Degradation of flame‐retardant ethylene‐propylene‐diene rubber by radiation and steam

IEEJ Transactions Elec Engng

2022

Self Cite

Measurement of the complex permittivity of a substance in a wide frequency range, for example, from 10 mHz to 100 kHz, provides useful information on the dielectric properties of the substance. In this regard, some of such dielectric properties can be analyzed more clearly if we pay attention to the frequency spectra of electric modulus, which is a reciprocal of complex relative permittivity. Especially, the imaginary part of the electric modulus can be a powerful tool for dielectric materials research. In this paper, this fact is demonstrated clearly by showing abundant examples acquired by the author's laboratory. First, the equations of electric modulus that hold for various dielectric relaxation processes, including their derivation, are shown in an easy-tounderstand manner. After that, for several insulating polymers, actual measurement examples of broadband complex permittivity spectra are shown, up to frequencies of about 100 kHz and a much broader range up to several THz. In the first example of showing permittivity spectra, polyamide exhibits an incredibly high relative permittivity value of 10 6 at high temperatures such as 200 • C at a frequency as low as 10 mHz. This is a result of the phenomenon often called electrode polarization. In other words, if electric charge carriers accumulate in an insulator facing a nearby electrode and have the polarity opposite to that of the electrode, the permittivity of the insulator goes up. In such a case, the dielectric loss factor usually shows a very high value in the low-frequency range and decreases in inverse proportion to the frequency. This reflects the fact that the charge carriers are transported, and the conductivity can be derived from this frequency dependence much more easily with apparently higher reliability than measuring the dc leakage current. Furthermore, it is shown that the coefficient of thermal expansion can be estimated from the permittivity spectra relatively accurately for several polymers that satisfy the necessary condition. Next, taking examples in electric modulus spectra measured in important insulating polymers such as engineering ones, it is demonstrated that the processes of dielectric polarization and relaxation, which are difficult to see in permittivity spectra, can be recognized obviously. Moreover, the effects of the addition of nanosized or microsized fillers to epoxy resin on its thermal property and filler-induced inhomogeneity are analyzed using the electric modulus spectra measured in these samples. The effects of thermal aging treatment and simultaneous aging treatment with heat and radiation on the degradation of cross-linked polyethylene are also analyzed by paying attention to the frequency spectra of its imaginary electric modulus.

“…The dominant change occurring in olefinic polymers when they are degraded is oxidation or the formation of carbonyl groups [30,31]. In this regard, Fig.…”

Section: Examples Of Experimental Results and Their Analysesmentioning

confidence: 99%

“…In Fig. 14, the samples aged purely thermally at 100 and 135 • C have very similar spectra to the unaged sample, whereas those aged thermally at 155 The dominant change occurring in olefinic polymers when they are degraded is oxidation or the formation of carbonyl groups [30,31]. In this regard, Fig.…”

Section: Examples (Iv)-degradation Of Polymermentioning

confidence: 86%

Broadband Complex Permittivity and Electric Modulus Spectra for Dielectric Materials Research

IEEJ Transactions Elec Engng

2022

Self Cite

“…In contrast, when LDPE or other polymeric insulation is degraded by being heated or concurrently irradiated by gamma rays, polar groups such as carbonyl groups and their derivatives are induced as a result of oxidation [13][14][15][16][17][18]. In addition, if the water ingress or formation of water trees occurs in the insulation, its permittivity increases due to the high permittivity of water.…”

Section: Discussionmentioning

confidence: 99%

Spatial Resolution between Two Abnormalities in a Cable by Frequency Domain Reflectometry

IEEJ Transactions Elec Engng

Hirai

2021

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The spatial resolution of a cable fault location method by frequency domain reflectometry was examined experimentally by giving two abnormalities intentionally along a 31.5‐m coaxial cable. As a result, two clear signal peaks appear, which can locate the presence of two hot spots perfectly if they are separated by more than 30 cm at a position about 15 m from the cable end with a measuring device. In addition, a clear signal peak with a small shoulder is recognized if the two hot spots are 1 cm away. Although the abnormalities were given in this research by heating the cable using tape heaters, the spatial resolution should be theoretically similar for any type of fault in a cable as long as it changes the characteristic impedance of the cable. © 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

“…Large-scale research projects are being conducted from this viewpoint in the USA [ 1 , 2 ] and Europe [ 3 , 4 ], as well as in Japan. During the course of the Japanese project, which has been entrusted to us by the Secretariat of the Nuclear Regulation Authority in Japan, we have been conducting much research into the degradation of cross-linked polyethylene (XLPE) [ 5 , 6 ], flame-retardant (FR) XLPE [ 7 ], FR ethylene-propylene-diene rubber (EPDM) [ 8 , 9 ], FR cross-linked polyolefin [ 10 , 11 ], silicone rubber [ 12 , 13 ], and hard and soft epoxy resins [ 14 , 15 , 16 ] as typical polymeric insulating materials used in safety-related cables and electrical penetrations. As a result of such extensive research, we have found that oxidation and hydrolysis, as well as cross-linking and the breaking of chemical bonds, are fundamental degradation factors common to all of the polymers studied.…”

Section: Introductionmentioning

confidence: 99%

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

Hirai

Okada³

2022

Polymers

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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.