Slow degradation dynamics under additional aging in EPR insulations sampled from BWR containment

Ethylene propylene rubber insulations under operation for 16 years in boiling water reactor containments were subjected to additional thermal aging at 110°C. 70 days were needed to decrease their mechanical property to the limit that they cannot secure the durability against design-basis events. This degradation rate is found to be three times slower than that predicted on the basis of an accelerated aging test in control samples. Infrared spectroscopic analysis reveals suppression in the formation of carbonyl products in the service-used cables. According to the gel fraction measurement, cross-linking is the prominent reaction in such service cables. The unique slow degradation dynamics observed in service cables could be attributed to such oxidation control and cross-linking reaction.