Fundamental Study of "The Third Phase" formed in Extraction Processes of Nucler Fuel Reprocessing

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Formation conditions, compositions and structures of precipitates or "the third phase" were systematically examined in the systems of Zr and radiation depleted products of TBP, such as HDBP, H_2MBP, H_3PO_1, by means of elemental analysis. X-ray diffraction, infrared spectra and ^1H-NMR. It was confirmed that one of the most important origins for the third phase is a complexation between Zr ion and the depleted products of TBP. Followings were also elucidated:(1) When the depleted products co-exist with each other, the cooperative effects on the pre- cipitate formation appear in low acid solutions. (2) Precipitate formation depends on the mole ratio of HDBP/Zr. The amount of precipitate reaches the maximum at the mole ratio of approximately 2 and decreases with increasing concentration of HDBP and finally disappeals at approximately 10. (3) Precipitate formed at the mole ratio of approximately 2 has the chemical formula, Zr(NO_3)_2(HDBP)_2(OH)_2. (4) Precipitates of the Zr-H_2MBP syste … More m begin to appear at the concentration of H_2MBP in one order of magnitude smaller than of HDBP in Zr HDBP system.EXAES (Extended X-ray Absorption Fine Structure) measurement was carried out for the study of the local structure around Zr in precipitates of Zr(IV)-monobutylphosphate and Zr(IV)- dibutylphosphate, which are one of the main species of so-called "The Third Phase". Both gelatinous and dried samples show the same Fourier transform spectra for Zr(IV)-monobutyl- phosphate and Zr(IV)-dibutylphosphate. The precipitates in Zr(IV)-monobutylphosphate have the bridging phosphate between Zr ions, Zr-phosphate-Zr. as well as the bridging hydroxyl between Zr ions, Zr-OH-Zr, while no bridging phosphate in Zr(IV)-dibutylphosphate. This fact indicates the layer structure with 16A^^ﾟ distance for Zr(IV)-monobutylphosphate described in Part (I) of the present reports. As for the re-dissolution of Zr(IV)-dibutylphosphate precipitates with increasing tributylphosphate observed in Part (I). the absence of the bridging phosphate in Zr(IV)-dibutylphosphate plays an important role. The chemical structures pospuiated by elemental analysis. infrared spectra. ^1H-NMR and X-ray diffraction in Part (I) were confirmed in detail from the present results of EXAFS. It was confirmed ZrO^<2->, zirconyl ion, does not exist in aqueous solutions. Less