Reliability of cable insulation under reaction- and diffusion-controlled thermal degradation

Cables transmit signals and power in nuclear power plants. The primary material for the cable insulation is cross-linked polyethylene, which inevitably degrades due to thermal stress. The degradation can become a safety issue, since the brittleness of degraded cross-linked polyethylene may render the exposure of the metal core in a cable. Elongation at break is a widely accepted measurement, evaluating the degree of the brittleness of the insulation. Reaction- and diffusion-controlled kinetics are proposed in this article to quantitatively predict the decrease of the elongation at break as a function of time and temperature. The proposed approaches are based on dichotomy model and Fick’s Law to respectively define the degree of reaction- and diffusion-controlled degradation. Probabilistic techniques are developed by Bayesian parameter estimation to determine the reliability of the cable insulation. These approaches are validated by experimental data.