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

熱,放射線の加速劣化によるケーブル絶縁材料の酸化分布

島田 明彦; 杉本 雅樹; 工藤 久明*; 田村 清俊*; 瀬口 忠男*

Shimada, Akihiko; Sugimoto, Masaki; Kudo, Hisaaki*; Tamura, Kiyotoshi*; Seguchi, Tadao*

架橋ポリエチレンに酸化防止剤を0, 0.1, 1.0%加え、厚さ2mmのシート状に成型した試料を熱,放射線劣化させ、劣化後の試料の機械特性と酸化防止剤濃度,酸化生成物濃度、及びこれらの厚さ方向の分布を調べ、相関関係について議論した。劣化とともに酸化防止剤濃度は、揮発や分解により減少していくが、十分な量の酸化防止剤(0.04%程度(限界値)以上)が架橋ポリエチレンに残っている場合には、劣化時間や線量に比例して酸化が進行し、厚さ方向に対する酸化防止剤の不均一な消費や酸化生成物の不均一な生成は見られない。しかし、酸化防止剤の濃度が限界値以下になると、酸化速度が著しく向上するとともに、試料表面の劣化が非常に激しくなるなど不均一な酸化劣化が起こることがわかった。したがって架橋ポリエチレンなどのケーブル絶縁材の加速劣化試験を行う際には、酸化防止剤の消失が引き起こす熱暴走(酸化速度が著しく向上すること)が起こらない条件で行うことが必要不可欠であると考えられる。

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. 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 limited 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.