L. Verardi

Electrical aging markers for low-voltage cable insulation wiring of nuclear power plants

This paper presents some preliminary results of test carried out to measure electrical properties of XLPE insulations used at nuclear power plants (NPP). These cables were aged under thermal and radiation stresses. In order to obtain significant results in a reasonable time, the dose rate and the temperature chosen were higher than the operation values. Electrical measurements were performed through dielectric spectroscopy and polarization/depolarization currents. First results show significant variations of electrical properties of aged cables which can be correlated to insulation aging. In particular, the real part of the permittivity increases noticeably with aging over the whole test frequency range and the imaginary part of the permittivity shows a polarization peak enhancing at very low frequencies.

The influence of accelerated aging procedures on the correlation of electrical and mechanical properties of LV nuclear power plant cables

This paper presents results of accelerated life tests having the purpose to single out the diagnostic characteristics of electrical and mechanical properties for XLPE low-voltage cables used in nuclear power plants. In this work, XLPE-insulated cables were aged under two different conditions, the first consisting in the simultaneous application of temperature and gamma radiation, the second involving thermal aging followed by irradiation at room temperature. The dielectric response in the frequency range 10-2-106 Hz was investigated through dielectric spectroscopy, while the degradation conditions of cable insulation was assessed by tensile testing and density measurements. Results show that sequential aging could be ineffective for the purpose of simulating the service insulation ageing, because it can lead to a very limited amount of cable degradation. Simultaneous aging, on the contrary, can affect significantly material properties. In particular, it has been found that the imaginary part of permittivity at 100 kHz could be a suitable aging marker, correlated with the degree of oxidation.

Aging investigation of low-voltage cable insulation used in nuclear power plants

This paper presents some preliminary results of mechanical and electrical tests carried out on CSPE/EPDM polymer insulation used in low-voltage nuclear power plant cables. The whole cables were exposed to operation stresses for 30 years, then the inner low-voltage insulations have been aged further in the lab under thermal and radiation stresses. Electrical measurements are performed on aged cable specimens using dielectric spectroscopy. Mechanical properties are investigated through tensile testing, i.e. elongation at break and Youngs modulus. These properties are aging markers commonly used to assess the conditions of low-voltage cables operating inside nuclear power plants containment area. First results have shown a significant increase of the real and imaginary parts of dielectric permittivity, especially at low frequencies, which can be correlated to bulk insulation aging. This paper represents only a part of a more extended work, which aims at identifying electrical aging markers able to correlate electrical and mechanical properties with aging.

Correlation of electrical and mechanical properties in accelerated aging of LV nuclear power plant cables

This paper presents the results of mechanical and electrical tests carried out to assess the condition of XLPE and EPR low-voltage cables installed in nuclear power plants. Cable samples underwent accelerated aging under simultaneous thermal and gamma-radiation stresses. In order to obtain significant results in a reasonable time, the dose rates were higher than the operation values, so that diffusion-limited oxidation could occur. Electrical measurements were performed through dielectric spectroscopy (frequency range: 10-2-106 Hz), while the degradation state of cable insulation was assessed by tensile testing and density measurements. Results on XLPE insulation show that elongation at break and electrical properties are not correlated when diffusion-limited oxidation strongly affects tensile testing and, in this case, the correlation with density is preferable. EPR-based insulation, more homogeneously aged, presents a good correlation between electrical and mechanical properties, as well as density.