Statistical analysis of aging trends in EPR-insulated safety cables sampled from BWR containments

Statistical analysis was performed on aging trends of ethylene-propylene rubbers used for safety cables within boiling water reactor (BWR) containments of nuclear power plants. In order to obtain a regressed aging trend curve, observed mechanical elongation at break data was converted for equivalent period in an environment of 60°C and 10 mGy/h. Dropping speed is found to be approximately two times slower in service-used cables than that predicted by an accelerated aging test. This newly found aging trend was used as a reference to fit to each elongation value, and the fitted curve is extrapolated to find available service durations. An Arrhenius plot shows that the duration predicted from the acceleration test is shorter than the actual duration. Activation energy is lower and the pre-exponential factor in the Arrhenius plot is higher in newly estimated service durations. Low-oxygen-concentration environment in BWR containments could be a contributor to such behavior.