The Use of Aliquat-336 in a Partitioning and Transmutation Process and a Kinetic Study of the Oxidation of U(IV) to U(VI)

Doctoral thesis, 1999

Spent nuclear fuel contains several long-lived radionuclides that must be stored in a safe manner for a long time. Transmutation of these radionuclides into short-lived or stable nuclides reduces the necessary storage time by several orders of magnitude. The nuclides most suitable for transmutation are those of the actinides. The products obtained from partitioning and transmutation are, however, radioactive. Partitioning and transmutation (P&T) should therefore be considered as a complement to a final storage. The connection between partitioning and transmutation, is presented. Generally, it can be concluded that the transmutation efficiency and the separation yield should be high if P&T should be a preferable option. Liquid-liquid extraction is one separation technique suitable to achieve the necessary separation yield. One group of extractants that can be used is the long-chained quaternary ammonium salts. In this work, the extractant Aliquat-336 has been investigated. The extraction behaviour of several elements, Sc, Fe, Zr, Nb, Tc, In, La, Ce, Pr, Nd, Pm, Sm, Eu, Th, U, Np, Pu, Am, Cm and nitric acid have been investigated. Aliquat-336 shows high extraction of tetravalent elements and anions. The extraction of multivalent cations was found to decrease in the order: . A separation process based on Aliquat-336 for separation of technetium, neptunium and plutonium in connection with the Reversed Talspeak process is presented. The process is designed with respect to the number of stages and volume flow ratios needed to achieve at least 99.9% separation. The crucial stage was found to be the stripping of plutonium(IV). Aliquat-336 has potential to achieve the necessary separation of technetium, neptunium and plutonium. A kinetic study of the oxidation of uranium(IV) to uranium(VI) by oxygen was done. It was found that the oxidation rate is a combination of a first order and an autocatalytic reaction mechanism. The reactive species in the acid interval investigated are the first and second hydrolysed uranium(IV) complexes. A kinetic investigation can be a method for determination of hydrolysis constants for metal cations with unstable oxidation states.