Several cables in safety-related systems in nuclear power plants (NPPs) are required to perform their safety functions in severe accident conditions even after they experience inservice aging. For such cables, the behavior of their insulation under the accidents is a matter of concern. In this research, after cables insulated with ethylene propylene diene rubber (EPDM) were preconditioned by exposing them to concurrent heat and radiation to simulate aging that would be incurred in their service life in normal operating condition of NPPs, they were exposed to radiation and high temperature steam that simulate harsh accident conditions postulated inside the primary containment vessel. The changes in chemical structures and thermal and mechanical properties were analyzed by Fourier transform infrared spectroscopy, thermogravimetry, and indentation hardness measurements. As a result, it has been confirmed that the pre-aging and radiation exposure induce chain scission by oxidation and cross-linking, whereas the steam exposure causes bleedout of additives compounded in EPDM.