Experimental Investigation of Sequential and Synergistic Ageing Effects in I&C Cables of NPP

Instrumentation and control cables are extensively being used in nuclear power plants for control and monitoring of process equipment/systems. Cable insulation materials in nuclear power plant environment must withstand radiation and thermal stresses to ensure that the cables perform their safety function during normal operation of the plant, and also during design basis accident conditions. It is important that the effects of service life degradation and accident conditions be assessed on the performance of the insulation materials. The sequential ageing assessment studies are well understood, and the effect of sequential ageing has widely been demonstrated in nuclear industry through equipment qualification guidelines. However, the studies on understanding the effect of synergistic ageing are limited due to challenges in setting up of synergistic ageing test facilities which require combined thermal and radiation environment. This paper presents the findings from experimental investigation of sequential and synergistic ageing effect in instrumentation and control cable insulation materials under identical test conditions. The widely used polymeric insulation materials such as ethylene propylene diene monomer and polychloroprene have been simulated under sequential and synergistic ageing environment consisting of thermal and radiation stresses via accelerated life testing procedures. The experimental investigation of ageing effect has been carried out by elongation-at-break measurement and weight loss measurement by tensile testing and thermogravimetry analysis respectively. The activation energies corresponding to weight loss of 5% have also been determined for various ageing sequences for remaining useful life estimation.