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Recent status of radiochemical analysis of irradiated trans-uranium targets

照射済超ウランターゲットにかかわる放射化学分析の現状

小山 真一; 逢坂 正彦; 三頭 聡明

Koyama, Shinichi; Osaka, Masahiko; Mitsugashira, Toshiaki

高速実験炉「常陽」で照射した超ウラン(TRU)核種の核変換挙動を評価するため、照射済TRU核種の化学分析手法(SRCS法)を開発した。試料を8M硝酸で溶解後、陰イオン交換法により3価アクチニド(An(III))及びランタニド(Ln)を含む核分裂生成物(FP),ウラン(U)の順で溶離する。溶液をヨウ化アンモニウムと濃硝酸混合液に変え、イオン交換樹脂に吸着したプルトニウム(Pu), ネプツニウム(Np)のうち、Puのみを4価から3価に還元して溶離する。An(III)とLnを含むFP溶液は、ピリジン樹脂型イオン交換法により、Lnを含むFP群とAn(III)に群分離する。その後、再度陰イオン交換によりAn(III)よりキュリウム(Cm), アメリシウム(Am)の順で溶離する。本分析法により、TRU核種の放射線計測, 質量分析が可能となったため、「常陽」で照射したMOX燃料及びAmターゲット試料分析を行い、Np, Am及びCmの照射条件依存性,核変換特性を評価することができた。

We have developed a sequential radiochemical separation method (SRCS) in order to isolate each trans-uranium element (TRU) contained in a TRU target (or fuel) irradiated in the experimental fast reactor JOYO. The chemical separation procedures for SRCS consist of the following steps. (1) Target dissolution with a 8M of HNO$$_{3}$$ solution including hydrogen peroxides as a valence control reagents for Pu and Np to their tetravalent states. (2) Sequential elution of main fission products (FPs) including trivalent actinides (An(III)) and lanthanides (Ln(III)) by washing the anion exchange resin column with 8M HNO$$_{3}$$. (3) Elution of U(VI) from the same column with 8M HNO$$_{3}$$ and the change the effluent to 11.6M HCl. (4) Elution of Pu(III) from the same column by reducing Pu(IV) to Pu(III) with 11.6M HCl-0.1M NH$$_{4}$$I solution. (5) Group separation for Ln(III) and An(III) by using tertiary pyridine-type anion exchange resin embedded in silica bead. (6) Mutual separation of Am(III) and Cm(III), also for each lanthanide, by anion exchange resin. For the step 5 and 6, the effluent used was 3/7(v/v) mixture of methanol and HCl and 9/1(v/v) mixture of methanol and 0.013M HNO$$_{3}$$, respectively. The decontamination factor of Pu for the isolated Np was much higher than 10$$^{6}$$. The isotopic composition of isolated trans-uranium was determined mainly by thermal ionization mass spectroscopy. The SRCS sheet was applied to the analyses of MOX fuels irradiated in the experimental fast reactor JOYO. On the basis of isotope analysis, the transmutation and incineration behavior (T-I behavior) of trans-uranium nuclides were elucidated.

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