Understanding the extraction behavior and mechanism of Th(Ⅳ) and U(Ⅵ) by the irradiated tri-iso-amyl phosphate with At-211 as an α-source

Abstract

Tri-iso-amyl phosphate (TiAP) is considered as a promising alternative to tri-butyl phosphate (TBP) for the reprocessing of spent nuclear fuels. However, how radiation alters the extraction properties of the extractant and what is the mechanism causing this phenomenon has not been determined, especially under alpha radiation. This study investigates the extraction of Th(IV) and U(VI) by 36 % TiAP - n-dodecane (n-DD) after exposure to alpha radiation from 211At, and compared with gamma radiation at an equivalent dose. The results indicate that the effect of alpha radiation on TiAP has a similar trend to gamma radiation, and the concentration of HNO3 plays a crucial role in the extraction of Th(IV) and U(VI) by irradiated TiAP. Even at low doses (10 kGy), radiolysis of the extractants remains observable. In addition, the stripping of extracted Th(Ⅳ) from the irradiated organic phase is more difficult than U(VI), which is consistent with previous investigations on gamma radiation. Theoretical calculations suggest that the [DiAP]− ligand formed by HDiAP as a radiolysis product of TiAP is closely related to the strong extraction ability of Th(Ⅳ) in the extraction system after radiation at low acidity, since the [DiAP]− ligand exhibits a higher coordination ability than TiAP and HDiAP, especially with Th(IV). The outcomes of this work not only provide a procedure for exploring the alpha radiation of extractions based on a short half-life nuclide, but also demonstrate the connection between the radiolysis products and the change of extraction performance, which are helpful for us to gain a better understanding of the potential of TiAP to be used in the separation and purification of Th(Ⅳ) and U(Ⅵ) in reprocessing plants.