Abstract:
99Tc is a major radioactive fission product of
235U and
239Pu during the operation of nuclear reactors, and with a half-life time of 2.11×10
5 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 HNO
3 and HTcO
4 coexist with U(Ⅵ) in aqueous solution, in addition to UO
2(TBP)
2(TcO
4)
2 and UO
2(NO
3)
2(TBP)
2 extracted complexes each just containing one type of anion, either \mathrmNO_3^- or \mathrmTcO_4^- , there might be another complex, (UO
2)(TBP)
2(TcO
4)(NO
3), being formed with mixed anions. While the UO
2(NO
3)
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, UO
2(TBP)
2(TcO
4)
2 and UO
2(TBP)
2(TcO
4)(NO
3). In the present work, the composition of the complex formed from the reaction of hydrated uranyl perchlorate UO
2(ClO
4)
2•(H
2O)
x with TBP is investigated with Raman spectroscopy to be UO
2(TBP)
4•(ClO
4)
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 UO
2(NO
3)
2(TBP)
2 and UO
2(TBP)
4•(ClO
4)
2 complexes are observed. In comparison of the Raman spectra of the sample from dissolving concentrated perchloric acid with that of UO
2(ClO
4)
2•(H
2O)
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 UO
2(ClO
4)
2•(H
2O)
x with different amounts of TBP, the Raman spectra suggest that only UO
2(TBP)
4•(ClO
4)
2 species is formed independently form the ratio of TBP to U(Ⅵ) used for preparing the samples. Dissolution of UO
2(ClO
4)
2•(H
2O)
x in the organic phase of 30% TBP-kerosene loaded with varying amounts of UO
2(NO
3)
2 yields light and heavy organic phases. It is confirmed by Raman spectra that UO
2(NO
3)
2(TBP)
2 is present in the system along with UO
2(TBP)
4•(ClO
4)
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^- .
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