Use of a heteropolyanion ligand as analytical reagent for off-line analysis of uranium(IV) in the PUREX process

This work was aimed to develop analytical method for the measurement of U(IV) in the U/Pu separation step of the PUREX process for irradiated fuel reprocessing. The study considered the possibility of stabilizing and analyzing U(IV) in the presence of U(VI), Pu(IV) and Pu(III), using the heteropolyanion P2W17O1061 - denoted ''PWO'' - as a selective complexing agent. The absorption spectrum of the UIV(PWO)16-2 limit complex, in the visible area, displays a typical and intense charge transfer band allowing sensitive spectrophotometric measurements. To develop the U(IV) analytical method using PWO, it was necessary to consider the complexation reaction of this ligand with metallic species of interest, present in the analytical samples, such as U(IV), U(VI), Pu(IV) and Pu(III), or to be added to the sample for analytical purpose, like Fe(III) and Fe(II). The redox reaction exhibited by these metallic species in the presence of PWO were also considered. We demonstrated that U(IV) could be stabilized in the presence of Pu species using a PWO/Fe(III) mixture: (1) the oxidation of U(IV) by Pu(IV) is thermodynamically impossible and (2) the reduction of U(VI) by Pu(III) is prevented in the presence of Fe(III), by the oxidation of Pu(III) to Pu(IV). The U(IV) analytical method, based on the spectrophotometric measurement at 510 nm (maximum absorption wavelength of the UIV(PWO)162 complex), was validated using samples taken during tests of the U/Pu PUREX partition operation, carried out in a pulsed column pilot facility. The new analytical method can be applied with aqueous or organic U(IV) samples. This new method appears to yield reliable results and to be readily feasible in hot environments. (orig./SR)