Nuclear power plants – Instrumentation and control important to safety – Management of ageing of electrical cabling systems Centrales nucléaires de puissance – Instrumentation et cont
General
Cabling systems in nuclear power plants are susceptible to degradation from ageing, necessitating regular testing to maintain operational safety Ageing can cause cables to become dry and brittle, leading to potential malfunctions during normal operations or in emergency situations, such as a Loss-of-Coolant Accident (LOCA) During a LOCA, cables face high-pressure hot steam, which can exacerbate issues related to insulation ageing, cracks, or other damage that allows moisture ingress The risk of cable failure is heightened in these conditions, as steam can penetrate smaller cracks more effectively than water The ageing of cable insulation materials is addressed in IEC 62392.
Cable insulation damage can lead to various issues, but problems may also stem from cable conductors, connectors, or accessories These issues can result in measurement errors, erratic signals, spikes, noise, and other anomalies that disrupt efficient plant operation and compromise safety This International Standard outlines the requirements and guidelines necessary to identify these problems.
Cable types
Cables in nuclear power plants are categorized into four main types: instrumentation and control cables, which include coaxial, triaxial, twisted pair, and shielded varieties; low voltage power cables operating at less than 1 kV; medium voltage power cables, typically under 30 kV; and general services cables, such as ground and communication cables.
This International Standard primarily addresses I&C cables, but many of its principles also apply to low voltage power cables due to their use of similar materials and shared degradation mechanisms The testing methods outlined are specifically utilized for control rod drive mechanism (CRDM) cables and comparable cables in nuclear power plants.
Instrumentation cables (including thermocouple extension wires) are normally low voltage
Cables typically used for digital or analog transmission of sensor or instrument signals operate at voltages below 1 kV Common configurations include shielded and twisted pairs for resistance temperature detectors, pressure transducers, and thermocouple extension leads, with many thermocouple extension wires made from mineral insulated cables Additionally, coaxial or triaxial shielded configurations are often employed in radiation detection and neutron monitoring circuits.
Licensed to Mecon Limited for internal use in Ranchi and Bangalore, supplied by Book Supply Bureau Auxiliary components like control switches, valve operators, relays, and contactors typically operate at low voltage and low current They often utilize multi-conductor cables with shielding for proximity to high voltage systems Low voltage power cables, rated under 1 kV, are designed to power auxiliary devices such as motors, motor control centers, heaters, and small transformers These cables can be either single or multi-conductor and are generally unshielded.
Typically, a cable consists of four to eight components For example, the main components for an I&C or a low voltage power cable are:
Control and low voltage power cables often feature a jacketing layer over the insulation of individual conductors for enhanced fire retardance, commonly known as a conductor jacket or inner jacket Typically, the term "jacket" refers to the outer layer of the cable construction Additionally, various other components may be included in the cable design.
• filler or bedding materials, which occupy the gaps between insulated conductors in multi- conductor (also known as multi-core) cables, to improve mechanical stability of the cable;
• tape wraps, which may provide additional electrical, mechanical or fire protection, or identify conductor groupings;
• armouring layers, which are sometimes used for mechanical protection under the outer jacket layer
Annex A provides a description of a typical cable and the components that are normally involved in the testing activity covered in this International Standard.
Reasons for cable ageing management
Cable testing in nuclear power plants is essential for troubleshooting issues like signal anomalies, establishing baseline measurements for predictive maintenance, and assessing cable aging.
In recent years, cable ageing management has become more important for two main reasons
Several plants have successfully renewed their licenses to operate cables for an extended qualified life Additionally, the nuclear power industry acknowledges the limitations of cable qualification testing, particularly regarding pre-aging and the application of models like the Arrhenius law for evaluating qualified life.
Managing the aging of cables presents challenges, particularly in detecting and locating hot spots These hot spots can arise from various factors, including radiation effects, electrical heating, ambient heat, and mechanical stress Unfortunately, there is currently no reliable in-situ technique for pinpointing these hot spots along cables, especially since they are often housed in conduits, making visual inspections ineffective for accurate diagnostics of cable conditions.
Cable stressors
The ageing and degradation of cables in nuclear power plants are primarily caused by prolonged exposure to environmental stressors such as radiation, heat, humidity, and vibration Additionally, contact with lubricants, chemicals, and contaminants can further compromise cable integrity Internal factors, including ohmic heating from electric currents, also contribute to the deterioration of both the insulation material and the conductor Table 1 illustrates various ageing stressors that pose a risk to cable performance.
Furthermore, mechanical stressors such as bending, squeezing, vibration, or a combination of
Licensed to Mecon Limited in Ranchi/Bangalore for internal use only, supplied by Book Supply Bureau The interaction of these effects with other environmental stressors can lead to synergistic changes that alter the aging characteristics of cables.
Cable testing techniques
To mitigate the risks associated with cable ageing and degradation in nuclear power plants, it is essential to conduct regular testing and condition monitoring, particularly for safety-critical cables Various techniques have been developed to assess ageing effects and identify effective maintenance strategies The International Standard outlines basic testing methods in Annexes B, C, and D, which can be employed to ensure reliable cable performance and safeguard nuclear plant safety against the impacts of cable deterioration.
NOTE This International Standard provides methods for assessing cable systems, including connectors and end devices Methods for assessing ageing degradation of cable insulation are covered in IEC 62392
Table 1 – Examples of stressors with potential to damage cables
Ageing stressor Affected component Consequence
Conductor Increased resistance and self- heating Corrosion/oxidation
Connector Increased resistance and self- heating
Vibration Conductor Increased resistance, reduced strength
Connector Reduced strength, reduced connection quality
Insulation Formation of cracks, reduced insulation resistance (IR) when subjected to humidity, loss of material
Heat and ionising radiation Insulation Changes in mechanical properties, changes in flammability characteristics, loss of additives (plasticisers, anti-oxidants, etc.)
Moisture and water can significantly impact insulation and conductor performance, accelerating radiation and thermal aging effects, leading to cable material deterioration The entry of moisture into cables can cause shorting and shunting effects, reduce insulation resistance (IR), and result in swelling Additionally, lubricants and contaminants can further deteriorate insulation materials and connectors, compromising overall system integrity.
General
The reliability of cables in operational conditions is crucial for the control and safety systems of nuclear power plants To ensure safety, the performance of safety-relevant cables must be periodically verified throughout the plant's lifespan This is especially important for cables that support safety functions, as their failure could affect qualified equipment and pose risks to plant safety.
This clause gives requirements for in-situ testing to verify that electrical cabling systems provide reliable service and to ensure safety
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Test methods
Test methods outlined in the annexes of this International Standard have been implemented in nuclear power plants These include in-situ testing methods applicable during plant operation and techniques for testing cable samples It is essential that any testing method for safety-related cables is validated in accordance with section 5.8 of this International Standard.
Application of cable testing requirements
This International Standard applies to instrumentation cable systems that provide data on temperature, pressure (including level and flow), and neutron flux It is also relevant for cable systems that supply power to Control Rod Drive Mechanisms (CRDMs), rod position indicators, motors, heater coils, Solenoid Operated Valves (SOVs), and Motor Operated Valves.
(MOVs), and similar components Cables used with test sensors such as accelerometers, humidity sensors, and the like may also be tested using the techniques described here.
Test interval
Test intervals shall be established to detect unacceptable performance The following factors should be considered in determining the test interval:
• manufacturer’s recommendation and other industry standards,
• margin between measured performance characteristics and desired performance,
• rate-of-change of performance characteristics with time,
• cable failure rates and target reliability.
Test location
Testing should be performed in-situ to the extent possible Cable removal for testing is not acceptable except for representative cables from cable deposits.
Calibration of cable testing equipment
Cable testing equipment shall have valid calibration traceable to national standards Written procedures shall be used to perform the calibration, and the results of the calibration shall be documented.
Test results
Cable testing results must be evaluated against the permissible performance limits when available If the results surpass these limits or indicate a trend that suggests future exceedance before the next test, appropriate measures must be implemented to resolve the issue.
Validation of test methods
Cable testing methods must be validated to ensure that the results accurately reflect the cable's conditions This validation process involves several preliminary phases to guarantee that the laboratory tests are both representative and reproducible Documentation of this validation is essential and should include a clear definition of the test setup.
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• definition of a set of test cases (cable types, defect types, various lengths, junctions, etc.),
• identification of defects to be diagnosed (a complete list should be written),
• laboratory characterization of the various cables and other devices (junctions, connectors, loads) with and without defects,
Environmental aging, influenced by factors such as radiation and temperature, significantly impacts test methods and their outcomes A thorough comparison of these methods with appropriate laboratory and in-situ tests is essential to validate their effectiveness Additionally, a solid theoretical foundation is necessary to support the chosen test method It is crucial to outline the assumptions and conditions applied to confirm the validity of the test method, ensuring reliable and accurate results.
Software and test tool validation
Software for data acquisition, qualification, or analysis in cable testing must be systematically designed and developed in accordance with industry standards for nuclear power plants Comprehensive Verification and Validation (V&V) testing is essential for all software packages, with documentation required for both the basis and results of these tests Additionally, any tools utilized in the testing process must be qualified through a systematic Quality Assurance (QA) program to ensure their proper functionality.
Qualification of test personnel
Properly trained personnel must conduct testing to verify the performance of nuclear power plant cables, with their training documented and updated regularly Key training topics are essential to qualify these test personnel.
• training on data acquisition and data analysis software,
• interpretation and documentation of cable testing results
6 Acceptable means for cable testing
Annexes B and C outline various methods for cable testing in nuclear power plants These methods must comply with the requirements of the International Standard and undergo validation according to the specified criteria for the methods, software, and equipment utilized in the testing process.
I&C cables typically connect to sensors like Resistance Temperature Detectors (RTDs), pressure transmitters, or neutron detectors It is essential to implement methods that differentiate issues within cable systems from those in the end devices An example of this is the Loop Current Step.
The LCSR technique, typically employed for in-situ response time testing of RTDs, is effective in differentiating cable issues from RTD problems Additionally, noise analysis is utilized for other sensors, such as neutron detectors and pressure transmitters, to detect anomalies For detailed information on the noise analysis technique and the LCSR method, refer to IEC 62385 Both LCSR and noise analysis serve as valuable methods for distinguishing sensor problems from cable issues, alongside other existing or developing techniques.
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Annex D provides a summary of a particular set of cable testing techniques that can be used together with the noise analysis technique to establish the ageing condition of Nuclear
Instrumentation Systems (NIS) in nuclear power plants This procedure is used in nuclear power plants to avoid premature replacement of neutron detectors or other NIS components
8 Relationship between initial qualification and cable ageing management
Cables used in nuclear power plants that impact safety are typically qualified according to established standards, such as IEC 60780 The primary focus of initial qualification is to assess the cables' survivability and reliability during accidents and throughout the plant's operational life However, while qualification tests are beneficial, they do not guarantee the long-term reliability of cables Therefore, it is essential to implement periodic cable testing, condition monitoring, and other measures as specified in the relevant International Standard to maintain plant safety For further details, consult IAEA TECDOC 1188.
9 Example of a nuclear power plant practice for cable ageing management
Annex E outlines a nuclear power plant's cable ageing management program that incorporates visual inspections, in-situ testing of cable systems, and condition monitoring of insulation materials These programs are essential for nuclear power plants, particularly for extending operations beyond their original licensing period.
10 Cable testing for long-term operation
Nuclear power plant operating licenses are being extended to allow continued operation to 60 years and discussions have been carried out for long life operation to 80 or more years
For long-term operation exceeding 80 years, various components, including pumps, valves, sensors, and larger elements like steam generators and reactor vessels, are often replaced However, wholesale cable replacement is typically avoided unless absolutely necessary Implementing cable testing and condition monitoring can offer alternatives by identifying which cables require replacement based on testing results, trends, and monitoring data.
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Typical components of an electrical cable
Electrical cables for industrial applications typically consist of the following components
Copper, aluminum, nickel, gold, and silver are effective conductors of electricity, making them suitable for use in cables Copper stands out as the preferred choice due to its superior conductivity and cost-effectiveness.
Cable conductors are typically made of stranded wires for flexibility or solid wires for strength
Cable conductors are insulated with dielectric materials that resist electrical currents and provide protection against water, chemicals, abrasion, and heat Additionally, these insulation materials are designed to retard flames in the event of a fire A variety of polymeric materials, such as polyvinyl chloride, are commonly used for cable insulation.
(PVC) compounds, polyethylene compounds, and elastomers
Shielding in cable construction enhances protection against noise and electromagnetic/radio frequency interferences (EMI/RFI) Cables may feature foil shielding, composed of a thin aluminum layer bonded to polyester film, often accompanied by a drain wire for grounding Alternatively, braided shielding, typically made from copper or aluminum, is also utilized for effective interference mitigation.
An overall jacket serves to protect cables by providing physical shielding and mechanical strength The choice of jacket material is typically influenced by the specific environment in which the cable operates Generally, cable jackets are constructed from materials that are similar to those used for cable insulation or dielectric.
Examples of cables that are covered by this International Standard are shown in Figure A.1
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Polymeric compound a) Power cable a) Triaxial cable IEC 841/10
Figure A.1 – Example of Cables Covered by this International Standard
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Conductor c) Twisted pair shielded instrumentation cable d) Co-axial instrumentation cable e) Multiconductor (also known as multi-core cables) shielded control cable
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There are passive techniques for cable testing and maintenance as well as active techniques
A few examples of these techniques are described below
The traditional "look, feel, and smell" approach to passive maintenance offers valuable techniques for identifying cable issues, which can be executed through several straightforward procedures.
Conduct a thorough visual and physical inspection of the cable, looking for any cracks, changes in texture, and variations in color Utilizing baseline information and experience is essential for effectively identifying potential issues during these inspections.
• Size: inspect the cable for swelling, shrinkage, and deformation
• Cleaning: remove dirt, lubricants, solvents, or any extraneous chemicals
• Environmental monitoring: monitor the ambient conditions around the cable such as temperature, humidity, radiation, or vibration
• Thermography: perform thermography to locate hot spots in cables, connectors, and other components of a wiring system This method is often useful at terminations (e.g., switchboards)
• Behaviour during plant operation: signal anomalies, spikes, and the like during plant operation often provide clues as to cable problems
Active testing and maintenance techniques for cables encompass electrical, mechanical, and chemical tests This section reviews fundamental electrical testing methods, while additional IEC standards detail mechanical and chemical tests for monitoring the condition of cable insulation materials.
Electrical tests of cables are essential for assessing the condition of conductors, connectors, and cable insulation Key electrical tests include DC resistance, AC impedance, and Insulation Resistance (IR) measurements Additionally, Capacitance (C), Inductance (L), and Resistance (R) measurements can be conducted using an LCR meter, which allows for efficient testing of these parameters.