Fig. 6-11 Radiation resistance of aromatic polymers (room temp., gamma-rays) The radiation resistance of polymers is evaluated with the dose when elongation at break of the irradiated polymers becomes the half value of the initial one. Aromatic polymers having aromatic rings in their main chain show very high radiation resistance compared with polyethylene, which has a simple molecular structure. In the case of the irradiation in air, they are degraded at a lower dose of 1/5 - 1/10 in vacuum, because of main chain scission induced by oxidation. It is revealed that for polymers, irradiation in air is a more severe environment than in vacuum.

Fig. 6-12 Development of electric parts for a remote maintenance robot working under very high radiation fields An example of high radiation-resistant electric parts made of aromatic polymers having high insulating performance. These can be used in an environment of 250 degrees cent., 100 MGy in nitrogen gas atmosphere.

Plastics and rubbers are used widely in daily life, owing to their intrinsic characteristics; i.e., excellent insulating capacity, high plasticity, high stiffness in a fiber form, lightweight and no-corrosion. However, since organic polymer materials usually are sensitive to energetic radiation and thermal stress, their use under such environment is limited. In contrast, the excellent characteristics of polymer materials mentioned above makes their use under severe radiation fields desirable. These fields are the insulator of super conducting magnets for nuclear fusion reactors, the electric devices and parts of maintenance robots for fusion reactors and nuclear fuel recycle facilities, and also the structural materials of solar cell paddles on artificial satellites. Polymeric materials in such environments are used in radiation fields of 10 - 100 MGy, under vacuum or nitrogen atmosphere at temperatures from -269 degrees cent. to 250 degrees cent. It is revealed from the studies of irradiation effects on many polymers that polymers containing aromatic structures in the main chain hardly degrade by irradiation and heat, because of the protection effect of aromatic rings for radiation and the more restricted molecular motion in these polymers. The deterioration in polyimide, liquid-crystal polymers and poly (biphenyl ether) is less in vacuum and in nitrogen atmosphere even at the irradiation of 200 - 400 MGy (Fig. 6-11). Their radiation resistance is about 200 - 400 times higher than that of conventional insulators, such as polyethylene. Their insulation resistance and break down voltage are hardly changed by the dose of the irradiation. The coaxial cable and the connector designed for the maintenance robot for ITER (fusion reactor), which is used polyimide and poly (biphenyl ether) polymers as the insulating materials, retained enough strength and flexibility to use after gamma-ray irradiation with 100 MGy at 250 degrees cent. in nitrogen gas atmosphere and also retained enough electric properties to use (Fig. 6-12).

Reference Y. Morita et al., Polymer Materials under Severe Radiation Environment, Genshiryoku eye, 44 (5), 40 (1998).

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