Radiation ageing technique for cable life evaluation of nuclear power plant

原子力発電所用ケーブル寿命評価のための放射線劣化法

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*

原子力発電所に敷設されているケーブルは事前に劣化試験が行われる。この劣化試験を短期間で精度よく行うため、ケーブルの実環境の低線量率,長時間照射によるケーブル材料の劣化の再現に必要な高線量率,短時間照射の条件を調べた。ケーブル材料として使われている高分子材料(架橋ポリエチレンとエチレンプロピレンゴム)を室温、0.25kGy/h、及び100C、1kGy/hで、400kGyまで照射した。劣化させた試料の酸化物の分布をFTIRで調べて、機械特性の変化と比較した。その結果、(1)劣化試験の際に照射温度を100Cまで上昇させてもその熱劣化は無視できること、(2)100Cで厚さ1mmの架橋ポリエチレンシートを0.8kGy/h、厚さ1mmのエチレンプロピレンゴムを10kGy/hで照射することにより、高分子材料全体を酸化劣化させることができ、室温での劣化試験と比べ、試験期間が1/15に短縮されることがわかった。

The life of cables installed in nuclear power plant was evaluated by the accelerated ageing tests of thermal and radiation degradation. For radiation degradation, the dose rate was the important points, because the oxidation distribution in the insulation was much affected by dose rate due to oxygen penetration limitation during irradiation. The oxidation distribution 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 profile was almost same between a low dose rate at room temperature and a high dose rate at elevated temperature (100 C), and the degradation was same for both irradiations. By increasing the irradiation temperature the oxygen penetration rate increased exponentially and the thermal degradation during irradiation was negligible at around 100 C. Therefore, the dose rate could be increased by 15 times by irradiation at 100 C instead of the irradiation at room temperature for the adequate oxidation throughout insulation. The experimental result was coincident with the theoretical analysis.