Thermal ageing predictions of polymeric insulation cables from Arrhenius plot using short-term test values

Ageing of systems and components in nuclear power plants is a well known phenomenon that must be managed to ensure the continued safe operation of these plants. Much of the degradation due to ageing is controlled through periodic maintenance and/or components replacement. However, there are components such as electric cables that do not receive periodic maintenance or monitoring once they are installed. Electrical cable insulations are subjected in service to many operating environmental conditions simultaneously. These include oxidative atmospheres and elevated temperature which cause deterioration due to thermal ageing, degradation of mechanical and electrical properties due to irreversible changes in chemical structure, and thus shortening of insulation service life. Arrhenius model is used for addressing time-temperature ageing effects. The model used in chemical kinetics to predict the rate process has been extended to reliability on the assumption that failure rate is proportional to reaction rate to estimate the Time-To-Failure (TTF) at temperature. Arrhenius model is useful in predicting the life-span and in choosing parameters for accelerated ageing also. In order to allow extrapolation of short-term data to predict long-term performance, appropriate curves must be drawn through the short-term values. The short-term points are obtained by selecting some degree of deterioration (i.e. 50 % retention of Elongation of Break or 75% retention of dielectric strength, or so on), then ageing a given material at several elevated temperatures until the desired extent of degradation is achieved. This paper covers the case study on generating Arrhenius plots by obtaining short-term test data on tensile testing for extrapolation to long-term performance of various types of cables used in operating power plants.