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The change of Np oxidation state in nitric acid and the effect of nitrous acid on the oxidation state were analyzed by spectrophotometry, solvent extraction, and electrochemical methods. The Np extraction with 30 vol.% TBP was enhanced by the adjustment of the Np oxidation state using a glassy carbon fiber column electrode system. The knowledge of electrolytic behavior of nitric acid was important because the nitrous acid affecting the Np redox reaction was generated during the adjustment of the Np oxidation state. The Np solution used in this work consisted of Np(V) and Np(VI) but no Np(IV). The ratio of Np(V) in the range of 0.5M∼5.5 M nitric acid was 32%∼19%. The electrolytic oxidation of Np(V) to Np(VI) in the solution enhanced the Np extraction efficiency about five times higher than without electrolytic oxidation. It was confirmed that the nitrous acid in a concentration of less than about 10−5M acted as a catalyst to accelerate the chemical oxidation reaction of Np(V) to Np(VI).

Abstract: A new static contactor was developed for solvent extraction using capillary phenomena induced among clearances formed within a highly packed fiber bundle. Feeding two immiscible phases cocurrently into the fiber bundle generated a very large liquid-liquid contact area for mass transfer within the fiber bundle without any flow turbulence or drop phenomena. In order to test the characteristics and stability of the fiber bundle contactor, continuous extraction experiments were carried out using the fiber bundle contactor with a TBP-uranyl ion-nitric acid system. The fiber bundle contactor had the same extraction performance as that of an ideal batch extractor with good reproducibility due to the sufficient liquidliquid contact area generated by the packed fiber bundle. A minimum residence time of the aqueous phase within the fiber bundle contactor was required for the extraction system to reach an extraction equilibrium state. In the TBP-uranyl ion-nitric acid system, the residence time was about 1.9 minutes. This contactor was confirmed to be effective enough to perform solvent extraction and to study the extraction kinetics because of the stable and large static liquid-liquid contact area.

Abstract: In order to remove U, Tc, and Np, which are positioning materials or target nuclides for transmutation, from the high-level radioactive waste, conditions of co-extraction and sequential stripping of the nuclides were studied by using 30 vol.% TBP. On the basis of the experiments performed on each element of U, Tc, and Np, a combination of co-extraction of U, Tc, Np rarr Tc stripping rarr Np stripping rarr U stripping was suggested. To enhance the Np extraction yield, the electrolytic oxidation of Np(V) was required at the co-extraction step. For the stripping of Tc 5M HNO3, of Np the electrolytic reduction of Np(VI) to Np(V), and of U 0.3M sodium carbonate were used. Phase ratios (O/A or A/O) were recommended to be of 2-3, for co-extraction and for stripping.