Mechanism elucidation of degradation dynamics of ethylene propylene rubber cable insulations sampled from nuclear power containment using additional aging

Ethylene propylene rubber insulation under operation for 16 years in boiling water reactor containments was subjected to additional thermal aging at 110°C. After 70 days of aging, the mechanical property of the insulation was decreased such that the limit guaranteeing durability against design-basis events was breached. This degradation rate is three times slower than that predicted on the basis of an accelerated aging test with control samples. Infrared spectroscopic analysis revealed suppression in the formation of carbonyl products in the service-used cables. According to the gel fraction measurement, cross-linking is the dominant reaction in such service cables. The unique slow degradation dynamics observed in service cables could be attributed to such oxidation control and the cross-linking reaction. The present study also conducts nuclear magnetic resonance analysis to elucidate the cross-linking point in the main chain.