ＭＯＸ燃料再処理における溶媒劣化　　Ｐｕ精製工程における溶媒劣化とその影響

概要

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A solvent including tributyl phosphate (TBP) has been used for extraction of uranium (U) and plutonium (Pu) from nuclear spent fuel at Tokai Reprocessing Plant (TRP). TBP is said to be degraded by radiolysis, hydrolysis, and reactions catalyzed by extracted elements. One of the problems caused by TBP degradation is transfer of Pu into U stream caused by strong chemical binding between Pu and dibutyl phosphate (DBP), which is a degradation product of TBP. TBP degradation kinetic equations were developed based on degradation experiments using Pu samples with different alpha-ray emission rates that were taken from actual processes at TRP. The influence of degradation on Pu stripping characteristics was investigated. Diluted nitric acid alone could not readily strip Pu when DBP concentration was on the order of 103 mg/L, but a reduction agent, i.e., U (IV) with hydrazine, drastically improved the efficiency of Pu stripping. TRP has recently started to reprocess mixed oxide (MOX) spent fuel from the heavy water reactor FUGEN that includes Pu with higher alpha-ray emission. However, DBP concentration was still sufficiently low for stable solvent extraction operation. In addition, even after retention of Pu in the solvent for 20 days during a temporary suspension of the solvent extraction process, no Pu transfer into U stream was observed.