Nuclear Plant Cable Aging Management in the U.S. with

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Nuclear Plant Cable Aging

Management in the U.S. with

Regard to Standards

Gary J. Toman

Senior Project Manager

Plant Support Engineering

704-595-2573 gtoman@epri.com

Topics

The status of implementation of nuclear plant cable aging management

Existing supporting information

Whether existing cable and environmental qualification standards adequately support industry needs

© 2011 Electric Power Research Institute, Inc. All rights reserved.

2

Introduction

• The average period of operation of the nuclear fleet is approximately 30 years, with the oldest plant in its 40 th year of operation

• Until recently, most plants did not have a formal cable aging management program

• In 2010, the industry committed to implementing cable system aging management via a communication between the Nuclear Energy Institute and the U.S. Nuclear

Regulatory Commission

• The Institute for Nuclear Plant Operations has added the assessment of cable aging management implementation to their assessment procedures

© 2011 Electric Power Research Institute, Inc. All rights reserved.

3

Implementation Guidance

• In 2010, EPRI issued three cable aging management program implementation guides:

1020805

– Medium Voltage Cable (4160 V+)

1020804

– Low Voltage AC and DC Power (<1000V)

1021629

– Instrument and Control Cable

• These guides describe the scope of the cable aging management program and how to assess the condition of the cables

• The guides focus on cables in adverse environments where aging before the end of plant life could result in significant cable system degradation

• A cable system includes the cable, it’s terminations and splices, and its support system (e.g., trays, conduits, ducts, vaults, and manholes)

© 2011 Electric Power Research Institute, Inc. All rights reserved.

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Scoping of Cable Aging Management Programs

Entire Plant Cable

Population

Wet Cables and Splices

Cable Supporting

Maintenance Rule functions, License Rule

Commitments, and other licensing commitments

Cables subject to hot spots

1 and radiant energy

Cables with hot conductors or splices

Cables covered by the

Cable Aging Management

Program

1

Includes adverse chemical and radiation environments

.

© 2011 Electric Power Research Institute, Inc. All rights reserved.

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Implementation Guide Topics

• Three separate guides were generated because different strategies apply to each cable type (medium voltage, lowvoltage power, and I & C)

• For medium voltage cables:

– A limited number of circuits exist (generally fewer than

100) and working circuit by circuit is appropriate

– Aging under wet-energized conditions is a key concern

– Ohmic heating of connections and conductors is a concern

– Radiation and thermal environments are low level concerns because medium voltage cables are generally in more benign areas

© 2011 Electric Power Research Institute, Inc. All rights reserved.

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Medium Voltage Cable Implementation

Guidance

• Nuclear plants typically have ethylene propylene rubber cables with helically wrapped copper shields

• The attenuation of the EPR and the shield with a slight tarnish eliminates partial discharge testing due to the low amplitude, high frequency signals being evaluated

• Off-line, elevated voltage, Tan δ or dielectric spectroscopy are recommended tests for the EPR insulated cables

• IEEE Std 400 and 400.2 provide Tan δ acceptance criteria for XLPE but not for rubber insulations

• The EPRI MV guide provides preliminary Tan δ acceptance criteria for EPR and butyl rubber insulation based on nuclear plant tests and expert opinion (Note:

IEEE Std 400.2 is being revised to cover cables with rubber insulations)

© 2011 Electric Power Research Institute, Inc. All rights reserved.

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Low-Voltage Power Cable Aging Management

Guidance

• The population of low-voltage power cables is very large

• Many cables are in benign conditions and have low duty cycles or are conservatively sized with respect to ampacity

• The recommended approach for low-voltage power cables is to identify adverse environments through a walkdown and then determine if the adverse environments are significantly affecting the cables

• Highly loaded cables and terminations would be assessed for ohmic heating

• Each adverse environment would be documented and assessed for their effect on the cables

© 2011 Electric Power Research Institute, Inc. All rights reserved.

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Low-Voltage Power Cable Aging Management

Guidance

• Actions such as replacement of damaged cable or continued periodic assessments will be taken

• Only when cables have been significantly affected would determination of the circuits affect have to be determined

• Individual circuit identification is necessary to allow replacement to be scheduled and any effects on connected component function to be assessed

• For wet and submerged cable, insulation resistance testing with a 100 megohm-1000 ft (30.4 megohm-1000 m) acceptance criteria for further action required is recommended

• For dry cable, insulation resistance will not indicate thermal or radiation damage

• Walkdowns and Line Resonance Analysis has been recommended for assessment of thermal and radiation damage

© 2011 Electric Power Research Institute, Inc. All rights reserved.

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I &C Cable Aging Management

• The scoping and assessment strategies are the same as for low-voltage power cable:

– Find the adverse environments

– Determine the effects of the adverse environments on the cables

• There is no concern for ohmic heating

• Jacket failure on wet shielded cables can create additional grounds and result in circuit noise

• Connectors on wide range radiation monitors have assembly quirks that must be understood

© 2011 Electric Power Research Institute, Inc. All rights reserved.

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Low-Voltage Cable and Wet Aging

• Unlike medium voltage cable there are no recognized wet aging mechanisms that cause deterioration of low-voltage cable insulation

• Little forensic information exists on low voltage cable failures

• Relatively few failures have occurred with respect to the size of the population of cables

• Further research is need to determine if there is truly a generic longterm degradation that is caused by long-term exposure to water for the commonly used EPR and XLPE insulations

• Only two failures are suspected of being associated with dc control circuits and wet aging; these were associated with insulations that are not in common use (an early radiation cured XLPE and high molecular weight polyethylene)

• Further research is needed to determine if there is an actual generic degradation mechanism or if failures are from manufacturing defects, installation damage, or other external causes

© 2011 Electric Power Research Institute, Inc. All rights reserved.

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Visual/Tactile Assessment

• Visual/tactile assessment can easily determine if cables are essentially unaged or are significantly degraded

• The cable jackets generally age before the insulation because they typically have ratings that are 15

°C lower than the insulation

• The rubber jackets that are used harden and discolor when thermally or radiation aged giving visual and tactile indications

• Visual/tactile assessment is an excellent screening tool

• However, precise degrees of aging cannot be determined with this method

© 2011 Electric Power Research Institute, Inc. All rights reserved.

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Visual Assessment of Neoprene and Hypalon

Thermal Aging

• Cracked brown to highly whitened cables are over aged neoprene jacketed cables

• Mostly black cables are

Hypalon jacketed

• Hypalon ages significantly more slowly than neoprene

• Both neoprene and Hypalon age much more rapidly than the XLPE and EPR insulations in use

© 2011 Electric Power Research Institute, Inc. All rights reserved.

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Formal Condition Monitoring Techniques for

Aging Assessment

• Numerous condition assessment methods exist for thermal and radiation affects

• Thermal and radiation aging data exist for these techniques and can be found in EPRI 1011874 (open to public)

• In-situ nondestructive methods include: Indenter modulus, line resonance analysis, and acoustic velocity assessment

• Line Resonance Analysis (LIRA) is an in situ electrical test that can locate thermal damage before failure of the insulation wall

• Laboratory tests requiring specimen removal include:

Oxidation induction time, oxidation induction temperature, micro-modulus, sol and gel assessment, nuclear magnetic resonance, and elongation-at break

© 2011 Electric Power Research Institute, Inc. All rights reserved.

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Observations on Further Standards for Cable

Aging Management

• IEEE Std 323 covers environmental qualification of all electrical equipment.

– Section 6.3.6 of IEEE Std 323 allows used of condition based qualification in which condition monitoring may be used as the basis for continued service

• IEEE Std 383 covers environmental qualification of cable for nuclear power plants

• IEEE Std 572 covers environmental qualification for connectors

• IEEE Std 400 and 400.2 covers tan δ testing for medium voltage cable testing but currently only provides acceptance criteria for XLPE

(a revision to cover EPR is under development)

• IEC is developing guidance on how to perform certain cable condition monitoring tests (Indenter and Elongation at break, others?); however, these standards do not include acceptance criteria

© 2011 Electric Power Research Institute, Inc. All rights reserved.

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Standards that Do Not Exist

• Submergence Qualification

– There is no standard related to non-accident submergence qualification for cables that may be immersed in water for a significant portion of the life of the plant

– IEEE Insulated Conductors Committee is in the process of forming a standards writing group; EPRI is beginning a submergence qualification for EPR cable

© 2011 Electric Power Research Institute, Inc. All rights reserved.

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Standards that Do Not Exist

• Acceptance Criteria for Condition Monitoring Methods

– A large amount of thermal and radiation aging data exists for commonly used insulation systems

– The data is not readily useful to plant personnel because it has not been reduced to acceptance criteria

– EPRI 1008211 is an initial attempt at reducing some of the information to practical use including acceptance criteria but is still a long way from standardized practice

• Developing an acceptance criteria standard is not a trivial task in that each manufacturer’s polymers are different from those of other manufacturers and from other polymers in the manufacturers product line. Uniform aging does not occur among the various polymers

• Cable design (beyond polymer considerations) can affect the aging rate.

© 2011 Electric Power Research Institute, Inc. All rights reserved.

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Guidance that Might Be Useful

• Power plant cables are different from distribution cables

• Cable installation practices in power plants are different from distribution cable practices

• Specific guidance (best practices rather than standards) on the longterm management of power plant cable systems would be helpful including practical methodology of operation

– Actions to be taken for ground indications (some systems can run for short periods with a ground in place)

– Testing concerns

– Splicing new cables to old (e.g., when replacing degraded wet sections)

– Suggestions for separable connectors to allow testing of cables independent from loads (and testing loads independent of the cable)

© 2011 Electric Power Research Institute, Inc. All rights reserved.

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Questions?

Together…Shaping the Future of Electricity

© 2011 Electric Power Research Institute, Inc. All rights reserved.

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