Lifetime predictions for semi-crystalline cable insulation materials: I. Mechanical properties and oxygen consumption measurements on EPR materials

Long-term accelerated aging studies (up to 7 years of aging) were conducted on four typical EPR materials used as cable insulation in nuclear power plant safety applications with the goal of establishing lifetime estimates at typical aging conditions of similar to 50 degrees C. The four materials showed slow to moderate changes in mechanical properties (tensile elongation) until just before failure where abrupt changes occurred (so-called induction-time behavior). Time-temperature superposition was applied to derive shift factors and probe for Arrhenius behavior. Three of the materials showed reasonable time-temperature superposition with the empirically derived shift factors yielding an approximate Arrhenius dependence on temperature. Since the elongation results for the fourth material could not be successfully superposed, consistency with Arrhenius assumptions was impossible. For this material the early part of the mechanical degradation appeared to have an Arrhenius activation energy E-a Of similar to 100 kJ/mol (24 kcal/mol) whereas the post-induction degradation data had an E-a Of similar to 128 kJ/mol. Oxygen consumption measurements were used to confirm the 100 kJ/mol Ea found from early-time elongation results and to show that the chemistry responsible before the induction time is likely to remain unchanged down to 50 degrees C. Reasonable extrapolations of the induction-time results indicated 50 degrees C lifetimes exceeding 300 years for all four materials. (c) 2006 Elsevier Ltd. All rights reserved.