Corrosion evaluation of uranyl nitrate solution evaporator and denitrator in Tokai Reprocessing Plant

東海再処理工場における硝酸ウラニル溶液蒸発缶及び脱硝塔の腐食評価

山中 淳至 ; 橋本 孝和; 内田 豊実; 白土 陽治; 磯崎 敏彦; 中村 芳信

Yamanaka, Atsushi; Hashimoto, Kowa; Uchida, Toyomi; Shirato, Yoji; Isozaki, Toshihiko; Nakamura, Yoshinobu

東海再処理工場(TRP)はPUREX法を採用し、1977年以降、1140tHMの使用済燃料を処理してきた。再処理プロセスでは、溶液中の硝酸濃度,U, Pu,核分裂生成物等のイオン濃度及び温度が異なることから、さまざまな腐食環境の中にあり、耐食性を考慮のうえ各機器の材料にステンレス鋼やチタン鋼を選定している。材料の腐食環境の厳しさは溶液の硝酸濃度と温度に依存し、溶液中のUは、ステンレス鋼の腐食に与える影響は小さく、チタン鋼の腐食速度を抑制するとされている。TRPで硝酸ウラニル溶液を取扱う機器は硝酸濃度も低く、これまで腐食故障を経験していない。しかしながら、U濃度の上昇に伴い、ステンレス鋼の腐食速度が若干増加する報告もある。TRPで硝酸ウラニル溶液を取扱う機器として、U濃度を最大約1000gU/lまで高める蒸発缶や約320Cの高温でUO粉末に転換する脱硝塔は、高濃度かつ高温のUを取扱うため、腐食の進行の程度を把握しておくべきであると考える。これらの機器について厚さ測定により評価を行った結果、その腐食速度はわずかであり、今後も健全に使用できることを確認した。

The Tokai Reprocessing Plant (TRP) adopted the PUREX method in 1977 and has reprocessed spent nuclear fuel of 1140 tHM (tons of heavy metals) since then. The reprocessing equipment suffers from various corrosion phenomena because of high nitric acidity, solution ion concentrations, such as uranium, plutonium, and fission products, and temperature. Therefore, considering corrosion performance in such a severe environment, stainless steels, titanium steel, and so forth were employed as corrosion resistant materials. The severity of the corrosive environment depends on the nitric acid concentration and the temperature of the solution, and uranium in the solution reportedly does not significantly affect the corrosion of stainless steels and controls the corrosion rates of titanium steel. The TRP equipment that handles uranyl nitrate solution operates at a low nitric acid concentration and has not experienced corrosion problems until now. However, there is a report that corrosion rates of some stainless steels increase in proportion to rising uranium concentrations. The equipment that handles the uranyl nitrate solution in the TRP includes the evaporators, which concentrate uranyl nitrate to a maximum concentration of about 1000 gU/L (grams of uranium per liter), and the denitrator, where uranyl nitrate is converted to UO powder at about 320C. These equipments are therefore required to grasp the degree of the progress of corrosion to handle high-temperature and high-concentration uranyl nitrate. The evaluation of this equipment on the basis of thickness measurement confirmed only minor corrosion and indicated that the equipment would be fully adequate for future operation.