Aging Mechanisms and Non-Destructive Aging Indicators of XLPE/CSPE Unshielded LV Nuclear Power Cables Subjected to Simultaneous Radiation-Mechanical Aging

Abstract

Low-voltage cable systems in nuclear power plants are key components that have a crucial role in the safe operation of nuclear facilities. Thus, the aging management of cable systems is of utmost importance as they cannot easily or economically be replaced or upgraded. Therefore, there is a continuous need to develop reliable non-destructive condition monitoring techniques, mostly based on the measurement of the dielectric properties of cable insulation. This paper introduces the changing of dielectric and mechanical properties of XLPE insulated and CSPE jacketed unshielded low-voltage nuclear power plant power cable in case of simultaneous mechanical and radiation aging. The cable samples were bent and exposed to 400 kGy gamma irradiation with a 0.5 kGy/hr dose rate. Dielectric response (real and imaginary permittivity) in the 0.1 Hz−1 kHz frequency range, extended voltage response (EVR), and the Shore D hardness test techniques were measured to track aging. The electrical and mechanical parameters have increased monotonically with aging, except the imaginary permittivity, which increased only at frequencies higher than 10 Hz. Furthermore, different quantities were deducted based on the frequency and permittivity data. The electrical parameters and deducted quantities correlation with aging and mechanical parameters were investigated. Since the deducted quantities and the electrical parameters are strongly correlated with absorbed dose and mechanical properties, the electrical measurements can be applied as a non-destructive aging indicator for XLPE/CSPE unshielded low-voltage nuclear power cables.

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

“…The values of CIP and CF are depicted in the Figure 10. For a more detailed analysis of the dependence of real and imaginary parts of permittivity curves on aging, and similarly to [30,45], four deducted quantities have been calculated, namely the central real and imaginary parts of permittivity (CRP and CIP), and the real and imaginary permittivities' central frequencies (RPCF and IPCF). The CRP is calculated by summing up the multiplication of the logarithm of the frequencies by the measured ε values at given frequencies and dividing this sum by the sum of the logarithm of the frequencies, as it can be seen in Equation (4).…”

“…The reason for the application of this technique in this study is that dielectric spectroscopy as condition monitoring for NPP cables has recently become a focus of interest because of its non-destructivity. Several studies show promising results, and their future expansion is likely [11,12,[24][25][26][27][28][29][30][31]. From the results of dielectric spectroscopy measurement, deducted quantities were introduced because assigning one measure to the whole low-frequency spectrum makes it easier to analyze the changing of the dielectric spectrum with aging.…”

Investigation of the Structural Dependence of the Cyclical Thermal Aging of Low-Voltage PVC-Insulated Cables

“…In investigating cable aging, single-core simple cables are usually used to eliminate the effect of the complex structure of multicore cables and the interaction among the polymeric components [20][21][22]. However, a typical LV cable contains layers such as conductors, core insulations, protective tape screens, a jacket, etc.…”

“…By executing the aging in this way, the asymmetric aging of the cable components was not affected by their different temperatures but by their different positions inside the cable cross section, causing, e.g., differences in contact with the atmosphere or the interdiffusion of additives among them. Previous studies have successfully adopted dielectric spectroscopy measurements on LV cables [21,[25][26][27][28]. Therefore, the dielectric parameters, tan δ, the capacitance, and the polarization and depolarization currents were measured after each aging cycle.…”

Investigation of Structural Dependence of Cyclical Thermal Ageing of Low Voltage PVC Insulated Cables

“…Then short-term artificial cyclic ageing was applied to simulate the short-term temperature changes due to the load variation. Previous studies have successfully adopted dielectric spectroscopy measurements on LV cables [21,[25][26][27][28]. Therefore dielectric parameters, tan δ, capacitance, polarization and depolarization currents were measured after each ageing cycle.…”