Dielectric and Mechanical Behavior of Thermally Aged EPR/CPE Cable Materials

Abstract

Power and instrumentation cables play a crucial role in the safe operation of Nuclear Power Plants (NPPs). Thermal and other stressors present in the reactor environment cause the cable materials to degrade. In this work, dielectric and mechanical properties of cable insulation and jacket materials are studied as they are thermally aged, supporting development of non-destructive evaluation sensors for monitoring cable aging. Materials selected for this study are found in certain types of single-core unshielded power cables. These utilize ethylene propylene rubber (EPR)-based insulation material and chlorinated polyethylene (CPE)-based jacket material. Flat mats of these materials were obtained from the cable manufacturer and thermally aged at 140 °C in an air-circulating oven. Elongation-at-break was measured on tensile specimens stamped from the aged mats, and dielectric properties were measured from 100 Hz to 100 kHz using a parallel plate capacitor and precision LCR meter. In the case of aged EPR-based materials, rapid decrease in elongation at break indicating end of useful life was accompanied by a significant increase in dissipation factor, D, measured at 100 kHz. Capacitive measurement of D shows promise, therefore, as a non-destructive indicator of corresponding mechanical property changes in thermally-aged EPR-based insulation materials.

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

“…Besides, it is observed that the cable jacketing material, CSPE undergo dehydrochlorination when exposed to higher temperatures. As a result, the dielectric relaxation phenomena which is proportional to the number and mobility of C-Cl dipoles of the vinyl chloride groups decreased [9], [47]. Therefore, the return voltage slope has decreased, Fig.…”

“…where: the real part εʹ(ω) refers to the stored energy within the insulation while the imaginary part εʹʹ(ω) is related to the energy loss "dissipation" [9], [17], [32], [33]. By employing the commercial dielectric response analyser type Dirana (OMICRON) and precision component analyzer type 6430A (Wayne Kerr Electronics), the complex permittivity has been explored over frequency band from 100 mHz to 1 kHz and from 2 kHz to 500 kHz, respectively.…”

“…Chain-scission causes backbone degradation which is opposed by inter-chain cross-linking. It is believed that the crosslinking results in a reduction in the permittivity while the chain-scission contribute to increase the permittivity [9]. Also, the mechanical bending stress involves two different stresses, compression and tension stresses.…”

“…These cables provide the link between the plant safety and control systems via signaling to control equipment, plant operators, and safety systems [7], [8]. Through the normal operation of the plant, cables are exposed to multiple stresses such as electrical, mechanical, thermal, and environmental stresses [9]. The presence of theses stresses stimulates the desire for a better understanding of cable insulation degradation [5].…”

“…Therefore, the oxidation process is accelerated, which leads to not only the formation of carbonyl and hydroxyl products [14] but, also changes in the morphological and macromolecular structures [15]. Consequently, the electrical, mechanical, physical, and chemical properties will degrade [9].…”