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

Report
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
[email protected]
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 40th
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.
4
Scoping of Cable Aging Management Programs
Entire Plant Cable
Population
Cable Supporting
Maintenance Rule
functions, License Rule
Commitments, and other
licensing commitments
Wet Cables
and Splices
Cables subject
to hot spots1 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.
5
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.
6
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.
7
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.
9
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.
10
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.
11
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.
13
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.
14
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.
16
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.
17
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.
18
Questions?
Together…Shaping the Future of Electricity
© 2011 Electric Power Research Institute, Inc. All rights reserved.
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