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緩衝材とオーバーパックの相互作用に関する腐食挙動評価研究(II)

Study on the Corrosion Behavior of the Overpack Material in the Buffer Environment(II)

鈴木 治雄*; 畑 祐二*; 島田 透*; 山田 真*

Suzuki, H.*; Hata, Yuji*; Shimada, T.*; Yamada, M.*

昨年度に引き続き遠隔定置概念に基づく緩衝材の施工で生ずると考えられる隙間を模擬した室内要素試験を実施した。本年度は、試験水の pH および緩衝材の隙間幅をパラメータとした試験、および昨年度と同じ試験条件でさらに長時間の試験を実施し、オーバーパックの腐食挙動に与える影響についてのデータを取得した。また、試験で取得したデータと、炭素鋼の腐食速度および腐食深さの測定結果に基づいて、緩衝材の隙間幅と環境条件が炭素鋼の腐食挙動に与える影響について検討した。その結果、隙間幅の腐食挙動への影響は小さいこと、また、環境条件の影響については、人工海水に比べて高pH となるセメント飽和人工海水の方が、腐食速度が減少する傾向が認められた。炭素鋼の腐食局在化の程度に関しては、昨年度取得したデータと合わせて、第2次取りまとめで用いたデータとの比較を行い、オーバーパックの腐食寿命への影響について検討を実施した。その結果、平均腐食深さと最大孔食深さの比である孔食係数は、今回得られたいずれのデータも、種々の溶液中あるいは土壌中での従来データに比べ、同等または小さくなり、本試験で実施した緩衝材の隙間の幅、試験期間および試験水溶液の条件の範囲では、顕著な局部腐食は生じないことが分かった。また、昨年度および本年度の成果をもとに、室内要素試験についての今後の課題、搬送定置装置への要求事項および大型試験実施の必要性を検討した。

Laboratory scale test was conducted in order to investigate the corrosion behavior of carbon steel overpack material by simulating gaps between buffer materials, which are supposed to be generated when the buffer material is set in the disposal facility. Fifty-eight day tests were performed under various aqueous solutions in the buffer material by changing the pH of the solutions and the width of the gaps simulated. The longer period of 150 day test was also conducted, and the data on the corrosion behavior of the overpack carbon steel were obtained. Based on the test results such as corrosion rates, depth profiles, electrode potential etc. of the carbon steel test specimens, the corrosion behavior was discussed taking into account of the aqueous solution environment and the gap width of the buffer material. As a test result, the influence of the gap width of the buffer material was very little and the higher pH condition of the aqueous solutions as being contact with cement gave the higher corrosion rate to the carbon steel than the ordinary seawater. The extent of local corrosion in the present test was compared with those of the former reports. The result showed the extent of the local corrosion, a ratio of maximum and average depth, was not influenced by the width of the gap between buffer material or by the aqueous environment. All the ratios obtained in the present study were below or within the data band of the former data taken in the various aqueous conditions and natural soils, implying that the corrosion safety was preserved even when the gaps were generated between the buffer materials. Furthermore, problems of the laboratory scale test and needs for a large scale test were also discussed.

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