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TBP-硝酸溶媒へのU, Pu混合酸化物の溶解

Dissolution of uranium and plutonium oxide using TBP-HNO$$_{3}$$ complex

宮原 幸子 ; 川崎 諭*; 柴 正憲*; 斉藤 和則*; 駒 義和  ; 菅沼 隆*; 青嶋 厚 

Miyahara, Sachiko; not registered; Shiba, Masanori*; not registered; not registered; not registered; not registered

硝酸を含むTBP(TBP-硝酸溶媒)を用いて混合酸化物(MOX)粉末からU及びPuを選択的に溶解する方法(粉化燃料抽出法)を検討している。この方法は硝酸による溶解工程およびTBP溶媒による抽出から成る工程(共除染工程)を一体化でき、比較的低温かつ大気圧で操作できるので、再処理工程の簡素化が期待できる。Pu酸化物はU酸化物と比較して硝酸に溶解しにくいことからPuの溶解挙動を把握する必要があり、TBP-硝酸溶媒へのPuとUの混合酸化物(MOXペレット焙焼粉)の溶解特性を調査した。U、Pu混合酸化物(Pu富化度(Pu/Pu+U)18%)の焼結ペレットを、空気雰囲気中400$$^{circ}C$$で4時間加熱し平均粒径約10$$mu$$mのU、Pu混合酸化物の粉末を調製した。この粉末(MOXペレット焙焼粉)を4.74mol/L硝酸を含むTBP溶媒に投入し300分撹拌した。MOXペレット焙焼粉6gを20mLに投入した試験では、300分経過後のTBP溶媒中のPu濃度は0.17mol/Lとなり、初期投入量の約9割が溶解した。MOXペレット焙焼粉は硝酸を含むTBP溶媒に溶解することが分かった。このときの初期の溶解速度は、同条件下でのMOX燃料の硝酸水溶液への溶解速度とほぼ一致した。Puの溶解速度は硝酸水溶液系の溶解速度から推測できるものと考えられる。また、MOX中に含まれるAmも、Puより溶解速度が遅いものの硝酸を含むTBP溶媒に溶解することが分かった。

The current technology for the selective separation of plutonium and uranium from spent nuclear fuel (MOX) using TBP-HNO$$_{3}$$ complex is being developed (Powdered fuel extraction process). It is promising to simplify the reprocessing process for the selective separation because of its potential to unite the chemical processes, dissolution process using nitric acid and co-extraction process using TBP solvent, and to operate under the ambient pressure and at relatively "mild" temperature. Plutonium oxide has reported to provide slower dissolution than uranium oxide in nitric acid. In this work dissolution behaviors of plutonium into TBP-HNO$$_{3}$$ complex from powdered plutonium and uranium mixed oxide were examined. The powdered MOX fuel (average particles size 10$$mu$$m) was prepared from PuO$$_{2}$$-O$$_{2}$$ pellets by heating for 4 hours at 400$$^{circ}$$C. The prepared powder was dissolved into TBP-4.74mol/L HNO$$_{3}$$ complex and was stirred for 300 minutes. In the test with 6 grams of powdered MOX fuel and 20 mL of the TBP-HNO$$_{3}$$ complex, the concentration of plutonium reached 0.17 mol/L and about 90 percent of plutonium was dissolved. It is experimentally confirmed plutonium was dissolved into the TBP-HNO$$_{3}$$ complex from plutonium and uranium mixed oxide. The early dissolution rate was almost the same as that obtained with nitric acid solution. It is likely to predict the dissolution rate from the rate for nitric acid solution. Americium that was contained in the MOX fuel was also dissolved into the TBP-HNO$$_{3}$$ complex, but was slower than plutonium.

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