Nuclear Power Plant Cables Research Articles

Improvement of electrical insulation performance of silicone rubber cables due to exposure to heat and radiation

AbstractThermal treatments, with and without co‐irradiation with gamma rays, were given to safety‐related cables insulated with silicone rubber for nuclear power plants (NPPs). To examine the effects of the above treatments on the insulation properties of the cables, DC voltages were applied to them and the induced changes in leakage current were measured by the current integration method. As a result, it has become clear that the leakage current is the largest in the pristine untreated cable. In addition, almost no leakage current flows in the cables heated at 200°C or 290°C for several hundred hours. The leakage current remains small when the co‐irradiation with gamma rays was given at 100°C or 200°C for relatively long periods exceeding 1000 h. The reason for these results seems to be that the silicone rubber insulation becomes hardened as a result of the thermal or concurrent treatment, and the transport of ionic carriers becomes difficult. The above results indicate experimentally that the degradation is not a matter of great concern for the insulation performance of silicone‐rubber‐insulated safety‐related cables in NPPs.

Study on Nonlinear Dielectric Properties of Micro Silica

Inorganic insulating powder can potentially be used in nuclear power plant cables, fire-resistant cables, and so on due to its high heat resistance and radiation resistance. It is of great academic and engineering value to study the dielectric properties of inorganic insulating powder. In this paper, we aim to study the nonlinear dielectric properties via the measurement of the time-domain polarization current spectrum under the application of a DC electric field. Three kinds of silica powders are measured by a measurement system with adjustable pressure. The effects of powder shape, particle size, and packing pressure and temperature on the dependence of relaxation polarization and electrical conductivity on the applied electric field are studied. The experimental results show that the relationship between electrical conductivity and the electric field of inorganic insulating powder presents two different characteristics, i.e., field-induced enhancement and field-induced weakening. The relationship between conductance and temperature shows an increase with temperature. That is, the electrical conductivity increases or decreases with the increase in temperature. The inorganic powder insulation can be regarded as a composite, which is composed of inorganic powder particles and air gaps. The interface between the powder particles and air gaps contributes a lot to the polarization of inorganic insulating powder. The phenomena (including the field-induced weakening characteristic between relaxation polarization and electric field and the decrease characteristic of polarization with increasing temperature) can be explained by a simplified interface polarization mechanism.

On the mechanism of radiation and thermal-induced degradation of the insulation cables in nuclear power plants-the presence of long-lived polyenyl C-centered radicals

Radiolytically and thermally produced long-lived polyenyl C-centered radicals in the nuclear power plants’ insulation cables are detected and measured after more than 45 months of aging since the irradiation and the thermal treatment. Electron Paramagnetic Resonance (EPR) results of the irradiated Cross-linked Polyethylene (XLPE) and Ethylene Propylene Rubber (EthProRu), used as primary insulation materials at Nuclear Power Plants (NPP), clearly demonstrate the presence of polyenyl radicals along with allyl and peroxyl radicals. The yields of these free radicals are dose-rate dependent. EPR results also show the presence of the long-lived polyenyl along with allyl and peroxyl free radicals in the thermally treated XLPE, and EthProRu. These free radicals can cause a major failure in the performance of these cables in NPPs. Besides, the results showed the enhancement in the oxidation reactions due to the migration of copper ions from the conductor to the bulk of the insulation.This study also shows that the oxidation index (OI) and Elongation at Break (EAB) increases and decreases respectively at higher doses and temperatures.

Fire Retardancy Analysis of the Aged Cables in Nuclear Power Plants: Flammability Modelling using Thermogravimetric Analysis and Differential Scanning Calorimetry

The first step towards fire protection in nuclear power plants based on the concept of ‘Defense in-depth’ is realized through the minimization of combustibles. Regarding the realistic methods for minimizing combustibles in nuclear power plants, the fire retardancy of cables as the highest number of combustibles should be kept for design lifetime as well as at construction. The flame test, according to IEEE-383 and IEEE-1202, was conducted to assess the fire retardancy of aged and unaged cables, both of 1E and non-1E classes. However, previous editions of IEEE-383 (such as the 2015 edition) did not require non-1E class cables to be fire retardant. Therefore, a method to validate the fire retardancy of non-1E class cables installed in nuclear power plants (that had) got the construction permission before 2015 should be developed and implemented. Additionally, it is still unclear as to how the accelerated thermal aging specified in IEEE-383 is close to that of the real aged cables in nuclear power plants. Therefore, flammability modelling using thermogravimetric analysis and differential scanning calorimetry is proposed for solving the above mentioned issues and comprehending the flammability of cables beyond the test codes such as IEEE-1202.

Implementation of Non-Destructive Condition Monitoring Techniques on Low-Voltage Nuclear Cables: II. Thermal Aging of EPR/CSPE Cables

Determining the aging state of low-voltage nuclear power plant cables using a nondestructive and reliable condition monitoring technique is highly desirable as the cables experience multiple aging stresses during the service period. This paper deals with the implementation and investigation of such nondestructive techniques, which can detect the overall aging state of low-voltage instrumentation and control (I&C) cables, which are subjected to accelerated thermal aging. The dielectric spectroscopy, extended voltage response, and polarization–depolarization current as nondestructive electrical aging techniques were used for the investigation purpose, while the elongation at break was also adopted as a mechanical measurement and for comparison. Prominent variations in the electrical parameters for the insulation and jacket were observed, whereas the elongation at break for both materials also decreased under thermal aging. Based on the electrical techniques, aging markers were selected that showed a strong correlation with the aging and elongation at break, proving the ability of the adopted electrical methods as a nondestructive condition monitoring technique.

Innovative Development and Application of a Stress-Strength Model for Reliability Estimation of Aged LV Cables for Nuclear Power Plants

As a part of the H2020 EU Project called “TeaM Cables” - which has, among its aims, modelling reliability of nuclear power plant cables - the goal of this paper is to develop a model for the prediction of residual reliability of low voltage cables for nuclear power plants subjected to gamma radiation stress. The model estimates the probability that such cables withstand random stress overshoots in-service, which can occur, e.g., in case of nuclear accidents.

Comparison of Mechanical and Low-Frequency Dielectric Properties of Thermally and Thermo-Mechanically Aged Low Voltage CSPE/XLPE Nuclear Power Plant Cables

During the service period of low-voltage nuclear cables, multiple stresses influence the aging of polymeric materials of cables. Thermal and radiation stresses are considered service aging factors in qualification tests, while the standards usually do not prescribe mechanical stress. CSPE/XLPE insulated nuclear cable samples were exposed to thermal and combined thermo-mechanical aging for more than 1200 h at 120 °C. The real and imaginary parts of permittivity were measured in the 200 μHz to 50 mHz range as dielectric properties. The Shore D hardness of the samples was measured to analyze the mechanical characteristics of the cable. To characterize the dielectric spectrum, derived quantities, namely central real and imaginary permittivities and real and imaginary permittivities’ central frequencies were calculated. The change of dielectric spectra did not show a clear trend with aging, but the imaginary permittivity’s central frequency was higher by 0.5 mHz in the case of thermo-mechanically aged samples. The Shore D hardness was also higher on the thermo-mechanically aged samples. These findings show the combined aging has a higher impact on the insulation properties. Hence, involving the mechanical stress in the aging procedure of cable qualification enables the design of more robust cables in a harsh environment.

Influence of Thermal Aging on the Combustion Characteristics of Cables in Nuclear Power Plants

In this study, the combustion characteristics and emission of toxic gases of a non-class 1E cable in a nuclear power plant were investigated with respect to the aging period. A thermal accelerated aging method was applied using the Arrhenius equation with the activation energy of the cables and the aging periods of the cables set to zero, 10, 20, 30 and 40 years old by considering the lifetime of a nuclear power plant. According to ISO 5660-1 and ISO 19702, the cone calorimeter Fourier transform infrared spectroscopy test was performed to analyze the combustion characteristics and emission toxicity. In addition, scanning electron microscopy and an energy dispersive X-ray spectrometer were used to examine the change in the surface of the sheath and insulation of the cables according to the aging periods. To compare quantitative fire risks at an early period, the fire performance index (FPI) and fire growth index (FGI) are derived from the test results of the ignition time, peak heat release rate (PHRR) and time to PHRR (tPHRR). When comparing FPI and FGI, the fire risks decreased as the aging period increased, which means that early fire risks may be alleviated through the devolatilization of both the sheath and insulation of the cables. However, when comparing heat release and mass loss, which represent the fire risk at the mid and late period, fire intensity and severity increased with the aging period. The emission of toxic gases coincided with the results obtained from the heat release rate, which confirms that the toxicity of non-aged cables is higher than that of aged cables. From the results, it can be concluded that the aging period significantly affects both the combustion characteristics and toxicity of the emission gas. Therefore, cable degradation with aging should be considered when setting up reinforced safety codes and standards for cables and planning proper operation procedures for nuclear power plants.

Analysis of Combustion and Smoke Characteristics According to the Aging of Class 1E Cables in Nuclear Power Plants

In this study, combustion and smoke characteristics according to the aging of class 1E cables in nuclear power plants were analyzed through a cone calorimeter test. In the case of combustion characteristics, during the early period, which was the first peak of the heat release rate, the peak value of the non-aged cable was higher by approximately 20-50 kW/m<sup>2</sup> than that of aged cables. However, in the mid-late periods, which was the second peak, the value of the aged cables were higher than the non-aged cable due to the decrease in flame retardant performance with aging deterioration. In addition, the duration of the char layer of the aged cables was shortened by 200 s than that of the non-aged cables due to the unstable formation of char layer. The total heat release measured was approximately 1.4 times higher in the aged cables than in the non-aged cables. In the case of smoke characteristics, the smoke production rate and total smoke release show a similar trend with the heat release rate and total heat release. The total smoke release of the aged cables was measured to be higher than that of the non-aged cables. The tendency of the smoke factor increased with aging deterioration, and the values of the smoke factor in the aged cables beyond 4 years were approximately 1.76-2.0 times different from those in the non-aged cables. Consequently, the smoke risk increased with aging deterioration. Therefore, the risk of heat and smoke release increased as aging progressed.

Electrical and Mechanical Condition Assessment of Low Voltage Unshielded Nuclear Power Cables Under Simultaneous Thermal and Mechanical Stresses: Application of Non-Destructive Test Techniques

Low voltage cables play a crucial role in the safe and reliable operation of nuclear power plants. This paper discusses the effect of simultaneous combined thermal and mechanical stresses on the overall insulation of unshielded low voltage power cables used in nuclear power plants. Cable samples based on XLPE/CSPE insulation were exposed to thermal-mechanical combined accelerated aging tests for an extended period lasted to 907 hours. Time and frequency domain dielectric spectroscopy were applied to reveal the degradation level of the core insulation and jacket due to combined stresses. The former technique was conducted through the measurement of the decay and return voltage slopes, while the latter technique was based on the measurement of the complex permittivity. Moreover, the mechanical properties of the cable insulation were investigated by applying the measurement of the Shore D hardness. The results showed that the combined aging affects both the electrical and mechanical properties of the overall insulation. The complex permittivity at 100 Hz, the decay voltage slope, and the hardness had an upward trend with aging, while the return voltage slope showed an inverse trend. In addition, the results of the electrical and mechanical measurements showed strong correlation with aging time and further correlation was observed between the dielectric and mechanical properties. The correlation between aging time and test results suggests that the presented test methods are promising tools for in situ condition monitoring of low voltage unshielded cables in nuclear power plants.

Effects of gamma irradiation on the degradation of silicone rubber by steam exposure

ABSTRACT Silicone rubber (SiR) is an important electrical insulating material for safety-related cables in nuclear power plants. This means that SiR must withstand the irradiation of gamma rays and the exposure to high-temperature steam. However, SiR is vulnerable to hydrolysis. Therefore, for the purpose of clarifying the behavior of SiR in such environments, sheets of SiR were exposed to steam at 171 or 200°C with and without pre-irradiation of gamma rays. As a result, it has become clear that SiR can withstand the exposure to high-temperature steam more robustly if it was irradiated by gamma rays to a proper dose in advance.

Industrial Applications of Cable Diagnostics and Monitoring Cables via Time–Frequency Domain Reflectometry

The demand for cable diagnostics and monitoring techniques has increased significantly in recent decades. Various diagnostic tests such as partial discharge, dielectric loss, and elongation-at-break tests are available in real-world applications. Among the cable diagnostic methods, reflectometry can be used to detect the location of a fault according to the reflected signal at the impedance-discontinuity point. Because it is a nondestructive method and can be applied regardless of the cable type, reflectometry is commonly used for monitoring real-world applications. Time-frequency domain reflectometry, an improved type of reflectometry, is more accurate than conventional reflectometry and employs time-frequency localized signals that are robust to noise. To compensate for the errors occurring in the time-frequency domain reflectometry, several algorithms have been reviewed. In the past, because the signal of time-frequency domain reflectometry contains both time and frequency information, a high-specification signal generator and measurement sensors were required, which made practical application of the method difficult. However, with recent developments in instrumentation and sensing technology, reduction in the instrumentation size and cost facilitates the use of time-frequency domain reflectometry in various industrial applications, e.g., control and instrumentation cables in nuclear power plants, superconducting cables, and submarine cables. This work reviews the use of time-frequency domain reflectometry for cable diagnostics and monitoring in real-world applications. The purpose and results of the experiments on industrial applications are introduced and analyzed. This study also suggests directions for further research to make time-frequency domain reflectometry a diagnostic standard.

Classification of Faults in Multicore Cable via Time–Frequency Domain Reflectometry

Owing to the increasing complexity of electrical systems, diagnostic techniques of cables used for connecting electrical elements are essential for system maintenance in order to prevent a failure that can cause significant impacts on the overall electrical systems. Multicore structures are typically used as control and instrumentation cables in nuclear power plants, and the failure of the control and instrumentation cables can result in a disaster such as a radiation leak. In this paper, a method for the diagnosis of multicore cables is proposed based on the reflectometry. The diagnosis relates to the classification of defective cores in joint, which is one of the weakest parts in cable systems. The reflected signals obtained through reflectometry are converted into images by an advanced image processing algorithm, and the images are classified using artificial neural networks. The proposed method is demonstrated by experimental data using a real-world multicore cable. In the experiment, the faults are emulated similar to real-world defects using a potentiometer. It is expected that the proposed technique will enhance the stability and reliability of multicore cable systems.

Condition Monitoring Uncertainties and Thermal - Radiation Multistress Accelerated Aging Tests for Nuclear Power Plant Cables: A Review

The low voltage cables in the nuclear power plant have the same importance as veins have in the human body. Since, the cables inside the containment are under a number of environmental stress, out which the thermal and radiation stresses are important and become more effective with the presence of oxygen. These stresses cause the degradation of the insulation and hence may lead to insulation failure. To study the behavior of the cables under the nuclear power plant environmental stress, the cables are subjected to accelerated aging tests in laboratories which helps in determining their withstand capacity and degradation behavior in the harsh thermal-radiation environment. In this paper, the accelerated aging tests conditions are discussed with the focus on the role of certain uncertainty factors which effect the aging markers during the condition monitoring of the insulation material of cable after the accelerated aging tests. This review has been put into a framework to better understand the aging process in the low voltage nuclear power plant cables. In the conclusion part, some future directions in the field of the thermal-radiation multi-stress accelerated aging tests and condition monitoring techniques are also identified.

Condition Assessment of I&C Cables in Nuclear Power Plants via Stepped-Frequency Waveform Reflectometry

A nuclear power plant (NPP) depends on instrumentation and control (I&C) systems to ensure its safe and efficient operation. In particular, I&C cables take on the pivotal role of measuring and controlling the critical equipment of the NPP. Thus, it is indubitable that the diagnostic technology of I&C cables for detecting faults and accurately assessing their health status is required for ensuring the safety and reliability of the NPP operation. We propose a diagnostic method that combines fault detection and evaluation algorithm for the I&C cables with stepped-frequency waveform reflectometry with signal propagation and reflection modeling. The signal modeling allows the assessment of the fault with an estimated reflection coefficient by separating the propagation and reflection effects of the measured signal. In short, cable faults are differentiated and quantified regardless of distance. The proposed algorithm is verified by characteristic impedance measurement, various fault detection/evaluation experiments, and the fault evaluation of local accelerated thermal aging cable.

Unusual Attenuation Recovery Process After Fiber Optic Cable Irradiation

At present, the number of optical cables in nuclear power plants has been increasing. Fiber optic cables are commonly used at nuclear power plants in instrumentation and control systems but they are usually used in environments without radiation. Nevertheless, currently, the number of applications in NPP containment with radiation is increasing. One of the most prevalent effects of radiation exposure is an increase of signal attenuation (signal loss). This is the result of fiber darkening due to radiation exposure and it is the main limitation factor in application of fiber optics in radiation environment. However, after the irradiation, the fiber optics go through a “recovery process” during which the optical properties improve again; i.e. attenuation decreases. However, we have found cable, where the expected healing process after few days changed its trend and the attenuation increased again to a value well above the attenuation just after the irradiation. This paper describes experiments that were carried out to explain this unusual recovery behaviour.

Electrical and mechanical diagnosis of aging 600 V rated STP cables in a nuclear power plant

This paper proposes electrical and mechanical diagnostic methods to estimate the aging effects of 14 American wire gage (AWG) 600 V rated shielded twisted pair (STP) cables used in nuclear power plant. The cables were accelerated to be degraded in a dry oven using Arrhenius's equation, and the properties of healthy and degraded cables were estimated electrically and mechanically as well. S-parameters were measured using a vector network analyzer (VNA) and were used to assess the aging effects of the degraded cable. An effective modulus was also used as an aging indicator of the mechanical properties. An indenter was designed and fabricated to measure the effective modulus of aged cables. Both methods' results were compared and found to be effective for diagnosing the deterioration of the aging cables. Therefore, the proposed methods could be very useful for maintaining power cables in nuclear power plants.

原子力発電所用安全系低圧ケーブルの難燃性に対する経年劣化処理の影響

Cables used in nuclear power plants are required to be flame retardant as a measure of fire protection. Cables with polymer insulation, such as silicone rubber (SiR) insulated cables, flame-retardant ethylene propylene diene rubber (FR-EPDM) insulated cables and flame-retardant cross-linked polyethylene (FR-XLPE) insulated cables, are commonly used as safety-related low voltage cables in nuclear power plants. This paper describes results of flame-propagation tests conducted in accordance with IEEE Std 383-1974 and IEEE Std 1202-2006, and oxygen index measurement tests conducted in accordance with JIS K 7201-2:2007, which were carried out to investigate the effect of pre-aging on flame retardancy of the cables. The samples employed in the tests were un-aged cables and pre-aged cables by accelerated simultaneous thermal and radiation aging to simulate aging in their service life under normal operating environment in nuclear power plants. In addition, decomposition temperatures were measured by thermogravimetry for pre-aged insulation samples of SiR, FR-EPDM and FR-XLPE to evaluate the results of flame-propagation tests and oxygen index measurement tests. As a result of flame-propagation tests and oxygen index measurement tests, flame retardancy of the cables was not degraded by the pre-aging process of this study. From the thermogravimetry, the significant decrease in decomposition temperature due to pre-aging was not observed.

Diagnostics and Life-Cycle Assessment of Medium-Voltage Cables in Nuclear Power Plants During Regular Overhaul Process

The reliability of operation of a nuclear power plant substantially depends on the reliability of its individual components. Electric cables, particularly medium-voltage cables, represent an important constituent of the nuclear island as well as of the balance of the plant. It cannot be considered as satisfactory that the redundancy of a particular subsystem is ensured. For this purpose, it is important to check the cable insulation condition, which is supposed to be carried out during the overhaul activities. The lifetime of medium-voltage cables is estimated to attain somewhere around 40 years. However, due to electrical, thermal, and mechanical stresses, it may be significantly reduced. In order to avoid its likely failures and following outages, the insulation must be tested regularly. Based on obtained testing results, the condition of a cable or cable termination can be assessed and repaired or a replacement made, if necessary. The paper presents some of the most important nondestructive diagnostic measurements on cable connections recommended to be done during the overhaul of a nuclear power plant following a good practice of asset maintenance management.

Effect of insulator environment on thermal ageing estimations for ethylene propylene rubber insulated nuclear power plant cables

ABSTRACTThermal ageing estimations merely focus on the degradation of cable insulator materials without considering their arrangement within the cable structure, by surrounding jacket or colorant pigments. This study reconsidered thermal ageing estimations based on the insulator elongation at break (EAB) degradation progress for flame retardant ethylene propylene rubber (FR-EPR) insulators within the whole cable structure. FR-EPR insulators were aged using two methods. In the first method, the insulators were heated, including jacket material at 125 °C, 150 °C, and 160 °C for 2208, 336, and 264 h, respectively. In the second method, the insulators were heated without jacket at 125 °C for 3480 h, and at 160 °C for 264 h. It was expected that the EAB degradation progress would be more severe when the FR-EPR insulators were aged without jacket. However, the results contradicted previous expectations, and the special heat-resistant polyvinyl chloride jacket accelerated the EAB degradation. In addition, the black FR-EPR insulators showed values that deviated from the predictions for the activation energy (Ea) and equivalent times compared to the white and red insulators due to the use of carbon black as a pigment. The reduced equivalent times for thermally aged FR-EPR insulators with jacket in this study were evident compared with the current degradation database for FR-EPR insulators degraded without jacket.