Raman Spectroscopic Study on Extraction of U(Ⅵ) From HNO₃-HClO₄ Solutions by Tributyl Phosphate

⁹⁹Tc is a major radioactive fission product of ²³⁵U and ²³⁹Pu during the operation of nuclear reactors, and with a half-life time of 2.11×10⁵ years it is also a long-term contributor to radiotoxicity of spent nuclear fuel. In PUREX spent fuel reprocessing, ⁹⁹Tc exists in the form of \mathrmTcO_4^- in solution of spent fuel dissolution in nitric acid. \mathrmTcO_4^- ion might be co-extracted with U(Ⅵ), Pu(Ⅳ), and other metal ions by TBP, then affects the efficiency of the uranium-plutonium separation cycle, giving a negative impact on the operation. Previous studies have shown that in PUREX, when HNO₃ and HTcO₄ coexist with U(Ⅵ) in aqueous solution, in addition to UO₂(TBP)₂(TcO₄)₂ and UO₂(NO₃)₂(TBP)₂ extracted complexes each just containing one type of anion, either \mathrmNO_3^- or \mathrmTcO_4^- , there might be another complex, (UO₂)(TBP)₂(TcO₄)(NO₃), being formed with mixed anions. While the UO₂(NO₃)₂(TBP)₂ complex has been structurally and thermodynamically studied with clear results, there is no clear experimental evidence for the composition and structure of the other two extracted complexes, UO₂(TBP)₂(TcO₄)₂ and UO₂(TBP)₂(TcO₄)(NO₃). In the present work, the composition of the complex formed from the reaction of hydrated uranyl perchlorate UO₂(ClO₄)₂•(H₂O)_x with TBP is investigated with Raman spectroscopy to be UO₂(TBP)₄•(ClO₄)₂ in which only the four TBP molecules directly bond to uranyl but the two perchlorate anions do not. When \mathrmNO_3^- and \mathrmClO_4^- coexist in the system, both UO₂(NO₃)₂(TBP)₂ and UO₂(TBP)₄•(ClO₄)₂ complexes are observed. In comparison of the Raman spectra of the sample from dissolving concentrated perchloric acid with that of UO₂(ClO₄)₂•(H₂O)_x into TBP, it has been found that the characteristic peak of \mathrmClO_4^- at 928 cm⁻¹ is independent of the existence of U(Ⅵ), and that U(Ⅵ) in the system does not directly interact with \mathrmClO_4^- , indicating that \mathrmClO_4^- only acted as a charge-balancing anion. For the complexes from dissolving UO₂(ClO₄)₂•(H₂O)_x with different amounts of TBP, the Raman spectra suggest that only UO₂(TBP)₄•(ClO₄)₂ species is formed independently form the ratio of TBP to U(Ⅵ) used for preparing the samples. Dissolution of UO₂(ClO₄)₂•(H₂O)_x in the organic phase of 30% TBP-kerosene loaded with varying amounts of UO₂(NO₃)₂ yields light and heavy organic phases. It is confirmed by Raman spectra that UO₂(NO₃)₂(TBP)₂ is present in the system along with UO₂(TBP)₄•(ClO₄)₂, but there is no U(Ⅵ), TBP, \mathrmNO_3^- , \mathrmClO_4^- quaternary complexes formed. The results obtained in the present work for the complexes formed from TBP with U(Ⅵ), \mathrmClO_4^- , and \mathrmNO_3^- will provides helpful information for determining the composition of the extracted complexes in PUREX formed from TBP with U(Ⅵ), \mathrmTcO_4^- , and \mathrmNO_3^- , due to the similarity in size and charge between \mathrmTcO_4^- and \mathrmClO_4^- .