Application of optical diagnosis to aged low-voltage cable insulation in nuclear plants

We have developed a novel non-destructive optical diagnosis technique for low-voltage cable insulations used in nuclear power plants. The key features of this diagnosis are the use of two wavelengths to measure the change in reflective absorbance (ΔA_R), and the use of polarized light to measure crystallinity. Then, chemical kinetics is used to predict the lifetimes of the cable insulations. When cable insulations darken and harden with age, the ΔA_R and depolarization parameters increase. This means that the cross-linking density in the cable insulations increases due to deterioration reactions. When the cross-linking density of an insulation increases, its elasticity, corresponding to the material´s life, increases. Similarly, as the crystallinity increases due to the change in the high-order structure of the insulating resin caused by irradiation, its elongation property decreases. The elongation property of insulation is one of the most important parameters that can be used to evaluate material lifetimes, because it relates to elasticity. The ΔA_R correlated with the elongation property, and the correlation coefficient of an accelerated experiment using model pieces was over 0.8. Thus, we concluded that this optical diagnosis could be applied to evaluate the degradation of cable insulations used in nuclear power plants.