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Abstract
The radio-thermal ageing of silane-crosslinked polyethylene (Si-XLPE) was studied in air under different dose rates (6.0, 8.5, 77.8, and 400 Gy·h−1) at different temperatures (21, 47, and 86 °C). The changes in the physico-chemical and electrical properties of Si-XLPE throughout its exposure were determined using Fourier transform infrared spectroscopy coupled with chemical gas derivatization, hydrostatic weighing, differential scanning calorimetry, dielectric spectroscopy and current measurements under an applied electric field. From a careful analysis of the oxidation products, it was confirmed that ketones are the main oxidation products in Si-XLPE. The analytical kinetic model for radio-thermal oxidation was thus completed with relatively simple structure–property relationships in order to additionally predict the increase in density induced by oxidation, and the adverse changes in two electrical properties of Si-XLPE: the dielectric constant and volume resistivity R. After having shown the reliability of these new kinetic developments, the lifetime of Si-XLPE was determined using a dielectric end-of-life criterion deduced from a literature compilation on the changes in R with for common polymers. The corresponding lifetime was found to be at least two times longer than the lifetime previously determined with the conventional end-of-life criterion, i.e., the mechanical type, thus confirming the previous literature studies that had shown that fracture properties degrade faster than electrical properties.
Highlights
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The analytical model, recently developed for accurately describing the radio-thermal oxidation kinetics of unfilled and unstabilized Si-XLPE insulation in nuclear power plants (NPPs) [8], has been completed with several structure–property relationships in order to predict the changes in its electrical properties
After an initial period where the oxidation kinetics occur at a constant rate, it allows predicting the auto-acceleration of the oxidation kinetics when hydroperoxide decomposition is no longer negligible
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Most nuclear power plants (NPPs) built in Europe in the 1980s and 1990s are reaching their originally planned end-of-life, and electric utility companies are wondering whether it is possible to extend their operating life by a few more decades. As low-voltage electric cables are among the most critical components in NPPs, the monitoring of the health of their polymer insulation throughout the NPP’s operating life rapidly emerged as one major issue. It should be mentioned that about 1500 km of low-voltage cable are used inside each NPP [1] to perform various functions, including power transmission, the control of equipment and instrumentation, and the communication of signal and data
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