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Oxidation characteristics of lead-alloy coolants in air ingress accident

空気侵入事故時の鉛合金冷却材の酸化特性

近藤 正聡*; 大久保 成彰; 入澤 恵理子; 小松 篤史; 石川 法人; 田中 照也*

Kondo, Masatoshi*; Okubo, Nariaki; Irisawa, Eriko; Komatsu, Atsushi; Ishikawa, Norito; Tanaka, Teruya*

鉛合金冷却高速炉の空気侵入事故におけるPb合金冷却材の化学的挙動を、種々の組成のPb合金の熱力学的考察および静的酸化実験によって調べた。鉛ビスマス(Pb-Bi)合金の性的酸化試験の結果、空気中の773KではPbOが優先的に形成され合金中からPbが減少したが、Biはこの酸化挙動には関与しなかった。その後Biが濃縮するとPb-Bi酸化物の他Bi$$_{2}$$O$$_{3}$$が形成された。合金の酸化速度は鋼の酸化速度よりもはるかに大きく、合金中のPb濃度が高いほど大きくなった。Pb-Bi合金とステンレス鋼との共存性は、合金中のPb濃度が低くなると悪化した。合金中のBi組成によって溶解タイプの腐食が促進されたためである。一方、Pb-Li合金は、Li$$_{2}$$PbO$$_{3}$$およびLi$$_{2}$$CO$$_{3}$$を形成しながら酸化が進行し、合金からLiが減少した。冷却材のこれらの酸化物は、空気侵入事故時の原子炉冷却系の酸素濃度増加後に低温領域で生成すると考えられる。

The chemical behaviors of lead (Pb) based coolants in the air ingress accident of fast reactors were investigated by means of the thermodynamic considerations and the static oxidation experiments for Pb alloys at various chemical compositions. The results of the static oxidation tests for lead-bismuth (Pb-Bi) alloys indicated that Pb was depleted from the alloy due to the preferential formation of PbO in air at 773K. Pb-Bi oxide and Bi$$_{2}$$O$$_{3}$$ were formed after the enrichment of Bi in the alloys due to the Pb depletion. The oxidation rates of the alloys were much larger than that of the steels, and became larger with higher Pb concentration in the alloys. The compatibility of Pb-Bi alloys with stainless steel was worse when the Pb concentration in the alloys became low, since the dissolution type corrosion was promoted by the Bi composition in the alloy. The Pb-Li alloys were oxidized as they formed Li$$_{2}$$PbO$$_{3}$$ and Li$$_{2}$$CO$$_{3}$$. Then, Li was depleted from the alloy.

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