Conference Paper

Aging investigation of low-voltage cable insulation used in nuclear power plants

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Abstract

This paper presents some preliminary results of mechanical and electrical tests carried out on CSPE/EPDM polymer insulation used in low-voltage nuclear power plant cables. The whole cables were exposed to operation stresses for 30 years, then the inner low-voltage insulations have been aged further in the lab under thermal and radiation stresses. Electrical measurements are performed on aged cable specimens using dielectric spectroscopy. Mechanical properties are investigated through tensile testing, i.e. elongation at break and Young's modulus. These properties are aging markers commonly used to assess the conditions of low-voltage cables operating inside nuclear power plants containment area. First results have shown a significant increase of the real and imaginary parts of dielectric permittivity, especially at low frequencies, which can be correlated to bulk insulation aging. This paper represents only a part of a more extended work, which aims at identifying electrical aging markers able to correlate electrical and mechanical properties with aging.

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... A typical nuclear power plant may compose of 1000 to 2000 km of cables. These cables are classified as Low Voltage (LV) & Medium Voltage (MV) Power cables, Instrumentation and Control (I&C) cables, special cables and general service cables [1][2][3][4]. These cables provide the communication between equipment, control and safety systems which monitor these plants. ...
... This testing technique named Elongation at Break (EaB) [11]. The advantage of this test is that, the aging can be effectively correlated but on the other hand, it requires sample scarification to be investigated and a proper laboratory is needed [3]. For lifetime extension, the cables must undergo a non-destructive test technique where some aging markers can be measured in-situ. ...
Chapter
This paper presents results of electrical tests carried out on nuclear power plant low voltage power cable samples based XLPE/CSPE polymer insulation. These cable samples have been subjected to accelerated thermal and mechanical stresses for 240, 480, 720 and 960 h. The effect of these aging stresses has been investigated through studying the real and imaginary parts of complex permittivity over frequency range from 20 Hz to 500 kHz. These two parameters are commonly used as aging markers to assess the insulation state of such kind of cables. The results show a non-uniform trend for the real and imaginary parts of permittivity. Also, it can be noticed that after the last two cycles, the imaginary part minimum values have been shifted to lower frequencies, 200 Hz.
... Throughout Nuclear Power Plants (NPPs), low voltage cables are extensively used for the purpose of signaling, data transmission, and control of equipment [1,2]. NPPs contains hundreds of kilometers of different types of cables such as Instrumentation and Control (I&C) and low voltage power cables [3]. Although some of these cables are fortunately placed on benign environmental and are less affected by the environmental conditions however there are large number of cables are being subjected to extensive multiple stresses such as high temperature (thermal stress), elevated radiation dose rates (radiation stresses), mechanical stresses and the humidity inside the containment area [4] . ...
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The insulation integrity of power and instrumentation & control cables plays a crucial role in the safe and reliable operation of nuclear power plants. Different stresses affect the insulating materials of these cables, which significantly contribute the structural changes results in the polymeric insulation. Consequently, the polymer material loses its mechanical and electrical properties causing cable insulation degradation. In this work, samples of low voltage nuclear power cable have been studied with accelerated aging tests. Combined simultaneous thermal and mechanical stresses have been applied to all samples under study. While the samples being placed in an air circulating oven at 120 °C temperature, they are coiled on a cylinder with diameter of 15 cm. The dielectric properties, capacitance and dissipation factor were measured by frequency swapping from 100 mHz to 1 kHz using a commercial dielectric response analyzer. Also, the shore D hardness was measured to investigate the changes in the mechanical property of the test specimens. Over the whole frequency range, the capacitance showed a downward trend with aging. In addition, the tan increased with aging for frequencies higher 10 Hz. A correlation between tan at 100 Hz and the insulation hardness was obtained.
... Lots of researchers focus on the life evaluation of power cables. G. C. Montanari studied the aging and breakdown characteristics of EPR under the combination of thermal and electrical stress, and established aging life model [5,6]. T. W. Dakin evaluated the aging state of insulating materials under thermal aging based on Arrhenius equation, and predicted the aging degree of insulating materials [7]. ...
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