Defective cables in the electric power systems of nuclear power plants can cause failure of essential components in control and instrumentation or power distribution subsystems, resulting in potential safety concerns. Due to this problem, a nondestructive and non-intrusive condition assessment technique is highly desirable to evaluate cable status and to predict the remaining useful life of a cable. The capability of joint time-frequency domain reflectometry (JTFDR) as such a condition assessment technique is studied in this paper. The health status of four popular insulation types utilized in nuclear power plants for control and distribution is monitored using JTFDR in thermal accelerated aging test including: Cross-linked polyethylene (XLPE), tree retardant cross-linked polyethylene (TR-XLPE), ethylene propylene rubber (EPR), and silicone rubber (SIR). The experimental results show that JTFDR can successfully monitor the aging process of all four insulation types. Further, the results from JTFDR are compared with the results from Elongation-at-Break (EAB). These results show that JTFDR technique is comparable with EAB, in some cases providing more linear characteristics with respect to aging time, and has great potential as a non-destructive and non-intrusive condition assessment technique.