Tadao Seguchi

Degradation mechanisms of silicone rubber (SiR) by accelerated ageing for cables of nuclear power plant

The degradation behavior of SiR for the cable insulation by accelerated thermal and radiation ageing was studied and the degradation mechanism was proposed. The degradation was observed by the change of tensile properties, the distribution of crosslinking, and the change of weight. The chemical reaction under the both ageing in oxidation conditions was crosslinking and the oxidation mechanism was found to be the same between thermal and radiation ageing. The yield of crosslinking was proportional to the ageing time and also the dose. The effect of irradiation temperature on oxidation was accelerated with an increase of temperature above around 120°C, which might be due to the specific radiation chemical reactions. Therefore, the degradation by simultaneous ageing at higher temperatures above 155°C was much higher than that for sequential ageing, such as irradiation followed by thermal ageing or thermal ageing followed irradiation. At a high temperature, the degradation by thermal ageing under vacuum (without oxidation) was more progressed than that for the ageing in air (with oxidation). The reason was assumed to be the thermal decomposition of crosslinks between SiR molecules formed by the chemical crosslinking agent. The hardness (Shore hardness) reflected well the degradation of the SiR material for any ageing conditions.

Degradation distribution in insulation materials of cables by accelerated thermal and radiation ageing

The degradation of cables by the accelerated thermal and radiation ageing for nuclear power plant was studied. The thermal oxidation of crosslinked polyethylene (XLPE) as a cable insulation showed the heterogeneous features along the depth of sheet specimens. The degree of oxidation was closely related to the distribution of antioxidant content due to the decay of antioxidant content by the evaporation and radiation decomposition during accelerated ageing. The specific relations were observed between the tensile properties and the yield of oxidation products and also between the oxidation products and antioxidant content in XLPE specimens. The yields and the content and their distribution were detected by FTIR spectra. The oxidation of XLPE containing antioxidant of enough content proceeded proportionally with ageing time or radiation dose at any ageing conditions. When the antioxidant content decreased less than a critical content by thermal and/or radiation ageing, the oxidation rate by thermal ageing increased sharply with ageing time. Therefore, the oxidation at surface of XLPE was much accelerated when the antioxidant decreased to result the heterogeneous oxidation. The mechanical properties depend closely on the degree of oxidation at the surface. The behavior of oxidation was also affected by the types of antioxidant.

Radiation ageing technique for cable life evaluation of nuclear power plant

The accelerated ageing of cables for nuclear power plant was studied for the life evaluation in the environmental conditions. For the accelerated radiation ageing, the dose rate is the important point, because the oxidation profile in the insulation is much affected by dose rate due to oxygen diffusion limitation during irradiation. The oxidation profile was observed by FTIR for crosslinked polyethylene (XLPE) and ethylene propylene rubber (EPR) of cable insulation degraded at various irradiation conditions and compared with the mechanical degradation. The oxidation profiles with low dose rate at room temperature and that with high dose rate at elevated temperature (100°C) were very much alike and the mechanical degradations of both irradiations were equal. By increasing the irradiation temperature the oxygen diffusion rate increased exponentially with temperature and the degradation by thermal during irradiation for several hundred hours was very little at around 100°C. Therefore, the dose rate can be increased by 15 times by irradiation at 100°C instead of the irradiation at room temperature for the adequate oxidation throughout the insulation. The experimental result was coincident with the theoretical analysis.

Degradation of cable insulation material by accelerated thermal radiation combined ageing

The degradation of ethylene propylene rubber (EPR) sheets as a cable insulation material for nuclear power plants (NPP) was studied by accelerated thermal ageing, radiation ageing and thermal - radiation combined ageing. The oxidation of EPR proceeded with ageing and the decay of mechanical property was closely related to the content of oxidation products. The antioxidant as a stabilizer in EPR was effective for the thermal oxidation, but not for the radiation oxidation. The distribution of oxidation products across the sheet was changed due to decay of antioxidant by evaporation during thermal ageing. The antioxidant was not effective for radiation oxidation, and it was decomposed by radiation. For the thermal and radiation combined oxidation, the mechanical property and the content of oxidation products were different among the treatment sequences, which was due to the decay of antioxidant. The lifetime of EPR cable is closely related to the remaining content of antioxidant, and the lifetime evaluation would be recommended by the reverse sequential combination (thermal ageing after radiation ageing).

EPR Application to Polymers: Radiation Induced Crosslinking and Graft Polymerization

EPR application to polymers has been closely related to the research and development of polymer modification by radiation processing. The free radicals induced in polymer chains by irradiation can initiate chemical reactions at rather low temperature ranges, therefore, the radical reactions can be followed by EPR measurement. EPR studies of polymers were applied mainly for the analysis of graft polymerization by the pre-irradiation method and the analysis of radiation crosslinking, which contributed to the development of polymer materials by radiation processing. For graft polymerization, the radicals trapped in the crystalline regions of polymers migrate to the surface and initiate the graft reaction. The concentration of the trapped radicals and their rate of decay are closely related to the graft yield and rate of the grafting reaction. As the radical migration rate in the crystalline part, namely the decay rate of the trapped radicals, is determined by the temperature, the graft polymerization is much affected by the temperature. And also the trapped radicals in any polymers can be conserved for a long time by cooling the irradiated polymers even in air. Therefore, the irradiation and the graft-polymerization can be separated in place and in time. For the crosslinking of polymers, polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN) fiber and polycarbosilane (PCS) fiber were selected for a specific application. The PAN fiber is a precursor of carbon fiber formation and PCS fiber is the precursor of SiC ceramic fiber. Especially the EPR studies for PCS fiber and the pyrolysis were useful for the development of radiation processing. The process of pyrolysis of PCS fiber at a high temperature involves radical reactions and could be followed by EPR measurements. A knowledge of the radical reaction mechanism contributed much to the development of a new method for SiC fiber synthesis.