ML15126A531

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

OFFICE OF NEW REACTORS

WASHINGTON, DC 20555-0001

September 24, 2015

NRC INFORMATION NOTICE 2015-09: MECHANICAL DYNAMIC RESTRAINT (SNUBBER) 

LUBRICANT DEGRADATION NOT IDENTIFIED 

DUE TO INSUFFICENT SERVICE LIFE 

MONITORING 

ADDRESSEES

All holders of and applicants for an operating license or construction permit for a nuclear power 

reactor under Title 10 of the Code of Federal Regulations (10 CFR) Part 50, “Domestic 

Licensing of Production and Utilization Facilities,” except those that have permanently ceased 

operations and have certified that fuel has been permanently removed from the reactor vessel.

All holders of and applicants for a nuclear power reactor early site permit, combined license, 

standard design certification, standard design approval, or manufacturing license under 

10 CFR Part 52, “Licenses, Certifications, and Approvals for Nuclear Power Plants.” All 

applicants for a standard design certification, including such applicants after initial issuance of 

a design certification rule.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert

addressees to potential degradation of the lubricant (grease) in mechanical dynamic restraints 

(snubbers) not identified due to insufficient service life1 monitoring (SLM). The NRC expects

that recipients will review the information for applicability to their facilities and consider actions, 

as appropriate, to avoid similar problems. Suggestions contained in this IN are not NRC 

requirements; therefore, no specific action or written response is required.

DESCRIPTION OF CIRCUMSTANCES

Limerick Generating Station, Unit 1

At the beginning of 2014, during the 15th refueling outage (1R15) at the Limerick Generating 

Station, Unit 1, Exelon Generation Company (licensee) functionally tested a sample of

safety- related snubbers in accordance with its technical specification (TS) surveillance 

requirements (SRs). The use of the TS SRs in lieu of American Society of Mechanical 

Engineers (ASME) Boiler and Pressure Vessel Code (BPV Code), Section XI, requirements for 

the Limerick Generating Station, Unit 1, was authorized by the NRC staff in response to Relief

___________________________

1 The term “service life” is defined by the ASME Code for Operation and Maintenance of Nuclear Power Plants

(OM Code) as, “the period of time an item is expected to meet the operational readiness requirements without 

maintenance.” The ASME OM Code is incorporated by reference in 10 CFR 50.55a, “Codes and standards,” with 

conditions.
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Request I3R-05 (Agencywide Documents Access and Management System (ADAMS) 

Accession No. ML080500584). Four mechanical snubbers were found not to meet the test 

acceptance criteria, which resulted in an increase in the testing scope as required by the TS SR. 

One snubber failed its test due to high drag, and three snubbers failed their tests due to high 

acceleration. An apparent cause evaluation, failure analyses, and comparison testing were 

performed. The apparent cause of the drag failure was confirmed to be due to dried grease 

throughout the snubber based on visual examination of the internal parts. The apparent cause 

of the high acceleration test failures was degradation of the test machine. 

Diablo Canyon Nuclear Power Plant, Unit 1

At the beginning of 2014, during the 18th refueling outage (1R18) at the Diablo Canyon Nuclear 

Power Plant, Unit 1, the Pacific Gas & Electric Company (licensee) functionally tested a sample

of safety-related snubbers in accordance with its Third Interval Snubber Program submitted to 

the NRC in its letter DCL-10-157, dated December 21, 2010 (ADAMS Accession No. 

ML103560316). For snubber inservice examination and testing, the licensee implemented the

requirements of ASME Code for Operation and Maintenance of Nuclear Power Plants

(OM Code), Subsection ISTD, and its Engineering Control Guidance 99.1, and identified five 

mechanical snubber failures during functional testing. These five failures, from two separate 

defined test plan groups, resulted in an increase in the testing scope of 51 additional snubbers.

These snubber failures were a result of grease degradation due to aging.

Browns Ferry Nuclear Plant, Unit 3

At the beginning of 2014, during the 16th refueling outage (U3R16) at the Browns Ferry 

Nuclear Plant, Unit 3, Tennessee Valley Authority (licensee) functionally tested a sample

of safety-related snubbers in accordance with its Technical Requirement Manual (TRM) 

TR 3.7.4. For snubber inservice examination and testing, the NRC staff authorized the 

use of Browns Ferry Nuclear Plant, Unit 3, TRM TR 3.7.4 in lieu of the ASME BPV Code, 

Section XI, requirements in response to Relief Request 3-ISI-2 (ADAMS Accession No. 

ML063330648). A total of four Pacific Scientific Arrestor (PSA) mechanical snubbers 

failed during their functional test to meet the specified acceptance criteria. These 

failures resulted in an expanded scope of functional testing of snubbers. Based on 

failure analysis, it was determined that two PSA-3 snubbers failed due to dried grease 

and damage to internal parts. One PSA-35 snubber failed due to sticky, tacky grease,

and damage to snubber internal parts. Another PSA-10 snubber failed due to loose 

internal parts. 

Arkansas Nuclear One, Unit 2

During the 22nd refueling outage (2R22) in 2012 at Arkansas Nuclear One, Unit 2, Entergy 

Operation, Inc. (licensee) functionally tested a number of safety-related snubbers in accordance 

with the 10 percent test plan in the 2004 Edition of the ASME OM Code, Subsection ISTD. 

During this functional testing, seven mechanical snubbers did not meet the specified 

acceptance criteria, resulting in a scope expansion. Two of the seven snubbers were later 

evaluated and it was concluded the functional test results were not failures. During scope 

expansion, all of a specific type large bore mechanical snubber model (that exhibited the failure) 

were tested. The predominant cause appeared to be dry or “caked” grease, which would imply 

a degraded condition of the grease. Similarly, during a previous Arkansas Nuclear One, Unit 2 
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refueling outage (2R19) in 2008, there were eight functional failures of mechanical snubbers. 

The failures resulted in five scope expansions. The primary issue was drag failures associated 

with over-greasing or dried grease.

Oyster Creek Generating Station

During the 23rd refueling outage (1R23) in fall 2010 at Oyster Creek Generating Station, Exelon 

Generation Company (licensee) functionally tested a sample of safety-related snubbers in 

accordance with its TS 4.5.M, “Shock Suppressors (Snubbers).” While performing the 

functional testing of selected snubbers, four functional failures caused Exelon to expand their 

inspection scope, and an additional 40 percent of the installed mechanical snubbers were 

tested. The apparent cause of the failures for all four snubbers was either hardened or missing 

grease. While inspecting four snubber testing failures that occurred during refueling outage 

1R23, the NRC inspector identified a non-cited violation of TS 4.5.M.1.f, “Service Life 

Monitoring.” Specifically, Exelon’s snubber testing program, contained in SP-1302-52-045, 

“Requirements for Functional Testing of Snubbers,” does not evaluate snubber maintenance 

and test records to identify common cause failures of snubbers due to environmental 

(temperature, vibration, and humidity) conditions and adjust snubber service life expectations 

accordingly, such that snubber service life reviews can be accomplished effectively without 

service life affecting reactor operations. Exelon took immediate corrective action to repair or 

replace the failed snubbers, performed an analysis to ensure the snubber failures had no 

impact on system operation, and entered this issue into its corrective action program (ADAMS 

Accession No. ML110390509).

BACKGROUND

Related NRC Generic Communications

NRC Information Notice (IN) 1994-48, “Snubber Lubricant Degradation in High-Temperature

Environment,” summarizes the degradation of the lubricants used in mechanical snubbers when 

snubbers are placed in a high-temperature environment (ADAMS Accession No. 

ML031060631). 

NRC Generic Letter (GL) 1984-13, “Technical Specification for Snubbers,” (ADAMS Accession 

No. ML031150710).

Related Guidance and Applicable Standards

ASME Standard QME-1-2007, “Qualification of Active Mechanical Equipment Used in Nuclear 

Power Plants,” as endorsed by Regulatory Guide 1.100, Revision 3, “Seismic Qualification of 

Electrical and Active Mechanical Equipment and Functional Qualification of Active Mechanical 

Equipment for Nuclear Power Plants,” provides one acceptable method for qualification testing 

of dynamic restraints such as snubbers, including the monitoring of lubrication degradation.

Nuclear power plant applicants and licensees may specify the use of ASME Standard 

QME-1-2007, as endorsed in Regulatory Guide 1.100, Revision 3, in procurement specifications 

for new or replacement dynamic restraints.

NUREG/CR-5870, “Results of LWR [light-water reactor] Snubbers Aging Research,” dated 

May 1992, provides aging research results and recommendations for snubbers used in nuclear 

power plants (NPPs) to enhance the understanding of snubber aging and its consequences 

(ADAMS Accession No. ML040340438). 
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DISCUSSION

At operating NPPs, safety-related snubbers are used as restraining devices to control the 

movement of piping systems and equipment during abnormal dynamic conditions (such as 

earthquakes, turbine trips, safety/relief valve discharge, and rapid valve closures). The design 

of a snubber allows free thermal movement of a component during normal operation conditions, 

but restrains the component during abnormal dynamic conditions. Adequate lubrication is 

essential to the proper functioning of snubbers.

The operational readiness of snubbers at an NPP is established by the combination of inservice 

examination, testing, and SLM of snubbers during refueling outages as required by 

10 CFR 50.55a and the applicable ASME BPV Code or ASME OM Code. Many snubbers were 

originally procured and installed with the expectation that they would be maintenance free for 

the duration of the manufacturer’s recommended design life. The physical condition of 

snubbers is evaluated by visual inspections which, depending on the number of unacceptable 

snubbers identified during the previous inspection interval, could be performed once every 

48 months. Also, if the licensee is using ASME OM Code Case OMN-13 after meeting the 

prerequisite requirements, the inservice visual examination can be performed once every 

10 years. 

Past industry experience with snubber failures has demonstrated that the failure modes of 

mechanical snubbers might not be identified by visual inspection or physical stroking. In 

particular, functional testing under static load conditions is more effective than visual inspection 

or physical stroking in confirming snubber operational readiness. In that only a small sample of 

the snubber population is selected for testing, SLM plays a very important role in maintaining

the operational readiness of snubbers at an NPP. 

During refueling outages, licensees typically select a small sample of snubbers for functional 

testing to demonstrate their operational readiness in accordance with the applicable ASME BPV 

Code or ASME OM Code, plant-specific TS/TRM, or NRC authorized relief or alternatives. In 

accordance with the specific sampling method, snubbers may be selected randomly or based

on the size, design, configuration, operating environment, load capacities and distribution of the 

snubber population, using various sample techniques considering test failure rates. With the 

small sample of snubbers selected for functional testing each refueling outage, it might take 

decades before all of an NPP’s snubbers are tested. Furthermore, some snubbers might never 

be tested during their service life. 

In the examples above, most of the snubber failures were determined to be caused by grease 

degradation, such as: (1) oil separation from grease; (2) dried or “caked” grease; (3) excessive 

grease; (4) sticky and tacky grease; and (5) hardened or missing grease. A well-planned SLM 

program for snubbers can minimize the number of snubber failures due to degradation of 

grease. An effective SLM program would include provisions for preventive maintenance (such 

as regreasing, partial disassembly for an internal inspection, or additional functional testing for 

SLM for mechanical snubbers) based on the results of performance monitoring and the 

evaluation of the service conditions for snubbers. 

In NRC IN 1994-48, the NRC staff alerted licensees to potential grease degradation due to high 

temperature. The high grease temperature might result from the installation area environmental 

conditions or internal conditions caused by vibration. In addition to high temperature, the NRC 

staff has observed that additional factors might contribute to degradation of the snubber 

lubricant (grease). For example, grease degradation might result from the following:
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• vibration

• elevated temperature for extended periods of time (e.g., high room temperature, high 

fluid temperature, and snubber installed in close proximity to high-temperature 

components)

• hazardous environment at the snubber location

• aging of the snubber and grease

• snubber installed from spare inventory without a recent preservice test

• shelf life of the grease exceeding the manufacturer’s recommendation.

The ASME BPV Code or ASME OM Code, plant-specific TS/TRM, or NRC-authorized relief or 

alternatives require licensees to perform SLM to evaluate service life of each safety-related 

snubber at least once each fuel cycle. An effective SLM program would consider these service 

conditions in evaluating the need for preventive maintenance for potentially affected snubbers 

prior to the end of their service life. In addition, the review of historical records or manufacturer 

recommendations may need to be supplemented with other information, such as plant 

conditions and operating experience, in evaluating the need for preventive maintenance of 

snubbers.

In addition, NUREG/CR-5870 concludes that aging may deteriorate mechanical snubber 

performance, particularly when snubbers are exposed to one or more environmental 

stressors. Performance is related to drag force, breakaway force, and acceleration threshold. 

Primary influences affecting degradation are elevated temperature, vibration, moisture, and 

dynamic transients. An effective SLM program would identify and monitor service conditions,

and implement appropriate preventive maintenance of snubbers. 

Alternatively, Section QDR, “Qualification of Dynamic Restraints,” in ASME Standard 

QME- 1- 2007 provides guidance for qualification testing of snubbers. Section QR-5000, 

“Qualification Principles,” and Nonmandatory Appendix QDR-A, “Functional Specification for 

Dynamic Restraints,” in ASME QME-1-2007 provide guidance for evaluation of aging effects, 

determination of qualified life (as defined in ASME QME-1, Section QR), and development of a 

replacement schedule for the lubricant, if applicable.

A key factor in the determination of service life of mechanical snubbers during SLM is the 

condition of the grease with consideration of the shelf life for the replacement grease. For 

example, the original lubricant Nuclear Radiation Resistance Grade (NRRG)-159 for mechanical 

snubbers was discontinued by the manufacturer around 1995, and was replaced with the new 

lubricant NRRG-2. Therefore, NRRG-159 that has been maintained in long-term storage may 

not be effective. The typical shelf life of grease is about 5 years under normal storage 

conditions, but some manufacturers have indicated that stored grease may be used for up to 

10 years. The lubricant suppliers or vendors may provide their own certified useful 

recommended shelf life. Grease for safety-related components that is used beyond its 

recommended shelf life is still subject to the regulatory requirements in 10 CFR Part 50, 

Appendix B, “Quality Assurance Criteria for Nuclear Power Plants and Fuel Reprocessing 

Plants.” 

CONCLUSION

Safety-related snubbers are important restraining devices to control the movement of piping 

systems and equipment during abnormal dynamic conditions. The design of a snubber allows 

free thermal movement of a component during normal operating conditions, but restrains the 
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component during abnormal dynamic conditions. In addition to appropriate operation of the 

mechanical parts of snubbers, adequate lubrication is essential to the proper functioning of 

snubbers. The operational readiness of snubbers is maintained by the combination of inservice 

examination, testing, and SLM as required by 10 CFR 50.55a and the applicable ASME BPV 

Code or ASME OM Code, the plant specific TS/TRM, or NRC authorized relief or alternatives. 

The NRC expects that addressees will review the information in this IN for applicability and 

consider actions, as appropriate, to avoid similar problems. Suggestions contained in this IN 

are not NRC requirements; therefore, no specific action or written response is required.

CONTACT

This IN does not require any specific action or written response. If you have any questions 

about the information in this notice, please contact the technical contact listed below or the 

appropriate NRC project manager.

/RA/ /RA/

Michael Cheok, Director Lawrence E. Kokajko, Director

Division of Construction Inspection Division of Policy and Rulemaking

 and Operational Programs Office of Nuclear Reactor Regulation

Office of New Reactors

Technical Contact: Gurjendra S. Bedi, NRR

301-415-1393

E-mail: Gurjendra.Bedi@nrc.gov

Note: NRC generic communications may be found on the NRC’s public Web site,

http://www.nrc.gov, under NRC Library/Document Collections.
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component during abnormal dynamic conditions. In addition to appropriate operation of the 

mechanical parts of snubbers, adequate lubrication is essential to the proper functioning of 

snubbers. The operational readiness of snubbers is maintained by the combination of inservice 

examination, testing, and SLM as required by 10 CFR 50.55a and the applicable ASME BPV 

Code or ASME OM Code, the plant specific TS/TRM, or NRC authorized relief or alternatives. 

The NRC expects that addressees will review the information in this IN for applicability and 

consider actions, as appropriate, to avoid similar problems. Suggestions contained in this IN 

are not NRC requirements; therefore, no specific action or written response is required.

CONTACT

This IN does not require any specific action or written response. If you have any questions 

about the information in this notice, please contact the technical contact listed below or the 

appropriate NRC project manager.

/RA/ /RA/

Michael Cheok, Director Lawrence E. Kokajko, Director

Division of Construction Inspection Division of Policy and Rulemaking

 and Operational Programs Office of Nuclear Reactor Regulation

Office of New Reactors

Technical Contact: Gurjendra S. Bedi, NRR

301-415-1393

E-mail: Gurjendra.Bedi@nrc.gov

Note: NRC generic communications may be found on the NRC’s public Web site, 

http://www.nrc.gov, under NRC Library/Document Collections.

ADAMS Accession No.: ML15126A531 *via email TAC MF5865

OFFICE NRR/DE/EPNB* Tech Editor* NRO/DE/MEB* NRR/DE/EPNB*

NAME GBedi JDougherty TClark DAlley

DATE 8/20/15 5/8/15 8/3/15 8/20/15

OFFICE NRR/DE* NRR/DPR/PGCB* NRR/DPR/PGCB NRR/DPR/PGCB*

NAME JLubinski ELee (ABaxter for/) MBanic SStuchell

DATE 7/23/15 9/01/15 09/02/15 09/15/15

OFFICE NRO/DCIP/D NRR/DPR/DD NRR/DPR/D

NAME MCheok AMohseni LKokajko

DATE 9/18/2015 9/21/2015 9/24/2015

OFFICIAL RECORD COPY