Radiolytic Stability of N-Donor Soft Ligands for Actinide(Ⅲ)/Lanthanide(Ⅲ) Separation

Abstract

The waste from nuclear power plants worldwide has to be isolated from humans and his environment for about 100 000 years to equal the levels of natural uranium. If, however, the long-lived actinides could be separated from the spent fuel and transmuted, then the isolation time could be shortened to about 1 000 years. It is known that one class of N-containing soft ligands has excellent extraction ability and selectivity for minor actinides (MA), the separation of MA/lanthanides(Ln) from high level waste has been expected. One of the limiting points to ensure a safe and stable long-term operation is the resistance against radiation of all chemicals involved in the systems due to the highly radioactive field and high nitric acid concentration. Derivatives of 2,6-bis(1,2,4-triazine-3-yl)pyridine(BTP), 6,6'-bis(1,2,4-triazin-3-yl)-2,2'-bipyridine(R-BTBP) and 2,9-bis(1,2,4-triazin-3-yl)-1,10-phenanthroline(R-BTPhen) were widely employed for trivalent minor actinoid and lanthanoid separation in the promising innovative Selective Actinide Extraction(i-SANEX) and Grouped Actinide Extraction(EURO-GANEX) processes. However, researches towards radiolytic stability of these N-donor ligands presented controversial results. Herein, researches on stability against γ-radiation or α-radiation of R-BTP, R-BTBP, R-BTPhen and 2,6-bis[1H-1,2,3-triazol-4-yl]-pyridine(PyTri) at different experimental conditions and solvent formulations are reviewed. In most cases, γ irradiations were performed using 60Co sources and α irradiation were performed using 241Am or 244Cm solution. The effects of molecular structure, dose and dose rate, contact between phases, oxygen content, different phase composition, acidity, diluent and phase modifier on the γ-radiation stability of these ligands are introduced. The performance and changes in the composition have been analyzed during the irradiation experiment by different techniques, such as Raman spectroscopy, HPLC-MS and ICP-MS, to determine the degradation of the organic or aqueous solvents. These data were used to provide critical insight into the fundamental radiolysis mechanisms responsible for the radiolytic stability of ligands. In general, degradation was slower in the presence of both organic and aqueous phases during irradiation for both the hydrophobic and hydrophilic ligands. However, different experimental conditions and solvent formulations may lead to different degradation paths and degradation rates, and these experimental results are not comparable. Moreover, it is generally accepted that simulate radiation conditions, usually 60Co, may exaggerate or underestimate the influence of radiation effects. The controversial results obtained demonstrate the importance of developing realistic irradiation experiments where different factors affecting the performance can be easily studied and isolated. The existing problems in the study of radiolytic stability are summarized, and the key research directions in this field are prospected.

Keywords

• ma/ln separation
• n-donor soft ligands
• btp
• γ radiation
• α radiation
• stability