- -- -1 1 -- -- -- -- --

Prairie Island 1 I- -- -- -- -- -- -- -- -- -- --

Prairie Island 2 -- -- -- -- -- -- -- -- -- --

Quad-Cities 1 7 -- -- 4 -- 2 -- - -- -- --

Quad-Citles 2 -- 9 4 5 5 2 3 -- -- -- 7

Rancho Seco I -- -- -- 36 14 5 8 9 3 10

Robinson 2 -- 1 -- -- -- 3 1 2 -- -- --

Salem1 -- -- -- 12 -- -- 3 -- 2 22 -

Salem 2 -- -- -- -- -- -- -- -- -- 4 11

San Onofre 1 -- -- -- -- 9 1 2 3 5 --

San Onofre 2 -- -- -- -- -- -- -- -- -- 5 --

San Onofre 3 -- -- -- -- -- -- -- -- -- 3

Sequoyah 1 -- -- -- -- -- -- -- -- -- 3 --

Sequoyah 2 -- -- -- -- __ __ __ __ __ 3 __

St. LucieI -- -- -- 11 -- -- -- -- 8 4

St. Lucle 2 -- -- -- -- __ __ __ __ __ __ __

Summer1 -- -- -- -- -- -- -- -- -- -- 5

Surry1 -- -- -- -- 1 12 22 4 1 11 27

Surry 2 -- -- -- -- -- 36 19 1 9 8 24

Three Mile Island 1 -- 15 16 2 45 2 -- 1 3 2

Three Mile Island 2 -- -- -- -- -- -- -- -- -- -- 2

Trojan -- -- -- -- -- -- -- -- -- 1 --

Turkey Point 3 7 -- 25 -- -- -- 1 -- -- I --

Turkey Point 4 5 6 -- -- -- 20 2 -- 1 --

Vermont Yankee -- -- -- -3 31 -- 2 3 1 --

Yankee-Rowe 1 I- -- -- -- -- -- -- -- -- --

Zion1 -- -- -- -- 6 5 -- -- 11 1 --

Zion 2 - -- -- -- 11 22 3 -- -- -- 3

The LER data provided no information about the total number of snubbers in

operation and no comprehensive list of.the snubber population was available.

It was, therefore, necessary to estimate this population. Five sets of

assumptions were used to estimate the population. The most conservative set of

assumptions assumed that an average power plant. commissioned before.1977

contained'100 snubbers while a plant commissioned after 1977 contained 300.

The least conservative set of assumptions assumed 1000 snubbers were present in

a pre-1977 plant and 2000 in a post-1977 plant. -

;In Tables 5.20 and 5.21,-only the most dangerous types of failures -(lock

up and would not activate) are included; oudf-of-tolerance-failures' have been

deleted. In Tables 5.22 and 5.23, all different types of failures are listed.

5.17
TABLE 5.12. Snubber Incidents by Plant and Year for Data Set B

IPlant 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 -~~~~__

Unknown

Arkansas I

Beaver Valley I

Big Rock Point I

Browns Ferry I

Browns Ferry 2

Browns Ferry 3

Brunswick I

Brunswick 2

Calvert Cliffs I

Calvert Cliffs 2

Cook I

Cook 2

Cooper Station

Cooper 2

Crystal River 3

Davis-Besse 1

Diablo Canyon I

Dresden 1

Dresden 2

Dresden 3

Duane Arnold

Farley I

Farley 2

FFTF

FitzPatrick

Fort Calhoun I

Fort St. Vraln

Ginna

Hatch I

Hatch 2

Indian Point 2

Indian Point 3

Kewaunee

LaSalle I

Maine Yankee

McGuire I

Millstone 1

Millstone 2

Monticello

Nine Mile Point I

North Anna 1

North Anna 2

Oconee 1

Oconee 2

Oconee 3

Oyster Creek 1

Palisades

Peach Bottom 2

Peach Bottom 3

-- - 10 22 -- -- -- -- 18 1 4

-- 6 4 - __ -- __ _

__ __ 19 4 - 2 -- - 6-- - 6 -

__ __ __ -- --. 3 7 __ __ _- -- 14 7 6 -- --.-- - 8 -- 1

-- -- -- -- 13 2 1

-- ~~ 6 1 19 -- 1

__ __ __ __ __ 1 __

__ __ __ -- -- 3 -

__ __ -- -- 3 - -

-- -- 32 6 2 28 --

- 10 9 12 2 -- 8 -- 1 2 -- 55 -- --

__ _- -- 8 57 -- 30 10 I 10 16 --

__ __ __ -- -- 30 9 __ -- -- -- -- 2 2 __ __ __ 5 -- 55 6

-- -- 15 1 3

__ __ __ -_ -- -- 2

__ ~~~~4

57 *8 5 8 -- -- --

52 *- ~~~I 5 -- -3 -

__, __ __ -- 4 1 1 -- -- -- -- -- 2 2

__ __ __ __ -- -- 3

-- 5

73 57 -- __ __ __ 1 -- 1 188 -- - 2 2 - . 1; 7 <25 6 5 --

. .147 2 10 15 1 --

3-- -- --- 3

23 -- -_ __ __ 8- -- ~ 1 1 __ __ 1 __ __

__ __ -- 3

28 3 5 -- --

23 5 132 1 --

-- 7 1 1 1

1 _ - -- 2 - -

__ _- 3

-- -- 3 2 6 __ __ -- 2 -

-- -- 88 1 --

1 -- -- -- 1

__ __ -- -- 45

__ __ 4 -- 10

-- 1 2 1 2 __ __ 4 10 --

135 -- -- I __ __ __ -- 1 2

43 -- -_ __ __

2 3 2 3 1

4 21 14 51 7

I __ __ __ I __ 50 4 5 1

1 4 -_ __ 2 - 2 3 --

35 -- -- 4 --

__ _ 4 4

__ __ -- 2

2 16 1 1 --

-- 4 -- -- _ -_

-- 3 -- 1 _

6 _ 1 1 __ 1. __

I 1,* 1 __ _

4 15 10 -- --

4 8 6 2 -

-'4 2 -- -4

__ __ 2 -- --

5.18
TABLE 5.12.- (contd) :

Plant

Pl Igrim .1 '''

Point Beach 1

Point Beach 2

Quad-Cities I - Quad-Cities 2

1973 1974- 1975 1976 1977 1978 -1979 -1980 1981 1982 1983 1984

...~;._

19 -=

7.

8

. 4

-6

21

1

Rancho Seco 1

Robinson 2

Salem 1

Salem 2

San Onofre 1

5 5 5 -2

-- -- ~~36 2

3 --12 - -

-- --. ~-9

7i-- -

San Onofre 2

San Onofre 3

Sequoyah I

Sequoyah 2

St. Lucle 1

1~

41-

3

3

1

2

2

_7

3

8

2

2_ 16 -

-- .-3 --

8 4 6-

Summer 1

Surry I

Surry 2

Three Mlle Is land 1

Trojan

Turkey Point 3

Turkey Point 4

Vermont Yankee 1

Yankee-Rove 1

Zion I -

Zion 2

__

.5 17, 2

8

90

. 4

4

2

2

3

2

2

9

4

1

11

__

10

8

1

__

4 __

21 .-- 24 __

I . -

1..: --

_ -, I

5

__I

8

3

-

-- 20

3 --

7 .- 5

1 , __-

. ... ,

TABLE 5.13. Snubber Failures by Year and Snubber Type for Data Set A .: ,-t .- ,

Year

1973

1974 -

1975

1976

1977

1978

1979

1980

1981

1982

1983

1984

Total

Unknown

Number Rate(a)

25 2.78

30 3;75

'1 -1.00

12 1.20

7 1.00

75 5.36

5 1.00

12 1.71

23 1.77

67 5.15

257 2.95

,Type of Snubber

Hydraulic

Number

272

472

54

161

385

248

308

;114

;, 330

134

66

2544

Rate

10.88

8*8.58

-2.84

.3.43

6.42

-3.76

-5.92

--2.43

-6.47

--3.35

2.00

5 .14

5.14

Number

14

- 17

21

- . 11

46

* 83

-- 20

49

. . 261

Rate Rate

-- 10.88

14.00 7.86 -- { >3.11

8.50 - -3.58

10.50 5.81

2.75:; 5.63

23;00 3.06

9.22 6.34

1.67 2.72

2.88 2.89

5.33 4.85 .. I. j.

> Mechanical

(a)"No. of failed snubbers reported per LER.

5.19
TABLE 5.14. Snubber Failures by Plant and Snubber Type for Data Set A

Type of Snubber

Plant

Arkansas 1

Arkansas 2

Beaver Valley 1

Big Rock Point 1

Browns Ferry 1

Unknown

Number Rate(a)

Hydraulic

4 4.00

Number

24

24

58

Rate

4.00

12.00

9.67

Mechanical

Number Rate

10 3.33

Browns Ferry 2

Browns Ferry 3

Brunswick 1

Brunswick 2

Calvert Cliffs 1

Calvert Cliffs 2

Cook 1

Cook 2

Cooper Station

Crystal River 3

Davis-Besse 1

Diablo Canyon 1

Dresden 1

Dresden 2

Dresden 3

Duane Arnold

Farley 1

Farley 2

FitzPatrick

Fort Calhoun 1

Fort St. Vrain

Ginna

Haddam Neck

Hatch 1

Hatch 2

Humboldt Bay

Indian Point 1

Indian Point 2

Indian Point 3

Kewaunee

LaSalle 1

Maine Yankee

McGuire 1

Millstone 1

Millstone 2

2

18

5

1

2

1

2

9

6

6

5

11

5

10

1

3

2.00

2.25

1.00

1.00

2.00

1.00

2.00

4.50

1.20

6.00

1.67

5.50

2.50

3.33

1.*00

1.50

14

6

26

216

9

19

30

4

33

105

19

4

20

34

28

137

3

68

66

142

6

12

72

4

4

1

197

38

2

1

7

98

10

2.33

6.00

2.89

7.45

1.80

4.75

3.75

1.33

2.36

13.13

3.17

2.00

2.86

3.09

3.11

45.67

1.00

1.00

9.43

4.44

1.50

4.00

8.00

1.33

15

9

6

3

13

45

4

7.50

2.*25

8. 00

3.00

1.00

13.00

45.00

2.00

4.00

1.00

10.37

9.50

1.00

1.00

1.40

9.80

1.25

14

3

74

2.33

1.50

24.67

5.20
TABLE 5.14. (contd)

Plant

Monticello 1

Nine Mile Point 1

North Anna 1

North Anna 2

Oconee 1

Unknown

Number Ratekd)

13 4.33

2 2.00

Type of Snubber

Hydraulic -

.Number Rate

42 - 21.00

25 2.50

12 1.20

3 1.00

- 7 1.40

- Mechanical

Number Rate

1 1.00

Oconee 2

Oconee 3

Oyster Creek 1

Palisades

Peach Bottom 2

Peach Bottom 3

Pilgrim 1

Point Beach 1

Point Beach 2

Quad-Cities 1

1

1

3

2

2

3

2

Quad-Cities

Rancho Seco

Robinson 2

Salem 1

Salem 2

2

1 1

3

4

1.00

1.00

3.00

1.00

2.00

3.00

1.00

1.00

1.00

3.00

2.00

1.00

5.00

2.00

1.00

7.50

11.00

2.40

9

1

137

24

39

174

:;24

4

2

13

28

84

6

22

2

17

14

33

53

74

8.29

3.43

1.00

1.00

3.25

3.00

1.00

6.52

1.41

3.00

1

2

San Onofre

San Onofre

San Onofre

Sequoyah 1

Sequoyah 2

1

2

3

1

5

2

1.00

2.00

2.15

4.67

2.00

22.00

2.00

4.25

7.00

2.36

4.82

3.89

7

14

9

2

1

3

3

9

7.00

I __

4.67

9.00

2.00

1;00

3.00

3.00

3.00

Z__

St. Lucie 1

Summer 1

Surry 1

Surry 2

Three Mile Island 1

5

45

44

12

Three Mile Island 2

Trojan

Turkey Point 3

Turkey Point 4

Vermont Yankee 1

Zion 1

Zion 2

Total

1

1

11

1

257

1.00

1.00

3.67

1.00

2.95

32

21

40

23

38

2544

: 10.67

10.50

4.44

2

1

2

'2.00

1.00

1.00

5.33

2.56 --

7.60 --

5.13- 261

(a) No. of failed snubbers reported per LER.

5.21
TABLE 5.15. Proportion of Failed Snubbers in Operating Nuclear Power

Plants for Data Set A

No.

Year Failed

76 44

77 178

78 91

79 24

80 7

81 214

82 5

83 --

Total 563

Hydraulic Snubbers

No. i lure .

Inspected Proportion, %

Mechanical Snubbers

No. No. Failure

Failed Inspected Proportion, %

531-

702

652

231

58

912

20

8

25

14

10

12

23

25

45

54

5

8

112

61

152

227

37

477

74

36 ' 2

22

3106 18 23

TABLE 5.16. Snubber Failure Data by Plant for Data Set A

Plant

Beaver Valley 1

Big Rock Point I

Brunswick I

Brunswick 2

Calvert Cliffs 2

No. of

Fal fed

Snubbers

23

2 .

2

145

16

No. of

Snubbers

Inspected

221 -

26

310

783

32

Fai lure

Proportion,

10

8

19

5

No. of Inspections

Snubber Type Hydraulic Mechanical

Unknown Snubber Snubber

2

3

2

2

Cook I

Cooper Station

Crystal River 3

Dresden 3

FitzPatrick

Fort Calhoun 1

GInna

Hatch I

Indian Point 2

Millstone I

Millstone 2

Oconee 3

Oyster Creek I

Quad Cities 2

Rancho Seco 1

St. Lucie 1

Surry 1

Surry 2

Three Mile Island 1

Turkey Point 4

Zion 2

19

6

75

I

57

25

2

45'

38

2

54

I.

12

7

52

4

47

19

45

20

20

156

200

261

4

230

184

8

61

125

to

152

1

99

24

198

214

199

30

259

20

24

12

3

29

25

25

2

14

25

74

30

20

2

2

I

36

100

12

29

26

2

24

63

17

too

83

2

2

4

2

2

5.22
TABLE 5.17. Snubber Failure Data by Failure Mode for Data Set A

No. of Fa Iled Snubbers/No. Inspected

Failure Mode

Unknown

Locked up

Out of tolerance

Would not lock up

No. of

Fai led

Snubbers

3

103

312

321

No. of

Snubbers

Inspected

88

427

1363.

1953

FaI lure

Proportion,

Snubber

Type

Unknown

Hydraulic Mechanical

Snubber Snubber

.3..

24"

23

- 16

3.18

; 5.83

7.00

1.95

4.50

1.33 - 11.27

4 4.08

t'.

1.67

7.20

4.75

1.75

S e F. D ; I

TABLE 5.18. Snub ber Failure Data by Failure Caus'e' for Data Set A

-No.Iof Fa led Snubbers/No. Inspected

Ultimate

Fa Ilure

Cause

Unknown

Design

Installation

Operation

No. of

Fa Iled

Snubbers

1

:74

11

653

No. of

Snubbers

Inspected

85

176

67

3503

Failure

Proportion,

1

16

19

Snubber

Type: - Unknown

3.00

1.75

2.30

3.52

Hydraulic

Snubber

4.50

8.02

2.73

4.77

Mechanical

Snubber

1.33

11.36

1.70

4.60

1,

I

-

5.23
TABLE 5.19. Number and Proportion of Snubber Failures by Manufacturer for Data Set A

Manufacturer

Unknown

Bergen-Paterson

Blaw Knox

Unknown Des

22 (1%) 104

__ 425

__ 1

Failure Cause

InstallIgn atlon

(6%) 168 (10%)

(63%) 46 (7%)

(25%) 3 (75%)

Failure Mode

Out of Would Not

Operation Unknown Frozen Tolerance Lockup

1450 (83%) 45 (3%) 150 (9%) 648 (37%) 901 (52%)

207 (31%) 1 (1%) 1 (1%) 57 (8%) 619 (91%)

__ __ -- -- 4 (100%)

U,

A.

ITT-Grinnell Corp.

Pacific Scientific

256 (51%)

9 (27%)

33 (7%)

6 (18%)

217 (43%)

15 (45%)

-- -- 215 (42%)

3 (9%) 13 (39%) 3 (9%)

291 (58%)

3 (9%) 14 (42%)

Anchor-Holth 3 __ __ -- 3

International

Nuclear Safeguards

Corp.

ITT Barton

McDowell Welmon

__ 91 __ -- 91

1

1

__ -- 1

__ -- 1

Power Piping Co. __ -- 1 I
TABLE 5.20. Snubber Failure Data by Year for Data Set A,

Assuming a Snubber Population

Year

1973

1974

1975

1976

1977

1978

1979

1980

1981

1982

1983

Total

No. of

Failures(a)

272

370

78

126

164

156

146

142

406

146

133

2139

100/300(b)

Snubber

Plant

77.71

77.08

13.68

18.26

19.29

16.08

14.17

12.03

32.74

11.50

9.78

21.41

300/500

Snubber

Plant'

25.90

25.69 -

4.56

6.60

7.63

6.64

5.96

5.26

14.71

5.20

4.49

8.81

500/800

Snubber

Plant

15.54

15.42

2.74

3.97

4.62

4.03

3.62

3.21

8.98

3.17

2.75

5.35

800/1000

Snubber

Plant

9.71

9.64

1.71

2.54

3 3.04

2.69

2.43

2.18

6.19

2.19

* 1.91

3.56

1000/2000

Snubber

Plant

7,77(c)

7.71

1.37

1.94

2.19

1.88

1.68

1.46

4.06

1.43

1.23

2.50

(a) No'"out-of-tolerance" failures are included.

(b) 100/300 means 100 snubbers in 1973-1976 and 300

(c) Failures/1000 snubber-years of operation.

snubbers in 1977-1983.

5.25
TABLE 5.21. Snubber Failure Data by Plant for

Assuming a Snubber Population

Data Set A,

Reactor

Arkansas 1

Beaver Valley 1

Big Rock Point 1

Browns Ferry 1

Browns Ferry 2

No of

Failures(a)

9

1

9

62

, 29

100/300

Snubb

Planter)

9.00

0.42

8.18

62.00

32.22

300/500

Snubber

Plant

3.00

0.25

2.73

20.67

10.74

500/800

Snubber

Plant

1.80

0.16

1.64

12.40

6.44

800/1000

Snubber

Plant

1.13

0.13

1.02

7.75

4.03

1000/2000

Snubber

Plant

0 .90(c)

0.06

0.82

6.20

3.22

Browns Ferry 3

Brunswick 1

Brunswick 2

Calvert Cliffs 1

Calvert Cliffs 2

Cook 1

Cook 2

Cooper Station

Crystal River

Davis-Besse 1

Diablo Canyon 1

Dresden 1

Dresden 2

Dresden 3

Duane Arnold

Farley 1

Farley 2

Fitzpatrick

Fort Calhoun 1

Fort St. Vrain

Ginna

Haddam Neck

Hatch 1

Hatch 2

Humboldt Bay

Indian Point 1

Indian Point 2

Indian Point 3

Kewaunee

LaSalle 1

6

44

218

8

12

13

4

41

107

5

8

4

22

44

46

2

9

16

61

131

6

4

58

5

4

1

156

3

2

25

2.86

20.95

242.22

8.89

5.71

14.44

2.22

41.00

50.95

2.78

5.00

20.00

40.00

51.11

0.95

10.00

17.78

61.00

87.33

5.45

10.00

64.44

3.33

5.00

1.25

156.00

1.25

2.00

1.71

12.57

80.74

2.96

3.43

4.81

1.33

13.67

30.57

1.67

1.67

6.67

13.33

17.04

0.57

6.00

5.93

20.33

52.40

1.82

3.33

21.48

2.00

1.67

0.42

52.00

0.75

0.67

1.07

7.86

48.44

1.78

2.14

2.89

0.83

8.20

19.11

1.04

1.00

4.00

8.00

10.22

0.36

3.75

3.56

12.20

32.75

1.09

2.00

12.89

1.25

1.00

0.25

31.20

0.47

0.40

0.86

6.29

30.28

1.11

1.71

1.81

0.67

5.13

15.29

0.83

0.63

2.50

5.00

6.39

0.29

3.00

2.22

7.63

26.20

0.68

1.25

8.06

1.00

0.63

0.16

19.50

0.38

0.25

0.43

3.14

24.22

0.89

0.86

1.44

0.33

4.10

7.64

0.4?

0.50

2.00

4.00

5.11

0.14

1.50

1.78

6.10

13.10

0.55

1.00

6.44

0.50

0.50

0.13

15.60

0.19

0.20

5.26
TABLE 5.21. i(contd)

100/300p 300/500

-No. of Snubb r Snubber

Reactor Failures(a) Plantib) Plant

Maine Yankee

McGuire 1

Millstone 1

Millstone 2

Monticello 1

8

3

93-

84

55,

7.27

3.33.

84.55

93.33

50.00

2.42

2.00

28.18

31.11

16.67..

500/800

Snubber

Plant -,

1.45

1.25

16.91

18.67

10.00

''Snubber

PI ant

0.91

1.00

10.57

11.67

6.25

Snubber

Plant

0.73(C),

0 0.50,

, 8.45 ,, 9.33

5.00

800/1000 1000/2000

Nine Mile

Point '

North Anna I

North Anna 2

Oconee,1

Oconee 2

; 22

11

3 - 8

10

20.00

6.11

* 2.50

7.27

10.00

6.67

3.67

1.50

, 2.42 .- 3.33

4.00

2.29

I *0.94 ,

,1.45

,2.00

2.50

1.83

0.75

- 0.91

1.25

2.00

0.92

0.38

0.73

1.00

Oconee 3

Oyster Creek 1

Palisades

Peach Bottom 2

Peach Bottom 3

Pilgrim 1

Point Beach 1

Point Beach 2

Quad Cities 1

Quad Cities 2

Rancho Seco 1

Robinson 2

Salermi 1

Salem'2

San Onofre 1

San Onofre 2

San Onofre 3

Sequoyah 1

Sequoyah 2

St. Lucie 1

2

133

23

32

39

; 22

2

2

. 28 -

48

7 7 t

. .- 17

4

20

I-,, 5

3

3

3

9

2.00

120.91

20.91

32.00

39.00

0.67

40.30

- 6.97

10.67 ,

13.00

0.40 0.25

24.18 15.11

4.18 - 2.61

,6.40,.- 4.00

7.80 4.88 . ... .. .

20.00 ;

1.82

' ' 1.82

11.82-

25.45

53.33,

6.36

' 8.10 '

4.44

18.18

0.20

12.09

,2.09

-3.20

3.90

6.67 4.00

0.61 '' 0.36

0.61 .,0.36

-3.94'' 2.36

8.48 5.09

17.78 10.67 '

2.12 , 1.27;-

4.86 ' 3.04',

2.67 1.67

6.06 3.64

2.50

0.23

0.23

1.48-

3.18

6.67

0.80

2.43

1.33

2.27-

6. ,0 .- 5.00

1.00

1.50

1.13

2.00

0.18

0.18

-1.18 -;

2.55

5.33 -

0.64

1.21.

0.67 " -1.82

2.50

0.50

0.75

0.56

16.67 -. 10.00-!', 6.25 ,

3.33

5.00

3.75

2.00

3.00

2.25

1.25

1.88

1.41

Summer 1

Surry 1

Surry 2

Three Mile

Island 1

Three Mile

Island 2

3

30

45

37

2

27.27

40.91

9.09

13.64

5.45

8.18

7.40

0.83

3.41

5.11

4.63

0.67

2.73

4.09

3.70

Q.33

37.00 12.33

2.22 1.33

5.27
TABLE 5.21. (contd)

Reactor

Trojan

Turkey Point 3

Turkey Point 4

Vermont Yankee 1

Zion 1

Zion 2

Total

No. of

Failures(a)

1

34

30

9

22

39

2139

100/300

Snubb9 r

Plantzb)

0.42

30.91

27.27

8.18

20.00

39.00

21.41

300/500

Snubber

Plant

0.25

10.30

9.09

2.73

6.67

13.00

8.81

500/800

Snubber

Plant

0.16

6.18

5.45

1.64

4.00

7.80

5.35

800/1000

Snubber

Plant

0.13

3.86

3.41

1.02

2.50

4.88

3.56

1000/2000

Snubber

Plant

0. 06 (c)

3.09

2.73

0.82

2.00

3.90

2.50

(a) No "out-of-tolerance" failures are included.

(b) 100/300 means 100 snubbers in 1973-1976 and 300 snubbers in 1977-1984.

(c) Failures/1000 snubber-years of operation.

TABLE 5.22. Snubber Failure Data by Failure Mode for Data Set A, Assuming a

Snubber Population

No. of

Failure Mode Failures

Unknown 49

Locked up 255

Out of tolerance 923

Would not activate 1835

Total 3062

100/300(a)

Snubber

,Plans~b (99 )9b)

0.49

2.55

9.24

18.37

30.65

Snubber

Plant

(242.9)

0.20

1.05

3.80

7.55

12.61

Snubber

Plant

(400.1)

0.12

0.64

2.31

4.59

7.65

Snubber

Plant

(600.4)

0.08

0.42

1.54

3.06

5.10

Snubber

Plant

(857.0)

0.06

0.30

1.08

2.14

3.57

300/500 500/800 800/1000 1000/2000

(a) Failures/1000 snubber-years of operation.

(b) Total number of snubber years of operation for each scenario measured in

units of 1000 snubber-years.

5.28
TABLE 5.23. Snubber Failure Data by Failure Cause for Data Set A, Assuming

a Snubber Population

Failure Mode

Unknown

Design

Installation

Operation

Total

No. of

Failures

25

886

257

1894

3062

10l/3oo(a)

Snubber

Plans~b (99 9)9b)

0.25

8.87

2.57

18.96

30.65

300/500

Snubber

Plant

(242.9)

0.10

3.65

1.06

7.80

12.61

500/800

Snubber

Plant

(400.1)

0.06

2.21

0.64

4.66

7.65

800/1000

Snubber

Plant

(600.4)

0.04

1.48

0.43

3.15

5.10

1000/2000

Snubber

Plant

(857.0)

0.03

1.03

0.30

2.21

3.57

(a) Failures/1000 snubber-years of operation.

(b) Total number of snubber years of operation for each scenario measured in

units of 1000 snubber-years.

5.29
6.0 PERSONNEL EXPOSURE TRENDS

The following section is a synthesis of actual radiation exposure data

occurring'during the maintenance and testing of snubbers, plus exposure

occurring during the ISI of piping and-other componentsin nuclear reactor.

systems. The data are insufficient to permit-statistical analyses; therefore,

only qualitative trends are possible (see;Table 6.1).'

The advantages and disadvantages of snubbers Oth emphasis on thevalue of

flexible systems are discussed in NUREG/CR-2136.2aJ .The:impact of removing

half of the snubbers from relatively new BWRs and PWRs is postulated. A basic

assumption is that the more flexible:systems,will;_have-a_-25%,.lower probability

of failure. The vital statistics of the two classes-of,plants.are shown in

Table 6.2. This table has been included for,-information only, and no attempt

has been made to validate it.

TABLE 6.1. Personnel Exposure Data . .. ~ . ..

I Plant

H. B. Robinson

Maine Yankee

Calvert Cliffs 1

Monticello

Vermont Yankee 1

Cooper, Station

Reactor ' Initial No. of

Type ,.Criticality ,Snubbers

PWR 9'20-70 ' 20

PWR 10-23-72 .- -75

PWR 10-07-74 352

BWR -12-10-70. - 92

BWR 3-24-72, - 50

BWR . 2-21-74 -269

* Occupational-Exposure.

.^ -; Tests, rem

100%-Visual ,-Functional

-0.50 Combined

- 3.6 Comhjgnd(a)'

0.30 nn

0.31 - 1.0 est.

1.2 -2 - Combined

0:50(c) .. NA : .

(a) -41 snubbers tested. -

(b) 33 snubbers tested.

(c) Dosage received each time on two inspections

(5/75; repairs on 5/76).

of dry well snubbers

(a) Landers, D. F., R. D. Hookway, and K. D. Desai. 1981. Effects-of Postulated Event Devices on Normal Operation.of Piping Systems in Nuclear

Power Plants. NlIREG/CR-2136,-Teledyne Engineering Services, Waltham,

Massachusetts.

6.1
TABLE 6.2. Relevant Snubber Data in NUREG/CR-2136

PWR BWR

Average snubbers per-plant 800 950

50% of the snubbers removed 400 475

Removal time for average snubber, man-hour 6 6

Removal time per plant, man-hour 2,400 2,850

Assumed exposure/snubber during removal, R/h 0.25 0.25

Exposure per plant, man-rem 1100 1100

Annual man-hour savings because removed snubbers 300 360

do not need to be examined

Improved ISI due to removed snubbers, man-hour 410 580

Maintenance and repair savings, man-hour 400 480

Cumulative savings, man-hour 1,100 1,420

Exposure savings/year for operation and 278 385

maintenance, man-rem

The preceding numbers can be adjusted for all plants now operating. For

example, at the time of this study, 41 PWRs and 16 BWRs were operating. If' a

simplistic assumption is made that all classes of plants have the same number

of snubbers and exposure per hour, a value of 6.8 x 104 man-rem is obtained for

the removal of 50% of the snubbers from all plants. Similarly, the annual

savings would be about 1.5 x 104 man-rem.

Similar calculations can be made for plants under construction in terms of

man-hours for removal and savings in exposure. In 1981, 43 PWRs and 20 BWRs

were under construction. Obviously, these numbers have changed over the past

several years. A prerequisite to removing snubbers is a prior analysis to

reestablish the design base; this analysis would require about 10,000 man-hours

per plant.

In early 1984, three utilities provided exposure information on their

plants over a period of years. One utility provided exposure figures covering

the maintenance and testing (5%) and removal and reinstallation (95%) of

snubbers on three plants. The following is a tabulation of annual exposure:

Annual Exposure Related to Snubbers, man-rem

Year Plant 1 Plant 2 Plant 3 Total

1983 47 42 29 118

1982 29 9 4 42

1981 122 56 9 187

1980 65 42 105 212

1979 8 9 0 17

Total 271 158 147 576

6.2
Another utility provided similar data for two plants for annual snubber

testing and maintenance.

Annual Exposure

Related to Snubbers, man-rem

Year Plant 4 Plant 5

1983 34 No data

1982 75 1.0

1981 158 0.1

1980 No data 1.0

1979 No data 1.0

1978 No data 2.0

A third utility provided data on one plant in its, first cycle of operation. The cumulative exposure was less than 1 man-rem for 100% visual inspection and 10% functional testing. These figures may berepresentative ofa new

plant; however, they should be different for older plants.

Exposure incurred in the NDE of welds or in the replacement of piping

should be reasonably representative of snubber maintenance and testing. 'Data

from the recent BWR intergranular stress corrosion cracking incidents were used

for comparison. Exposures incurred during both examination and removal were

collected from several sources and are summarized in Table 6.3. These data

were presented at a Committee on the Safety of Nuclear Installations-sponsored

meeting related to intergranular stress corrosion cracking and have not been

published.

6.3
TABLE 6.3. Exposure Values (in man-rem) Cited by Country for BWRs in Past 2 to 3 Years

Country

U.S.

NDE

0.5 to 0.75/weld

Replacement

<2000/plant

R aa Comments

Replacement assumed

Japan 3.8 to 4.5/recirculation

weld

752 Fukushima

141 Tokai 2

Recirculation loop

Reactor water cleanup

Reactor heat -removal

Reactor pressure vessel nozzle

Man-rem/h

100 to 300

200.to 700

100 to 400

20 to 10,000

Sweden 22.5/pl ant

on Italy 120/1

1984

1983

1981

1983

1981

1979

plant 1

85

16

6.5

TVO

90

60

20

(Garigliano)

Caorso

Caorso

Caorso

91

1982

1980

1978

Plus 18 for isolation, etc., or 130 total

60 for support activities

145 estimated

18

26

Finland Loviisa

130

110

140

TVO

100

50

3

Loviisa

230

220

110

Switzerland 2.2 to 3/weld

1.1 to 1.5/weld repeat

30? per weld?

Spain 30/plant

Germany

27 repair

Plant B 170

Plant C 570

Plant E 650

53 auxiliaries?

Includes dismantling (20% to 30%), construction

(45% to 65%), inspection (5% to 10%), miscellaneous (10% to 15%). Typical collective

dose/plant is 400 to 500 man-rem.
,j7.0 COSTS AND VALUE-IMPACT .,,

The information in Section 7.1 wasgathered during a preliminary study

conducted in 1982-1983.J--Thedata arepresented.to show the diversity and range

of information that has-been compiled in the literature., The data are typically presented in the raw or "as-reported' form, and no attempt was made to

justify the data. In Section 7.2, selected data are organized in a consistent

format to enable a value-impact'analysis to be performed. -

7.1 PRIOR VALUJE-IMPACT STUDIES ' -

The following information was supplied-inresponse-to the draft Regulatory

Guide on snubber. testing;,,no attempt has been made to confirm or, validate the

information: ,

* snubber costs: 1,000 to 10,000 lb $1000 to $2000

10,000.to 20,000 lb r $2000 to $2500

-20,000:to 30,000 l1b $2500,to $3000+' -

' 30,'000 to 50,000 lb .' -$3000'to $6000-

* installation costs per -snubber, .- $16,000

* annual maintenance inspection costs:

650-snubber plant ' $250,000

1000-shubber plant ; ' ' '$, 0' 'Ooo"

One source compares two plants as follows:-,

- Plant A - 10 years 'old. All hydraulic snubbers'. Ins'pcted every

6 months, requiring 600 man-hours plus 20 man-hour's to reinspect

questionable snubbers. Maintenance and inspection costs'f6r a

1000-snubber plant were $1,000,000 per year or about $1000/snubber.

Assumed that enough'snubbers were stockpiled to 'permit replacement

without extended shutdown.'. .

* , ~~~- , ,r :. :

* Plant B -6 years old. AVmix of hydraulic and mechariical' snubbers'

(650). Maintenance 'and repairs costs 'per year~estiaated to be ''

' with -$250,000 about 150 man-hours' for 100% inspection per year. '

It should be 'emphasize' that these 'numbers' were provided by individual

utilities as rough estimatesonly., Therefore, care should beitaken;when

extending these,-costs-,to-other 'plants-.-, ';This fact becomes evidentin, the,

maintenance and inspection costs :cited abo've ', Th 6 old er plant (all hydraulic

snubbers) reports an annual mainten'ance-and'inspection' cost of $1000/snubber. . ' '. 7, .. ,, , ''- I ' -, ;- , * t

7.1
The newer plant (mixture of hydraulic and mechanical snubbers) reports an

annual maintenance and inspection cost of only $385/snubber. There are several

possible explanations for this difference in costs; however, a probable factor

is the early problems that hydraulic snubbers had with polyurethane seals.

Although the seal problem has apparently been alleviated, the earlier high

failure rates for hydraulic snubbers may still impact the inspection and

replacement schedules for older plants.

In NUREG/CR-2136, three plants were examined with regard to snubbers. A

figure relevant to value-impact is the estimate pertinent to removal, testing,

and rebuilding. It was estimated that two 4-man crews would spend a 12-h shift

to remove 8 snubbers; a 2-man crew could test and rebuild about 12 snubbers per

shift. Thus, about 13 man-hours per snubber would be required for removal,

testing, rebuilding, and replacement. In terms of exposure, each man would

spend about 1.5 h in a field of 250 millirem/h, which would amount to

375 millirem/man.

A Battelle, Pacific Northwest Laboratories study assessed the implication

of removing some snubbers as an end product of a decoupling loss-of-coolant

accident plus a safe shutdown earthquake. The implications of reducing snubbers appears to be quite arbitrary. For example, such reductions in numbers

were assumed to reduce the probability of pipe failure by 25%, which probably

has no basis in fact.

Miscellaneous cost and quantity estimates relevant to snubbers are

compiled in Table 7.1. The information in Table 7.1 (which is an expansion of

Table 6.2) was drawn from various sources, primarily NUREG/CR-2136, NUREG/CR2800,() ~and NUREG 0933.(b) A "typical" PWR containing 800 snubbers and a

"typical" BWR containing 950 snubbers cited in NUREG/CR-2136 were used. A

removal time of.6 man-hours/snubber was assumed. Pipe restraints (100 per PWR

and 140 per BWR) were also considered; pipe restraint removal was assumed to

require 40 man-hours.

At best, the figures in Table 7.1 are averaged estimates; however, there

are obvious trends related to reduced exposure for snubber testing as well as

fewer snubbers and restraints. Results would depend on a particular model and

the age of that model. For example, testing of an older hydraulic snubber containing the early type of seals would confirm that they degrade rapidly; however, active programs on hydraulic snubbers resulted in regular changes in seal

materials and any conclusions drawn would be invalid for current snubbers. A

similar situation could exist with mechanical snubbers. Those of one

(a) Andrews, W. B., et al. 1983. Guidelines for Nuclear Power Plant Safety

Issue Prioritization Information Development. NUREG/CR-2800, PNL-4297,

Pacific Northwest Laboratory, Richland, Washington.

(b). Emrit, R., et al. 1983. A Prioritization of Generic Safety Issues.

NUREG-0933, U.S. Nuclear Regulatory Commission, Washington, D.C.

7.2
TABLE 7.1. Value-Impact Data for Snubbers

Assumptions

50% of pipe snubbers removed

50% of pipe restraints removed

Snubber removal time, man-hours

Restraint removal time (above backfit;

forward fit represents similar savings),

man-hours

PWR

400

50

2,400

2,000

BWR

475

70

2,800

2,800

Exposure per plant, man-rem - 1,100 1,400

Man-hour savings/year from changes:

For in-service testing

For pumps and valves (restraints)

Maintenance/repair

Piping reanalysis item (backfit plants),

man-hours

Overall design and incorporation

(50 h/week), man-hours

300

410

400

, 10,000

18,000

360

580 . 480

10,000,,

* -12, 500

Cost savings from removal/plant (assumes

savings of about $10,000/snubber or

restraint): .I

Backfit plants

Forward fit plants

Improved in-service testing and

maintenance

$8.2 x 106

$4.5 x 106

$53,800/yr

$8.9 x i05

$5.2 x 1o6

$69,'000/yr',

Man-rem savings/year:

Operation and maintenance.

Improved in-service testing(a)

(y. rI .

(a)' May be for plant life.. :'

:.. I I 280

-4 . 1 o

,- .

- 390

1,400

manufacturer did not operate correctly from initial installation; however,

extrapolation to other manufacturers is not justified because the failure

behavior was unique to one company.

A collection of cost data was provided in response to the.request for

comments to the draft RegulatoryGuide., Not-all people agree that the costs

7.3
cited for testing snubbers will be so high. Because of their variability and

incompleteness, they are not cited here.

A recent paper(a) cites cost factors on hydraulic snubbers using ethylenepropylene seals. The authors examined the economic impact of the snubber testing program in the Technical Specifications. Assuming $250,000/day power

replacement costs for the 690-liWe plant and outages of 2.5 to 3 days, costs

were estimated at $325,000to' $500,000 for the testing phase. The utility

estimates personnel exposure costs of $6000/man-rem, which is higher than some

other utilities. Costs of rebuilding and testing snubbers were set at

$4500/snubber. This BWR has substantial changes in ambient temperature..,

depending on location. For example, the impact on seal life as a function of

temperature is: 150'F, -10 years; 200'F, -3 years; and 250'F, -1.5 years.

These values indicate that relocating snubbers may have substantial cost

advantages.

7.2 VALUE-IMPACT ANALYSIS

This value-impact analysis examines the per unit costs associated with

snubbers installed at nuclear power plants. Due to the limited available data,

the costs used in this analysis must be regarded as rough estimates. The cost

of snubbers over the expected operating life of the plant is evaluated, and the

impact of reducing the number of these dynamic restraints is briefly discussed.

7.2.1 Snubber Unit Costs

The costs that are considered in this analysis include:

* initial or capital cost

* installation cost

* inspection and maintenance costs

* employee exposure cost.

All costs are reduced to-unit costs using weighted averages to account for cost

variations due to snubber size and type differences. When evaluated on a per

unit basis, cost savings that may result from manufacturer quantity discounts,

increased efficiency, and learning curve effects are ignored. For this analysis, these costs will be considered variable (i.e., directly proportional to

the number of units.) This approach will provide a good estimate of the direct

(a) Roberts, K., R. N. Kelly, and T. R. Branca. 1983. "Overview of a

Comprehensive Snubber Surveillance Program Implemented at the. Pilgrim

Nuclear Power'Station." Presented at 6th International Conference on

*-NDE in the Nuclear Industry, November 28-December 2, 1983, Zurich,

Switzerland.

7.4
cost associated with increasing or decreasing the number of snubbers at a

nuclear plant and is consistent with the overall accuracy of the rest of the

analysis.

Capital Cost

To estimate the initial cost of snubbers, letters were sent to the 62 foreign'and dome stic companies listed under "SNUBBERS" in the Nuclear News Buyers

Guide 1984, (a)requesting vendor data. A copy of the letter is shown in Figure 7.1. Responses were received from 18 domestic companies and 5 foreign

companies. Of these 23 responses, 16 indicated that they did not manufacture

snubbers. The seven remaining companies are listed in Table 7.2. These five

domestic firms and two foreign firms provided data on a total of 75 snubber

models (Table 7.3). One manufacturer estimated the size distribution of

snubbers in nuclear power plants to be as shown in Table 7.4. Using the data :

from Tables 7.3 and 7.4, the estimated average per unit capital cost is $3,250,

using a simple Weighted average-cost'.:

An-inventory of spare snubbers is'typically maintained to minimize'the -

risk-and duration of unscheduled plant outages due to snubber failure's.- The

inventory cost may be considered a part 'of the capital cost for the in-place

snubbers. An inventory of .30% raises the.total, capital cost per snubber to

$4,225. Warehousing.and incr mental administrativ6 costs for maintaining a

spare inventory will be ignored in this analysis.

Installation Cost

Because snubbers are dynamic devices, installation requires more effort to

ensure proper placement and alignment. Improper installation has been a major

cause of snubber failures. It has been estimated that 200man-hours are

required to install a typical snubber. A bur~dened labor rate of $25/h(b)

(based on Meane's Guide to Labor.,Rates-.in the Construction Industry-Pipe Fitters) results in an installation c6stf of $5',f00l/snubber. This study ignores -

ancillary costs (such as special tooling requirements) because it was felt that

they were beyond the accuracy of the analysis.

Inspection and Maintenance Costs

The costs associated with inspection and maintenance of snubbers in two

plants were given in "Snubber Cost and Test Information" (Guzy'June-4, 1982).

One of the plants had only hydraulic snubbers, and the second had both mechanical and hydraulic snubbers. The second plant appears to be more typical in - -

quantity of snubbers (650) and incorporates bothitypes *of snubbers. Therefore,

(a) Volume 27, No. 4.

(b) Discussions with an engineer for the Washington Public Power Supply System

indicated that this value may be low by as much as a factor of two.

7.5
~BanlleI~

Pacific Northwest Laboratories

P.O. Box 999

Richland. Washington U.S.A. 99352 October 4, 1984 Telephone 609)

Telex 15-2874

Dear Mr.

Battelle-Northwest has undertaken a project for the United States Nuclear

Regulatory Commission that involves gathering data on snubbers currently

available to the U.S. commercial nuclear power industry. Data from the

major manufacturers of snubbers will Be assimilated to provide a data base

of equipment characteristics and suppliers.

To assist us in'this project, and to have your equipment included in the data

base, please send information describing your snubber product line, including

load capacities and prices to:

Battelle, Pacific Northwest

P. 0. Box 999

Richland, Washington 99352

Attn: Ryan E. Dodge

Research Engineer

Laboratories

We appreciate your assistance in this

you in the near future.

effort and look forward to hearing from

Sincerely,

Ryan E. Dodge

Research Engineer

RED:ri

FIGURE 7.1. Sample Letter Sent to 62 Companies Requesting Snubber Information

7.6
TABLE 7.2. Snubber Manufacturers Responding to Request for Information

Manufacturer

Bergen-Paterson Pipe Support Corp.

(BPPC)

Lisega-Kraftwerkstechnik GmBH (LIS)

Western Piping & Engineering Co., Inc.

(WP&E)

Sanwa Tekki Corporation (STC)

Paul-Munroe Energy Products (PMEP)

Anchor/Darling Industries, Inc. (ADII)

Pacific Scientific - Kin Tech Division'

(PSA)

Address

74 Commerce Way

P.O. Box 4011

Woburn, MA 01888

Postfach 1340, D-2730

Zeven, Germany

1485 Yosemite Avenue

San Francisco, CA 94124

6-5-19, Minami Shinagawa-ku

Tokyo, Japan

1701 W. Sequoia Ave.

P.O. Box 5900

Orange, CA,-92667

Wambold Rd.

P.O. Box 300

Kulpsville, PA 19443

1346 S. State College Blvd.

Anaheim, CA 92803

data from that plant were used in this analysis. The plant reported a total

cost of $250,000/year for inspection and maintenance of 650 snubbers

($385/snubber/year). When reducing the'total cost to a per unit cost, it may

be helpful to think-of the reduced valueas.the sum of the inspection costs and

the average maintenance costs times the probability of requiring maintenance;

for a single snubber.

Employee Exposure Costs

It is difficult to associate dollar costs with increased employee expos-.

ure. The actual costs that might be incurred due to worker "burn up," longterm medical expenses, and unfavorable public opinion are-ambiguous and not'

amenable to quantification. A rule of thumb that is often used to evaluate the

cost effectiveness of implementing exposure mitigation measures is that a

$1,000/man-rem reduction in exposure is'cost effective (see, for example,'

10 CFR 50, Appendix-I). Determining the appropriate cost to assign to employee

exposure is certainly a subjective process. However, it is important to

recognize employee exposure as a true cost to the utility and to include it in

the value-impact analysis. The $1,000/man-rem rule of thumb is reasonably

7.7
Model

AD-40

AD-70

AD-150

AD-500

AD-1600

AD-5500

AD-12, 500

PMH 2101

PMH 2103

PMH 2200

PMH 2300

PMH 2400

PMH 2500

SMS-01K

SMS-04K

SMS-1K

SMS-3K

SMS-9K

SMS-16K

SMS-25K

SMS-40K

SMS-65K

SMS-1OOK

SMS-160K

SMS-250K

WPE 50

WPE 100

WPE.35

WPE.i65

WPE 1. 5

WPE 6.0

WPE 15

303251

304251

305251

306251

307251

308251

l309259

300259

322259

323253

TABLE 7.3.

Manufacturer(a)

ADII

ADII

ADII

ADII

ADII

ADII

PMEP

PMEP

PMEP

PMEP

PMEP

PMEP

STC

STC

STC

STC

STC

STC

STC

STC

STC

STC

STC

STC

W1P&E

WP&E

WP&E

WP&E

WP&E

WP&E

WP&E

LIS

LIS

LIS

LIS

LIS

LIS

-LIS

LIS

LIS

LIS

Manufacturer

Load, lb

400

700

1,500

5,000

16,000

55,000

125,000

1,000

3,000

10,000

35,000

53,000

105,000

110

400

1,000

3,000

9,000

16,000

25,000

40,000

65,000

100,0oo

160,000

250,000

50,000

120,000

350

650

1,500

6,000

15,000

1,800

3,600

7,200

22,500

45,000

78,700

112,400

225,000

900

1,800

Data on Snubbers

Type(b) Stroke,in

M 3

M 4

M 4

M 5

M 6

M 6

14 6

H 5

H 5

H 5

H 5

H 5

H 5

H 4

M 4

M 4

H 4

4

M 4

tfl 4

M 4

M 4

M 4

M 4

M 4

H 6

M 6

M 4

M 2.5

14 4

H 5

H 6

H 5.9

H 5.9

H 5.9

H 5.9

H 5.9

H 5.9

H 5.9

H 5.9

M 5.9

M 5.9

Price, $

790

815

1,200

1,565

1,860

6,100

11,160

NA(c)

NA

NA

NA

NA

NA

575

600

625

815

865

1,205

2,14n

2,745

3,790

7,400

12,400

18,400

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

7.8
-TABLE 7.3. (contd)-

Model Manufacturer(a) Load, lb Type(b)

325251

326251

311153

312153

313153

2500-3-6t

2500-10-6

2500-20-6

2500-30-6

2500-50-6

2525-3-6

2525-10-6

2525-20-6

2525-30-6

2525-50-6

2530-3-6

2530-10-6

2530-20-6

2530-30-6

2530-50-6

PSA 1/4

PSA 1/2

PSA 1

PSA 3

PSA 10

PSA 35

PSA 1o0

Stroke, in. . Price,- $

ILIS

LIS

LIS

LIS

BPP

BPPC

B PPC

BPPC

BPPC

BPPC

.BPPC

.BPPC

BPPC

BPPC

' BPPC

BPPC

BPPC

BPPC

BPPC

PSA

PSA PSA PSA

PSA

PSA

PSA

PSA

.7,200

22,500

281,-000

562,000

843,000

23,000

30,000

50,000

30,000.

10,000

20,000

30,000

50,000

3,000

;10,000

20,000

30,000

,~50,000

350

650

1nn50

6,000

15,000

50,000

120,000

M

* H

H

H

H

H

* H

H

H

H

H

H

H

H

H

H

H

M

M

H

M

'H

M

5.9

5.9

3.93 ': fi 3.93.

3.93

~6

6

6

6

6

6

6

6

6.

6

6

6.

6

4

2.5

4

5

6

6

6

NA

NA

NA.

NA

;3,355

-3,590

- 4,964

5,619

8,236

3,624

3,880

5,303.

6,173

9,278

3,874

4,222

5,887

6, 776

11,098

845

870

1,276

1,665

1,975

6,473

11,838

I

(a) See Table 7.2 for manufacturer names.

(b) M - mechanical; H - hydraulic.-

(c) NA - not available.

TABLE 7.4. Estimated Size Distribution of Snubbers

Size, kips

1

3

10

25

45

~ _- 70 - ;

110 -

200 to 2000

% of Snubbers

.8

40

28

10

5

3

2

4

I , 2

7.9
conservative and provides consistency with some other published reports. This

number was therefore used in this analysis to quantify the exposure costs

relevant to snubbers.

Employee exposure presents an additional level of uncertainty into the

analysis. As before, the direct costs must be estimated. In addition, the

number of snubbers to be inspected must be estimated.

Table 6.2 indicates an average removal time of 6 man-hours per snubber and

an average exposure level of 0.25 rem/h during removal.(a) If it is assumed

that a similar effort is required to reinstall snubbers, then the average

exposure for removing and replacing a single snubber is 3.0 man-rem.

However, not every snubber will need to be removed for inspection. A

simplified version of the testing schedule outlined in ASME/ANSI OM4 is used in

this analysis to estimate the fraction of snubbers that would be inspected each

year. It was assumed that 10% of the snubbers must be inspected each year. If

a failed snubber is found it must be repaired or replaced; and a second sample

of 10% must be inspected.(b) If a failed snubber is found in the second

sample, it must be repaired or replaced; and another sample of 10% must be

inspected. This sampling inspection is repeated until a sample passes

inspection with no failures or all snubbers have been inspected.

The average sample size each year may then be estimated as:

co ~~~~~~i

S = S i1(xN)

where x = sample size = 10%

N = total snubber population at a given plant

f = failure rate/snubber/year.

Several assumptions and simplifications are incorporated into the above

estimate including:

* failures occur randomly

* samples are chosen randomly

(a) Discussions with an engineer for the Washington Public Power Supply System

indicated that this man-hour estimate may be high by as much as a factor

of two.

(b) A proposed revision to ANSI/ASME OM4 would require a reinspection sample

size of half the original sample size (or 5%).

7.10
* impact of failures found in preceding samples.are not taken into :'s

account. '

,A mean failure 'rate of 1.91 failures/1000 snubbers/year is. given in

Table 5.20 for 1983, assuming a population of 800 to 1000 snubbers per plant.

Using an annual per snubber failure rate of f = 0.00191 and population, the

average annual sample size is approximately 124 snubbers (12.4%).' The-resulting annual exposure rate per snubber can therefore be estimated as the product

of the average exposure per snubber'removed times the average sample size

(3.0 x 0.124), giving an'annual exp'osurie: rate of 0.37 man-rem/snubber/year.' -'

Using th6 $1000/man-rem rul'eof thumb discussed earlier, the exposure'cost'

would be $370/snubber/year. -

Unit Cost Overthe -Operating Life of the Plant.

.The snubber unit costs over the operating-life of.a new plant may.be cal--

culated using the preceding cost estimates.., Aplant operating life-of-,40:years

will be assumed and the present value of the-cash flow will be evaluated-using;

discount rates of 2%, 5%, and 10% (Table 7.5).

TABLE 7.5.'-Summary-of Snubber Costs Assuming a 40-Year Plant Life and .

Discount Rates of 2%- 5%i and 10% .

Cost Category

Capital cost

Installation cost

Inspection/maintenance costs

Employee exposure cost

Total

I.,

-!,-2% Real.Discount Rate

Cost,,$ Timing Total Cost,$

''4^225 0 4,225

5,000 0 5,000

385 Annual 10,532

370 Annual 10A122'>'

29,879

-, ';5% Real Discount Rate'

Cost; $ Timina - Total Cost.-$.

Capital cost

Installation'cost

Inspection/maintenance

Employee exposure cost

Total

cost

4,225 O' 4,225

5,000 , 0 5,000

:385 Annual''1 6,606

370 Annual 6,349

:-:- 22,180-:-

"Capital cost

Installation cost

.,,Inspection/maintenance

Employee' exposure'- cost

Total

I _ * *IX !; I , :

-' ", Cost, $

4,225

5,000

costs- ;,,385.

- . - 370

10% Real Discobunt'Rate'

Timing Total Cost,,$

0 4,225

0 .5,000

,Annual 3,765

Annual _. 3,618,

16,608

7.11
Based on the estimates and assumptions discussed in this report, the total

discounted cost per snubber over the operating life of a reactor is between

$29,879 and $16,608, depending on the applied discount rate. For a plant using

1000 snubbers, the total cost of the installed snubber would be from $16 to

$30 million.

'Other Factors

The risk of increasing the number or length of plant outages is another

factor that impacts the total cost of snubbers. The NRC has stated that when

inoperable snubbers are found they must be replaced in a timely fashion or the

plant must be shut down. In addition, the various inspection schedules proposed generally require testing more samples at shorter intervals when inoperable snubbers are found. These schedules could conceivably require testing all

snubbers in a plant. For a nuclear plant designed with 1000 snubbers, such a

requirement could represent a sizable endeavor. With an approximate cost of

$1,000,000 per day for plant shutdown, any downtime imposed due to snubber

inoperability would seriously impact the above analysis.

The potential for failed and locked snubbers to cause a pipe failure was

another factor that was not considered in this analysis. The potential for

damage to the plant and/or for plant shutdown could result in very high costs.

A factor that could decrease the estimated snubber costs is the advent of

in situ-testing equipment for snubbers. Although several manufacturers are

currently offering such equipment, in situ testing has not yet been approved by

NRC.

7.2.2 Reducing the Number of Snubbers

Much attention has been given to the possibility of decreasing the number

of snubbers used in nuclear power plants. The number of snubbers could be

reduced through increased attention to design and through a relaxation in

design requirements. Several studies, including recent work by Lu and Chou~a)

at LLNL, have indicated that appropriately decreasing the number of snubbers in

use at nuclear power plants may actually increase piping reliability.

Savings from Reducing the Number of Snubbers

Since the above analysis was done on a per snubber basis, cost savings

attributable to snubber reductions can be evaluated directly. For example, if

a proposed plant with 1000 snubbers were to reduce the number of snubbers by

(a) Lu, S. C., and C. K.-Chou. 1984. Reliability Analysis of Stiff Versus

Flexible Piping - Interim Project Report. NUREG/CR-3718, Lawrence

Livermore National Laboratory, Livermore, California.

7.12
30%, a cost savings of $9,732,900 would be indicated. However, this savings

would be expected only for plants not yet designed or built because no redesign

or backfit costs have been considered. '

Cost of Reducing the Number of Snubbers in Existing Plants 11 .1,

For backfit applications, the cost of redesigning and implementing a more

flexible system must. be estimated and then subtracted from the/predicted cost

savings. The cost of backfitting plants to reduce the number of snubbers has

been estimated to be from $820,000 to $890,000 per plant. This cost does-notinclude the increased employee exposure-that would result from repiping and

removing unnecessary snubbers. However, this exposure rate might be expected

to be on the same order as that incurred from inspecting and maintaining a

snubber (estimated to be 2.5-man-rem/snubber).' As before, the $1000/man-rem

rule of thumb was used to quantify-this cost. *The cost savings would be

$2,500/snubber for each snubber eliminated.- -

Present Value of Reducing the Number of Snubbers

In a plant containing 1000 snubbers,-a-30% reduction in the number of

snubbers could be expected to result in a total cost reduction of from $526,000

to $3,500,000 (Table 7.6). All of the previous caveats regarding the impact of

hidden costs such as increased plant downtime or damage to piping still

apply. -The exposure/implementation costs were obtained by multiplying th'e unit

cost ($2,500) by the reduction in snubbers (300).

In Situ Testing

The implications of in situ testing of large snubbers that are

ble to removal have been recognized in the context of valjue-impact.

tion on in situ testing is currently being compiled and-evaluated.

not amenaInformaTABLE 7.6. Estimated Cost Savings Resulting from Reducing the Number of

Snubbers in an"Existing Plant(a

Total Costs. S

Cost Cateqory

Capital

Implementation , -

Inspection/maintenancei

Exposure/Implementatlon

Exposure savings

Total

Cost, $ YTinminq

0

855,000 0

(115,100) Annual

-750,000 0

(111,000) Annual

2% Discount

Rate

0

855,000 -

(2,577,832)

- '750.000 -

(2,486,007)

(3,458,839)

5% Discount 10 Discount

Rate Rate

0 0

855,000 " -855,000

(769,369) (1,085,038)

.:750,000 750,000

(1,706,342) (1,046,388)

(1,870,711) (526,426)

(a) The following assumptions were used: 30-year remaining plant life; 2% real

discount rate; 300 unit reduction In snubbers (and a representative

population).

7.13
7.2.3 Increasing Snubber Reliability

The savings that may be realized through improved snubber reliability can.

be estimated, or, equivalently, the sensitivity of the current analysis to the

estimated mean failure rates may be investigated.

Reducing the mean failure rate for snubbers will primarily impact the

average sample size that must be inspected. In the preceding section, an

average sample size of 1.8% was estimated for a mean failure rate of 0.0036.

If the mean failure rate is reduced by 25% and 50% (to 0.0027 and 0.0018,

respectively), then the corresponding sample sizes may be calculated

(Table 7.7).

The average sample size should impact the inspection/maintenance and

employee exposure costs directly. If these costs are scaled directly with the

sample size, the net present value of the total costs of snubbers can be

estimated assuming a 40-year plant life. These results are shown in Table 7.8.

TABLE 7.7. Average Total Sample Size for Specified Mean Failure Rates

Mean Failure Nominal Sample Assumed Snubber Average Sample

Rates Size, X Population Size,

0.0036 10 900 14.8

0.0027 10 900 13.2

0.0018 10 900 11.9

TABLE 7.8 Summary of Net Present Value of Snubber Costs for Various Mean

Failure Rates

Assumed Failure Rate

Cost Category Timing 0.0036 0.0027 0.0018

Capital 0 4,225 4,225 4,225

Installation 0 5,000 5,000 5,000

Inspection/Maintenance Annual 385 343 310

Exposure Annual 370 330 298

Net Present Value, 2%, 40-yr life 29,879 27,635 25,857

Net Present Value, 5%, 40-yr life 22,180 20,773 19,658

Net Present Value, 10%, 40-yr life 16,608 15,806 15,171

7.14
8.0 IMPLICATIONS OF AGING AND DEGRADATION OF SNUBBERS

A review of Tables 5.3, 5.4, and 5.5 reveals the large number of failure

mechanisms,'many of which are related to-aging/degradation.. In the case of

fiyarauTic snubbers, several of the failure mechanisms result in a loss of.fluid

and a consequent loss of function. <This.occurrence.will be a fail-safe-mode in

that there should be no lockup and overstressing of the pipe. Based on early

arguments on'flexible versus stiff piping, the lack of operation.of hydraulic

snubbers during a seismic event should have limited-effects. Whether,-the same

can be said concerningmore severe dynamic loads depends'on analysis or testing. It can be concluded that the loss of function of some hydraulic-snubbers

should have a limited effect on the probability of failure of piping during a

seismic'event (see Section 1). - -

With mechanical snubbers, the situation is not as clear. One obvious

aging/degradation mechanism--corrosion of internojyS-er-a period of time--has.

been known to lead to loss of function and lockup. In this instance, the

situation is not fail-safe; and mechanical snubbers located in regions of high

thermal expansion of-piping during heatup and cooldown may severely stress.

snubbers or piping or'both. If sufficiently high stresses occur,-the snubber

may be torn from the wall or the pipe may be damaged.. In regions of lower

thermal expansion, the locked up snubber may exist for a substantial time. If

a dynamic load occurs, the snubbers will behave as rigid members and they or.

the pipe may be damaged. Mechanical snubbers have locked up without being

detected for. several days or weeks. Obviously, this condition is not

desirable.

;Several utilities have extensive snubber replacement programs,.replacing

one form of hydraulic for another or replacing mechanical for hydraulic. Thus,-

the age factor cannot be quantified using an assumption that the samesnubbers

were originally in the system. The same can.be said for seals and seal materials in hydraulic snubbers where several-changes have occurred .in original,-

materials as well as replacement materials.

Obvious degradation mechanisms include seal j.gig.-in..hydr-aiii nbberg

and vibrations in mechanical,snubbers. Several load conditions,-such as water-

'and steam hammer or-valve closure,.can render.snubbers inoperable. Under these

conditions, loss of function can occur whether the snubbers are new or old.

The following paragraphs represent suggested actions that could, in the

opinion of the authors, improve the status of snubbers. These suggestions

should not be considered to be current ASME or NRC positions.

If the following three questions can be answered affirmatively, there is

some assurance that snubbers will function as desired:

8.1
* Has an adequate inspection program been conducted to assure that the

snubbers functioned correctly when installed?

* Has there been an appropriate environmental qualification program

carried out that establishes the aging/degradation mechanisms in

snubbers or the anticipated life span before loss of function?

* Does an adequate ISI and testing program exist to permit the detection of-loss of function in a reasonable time frame or, more importantly, the detection of the onset of generic failure mechanisms at

an early stage.

If the draft Regulatory Guide were converted to an active Regulatory

Guide, the aspect of an inspection program should be resolved. While it could

be argued that further modifications could result in improvements, it appears

that the draft Regulatory Guide covers the salient aspects pertinent to an

inspection program.

Environmental qualification-has been handled on a case-by-case basis, and

there is no assurance that a generic program exists. Furthermore, there are

several aspects for such a program. Under normal operating conditions, ant

ocean site will require a different environmental qualification program than an

inland site. In-addition, a high-temperature dry well of a BWR will be

markedly different from reactor regions near ambient temperature. In essence,

the areas of environmental qualification are not well defined and specific

criteria are needed.

With regard to IS1 and testing, there should be a reassessment of existing

requirements to establish what modifications may be necessary. The existing

visual examination in ASME XI may not be adequate, and the bench testing program probably requires review and-modification. Currently, only smaller

snubbers are covered by ASME XI; expansion will be required due to the failures

of larger snubbers, particularly hydraulic ones. It may be necessary to'test

in situ in recognition of the massive size of these larger snubbers. In this

case, a simpler test-may be necessary to determine the functionality of these

snubbers. ASME OM4 represents a positive step toward improving testing; however, it may not be totally adequate. Some of these aspects are discussed in

Section 10.

8.2
9.0 'PHASE II STUDY

Several generic aging problems exist that provide a basis for'a'Phase II

program. ..There is a finn tine-between tr ue a.p.ging effecta.angggrca ated

effects that-arp-not repruesentAtti:venf aging_ Some obvious effects would be

mechanical snubbers in a vibrating environment where snubbers could fail in

less than 100 h. While vibratory failure is important, the mechanism is

directly related-to amplitude and number of cycles. .

Table 3.3 is an obvious basis 'for developing a program on older hydraulic

and mechanical snubbers. It -conta'ins the'parameters cited.'in the 'draft

Regulatory Gu'ide, ASME 0M4, and ASME XI. The table'is' aqualitative presentation of aging factors such as wear; corrosion, and contamination. A selection

of both hydraulic and 'mechanical snubbers' could have a series of tests covering

spring rates, release rates,'dead band, drag, etc.', that would note deviations

from normal. The snubbers-could then be disassembled.to ascertain the causes

for the deviations. Any quantification will-depend on the snubbers that are

selected. ASME 0M4 is suggested as a benchmark for such testing.

A specific position has been included'in'the NRC NPAR Program. It is

repeated below'as an appropriate scope for-a Phase II study:

Aging Assessment and Analysis of Snubbers'and Recommendations for

Inspection, Surveillance and Maintenance

Objective: To evaluate and assess the performance of hydraulic and

mechanical snubbers. Establish failure~mechanisms and causes and

provide recommendations for practical and cost effective inspection,

surveillance and maintenance 6f-snuibbers in nuclear power plants.

Conduct a Phase'-II assessmentof fsnubbers in accordance with the

NPAR 'strategyfand involving the f6l'lowing'specific research

elements: ' ' i;., - . ,

A. Comprehensii'e Aging Assessme`t - Based upon the reviews of the

operating experiences and analysis performed,'in Phase 'I

(NUREG/CR-4279) conduct a comprehensive aging-assessment

including, as necessary, postservice examinations and laboratory testing of naturally-aged snubbers. To be included are

effects of accident conditions (e.g., seismic and LOCA effects)

on the behavior of aged snubbers. Both mechanical and hydraulic snubbers will.be included in the assessment. Effects of

aging factors such as wear=,corrosion and contamination will be

assessed in snubber examination and testing Testing considerations will include spring rate, release rate, dead band and

9.1
drag, assessing deviations from expected norms. Effects of

size and manufacturer will be considered in selection of

snubbers for testing, recognizing the limitations to address a

major cross section of the potential alternatives. A significant task during the testing phase will be to evaluate potential performance indicators for snubbers.

B. Review and Verification of Inspection, Surveillance and Monitoring Methods and Technologies - The Draft Regulatory Guide,

"Qualification and Acceptance Tests for Snubbers Used in Systems Important to Safety," deals primarily with qualification

testing and acceptance testing. Pre-service and in-service

inspection aspects for snubbers have been addressed in codes

and standards ASME-XI and ANSI-OM4-1982, "Examination and Testing of Nuclear Power Plant Dynamic Restraints (Snubbers)."

Also, the plant specific or standardized technical specifications deal explicitly with this area. Based upon the reviews

and analysis performed in Phase I (NUREG/CR-4279) and in conjunction with the aforementioned codes and standards identify

and recommend inspection, surveillance and monitoring methods

and technologies considered useful in detecting and trending

aging and service wear effects in snubbers, which will remain

in use on safety related piping and components. The recommended methods must-complement the present practices and

requirements. The study should further consider whether

advanced methods for snubber maintenance and in-service evaluation are feasible and cost effective.

C. Application Guidelines - The recommendations for inspection,

surveillance and monitoring methods and the, guidelines for

their applications will be based upon a coordinated effort with

the codes and standards committees and through proper interaction with the NRC staff. Application guidelines will also

include considerations for maintenance and service lifetime

prediction methods for snubbers.

9.2
10.0 SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS

The Nuclear Plant Aging Research (NPAR) program strategy (Figure 10.1) is

used to determine the current status of these investigations and to determine

additional requirements. The current status is:

* Select Equipment and Define Boundary - Expert opinion has been

applied; specific suggestions have been made concerning a redefinition of how many snubbers should be used. As'indicated, there could

be a substantial reduction in numbers using the current state-of-theart. In the context of a prioritization study, systems, risk, and

agingteffects have been considered.- -The first priority is to reduce

the number of snubbers. The second priority is to place definitive

tests such as those cited in Section'3 into practice to evaluate

aging before it becomes a controlling factor.

* Evaluation of Operating Experience - A major effort has been expended

evaluating operating experience. Several sources were examined to be

sure that the most significant information was reviewed. The

principal source was the LERs;"however, other sources were checked

and applicable industry practices were examined.

* Comprehensive ACin' Assessment - -The LERs were used to establish

aging/degradation mechanisms; While it is possible that some failure

mechanisms were missed, we are confident that most were detected.

Parameters such as design specifications,'operation, and to a limited

degree ongoing research were examined. It 'appears that more work is

required.

o Review and Recommendations for Inspection, Surveillance, and

Monitoring - This aspect has been examined in the context of existing

and proposed codes, standards, regulatory guides, and industry

practice. These factors have. been examined in considerable depth.

The aspect of cost-effective performance indicators has -been considered, primarily with respect to its implications to snubbers.

There is very little guidance that appears relevant to the review and

verification of advanced methods such as in situ testing.

* Application Guidelines -'Most factors have been 'considered to some

degree; for example, a value-impact analysis was conducted. We have:

had active interactions with ASME XI and the Board of Nuclear Codes

and Standards as they relate to snubbers. Furthermore, ongoing

activities within'the PVRC and the NRC Piping Review Committee relate

10.1
I0)

FIGURE 10.1. Nuclear Plant Aging Research Program Strategy
directly to snubbers. With regard to life extension, there has been

substantial progress in the past 10 years. Further work is required

and should be pursued.

Maintenance is a significant issue. While progress has been made, maintenance often leads to further degradation rather than improvement (see

Table 5.5). Obviously, this area requires further action. No attempt has been

made to develop guidelines to predict-service life. -The diversity of failure

mechanisms militate against a definite position. The recommendation section

notes further suggested actions with regard to codes and standards.

This report is an extensive overview of the status of snubbers; and, as

such, it represents a dissemination of technical results. With regard to innovative materials and designs, efforts were limited to the existing status.

As cited previously, there have been extensive interactions with NRC staff

either with regard to technical specifications through the Piping Rev'iew

Committee or directly with the author',of. the draft Regulatory Guide. Finally,

interaction with the nuclear industry has occurred through the literature,

through direct contacts regarding ongoing snubber activities, or through a

series of contacts related to the reduction of the number of snubbers. An

obvious interface is the PVRC Steering Committee on Piping, which permits

interaction between NRC, utilities, and nuclearsteam supply system (NSSS)-

suppliers. The NPAR program strategy has been considered and actively used in

developing and enunciating this program.'

The following major recommendations are made:

* The PVRC suggestions include incorporation into ASME III as well as

approval of NRC on a case-by-case basis and ultimately on a generic

basis to provide utilities the option to markedly reduce the total

number of supports, particularly snubbers now used on piping.

* -A large number of aging/degradation mechanisms have'been cited. In

fact, there are far too many to'permit'concentration on any one

mechanism. Therefore, it is imperative to'pursue qualification-test-'.-

-ing to minimize failure and to provide'an early warning of failures.

* The draft Regulatory Guide on snubbers'should be activated as'soon as

possible to provide a definitive basis for qualification testing.

Existing criteria within ASME XI are inadequate with regard to test-,

ing of snubbers 'and are nonexistent for larger'snubbers. Furthermore, the criteria for examination may,'not be sufficient. Technical

specifications-should be standardized, and the suggestions Included

in the report 'should meet this' require'ment.

10.3
• A definitive standard is required for environmental qualification

testing.

The following additional recommendations are specific to given tests:

* Breakaway force is sensitive to both vibration and extended periods

of inactivity; inactivity may increase breakaway force levels substantially. Both conditions may exist and should be considered.

* The usual tendency is to minimize the dead band level to minimize

impact loads in the snubbers and attached components. This tendency

needs to be counterbalanced against the increased tolerance to higher

vibration levels at higher dead band levels.

* The spring rate or load displacement is an indication of the stiffness in the snubber; however, stiffness is controlled by the associated hardware attached to the structure, the snubber, and the component. Therefore, the spring rate of the snubber is only a part of

the picture and evaluations based on the cited values may not be

valid.

* The various measured parameters are quite sensitive to the type of

test and the test procedures. An acceptable snubber may be rejected

or an unacceptable snubber may be accepted due to variability in test

equipment and test procedures. This factor is not recognized in the

various codes and standards. A definitive set of criteria should be

developed to control this variable.

It is apparent that acceptance and qualification of snubbers is based on

some or all of the following parameters: activation, breakaway, dead band,

drag, release rate, and spring rate. Therefore, any consideration of preservice and in-service testing should consider these same parameters and determine

if they will detect the various aging/degradation mechanisms to which snubbers

are subjected. 1Measurement of these parameters can detect wear QrrQ~ioa,

-cffamFibiti`bon, and changes in hydraulic fluid caused by temperature, changes in

ViSC(-sffy, or entrapped air. Loss of fluid in hydraulic snubbersTi-5onvious,-

and-mechanicZal snubbserefo-iipcan etec t veral tests,,

In hydraulic snubbers, the hydraulic fluids should have adequate lubricity

to minimize galling, be compatible with other materials (e.g., seals), be

stable under operating conditions, provide corrosion protection, resist fire,

resist radiation damage, and be capable of cleansing by filtration. The

effects of entrained or dissolved gases should also be considered.

10.4
An analysis of the impact of in situ testing of large snubber should be

completed, and the results should be compared with the current ASME XI position

of not requiring testing. The position developed should be made available in

an appropriate document.

10.5
11.0 BIBLIOGRAPHY

American Society of Mechanical Engineers.' Rules for Construction of Nuclear

Power Plant Components. ASME III, Articles NF-1000 through NF-1112 and NF-3400

through NF-3423. New York. 7,

American Society of Mechanical Engineers. 1980. Component Support Snubbers -

Design, Application and Testing. PVP-142, presented at the Pressure Vessel and

Piping Conference, ASME Century 2 - Emerging Technology Conferences, San

Francisco, California.

i $ .

American Society of Mechanical Engineers. 1980. Criteria for Nu'clear Safety

Related Piping and Component Support Snubbers. PVP-45, presented at the Pressure Vessel and Piping Conference, ASME Century 2 - Emerging Technology

Conferences, San Francisco, California.

American Society Mechanical Engineers. 1981. Snubber Design Applications and

Minimization Methods. PVP-55, New York. -

American Society of Mechanical Engineers. 1982. Examination and Performance

Testing of Nuclear Power Plant Dynamic Restraints (Snubbers). ANSI/ASME 0M4,

New York. -

American Society of Mechanical Engineers. 1982. Requirements for Preoperational and Initial Start-up Vibration Testing of Nuclear Power Plant Piping

Systems. ANSI/ASME DM3, New York.

American Society of Mechanical Engineers. 1983. Rules for Inservice Inspection of Nuclear Power Plant Components. In ASME Boiler and Pressure Vessel

Code, Section XI, New York.

Andrews, W. B., et al. 1983. Guidelines for Nuclear Power Plant Safety Issue

Prioritization Information Development. NUREG/CR-2800, PNL-4297, Pacific

Northwest Laboratory, Richland, Washington.,

Brown, E. J., to K. V. Sefrit, memo. September 20, 1984. Failure of Large

Hydraulic Snubbers to Lockup. AEOD/E423, Office of the Analysis and Evaluation

of Operational Dataj U.S. Nuclear-Regulatory Commission, Washington, D.C.

Butler, J. H. 1977. Analysis of Owner Design Specifications for Snubbers,

Report;2.. ORNL/NUREG/TM-116, Oak Ridge-National Laboratory,-Oak Ridge,

Tennessee.

'11. 1
Butler, J. H. 1978. Analysis of Design Practices for Snubbers, Report 3.

ORNL/NUREG/TM-145, Oak Ridge National Laboratory, Oak Ridge, Tennessee.

Butler, J. H., and F. M. O'Hara, Jr. 1976. Analysis of Abnormalities of

Snubbers in Nuclear Reactor Service (Report 1). ORNL/NUREG/TM-53, Oak Ridge

National Laboratory, Oak Ridge, Tennessee.

Chang, T. Y. 1983. Seismic Oualification of Equipment in Operating Plants.

NUREG-1018, IU.S. Nuclear Regulatory Commission, Washington, D.C.

Chuang, T. Y., et al. 1984. Impact of Changes in Damping and Spectrum Peak

Broadening on the Seismic Response of Piping Systems. NUREG/CR-3526, Lawrence

Livermore National Laboratory, Livermore, California.

Eidinger, J., and G. Hau. 1985. "Snubber Reduction by Reduction of

Unnecessary Seismic Margins." Presented at ASME Pressure Vessel and Piping

Conference, June 24-27, 1q85, New Orleans, Louisianna.

Eisenhut, D. G., NRC, to H. R. Denton, NRC, memo. April 23, 1984. Elimination

of Tabular Listing of Snubbers in Technical Specifications. IJ.S. Nuclear

Regulatory Commission, Washington, D.C.

Emrit, R., et al. 1983. A Prioritization of Generic Safety Issues.

NUIREG-0933, U.S. Nuclear Regulatory Commission, Washington, D.C.

Gurdal, R. J. 1985. "Snubber Lockup Accelerations and Acceptable Dead-Bond

Displacement by Extension of the Response Spectra Method." Presented at ASME

Pressure Vessel and Piping Conference, June 24-27, 1985, New Orleans,

Louisianna.

Hendrickson, L. A., and K. E. Schwartztrauber. July 1984. Examination of

TMI-2 Snubbers: Phase 2. EPRI NP-3593, prepared for the Electric Power

Research Institute by PENTEK, Inc.

Hsu, C. March 23, 1984. Mechanical Snubber Failure. AEOD/E406, Office of the

Analysis and Evaluation of Operational Data, U.S. Nuclear Regulatory

Commission, Washington, D.C.

IE IN 84-67. August 17, 1984. Recent Snubber Inservice Testing with High

Failure Rates. U.S. Nuclear Regulatory Commission, Washington, D.C.

IE IN 84-73. September 14, 1984. Downrating of Self-Aligning Ball Bushings

Used in Snubbers. U.S. Nuclear Regulatory Commission, Washington, D.C.

11.2
IE IN 81-01. January 27, 1981. Surveillance of Mechanical Snubbers. IJ.S.

Nuclear Regulatory Commission, Washington, D.C.

Jimenez, P., and A. Requena. 1985. "Snubber Optimization Program." Presented

at ASME Pressure Vessel and Piping Conference, June 24-27, 1985, New Orleans,

Louisianna.

Kohler, E. G., J. J. Smerke, III, and H. F. Dobson. 1983. "

Atomic Power Station Hydraulic Snubber Failures." ANS Trans.

Peach Bottom

45:565.

Landers, D. F., R. D. Hookway, and K. D. Desai. 1981. Effects of Postulated

Event Devices on Normal Operation of Piping Systems in Nuclear Power Plants..

NUREG/CR-2136, Teledyne Engineering Services, Waltham, Massachusetts.

Levin, H., and R. Cudlin. 1978. Operating'Experience with Snubbers.

NUREG-0467, U.S. Nuclear Regulatory Commission, Washington, D.C.

Lu, S. C., and C. K. Chou. 1984. Reliability Analysis of Stiff Versus

Flexible Piping - Interim Project Report. NUREG/CR-3718, Lawrence Livermore

National Laboratory, Livermore, California.-

Murphy, G. A., et al.

Identify Aging Trends.

Laboratory, Oak Ridge,

1984. Survey of Operating Experience from LERs to

NUREG/CR-3543, ORNL/NSIC-216, Oak Ridge National

Tennessee.

Olshinski, J. A., NRC, to D. G. Eisenhut, NRC, memo.- October 24, 1984. Inconsistencies in Snubber Technical Specification and Recommended Changes. U.S.

Nuclear Regulatory Commission, Washington, D.C.

Pacific Scientific., August 1984. ;-Data Report 1700, Anaheim, California.

Pacific Scientific. 1984. A Simulated 40-Year Service Life Test for a PSA-3

Shock Arrestor with 2% to 3% Friction. Test Report-871, Anaheim, California.

PVRC Technical Committee on Piping Committee, Task Groups. December 1984.

"Damping Values, Seismic Spectra, Industry Practice, Dynamic Allowables."

Technical Position on Criteria Establishment. WRC Bulletin 300, ISSN

0043-2326, New York. K

Palo Verde Unit 1, Plant Systems 3/4.7.9 Snubbersi-Regulatory Guide Final

Draft. 2 -

* t l . . . ; , 0

11.3
Rahn, R. C. 1978. Probabilistic Stress Evaluation of Piping Systems Assuming

Single Snubber Failures. ASME Publication 78-PVP-107, presented at the Joint

ASME/CSME Pressure Vessel and Piping Conference, Montreal, Canada, June 25-30,

1978.-

Renkey, E. J. 1981. "Low Level Vibration Effects on Snubber Life." HEDL-SA2231, presented at ASME Pressure Vessel and Piping Conference, June 21-26,

1981, Denver, Colorado.

Severud, L. K., and G. D. Summers. 1980. "Design Considerations for

Mechanical Snubbers." HEDL-SA-1991, presented at ASME Pressure Vessel and

Piping Conference, June 21-26, 1981, Denver, Colorado.

Stevens, A. J., and M. Braccio. 1982. Snubber Reliability Improvement Study.

EPRI NP-2297, prepared for Electric Power Research Institute by Babcock &

Wilcox Company, Lynchburg, Virginia.

Task Group on Damping Values of the Technical Committee on Piping Systems under

the guidance of the Steering Comrnmittee on Piping Systems of the PVRC Committee.

December 1984. Technical Position on Damping Values for Piping-Interim Summary

Report. WRC Bulletin 300, ISSN 0043-2325, New York.

Task Group on Industry Practice of the Technical Committee on Piping Systems

under the guidance of the Steering Committee on Piping Systems of the PVRC

Committee. December 1984. Technical Position on Industry Practice. WRC

Bulletin 300, ISSN 0043-2325, New York.

Task Group on Spectra Development of the Technical Committee on Piping Systems

under the guidance of the Steering Committee on Piping Systems of the PVRC

Committee. December 1984. Technical Position on Response Spectra Broadening.

WRC Bulletin 300, ISSN 0043-2325, New York.

Tuttle, E. B., and A. D.-DuBose. 1985. "Snubber Optimization Using Multiple

Response Spectra and Increased Damping Techniques." Presented at ASME Pressure

Vessel and Piping Conference, June 24-27, 1985, New Orleans, Louisianna.

U.S. Department of Energy. September 1984. Mechanical and Hydraulic Snubber

for Nuclear Applications. NE E7-9T, Washington, D.C.

U.S. Nuclear Regulatory Commission. February 1981. Qualification and

Acceptance Tests for Snubbers Used in Systems Important to Safety. Draft

Regulatory Guide SC 708-4, Rev. 1, Washington, D.C.

11.4
U.S. Nuclear Regulatory Commission. July 1981. IJSNRC Standard Review Plan for

3.9.3 ASME Code Class 1, 2, and 3 Components, Component Supports, and Core

Support Structures. NUREG-0800, Washington, D.C.

U.S. Nuclear Regulatory Commission. May 3, 1984. Technical Specifications for

Snubbers. Generic Letter 84-13, Washington, D.C.

U.S. Nuclear Regulatory Commission. November 1984. Report of the U.S. Nuclear

Regulatory Commission Piping Review Committee, Evaluation of Potential for Pipe

Breaks. NUREG-1061, Vol. 3, Washington, D.C.

U.S. Nuclear Regulatory Commission. December 1984. Report of the U.S. Nuclear

Regulatory Commission Piping Review Committee, Evaluation of Other Dynamic

Loads and Load Combinations. NUREG-1n61, Vol. 4, Washington, D.C.

U.S. Nuclear Regulatory Commission. April 1985. Report of the U.S. Nuclear

Regulatory Commission Piping Review Committee, Evaluation of Seismic Designs -

A Review of Seismic Design Requirements for Nuclear Power Plant Piping.

NUREG-1061, Vol. 2, Washington, D.C.

U.S. Nuclear Regulatory Commission. April 1985. Report of the U.S. Nuclear

Regulatory Commission Piping Review Committee, Summary and Evaluation of

Historical Strong-Motion Earthquake Seismic Response and Damage to Aboveground

Industrial Piping. NUREG-1061, Vol. 2 Addendum, Washington, D.C.

U.S. Nuclear Regulatory Commission. April 1985. Report of the U.S. Nuclear

Regulatory Commission Piping Review Committee, Summary - Piping Review

Committee Conclusions and Recommendations. NUREG-1061, Vol. 5, Washington,

D.C.

W-STS, Plant Systems 3/4.7.9 Snubbers, Regulatory Guide Draft.

Wadsworth, F. L., et al. 1983. Examination of Mechanical Snubber from TMI-2.

EPRI-NP-2966, Electric Power Research Institute, Palo Alto, California.

11.5
APPENDIX A

CONTENTS OF A FUNCTIONAL SPECIFICATION FOR SNUBBERS

USED IN SYSTEMS IMPORTANT TO SAFETY
APPENDIX A

CONTENTS OF A FUNCTIONAL SPECIFICATION FOR SNUBBERS USED IN SYSTEMS

IMPORTANT TO SAFETY

1. SCOPE

This document establishes requirements for a functional specification for

mechanical and hydraulic snubbers-for applications in-systems important to the

safety of nuclear power plants.

2. DESIGN SPECIFICATION RELATIONSHIP

The functional specification provides detailed definition of functional

requirements applicable to snubbers for components and piping systems important

to safety. The requirements of the functional specification may be provided

as part of the snubber design specification or as part of an equipment or purchase specification that also includes the design specification. If this functional specification is prepared by the snubber manufacturer, an application

report prepared for the licensee shall be made part of the design specification.

The application report shall be reviewed and certified by one or more registered

professional engineers to be correct and complete in accordance with the functional parameters developed from the safety-related functional requirements with

those in the functional specification prepared by the manufacturer. Compliance

with these requirements for this functional specification is intended to'ensure

that the operating conditions and safety-related'functions of the snubber have

been adequately defined;"permittinb the snubber manufacturer to demonstrate the

adequacy of both the design of the snubber and the materiails used in its construction for the'intended service. '

3. DEFINITIONS

Activation level - The axial velocity or acceleration'that'-causes the activation of the snubber. - -

Dead band - The free axial movemenit of the 'snubber'between the two activations in opposite directions.

A. 1
Drag - The breakaway load that overcomes the internal snubber friction or

the load maintaining the snubber movement at a specific velocity.

Emergency load - The design load capacity for the snubber based on the

use of Level C Service Limits defined by Section III of the ASME B&PV Code.

Faulted load - The design load capacity for the snubber based on the use

of Level D Service Limits defined by Section III of the ASME B&PV Code.

Rated load - The design load capacity for the snubber based on the use of

Level A Service Limits defined by Section III of the ASME B&PV Code.

Release rate - The rate of the axial snubber movement after the activation of the snubber under a specified load.

Spring rate - The linear approximation of the relationship between the

peak force range and the peak displacement range. The peak displacement range,

including the dead band, should be obtained during the dynamic cyclic test

through the peak force range. The peak force range should include the rated

load or emergency load applied in opposite directions.

4. FUNCTIONAL SPECIFICATION

It is the responsibility of the owner or his agent to identify the safetyrelated functional requirements of the snubber and provide for the delineation

of the following:

a. Application characteristics (see Section 4.1),

b. Design requirements (see Section 4.2),

c. Operational requirements (see Section 4.3),

d. Functional parameters (see Section 4.4),

e. Special material requirements (see Section 4.5),

f. Installation requirements (see Section 4.6),

9. Maintenance and inspection requirements (see Section 4.7),

h. Other requirements (see Section 4.8).

4.1 Application Characteristics

The application characteristics of each snubber shall be identified by

listing whichever of the following descriptive terms are appropriate:

a. Seismic restraint,

b. Dynamic force restraint,

A.2
c. Pipe whip restraint,

d. Relief valve restraint,

e. Others, including combinations of the above.

4.2 Design Requirements

The following information shall be specified:-

a. The design operating temperature,.

b. Time-temperature data for design thermal transients with the number

of cycles indicated, - I

c. The seismic acceleration and dynamic loadings that the snubbers must

be capable of withstanding transverse to the line of action without loss of

functional capability,

d. The seismic acceleration and dynamic loadings that the snubbers must

be capable of withstanding along the line of action without loss of functional

capability,

e. Limits on the acceptable range of the fundamental frequency of the

snubber assembly,

f. Limits on acceptable angular offset from the line of action of-the

load.

4.3 Operational Requirements

-Anticipated modes of snubber operation, including those related to seismic

events, water hammer, etc., shall be specified.- The operating conditions and

environmental conditions shall be identified.

4.3.1 Operating Conditions,. The number of operational cycles, the imposed

loading or movement (number, amplitude, and direction), and the environment,

including temperature, for each of the following operational categories shall

be specified: - -

a. Installation testing,

b. System hydrostatic testing,

c. Preoperational testing,

d. - Startup testing, . -

e. Normal and abnormal plant operations (including postulated accident

conditions, shock, or pulsating loads),

A.3
f. Inservice testing,

g. Vibration,

h. Others.

4.3.2 Environmental Conditions. A histogram of the environmental conditions that are postulated to exist shall be provided.

The need of snubbers to survive normal and abnormal environmental conditions with or without maintenance shall be stated. Since the attaching hardware can influence the survival of the snubber, it shall also be considered.

The following factors are considered relevant:

a. The atmosphere, including chemistry, temperature, humidity, and radioactivity, in which the snubbers will be installed and must operate under normal

plant conditions.

b. The atmosphere, including chemistry, temperature, humidity, and radioactivity, in which the snubbers must operate under upset, emergency, and faulted

plant conditions. The duration of these conditions shall be specified.

c. The vibration environment under normal, upset, emergency, and faulted

conditions.

4.4 Functional Parameters

As a minimumi the following functional parameters shall be specified.

-a. Acceptable limits for both the breakaway drag force and the drag

force-associated with moving under a specified velocity at the maximum and

minimum working temperatures,

b. Acceptable limits for the dead band at the maximum and minimum working

temperatures for the range of working loads and piston locations,

: c. Activation level (when applicable) and tolerance at rated load at

the-maximum and minimum.working temperatures,

d. Release rate and tolerance at 5%, 10%, 25%, 50%, and 100% of rated

load and at emergency load for the maximum and minimum working temperatures,

e. Acceptable range of spring rates at the maximum and minimum working

temperatures and the frequency, the load range, and the classification of the

load (i.e., normal or emergency) at which the spring rate is to be determined

with piston locations at 1/4, 1/2, and 3/4 stroke locations. -

A.4
4.5 Special.Material Requirements

Special material requirements of the snubber shall be specified. Items

to be considered shall include, but not be limited to, the following:

a. Hydraulic fluid (including its potential for electrolytic corrosion),

b. Seals,

c. Springs,

d. Special surface preparations or coatings,

e. Lubricants.

4.6 Installation Requirements

The following requirements for the installation of the snubber shall be

specified:

a. Orientation of the hydraulic snubber and relative position of the

hydraulic reservoir to the snubber if any limitations exist,

b. Orientation of the mechanical snubber installation if any limitations

exist,

c. The available space for installation and removal if any limitations

exist,

d. Piston location in the snubber as installed,

e. The range of transverse movement provided,

f. Any special mounting provided or required.

4.7 Maintenance and Inspection Requirements

An acceptable hydraulic fluid leakage rate shall be specified for hydraulic

snubbers. Other special provisions for snubber maintenance shall be specified.

Where requirements are established for in situ inservice testing, requirements

should be included for demonstrating the feasibility of performing the required

inservice tests (i.e., drag test, activation level test, and release rate test)

with specific test equipment.

4.8 Special Performance Requirements

Other requirements for special performance or loading conditions, as applicable, shall be specified.

A.5
APPENDIX B

QUALIFICATION OF FUNCTIONAL PARAMETERS FOR SNUBBERS

USED IN SYSTEMS IMPORTANT TO SAFETY
APPENDIX B

QUALIFICATION OF FUNCTIONAL PARAMETERS

FOR SNUBBERS USED IN SYSTEMS IMPORTANT TO SAFETY

1. SCOPE

This document describes the basic requirements for the qualification of a

design for snubbers used in nuclear power plant systems important to safety.

2. PURPOSE

The purpose of this document is to provide guidance for demonstrating the

capability of a snubber design to satisfy the'specified functional requirements.

Testing is required to demonstrate the capability-of the design to satisfy all

specified functional requirements except the ability to withstand ultimate load,

for which an analysis may be used instead of testing.

3. QUALIFICATION PLAN

A qualification plan shall be developed for each snubber design'and rating,

and this plan shall be submitted to the owner or his agent to be reviewed for

consistency with the design specification and foriapproval.oriacceptance as

the basis for an application report. The-qualification plan for functional

parameters for snubbers shall, asia minimum, contain the following information:

a. Snubber descriptions and specifications (see Section 3.1),

b. Snubber sampling(requirements (see Section 3.2),

c. Material data requirements,(see Section 3.3),

id. Mounting requirements (see Section-3.4),

e. The aging-simulation requirements (see Section 3.5), ,

f. The servicecondition simulation requirements (see Section 3.6),

--. Functional parameters and environmental variables to be measured

-.(see Section,3.7), ; .

'h. Test and monitoring-equipment requirements (seeSection 3.8),

i. The test sequence (see Section 3.9),

B.1
j. Special tests (see Section 3.10),

k. Limits or failure definition for the test (see Section 3.11),

1. Ultimate load capacity (Section 3.12).

3.1 Snubber Descriptions and Specifications

The snubber design to be qualified shall be described in full by its model

number, drawing numbers, and total weight. Serial numbers of tested snubbers

shall be recorded. The qualification requirements shall be specified in the

functional specification that covers the functional parameters described in

Appendix A.

3.2 Snubber Sampling Requirements

Two snubbers, either two prototypical units manufactured for qualification

purpose or two production units selected randomly from a population of not less

than four, shall be used to qualify a specified design and rating. This includes

the use of the same materials, parts, and general arrangement. Snubbers used

for qualification testing shall not be used for functions important to safety

in:nuclear power plants.

3.3 Material Data Requirements

The following material data shall be included to ensure that the snubber

was manufactured according to the specification:

a. The data from or reference to tests conducted to prove the adequacy

of the basic material selection, including, for example, tests for compatibility between the seal material and the hydraulic fluid and between the seal

material and the working environment with special consideration given to the

combined effects of temperature and radiation on material performance.

b. The data on material and process traceability for the snubber. These

data shall be included to demonstrate that the material of the snubber that

was tested and the materials called out in the manufacturing specification have

the same specification as the materials whose selection was justified in 3.3.a.

B.2
3.4 Mounting Requirements

The qualification plan shall specifythe waythe snubber is to be mounted

for testing. *The plan shall require that the snubber unitbe mounted in a manner

(pins, bolts,,welds, clamps, etc.) and position (orientation withrespect to

gravity) that simulate its expected installation with attachments for service

application when in actual use unless an analysis can be performed to show that

its performance would not be altered by other means of mounting._,Such an

analysis should be based on a demonstration using service experience and previous

test results that all relevant failure modes related ,to mounting have been

adequately considered in the analysis and that assumptions made to provide a

basis for the' analysis are reasonable.

3.5 Aging Simulation Requirements'' ''

The qualification plan shall specify the aging simulation for the snubber

e design based on requirements in the functional, specification. Aging simulation

procedures to put the snubber-units in a condition equivalent to the end of life

condition shall be conducted, including sand and dust simulation anda salt

spray test similar to Mil-E-5272c* if the specifiedworking environment requires

it.

3.6 Service Condition Simulation Requirements

The qualification plan shall specify a steam humidity simulation of 350'F

(1770C) saturated steam and for a duration of 72 hours if the snubber service

area is inside.the containment. It shall specify a submergence in 200'F (930C)

water for a duration of 72 hours-if the snubber service is in a water

environment.,,;, , ;

3.7 Functional Parameters and Environmental Variables To Be Measured

.The qualification plafixshall specify the;.functional parameters and environmental variables to be-measured for the-snubber test.. The-functionalparameters

shall include, asa:-minimum, .the'drag, the-dead band,'.the activation level (when

- .'- . .. '' % t.-' .' '- t'3: '.! - - . .

"Environmental Testing;, Aeronautical and Associated Equipment,-General Specifications for" may be obtained from the Naval Publications and Forms Center,

5801 Tabor Avenue, Philadelphia, Pa. 19120, Phone: (215)697-3321.

B.3
applicable), the release rate, and the spring rate. The environmental variables'shall include temperaiure, humidity or steam-water condition, special

thermal transients, and radiation (when applicable).'' For hydraulic snubbers,

the hydraulic fluid leakage during tests shall be recorded.' The application

of a low-amplitude axial vibration to'snubbers for not less than 5 x 10i cycles

shall be included as an environmental requirement.

3.8 Test and Monitoring Equipment

The test shall be conducted and monitored using equipment that provides

resolution for detecting meaningful changes in the variables. The qualification plan shall specify the test and monitoring equipment to be used for the

qualification of the snubber and describe the accura6' withi-n the range anticipated for use on the proposedftests. The test equipment and 'monitoring equipment shall be calibrated against auditable calibration standards and shall have

documentation to support'such'calibration. The data-recording equipment'shall

have sufficient speed and capacity to permit the time dependence of each measured

variable to be determined.

3.9 The Test Sequence

The qualification test shall include the following tests in the described

sequential order as a minimum requirement:

a. All five parameters described in Section 3.7 shall be determined at

the recorded room temperature (or the specified lowest design service temperature ±100F (5.50C), whichever is lower). Temperatire shall be' recorded at the

beginning and end of each of the tests. The tests shall be performed with the

snubber at the 1/2 stroke location unless otherwise required.

1. ' The drag shfall be determined for both directions both for the

breakaway condition and for the condition of moving-at a specified velocity.

The values of the drag and the velocity shall befrecorded.

2. The-activation level (when applicable) shall be tested for both

directions by rapid cyclic application of a force equal to 5%, 10%, 25%, 50%,

and 100% of the rated load. The acceleration and velocity of the piston motion

-shall' be recorded as a function of time. The activation level shall be determine'd'from this recording for each force level. '

B.4
or

3. The dead band shall be-recorded during the activation level

testing described in Section 3.9.a.2.

4. The release rate shall be tested and recorded for both directions

at 5%, 10%, 25%, 50%, and 100% of rated load and at the emergency load.

5. The spring rate shall be tested by a dynamic cyclic loading equal

to the rated load or other specified load and with snubber movement centered

about the 1/4, 1/2, and 3/4 stroke locations according to the requirement of

the functional specification. The testing frequency shall be from 3 Hz to 33

Hz-at intervals of approximately 3 Hz. Each frequency shall last not less than

10 seconds. Response'at each frequency shall be recorded as load-displacement

traces. .

6. One-cycle dynamic loading tests with snubber movement centered

about the 1/4, 1/2, and 3/4 stroke locations and a loading amplitude equal to

the faulted loading shall be performed to demonstrate adequacy of the response.

The force, displacement, and velocity shall be recorded for this test. Any

damage to the snubber resulting from this test may be evaluated separately or

by performance in tests of b. or c. below.

7. Hydraulic fluid leakage during the testing shall be recorded.

b. Repeat tests'described in a. but-with'snubber temperature at'2000 F

'(930C) (or the specified highest design service temperature, whichever'is

higher). '

c. The aging simulationfprocedure described in Sections 3.5 and 3.6 shall

be conducted, including the application of at least 5 x 106 cycles of an axial

vibration with an amplitude not less than 100% of the dead band at the applied

frequency.

d. Repeat tests described in a. after completion of the aging simulation

procedures described in c.

3.10 Special Tests

The qualification plan shall specify special tests for the snubber design

according to the design specification requirements.- :These are the tests

demonstrating the ability of the snubbers to meet special specified requirements

such as load-sharing arrangements or an in situ inservice activation test.

Test setup and equipment used shall closely simulate the required condition so

the feasibility can be illustrated and correlation between results can be

established. ; 5 -

B.5
3.11'Limits or Failure Definition-for the Test

The snubber design shall be considered to have failed the qualification

test if any one of the following occurs:

a. The drag exceeds the specified limits in either direction.

b. The dead band exceeds the specified limit.

c. The activation level exceeds the specified range in either direction.

d. The release rate exceeds the specified range in either direction under

the specified loadings.

e. The spring rate is less than the specified range for all frequencies

at the specified stroke location.

f. The hydraulic fluid leakage rate exceeds the specified limit.

g. Any failures except those caused by testing equipment or procedure

abnormalities occur.

Revisions to the design must be made before the new design can be qualified

in accordance with this Appendix.

3.12 Ultimate Load Capacity

The ultimate load capacity of a snubber design shall be determined by analysis. The analysis. report shall follow the stress report requirement described

by Section III of the ASME B&PV Code. The mode of failure, whether the snubber

movement will be. frozen or free after an ultimate failure, shall be determined

by the analysis.

4. DOCUMENTATION

4'.1 General

The qualification documentation shall provide a basis'for demonstrating'

that the snubber design is qualified for its application and meets its specified performance requirements. The basis for the qualification shall be described to show the relationship of all facets of;analytical'and test results

to the specified requirerments'in order to support the adequacy of the snubber

design.

4;.2-Documentation-Files- -.

The manufacturer shall maintain a file of the qualifidation plan and test

data for each specific snubber design tested. The file shall contain the inforB.6
mation listed in Section 3. Test data shall be recorded as described in Section 4.3.

4.3 Test Data - -

The test data shall contain:- ;- - -

-a. -The-snubber functional specification,

b. The qualification plan,

c. Identification of the specific functional parameter(s) to be demonstrated by the test,

d. The relative position of the test in the test sequence,

e. Report of test results. The report shall include:

1. Objective of the test,

2. The snubber tested,

3. Description of test equipment (test setup) and instrumentation

used including calibration records reference,

4. Deviations from the qualification plan, if any,

5. Test procedures,

6. Test results,

7. Summary, conclusions, and recommendations,

8. Approval signature and date (certification by a professional

engineer competent in the field of functional and environmental

testing).

4.4 Test Evaluation

After the snubbers have successfully passed or failed the qualification

tests, a posttest analysis shall be conducted. The results of this analysis

shall be a part of the report of test results and shall contain the following

information:

a. Identification of the snubber tested,

b. The last test conducted on the snubber in the test sequence,

c. Analysis of the posttest snubber condition,

d. Summary, conclusions, and recommendations,

e. Approval signature and date (certification by a professional engineer

competent in the field of functional and environmental testing),

f. Disposition of snubber.

B.7
4.5 Other Information

The following information, as applicable, shall be included in the report

of test results:

a. A statement of inapplicable portions of the specification,

b. A description of any conditions peculiar to the snubber test that

are not covered above but that would probably affect the performance

of the snubber during testing.

B.8
APPENDIX C

ACCEPTANCE TESTING CRITERIA FOR THE PRODUCTION OF

SNUBBERS USED IN SYSTEMS IMPORTANT TO SAFETY
APPENDIX-C' - - i.

ACCEPTANCE TESTING CRITERIA FOR THE PRODUCTION

OF SNUBBERS USED IN SYSTEMS IMPORTANT TO SAFETY

1. SCOPE

This document describes the basic requirements for the acceptance test - 'I , 2, - , . C . , ' .'I

for the production of snubbers used in systems important to safety in nuclear

power plants. , 2 ,

2. PURPOSE .2.25 , -

-The purpose of this document is;to provide guidance for demonstrating-the

adequacy of fabrication in light-of functional requirements for the-acceptance

of snubbers fromthe production line.- Acceptance testing is required for all

snubbers that are used in systems important to safety.

3. PRODUCTION-ACCEPTANCE TEST PLAN.+' t -' -.

A production acceptance test plan shall be developed~fo'r"each mo'deland

size of snubber by the snubber manufacturer and shall be submitted to the owner

or his agent to be reviewed for consistency with theJdesign specification and

for. approval. -Theproduction acceptance test plan forsnubbers shall contain

the following information: ,

- a.- : Snubber descriptions and;specifications'(see Section 3.1), 'I2

b. Manufacturing data (seecSection-3.2), 2 - i

c.-: Test mountingzrequirementsr(see:Section 3;3), -;

.' d.; Functional parameters.to be measured (see Section 3.4), 2 * '

e. Test and monitoring equipment requirements (see Section 3.5), .

f. Test sequence (see Section 3.6),

g. Limits or failure definition fortthe'test (see.Section:3.-7);: 2i

2 '~'-. ' I :'. . 2

- 2 , 2 * .' - ia, '. i . s.: . I .., :' : " -2 ; 2 , :2 .1 2

- .. _2 _. : - . ! . ';* * . - ' 2f ; , .. ; . ; ; 2.,; 

2 . -- 5.'.'_ . - i. 2 .. _; 2 .t-I

C;1
3.1 Snubber Descriptions and Specifications

The snubber shall be described in full by its model number, serial number,

drawing numbers, and total weight.

3.2 Manufacturing Data

The manufacturing data identified below shall be included to ensure that

the snubber was manufactured according to the specification and that materials

and processes have the same specification as those for the snubbers used in

the qualification tests (Appendix B).

a. The data from or reference to tests conducted to prove the adequacy

of the basic design. This should include, for example, the compatibility test

between seal material and the hydraulic fluid and the working environment with

special consideration given to the combined effects of temperature and radiation on material performance. A master file of material properties can be

referred to.

b. The data on material and process traceability of the snubber. These

data shall be included to demonstrate that the snubber has successfully met

the fabrication requirements.

3.3 Test Mounting Requirements

The test plan shall specify the way the snubber is to be mounted for testing.

The plan shall require that the snubber be mounted in a manner (pins, bolts,

welds, clamps, etc.) and position (orientation with respect to the gravitational

field of the earth) that simulate its expected service installation using standard interface hardware when in service unless an analysis can be performed and

justified to show that the snubber's performance would not be altered by other

means of mounting..

3.4 Functional Parameters To Be Measured

The test plan shall specify the method of testing for all functional parameters as specified in the functional specification. As a minimum, the following

four functional parameters shall be measured for the snubber test under a recorded

temperature ambient. They shall include the drag, the dead band, the activation

level (when applicable), and the release rate. The plan shall also specify

C.2
that for hydraulic snubbers, the,,hydraulic fluid leakage during'_tests shall be

measured -and.recorded. , , ,

3.5 *Test.and Monitoring Equipment.: , , ,

;The test plan shall indicate the equipment, and types of sens'ors used to

measure the parameters for the testiAnd describe the accuracy within'the range

anticipated for use!on'the tests.-' They should provide'sufficient resolution

to detect meaningful changes in the variables. The test equipment and moni--

toring equipment shall be calibrated :against auditable calibration standards

and shall have documentation to support such calibration. The time interval

.between measurements.shall:be specified .in the test plan so that the time

dependence of each measured variable can be determined.;

3.6 Test!!Sequence ' 'i ":!' '' ' ;. ' ' -

The te'stifig shall include the' following te'sts 'in'the describ'ed'seq'uential

order as a minimum requirement unless the design specification requires otherwise. Iniervice' test 'behavior sh'all'be considered'if it'is a requirement.

'a. 'A free'(without load)-exercising of not less than'30 full-stroke

cycles shall be made to'id6monstrate that the snubber unit is in working 'order.

b. All 'four parameters described in Section 4'.4 'shall be-determined and

ambient and snubber temperature recorded with the snubber movement centered

about the 1/2 stroke position.

1. The drag shall be determined both for the breakaway condition

-,and the *~~~~~~~ 

conditionof movingunder a specified velocity. Tests shall be made a ,..;.tA -, 

forboth directions., i - . . - i ia

2. The activation level shall be tested for 5%,,10%, 25%, 50%,and

100% of the rated load for both directions.

3. The dead band of the snubber shall be measured and recorded

during the tests described in Section 3.6.b.2.

4. The' release riate shall be tes'ted'at5%,'10%,j'25%;"50%, and 100%

ofthe rated load. Tests shail be made for'both directions.

5 .' ~ Hydiraui fluid e'akage'during'the'testing'shall be recorded. '

- . . - -, .

C. 3
3.7 Limits or Failure Definition for the Test

a. As a minimum, the test plan shall specify the limits for the parameters identified below. The snubber unit shall not be accepted for applications

in systems important to safety if any one of the functional parameters fails to

meet the specified limits, in particular, if any one of the following occurs:

1. The drag exceeds the specified limits in either direction.

2. The dead band exceeds the specified limit value for the

associated loading.

3. The activation level exceeds the specified range in either direction.

4. The, release rate exceeds the specified ranges in either direction

under the specified loadings.

b. The test plan shall specify a hydraulic fluid leakage limit for the

test of hydraulic snubbers based on the design specification requirements.

The unit shall not be accepted from the production line if the hydraulic fluid

leakage rate exceeds the specified limit.

c. A posttest analysis shall be made for those snubber units that failed

the test. Rework to correct the deficiency identified by this analysis is allowed

only if the deficiency is not caused by the basic design. The reworked snubber

unit may be retested and accepted if it meets the test requirement.

4. DOCUMENTATION

4.1 General

The documentation of the test results shall provide a basis for demonstrating that each snubber accepted from the production line has met its specified

performance requirements.

4.2 Documentation Files

The manufacturer shall maintain a file with the production acceptance test

plan and test data for each specific snubber tested. The file shall contain.

the information listed in Section 3. Test and analysis data shall be recorded

as described in Section 4.3. If a snubber failed to pass the test requirements,

a post-test analysis shall be conducted. The results of this analysis shall

be a part of the documentation as described in Section 4.4.

C.4
4.3 Test Data and Records

The test data shall contain:

a. The reference production acceptance plan,

b. Report of test results. The report shall include:

1. Objective of tests,

2. Identification of the snubber tested,

3. Description of test equipment (test setup) and instrumentation

used, including reference to calibration records,

4. Deviations from the test plan, if any,

5. Test procedures,

6. Test results,

7. Summary, conclusions, and recommendations,

8. Approval signature and date indicating acceptance or

nonacceptance.

4.4 Test Evaluation and Records

The posttest analysis data shall contain:

a. Identification of the snubber tested,

b. Tests in which the snubber failed to meet the requirement,

c. Analysis of the posttest snubber condition,

d. Summary, conclusions, and recommendations,

e. Approval signature and date.

C.5
APPENDIX D

WESTINGHOUSE STANDARDIZED TECHNICAL SPECIFICATIONS
APPENDIX D

WESTINGHOUSE STANDARDIZED TECHNICAL SPECIFICATIONS

PLANT SYSTEMS W-STS

3/4.7.9 SNUBBERS

LIMITING CONDITION FOR OPERATION

3.7.9 All snubbers shall be OPERABLE. The only snubber excluded from the

requirements are those installed on nonsafety-related systems and them only if

their failure of failure of the system on which they are installed would have

no adverse affect on any safety-related system.

APPLICABILITY: MODES 1, 2, 3, and 4. MODES 5 and 6 for snubbers located on

systems required OPERABLE in those MODES.

ACTION:

With one or more snubbers inoperable on any system, within 72 hours replace or

restore the inoperable snubber(s) to OPERABLE status and perform an engineering

evaluation per Specification 4.7.9g. on the attached component or declare the

attached system inoperable and follow the appropriate ACTION statement for that

system.

SURVEILLANCE REQUIREMENTS

4.7.9 Each snubber shall be demonstrated OPERABLE by performance of the following augmented inservice inspection program in lieu of the requirements of

Specification'4.0.5.

a. Inspection Types

As used in this specification, type of snubber shall mean snubbers of

the same'design and manufacturer, irrespective of capacity.

b. Visual Inspections'

Snubbers are categorized as inaccessible or accessible during reactor

operation. Each'of these groups (inaccessible and accessible) may be

D.1
inspected independently according to the schedule below. The first

inservice visual inspection of each type of snubber shall be performed after 4 months but within 10 months of commencing POWER

OPERATION and shall include all snubbers. If all snubbers of each

type (on any system) are found OPERABLE during the first inservice

visual inspection, the second inservice visual inspection (of that

system) shall be performed at the first refueling outage. Otherwise,

subsequent visual inspections (of a given system) shall be performed

in accordance with the following schedule:

No. of Inoperable Snubber of Each Type Subsequent Visual

(on Any System) per Inspection Period Inspection Period*,**

0 18 months t25%

1 12 months ±25%

2 6 months ±25%

3,4 124 days ±25%

5,6,7 62 days ±25%

8 or more 31 days ±25%

* The inspection interval for each type of snubber (on a given

system) shall not be lengthened more than one step at a time

unless a generic problem has been identified and corrected;

in that event the inspection interval may be lengthened one

step the first time and two steps thereafter if no

inoperable snubbers of that type are found (on that system).

** The provisions of Specification 4.0.2 are not applicable.

c. Visual Inspection Acceptance Criteria

Visual inspections shall verify that: (1) there are no visible indications of damage or impaired OPERABILITY, (2) attachments to the

foundation or supporting structure are functional, and (3) fasteners

for attachment of the snubber to the component and to the snubber

anchorage are functional. Snubbers which appear inoperable as a

result of visual inspection may be determined OPERABLE for the purpose of establishing the next visual inspection interval, provided

that: (1) the cause of the rejection is clearly established and

remedied for that particular snubber and for other snubbers irrespective of type (on that system) that may be generally susceptible; and

(2) the affected snubber is functionally tested in the as-found

condition and determined OPERABLE per Specification 4.7.9f. All

D.2
snubbers connected to-an inoperable common hydraulic-fluid reservoir

shall be counted as inoperable-snubbers. (For those snubbers common

to more than 'one system, the OPERABILITY of such snubbers shall be

considered in assessing the surveillance schedule for each of the

related systems.)

d. Transient Event Inspection

An inspection shall be performed of all snubbers attached to sections

of systems that have experienced unexpected, potentially damaging

transients as determined from a review of operational data and a

visual inspection of the systems within 6 months'following such as

event. In addition to satisfying the visual inspection acceptance

'criteria, freedom-of-motion of mechanical snubbers shall be verified

using at -least one of the following:, (1) manually induced snubber

movement; or (2) evaluation of in-place snubber piston setting; or

(3) stroking the mechanical snubber through its full range of travel.

e. Functional Tests

During the first refueling shutdown and at least once per 18 months

thereafter during shutdown, a representative sample of snubbers of

each type shall be tested using-one of the following sample plans.

The sample plan for each type shall be -selected prior to the test

period and cannot-be changed during the test period. The -NRC

Regional Administrator shall-be'notified in writing of the sample

plan selected for each snubber type prior to the test period or the

sample plan used in the prior test period-shall be implemented:

1) At least 10% of the total of each type of snubber shall be

functionally tested either in-place or in a bench 'test. For

each snubber of a type that does not meet-the functional test

-acceptance criteria of Specification 4.7.9f., an additional 10%

of that-type of snubber shall be functionally tested until no

more failures are found or until all snubbers of that type have

been functionally tested; or ' -

2) A representative sample of each type of snubber shall be functionally tested-in accordance with'Figure 4.7-1. "C" is the

total number of snubbers-of'a type found not'meetiig the acceptance requirements of Specification 4.7.9f. The cumulative

number of snubbers of a type tested is denoted by "N". At the

end of each day's testing; the new values of "N" and "C" (previous day's total plus'current'day's increments) shall be

plotted on Figure-4.7-1. If at any time the point plotted falls

in the "Reject" region, all snubbers of that'type shall be

functionally tested. If'at-any time the pointlplotted falls in

the "Accept" region, testing of snubbers of that type may be

terminated. When the point plotted lies in the "Continue

D.3
Testing" region, additional snubbers of that type shall be

tested until the point falls in the "Accept" region or the

"Reject" region, or all the snubbers of that type have been

tested; or

3) An initial representative sample of 55 snubbers shall be functionally tested. For each snubber type which does not meet the

functional test acceptance criteria, another sample of at least

one-half the size of the initial sample shall of at least onehalf the size of the initial sample shall be tested until the

total number tested is equal to the initial sample size multiplied by the factor, 1 + C/2, where "C" is the number of snubbers found which do not meet the functional test acceptance

criteria. The results from this sample plan shall be plotted

using an "Accept" line which follows the equation N = 55(1 +

C/2). Each snubber point should be plotted as soon as the

snubber is tested. If the point plotted falls on or below the

"Accept" line, testing of that type of snubber may be terminated. If the point plotted falls above the "Accept" region or

all the snubbers of that type have been tested.

Testing equipment failure during functional testing may invalidate

that day's testing and allow that day's testing to resume anew at a

later time provided all snubbers tested with the failed equipment

during the day of equipment failure are retested. The representative

sample selected for the functional test sample plans shall be

randomly selected from the snubbers of each type and reviewed before

beginning the testing. The review shall ensure, as far as practicable, that they are representative of the various configurations,

operating environments, range of size, and capacity of snubbers of

each type. Snubbers placed in the same location as snubbers which

failed the previous functional test shall be retested at the time of

the next functional test but shall not be included in the sample

plan. If during the functional testing, additional sampling is

required due to failure of only one type of snubber, the functional

test results shall be reviewed at that time to determine if additional samples should be limited to the type of snubber which has

failed the functional testing.

f. Functional Test Acceptance Criteria

The snubber functional test shall verify that:

1) Activation (restraining action) is achieved within the specified

range in both tension and compression;

2) Snubber bleed, or release rate where required, is present in

both tension and compression, within the specified range;

D.4
3) For mechanical snubbers, the force required to initiate or

maintain motion of the snubber is within the specified range in

both directions of travel; and

4) For snubbers specifically required not to displace under

continuous load, the ability of the snubber to withstand load

without displacement.

Testing methods may be used to measure parameters indirectly or

parameters other than those specified if those results can be

correlated to the specified parameters through established methods.

g. Functional Test Failure Analysis

An engineering evaluation shall be made of each failure to meet the

functional test acceptance criteria to determine the cause of the

failure. The results of this evaluation shall be used, if applicable, in selecting snubbers to be tested in an effort to determine

the OPERABILITY of other snubbers irrespective of type which may be

subject to the same failure mode.

For the snubbers found inoperable, an engineering evaluation shall be

performed on the components to which the inoperable snubbers are

attached. The purpose of this engineering evaluation shall be to

determine if the components to which the inoperable snubbers are

attached were adversely affected by the inoperability of the snubbers

in order to ensure that the component remains capable of meeting the

designed service.

If any snubber selected for functional testing either fails to lock

up of fails to move, i.e., frozen-in-place, the cause will be

evaluated and, if caused by manufacturer or design deficiency, all

snubbers of the same type subject same defects shall be functionally

tested. This testing requirement shall be independent of the

requirements stated in Specification 4.7.9e. for snubbers not meeting

the functional test acceptance criteria.

h. Functional Testing of Repaired and Replaced Snubbers

Snubbers which fail the visual inspection or the functional test

acceptance criteria shall be repaired or replaced. Replacement

snubbers and snubbers which have repairs which might effect the

functional test results shall be tested to meet the functional test

criteria before installation in the unit. Mechanical snubbers shall

have met the acceptance criteria subsequent to their most recent

service, and the freedom-of-motion test must have been performed

within 12 months before being installed in the unit.

D.5
i. Snubber Service Life Program

The service life of hydraulic and mechanical snubbers.shall be

monitored to ensure that the service life is not exceeded between

surveillance inspections. The maximum expected service life for

various seals, springs, and other critical parts shall be determined

and established based on engineering information and shall be

extended or shortened based on monitored test results and failure

history. Critical parts-shall be replaced so that the maximum service life will not be exceeded during a period when the snubber is

required to be OPERABLE. The parts replacements shall be documented

and the documentation shall be retained in accordance with

Specification 6.10.3.

D.6
NUREG/CR-4279

PNL-5479

RV

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General Dynamics

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General Electric Company -

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Public Service Electric & Gas

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Union Electric Company

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Hanford Engineering

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Sargent and Lundy Engineers

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Chicago, IL 60603

0. Hafley

Sequoyah Nuclear Power Plant

P.O. Box 2000

Soddy-Daisy, TN 34701

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Southern Company Services

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NRC FORM 335 U.S. NUCLEAR REGULATORY COMMISSION I REPORT NUMBER IAswgqedby TIOC. dad Vol to. 04,,"y 12 841

3201,3202' BIBLIOGRAPHIC DATA SHEET NUREG/CR-4279, Vol. 1

SEE INSTRUCTIONS ON THE REVERSE PNL-5479 2. TITLE AND SUBTITLE 3 LEAVE BLANK

Aging and Service Wear of Hydraulic and Mechanical Snubbers

Used on Safety-Related Piping and Components of Nuclear

Power Plants 4 DATE REPORT COMPLETED

Phasp StudH I YEAR 5 AUTHORIS) -January 1986

SH Bush RE Dodge MONTH ATE REPORT ISSUEDAR

PG Heasler February 1986

7. PERFORMING ORGANIZATION NAME AND MAILING ADDRESS (ImcuaZp ModCI B PROJECTITASKrWORK UNIT NUMBER

Pacific Northwest Laboratory

P.O. Box 999 9 FIN OR GRANT NUMBER

Richland, WA 99352 B2865

10. SPONSORING ORGANIZATION NAME ANn MAIUNG ADDRESS (1chd.Zwp Code# Il. TYPE OF REPORT

Division of Engineering Technology Topical Report

Office of Nuclear Regulatory Research TopicalReport

U.S. Nuclear Regulatory Commission b PERIODCOVERED ll,&Us,.'edl

llashington, DC 20555

12. SUPPLEMENTARY NOTES

13. ABSTRACT (ZUU. w. a, foaJ

This report presents an overview of hydraulic and mechanical snubbers used on nuclear

piping systems and components, based on information from the literature and other sources.

The functions and functional requirements of snubbers are discussed. The real versus perceived need for snubbers is reviewed, based primarily on studies conducted by a Pressure

Vessel Research Committee. Tests conducted to qualify snubbers, to accept them on a caseby-case basis, and to establish their fitness for continued operation are reviewed.

This report had two primary purposes. The first was to assess the effects of various

aging mechanisms on snubber operation. The second was to determine the efficacy of existing tests in determining the effects of aging and degradation mechanisms. These tests

include breakaway force, drag force, velocity/acceleration range for activation in tension

r compression, release rates within specified tension/compression limits, and restricted

hermal movement. The snubber operating experience was reviewed using licensee event

reports and other historical data for a period of more than 10 years. Data were statistically analyzed using arbitrary snubber populations. Value-impact was considered in

Iterms of exposure to a radioactive environment for examination/testing and the influence

,of lost snubber function and subsequent testing program expansion on the costs and opera- tion of a nuclear power plant. The implications of the observed trends were assessed;

recommendations include modifying or improving examination and testing procedures to

nhance snubber reliability. Optimization of snubber populations by selective removal of

unnecessary snubbers was also considered. 14 DOCUMENT ANALYSIS -a KEYWORDS/OESCRIPTORS

Mechanical Snubbers

Hydraulic Snubbers

Safety-Related Piping

Nuclear Plant Aging Research Program

b. IOENTIFIERSIOPEN-ENDED TERMS

15 AVAILABILITY

STATEMENT Aging

Degradation

Testing Requirements

Unlimited

16 SECURITYCLASSIFICATION

Unclassified

IT,. rftegMOrt

Unclassified

17. NUMBER OF PAGES

18 PRICE
I I I

UNITED STATES

NUCLEAR REGULATORY COMMISSION

WASHINGTON, D.C. 20555

OFFICIAL BUSINESS

PENALTY FOR PRIVATE USE, $300

11

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7

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.SPECIAL FOURTH-CLASS RATE.. , POSAGEEtESPAID

USNRC.

WASH. D.C. PERMIT No G-J7.

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