Studies On The Separation Of U、Np、 Eu、Am By Organophosphorus Ligand

The safe treatment and disposal of spent nuclear fuel reprocessing, especially HLLW(high-level liquid waste) has been received extensive attention all over the world. Currently, solvent extraction has become the mainstream technology of actinide separation from HLLW. Because of the radioactivity and chemical toxicity of actinides, there are numerous difficulties in studying actinide species by experimental researches. Theoretical calculations have been considered to be an effective compensatory approach for exploring actinide complexes at the molecular scale. In this thesis, the extraction behavior of U(V, VI), Np(V, VI), Am(III) and Eu(III) with the organophosphorus ligand HDEHP(di-(2-ethylhexyl) phosphoric acid) have been investigated by using density functional theory. We mainly focused on the structure properties and bonding natures as well as thermodynamic stabilities of the extraction complexes, and the complexation mechanism of actinides/lanthanides and HDEHP ligands were also studied. The m ain results are summarized as follows:(1)Theoretical studies on the extraction of U(V, VI) and Np(V, VI) with HDEHP suggested that the HDEHP ligands usually form hydrogen bonded dimers in the nonpolar diluents and all the dimers coordinate to the uranyl and neptunyl ions as bidentate ligands with two phosphoryl groups in the equatorial plane. For all the 1:1 and 1:2-type(metal : ligand) complexes, the bonds between metal and ligands are predominately ionic and HDEHP ligands show higher extraction ability for An(VI)(An=U, Np) over An(V). In addition, the formation of Np(VI) complexes is slightly more favorable than those of U(VI) analogues, which is mainly attributed to the stronger donor-acceptor interaction in Np(VI) complexes. The intramolecular hydrogen bonds play an significant role in the stability of the 1:1 type complexes An O2(HL)2(NO3)2(L=DEHP-). Moreover, An O2(HL)2(NO3)2 are the most stable species in nitrate-rich acid solutions, while at low nitric acid concentrations, An O2(HL2)2 are the most stable species in the extraction process.(2) For the theoretical studies on the extraction of Eu(III) and Am(III) with HDEHP, from the electronic structural and bonding properties, HDEHP ligand coordinate to the metal in the form of HDEHP dimers and the phosphoryl oxygen atom is the preferred site of electrophilic attack by metal ions compared to the oxygen atom in the P-OH group. For all the extraction complexes, the metal-ligand bonds are mainly ionic in nature. The 1 : 3(metal : ligand) stoichiometric species M(HL2)3 are predicted to be the most favorable complexes in the extraction process. Although Eu(III) complexes have higher interaction energies, the HL2- dimer shows comparable affinity for Eu(III) and Am(III) according to thermodynamic analysis, which may be attributed to the higher stabilities of Eu(III) nonahydrate.