Distributed capacitance sensing for characterization of nuclear power plant cables

Depending on their location in a nuclear power plant, cables are exposed to mechanical stresses, elevated temperature, radiation, humidity and environmental stresses which can lead the cables to an earlier degradation if not well designed. For this reason, it is crucial to understand how materials that insulation and jacket are made of, installed in NPPs, will age during their service life and to develop condition monitoring techniques to assure continued safe operation under the normal operating condition and under Design Basis Event (DBE). Nowadays methods based on tensile tests like Elongation at Break (EaB) are the most used as condition monitoring technique since their direct correlation with the ageing effect. However, these destructive methods consist in the destruction of the cables and their replacement in their initial position which means a decrease of reliability of the entire system. Non-destructive techniques such as Dielectric Spectroscopy (DS) are showing good correlation with ageing. In this work, DS on a three-conductor long cable with a thermally-aged middle section is carried out. Starting with measurements of the electrical properties of a pristine cable and the same type of cable with the aged section for a first preliminary assessment, the latter is made successively shorter and the variation of the capacitance (here considered as specific capacitance, pF/m) and dissipation factor are studied with the increase in Damage Ratio (R), the ratio between the length of the damaged section and the total length of the cable. The goal is to figure out if it is possible to detect ageing effect by measuring electrical properties of the pristine section of the same cable, i.e. if the DS can distinguish between a pristine cable and the same cable containing a middle thermally-aged section.