Penetration Routes of Oxygen and Moisture into the Insulation of FR-EPDM Cables for Nuclear Power Plants

Abstract

The polymeric insulation used in nuclear power plants (NPPs) carries the risk of molecular breakage due to oxidation and hydrolysis in the event of an accident. With this in mind, tubular specimens of flame-retardant ethylene-propylene-diene rubber (FR-EPDM) insulation were obtained by taking conductors out of a cable harvested from an NPP. Similar tubular specimens were made from a newly manufactured cable and those aged artificially using a method called the “superposition of time-dependent data.” The inner and outer surfaces of each tubular specimen were subjected to various instrumental analyses to examine their oxidation, moisture uptake, and cross-linking. As a result, it has become clear that oxygen penetrates the cable through gaps between the twisted conductor strands. Meanwhile, water vapor diffuses more often through the sheath than through gaps between the conductor strands. Of the two methods used to simulate design-based accidents in NPPs, the one used to simulate the designed loss-of-coolant accident is more severe to FR-EPDM than the one used to simulate the designed severe accident. In addition, the validity of the method called the “superposition of time-dependent data,” which is used to give artificial aging treatments to cable samples, was confirmed. Measurements of spin-spin relaxation time and residual dipolar coupling using time-domain nuclear magnetic resonance were found suitable to use to obtain information on the cross-linking of FR-EPDM insulation.

Reasons for Resistivity Increase in FR‐EPDM Insulation of Cables Aged in Nuclear Power Plants

“…It probably does not contain pigments or additives necessary for cable manufacturing. As reported in [22,23], the 60 Co gamma‐ray irradiation at a rate of 100 Gy/h at 100°C to D0 has been known to make the condition of D0 equivalent to that of A1 and C1, when irradiated for 1997 and 1289 h, respectively. With this regard, the sheet was irradiated with gamma rays at a rate of 100 Gy/h at 100°C for the periods of 100, 1200, and 2000 h. Then, the leakage current flowing through the sheet was measured upon the application of a DC voltage in the same manner as in the previous section.…”

“…In addition, the insulation degradation of low-voltage cables insulated with FR-EPDM, removed from NPPs, was investigated [20][21][22][23]. As a result, it has become clear that the electrical resistance of the insulation is significantly higher in cables removed after many years of installation in NPPs than that of an unused new cable [20,21].…”

Improvement of electrical insulation performance of silicone rubber cables due to exposure to heat and radiation

“…1 The reduction of leakage current in the SiR cables and sheets due to the thermal or simultaneous thermal and radiation treatment, found in the present research, seems due to the same reason. In our previous study, 17,19 we analyzed the structural changes induced in SiR using Fourier transform infrared absorption spectroscopy, terahertz time-domain spectroscopy (THz-TDS), and nuclear magnetic resonance (NMR) spectroscopy using 29 Si and 13 C as nuclides. As a result, it has become evident that SiR detaches pendant CH 3 groups and forms a chemical structure shown in Figure 20 when treated thermally or concurrently with heat and radiation in the presence of oxygen.…”

“…Further, assuming that each cable sample is in the containment vessel of an in‐service NPP, the estimated cable laying period corresponding to each treatment condition is also listed in Table 1. Here, we used a method called “superposition of time‐dependent data” for the estimation, 28,29 and the ambient condition of each sample is assumed to be 60°C and 0.34 Gy/h for PWR and 66°C and 0.50 Gy/h for BWR 28…”

Study on the direct current properties of ultraviolet crosslinked epdm rubber

“…At the same time, EPDM is also one of the commonly used insulation materials for cable accessories [1]. Due to its rich sources of raw materials, small specific gravity, and low consumption per unit weight of the product, EPDM rubber has great development prospects [2][3][4][5][6].…”