Effects of radiation and thermal ageing on physico-chemical properties of elastomeric materials (EPR-CSPE) for cable insulation and sheathing

In order to estimate cable service life in nuclear power plants, a research program has been established and performed on EPR/CSPE LOCA (Loss Of Cooling Accident) cables. This program is based on study of dose (5 to 100 kGy), dose rate (10 to 2500 Gy/h) and temperature (40 to 135°C) effects on these materials. This paper presents the results of physico-chemical methods (dielectric and dynamic mechanical analysis) carried out on materials after accelerated ageing to characterize the degree of degradation. Dynamic mechanical and dielectric spectra show that these materials are highly sensitive to thermal ageing, particularly CSPE. Indeed, both loss factor (tg δ) and elastic modulus (G´) are affected by temperature. Elastic modulus of the rubbery state allows crosslink densities which decrease with increasing temperature ageing to be calculated. Radiative ageing effects are the weakest. EPR is more affected than CSPE especially in the range of 30-70°C where a short decrease of G´, related to microcrystallinity, is modified by dose rate. Volume resistivity measurements prove higher sensibility for radiation effects