[en] With an increase in the number of nuclear power plants operated for a long period, the assurance of integrity is of prime importance for all the safety-related components in the plants. Polymer-insulated electric cables play key roles such as power supply and information transmission. In order to secure even higher reliability for cables in nuclear power plants, several research projects have been conducting in Japan. In this paper, important research results obtained through three such projects are described. The first one is a seven-year project inaugurated in April 2001. Based on the results obtained in simultaneous thermal and radiation aging tests done in the project, the activation energy values calculated from the experimental data were found to be smaller than those currently used. In the second project, research was conducted to examine whether we can locate the position of a degraded portion in a cable by measuring the magnitude and phase angle of the cable's impedance as functions of frequency in a very wide frequency range and by carrying out inverse fast Fourier transform analyses on the spectra. As a result, this method was found to be much superior to the time domain reflectometry in the locating ability of the degraded portion in all the three kinds of cables examined. In the third project, research has been carried out to clarify the degradation mechanism of polymeric insulating materials under thermal and irradiation stresses, aiming at supporting the inspection and maintenance procedures of safetyrelated cables. (author)