[en] Among the fission products present in the spent nuclear fuel, technetium exhibits a singular behavior in reprocessing operations performed by solvent extraction. Indeed, this strong acid readily dissociates to form the oxo-anion TcO₄^- that may interfere with uranium(VI), plutonium(IV), and zirconium(IV) in the extraction cycles of the PUREX process. This paper focuses on the uranium-technetium complex with TBP and on its non-radioactive rhenium surrogate. Despite the large set of distribution data available for rhenium and technetium extraction with TBP, the structures of the co-extracted complexes remain largely unknown. However, it is important to understand clearly the extraction mechanism of technetium with TBP in the PUREX process to optimize the separation process and to model its behavior during the extraction steps. Based on distribution data available in the literature, a thermodynamic model was developed for the extraction of technetium with TBP for a large excess of uranium(VI) in organic phase. A good representation of uranium and technetium distribution data was thus obtained when considering the formation of (HTcO₄)(TBP)_n complexes, as well as mixed UO₂(NO₃)(TcO₄)(HNO₃)_x(TBP)_n complexes. In the complex UO₂(NO₃)₂(HNO₃)_x(TBP)_n, one pertechnetate anion replaces one nitrate in the uranium coordination sphere. Combination of complementary spectroscopic techniques (FT-IR and X-ray absorption) supported by theoretical calculations (density functional theory) with organic phases containing a large excess of technetium(VII) or rhenium(VII) enabled full characterization of the limit mixed uranium-technetium species and also of mixed uranium-rhenium species. Details on the coordination of the uranium-technetium complex are provided with the help of DFT calculations and XAS measurements. (authors)

[en] The N,N-di-alkylamide DEHiBA (N,N-di-2-ethylhexyl-isobutyramide) is a promising alternative extractant to TBP (tri-n-butylphosphate) to selectively extract uranium(VI) from plutonium(IV) and spent nuclear fuel fission products. Extraction of technetium, present as pertechnetic acid (HTcO₄) in the spent fuel solution, by DEHiBA was studied for different nitric acid and uranium concentrations. The uranium(VI) and technetium(VII) co-extraction mechanism with DEHiBA was investigated to better understand the behavior of technetium in the solvent extraction process. Uranium and technetium distribution ratios were first determined from batch experiments. On the basis of these data, a thermodynamic model was developed. This model takes into account deviations from ideality in the aqueous phase using the simple solution concept. A good representation of uranium and technetium distribution data was obtained when considering the formation of (DEHiBA)_i(HNO₃)_j(HTcO₄)_k complexes, as well as mixed (DEHiBA)₂(UO₂)(NO₃)(TcO₄) and (DEHiBA)₃(UO₂)(NO₃)(TcO₄)(HNO₃) complexes, where one pertechnetate anion replaces one nitrate in the uranium coordination sphere in the two complexes (DEHiBA)₂(UO₂)-(NO₃)₂ and (DEHiBA)₃(UO₂)(NO₃)₂(HNO₃). Combination of complementary spectroscopic techniques (FT-IR and X-ray absorption) supported by theoretical calculations (density functional theory) enabled full characterization of the formation of mixed uranium-technetium species (DEHiBA)₂(UO₂)(NO₃)(TcO₄) in the organic phase for the first time. The structural parameters of this complex are reported in the paper and lead to the conclusion that the pertechnetate group coordinates the uranyl cation in a monodentate fashion in the inner coordination sphere. This study shows how combining a macroscopic approach (distribution data acquisition and modeling) with molecular-scale investigations (FT-IR and X-ray absorption analysis supported by theoretical calculations) can provide a new insight into the description of a solvent extraction mechanism. (authors)