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超臨界水中でのステンレス鋼の高温酸化に$$gamma$$線照射が及ぼす影響

Effects of $$gamma$$-ray irradiation on the high temperature oxidation of stainless steel in supercritical water

中原 由紀夫 ; 山本 正弘 ; 加藤 千明  ; 木内 清; 唐澤 英年*; 勝村 庸介*

Nakahara, Yukio; Yamamoto, Masahiro; Kato, Chiaki; Kiuchi, Kiyoshi; Karasawa, Hidetoshi*; Katsumura, Yosuke*

超臨界圧水冷却炉を念頭においたステンレス鋼の腐食特性評価のために、$$gamma$$線照射が行える超臨界水ループ試験装置を用いた腐食試験を行った。試験には、3種類の汎用ステンレス鋼,SUS304L,SUS316L、及びSUS310Sを用いた。超臨界水の圧力及び温度は25MPa, 550$$^{circ}$$Cで、電導率$$<$$0.1$$mu$$Scm$$^{-1}$$,脱気条件(DO$$<$$5ppb)の純水を流量4Lh$$^{-1}$$で循環させた。$$gamma$$線の吸収線量率は、試験片の表面近傍で5$$sim$$15kGyh$$^{-1}$$と推定された。腐食試験の結果を酸化皮膜の成長による重量増として評価した。SUS304L及びSUS310Sでは$$gamma$$線照射により増量が大きくなり、重量増加の時間依存性は放物線則に従っていた。重量増加の速度定数は$$gamma$$線の線量率の増加に伴って減少する傾向が認められたが、これは溶解による重量減少の影響も含む可能性があり、より詳細な検討が必要である。SUS316Lの重量は、非照射条件では放物線則に従い増加したが、照射条件では酸化皮膜の剥落が見られ大きく減少した。すべての試験片において、多孔質の鉄酸化物の外層と、鉄,クロム及びニッケルを含むち密な内層とによって構成される二層の酸化皮膜が形成されていた。外層の鉄酸化物は、非照射条件ではマグネタイト(Fe$$_{3}$$O$$_{4}$$)だったものが、$$gamma$$線照射によりマグネタイトに加えてヘマタイト($$alpha$$-Fe$$_{2}$$O$$_{3}$$)も形成されていた。また、SUS304Lでは、$$gamma$$線照射により、酸化皮膜内層の金属界面近傍で顕著なCr濃度の増加が見られた。

Corrosion experiments of stainless steel (SS) in supercritical water (SCW) have done with a flowing SCW loop including a $$gamma$$-ray irradiated autoclave. Three types of commercial-grade austenitic SS, Type 304L, Type 316L, and Type 310S, were examined. The condition of SCW was adjusted to the core environment of a SCW reactor system. Test samples of the SS were immersed in deaerated pure SCW (25 MPa, 550 $$^{circ}$$C). Feed water was circulated at a flow rate of 4 L h$$^{-1}$$. Absorbed dose rates near the surface of the test samples in SCW were estimated at 5-15 kGy h$$^{-1}$$. The time dependence of net weight gains followed parabolic rate law except $$gamma$$-ray irradiated Type 316L SS. The net weight gains of $$gamma$$-ray irradiated Type 304L SS and Type 310S SS were larger than those of the non-irradiated. Rate constants of parabolic rate law in net weight gains of Type 304L SS and Type 310S SS were decreased as the $$gamma$$-ray exposure rate was increased. The net weight gain of $$gamma$$-ray irradiated Type 316L SS was fluctuating and decreasing widely, because the exfoliation of the surface oxide occurred. On the surface of all the samples, the formation of two-layer oxide film in which the outer layer was porous iron oxide and the inner layer was dense iron-chromium-nickel oxide occurred. Iron oxide in the outer layer was magnetite (Fe$$_{3}$$O$$_{4}$$) with non-irradiated samples. Hematite ($$alpha$$-Fe$$_{2}$$O$$_{3}$$) was additionally formed in the outer layer with $$gamma$$-ray irradiated samples. A chromium-rich part in the inner layer was formed along to the oxide/metal interface of $$gamma$$-ray irradiated Type 304L SS.

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