Assessment of the insulation degradation of cables used in nuclear power plants

Cable insulating materials are usually, during their operational lifetime, exposed to a high number of various deteriorative environmental effects resulting in their degradation. In the case of cables used in the nuclear power plant (NPP) hermetic zone these factors consist predominantly of long-term irradiation (at rather low dose rates, in the presence of oxygen) and enhanced temperature. Hence, all cables assigned for use in NPP have to be qualified for use under such severe conditions. However, not only the initial qualification but also monitoring of the actual state of the installed cables in regular intervals is now recommended. Monitoring of the actual state of the cable insulation and the prediction of their residual service life (i.e., the on-going qualification) consist of the measurement of the properties that are directly proportional to the functionality of the cables (usually the elongation at break is used as the critical parameter). For the cables installed in the NPP hermetic zone a method based on the measurement of the thermo-oxidative stability by the differential scanning calorimetry has been developed