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Effect of decontamination factor on core neutronic design of light water reactors using recovered uranium reprocessed by advanced aqueous method

先進湿式再処理により回収したウランを装荷した軽水炉の炉心核設計に除染係数が与える影響

中野 佳洋; 大久保 努; 駒 義和

Nakano, Yoshihiro; Okubo, Tsutomu; Koma, Yoshikazu

軽水炉に先進湿式再処理で回収したウランを装荷した場合に、除染係数(DF)が炉心核特性に与える影響を評価した。再処理においては、DFの値に対応した量の超ウラン元素(TRU)や核分裂生成物(FP)が、回収されたウラン製品に混入する。先進湿式再処理の代表的なDF値100と、これに加えて1000及び無限大とした3つのケースでTRUやFP混入量を評価し、これら不純物と$$^{235}$$Uの中性子捕獲により生成される$$^{236}$$Uが炉心の核特性に与える影響を評価した。計算はAPWRについて行った。初期$$^{235}$$U濃縮度は4.6%,取出燃焼度を49GWd/tとし、使用済燃料中のU組成を求めた。DFが無限大の場合で、同じ燃焼度49GWd/tを達成するためには回収ウランを5.24%まで濃縮する必要があることがわかった。DFを100としても必要な濃縮度の増分はわずかであり、濃縮度に与えるDFの影響は少ないことがわかった。減速材温度係数に対しても検討を行い、これについてのDFは有意な影響を与えないことがわかった。

In the case where uranium recovered by an advanced aqueous reprocessing is utilized in light water reactors (LWRs), the effects of the decontamination factor (DF) of the reprocessing on core neutronic characteristics were examined. The amounts of transuranium (TRU) elements and fission products (FP) contained in the recovered uranium depend on the DF of the reprocessing process, and also $$^{236}$$U is generated by neutron capture of $$^{235}$$U. These all act as poisons in the fuel. Therefore, in this paper, the additional $$^{235}$$U enrichment necessary to compensate for the produced TRU, FP and $$^{236}$$U was evaluated for three cases of representative DF values: 10$$^{2}$$, 10$$^{3}$$ and infinity. The low value, 10$$^{2}$$, corresponds to the advanced aqueous reprocessing process investigated here. An APWR core with a discharge burnup of 49 GWd/t when the initial $$^{235}$$U enrichment is 4.6% was considered as the reference core. It was calculated that the recovered uranium has to be re-enriched up to 5.24% even when DF is infinity in order to achieve the same burnup of 49 GWd/t as the reference core. On the other hand, it was also found that the necessary $$^{235}$$U enrichment after the advanced aqueous reprocessing studied here with the low DF value 10$$^{2}$$ is only slightly different. The effect of the DF value on moderator reactivity coefficient was also studied, and no effect was found.

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