Several cables in safety-related systems in nuclear power plants are required to maintain their intended electrical insulating function while they are exposed to harsh environmental conditions in the primary containment vessel during assumed accidents. This must be satisfied even after the cables are operated for their full service period. For such cables, the behavior of their insulation under accidents is a matter of concern. In this research, after silicone rubber insulated cables were pre-aged by simultaneous thermal and radiation exposure, they were irradiated or exposed to high temperature steam to simulate the environmental conditions during the accident, or subjected to the combination of these conditions. The resultant changes in various properties were analyzed by instrumental analyses. As a result, it has been confirmed that the pre-aging and radiation exposure induce cross-linking and depolymerization of siloxane chains to produce cyclic siloxanes, whereas the steam exposure causes significant depolymerization and the generation of low-molecular cyclic siloxanes.