The displacement of metals from butylphosphate complexes

A study of chemical aspects associated with the nuclear fuel cycle is described. In particular, attention is paid to contaminants which accumulate in tributylphosphate - odourless kerosene (TBP/OK) during its use as an extractant in the primary separation of spent nuclear fuel. The contaminants, di- and monobutylphosphate (HDBP and H2MBP respectively), and the complexes they form with metals present in the system need to be removed from TBP/OK in order to render it suitable for re-use. A number of metal butylphosphate complexes have been prepared. Characterised and their solubility and stability data determined. The metals studied were zirconium (Zr4+), uranium (as UO22+), iron (Fe3+), thorium (Th4+), plutonium (Pu4+) and molybdenum as (MoO22+). The complexes were prepared in 3 M nitric acid (representing a typical reprocessing medium). The compositions of complexes produced under these conditions were determined by elemental and spectroscopic analysis. Solubilities of the complexes were determined in nitric acid media (usually 3 M) and the results combined with those of ligand competition studies (in the same medium), and used to evaluate the stability constants and solubility products of the complexes. These results were used to predict the effectiveness of metal displacement of the type M(DBP)x + xL- ↔ MLzub(x) + xDBP- when used in the TBP/OK solvent refurbishment scheme. Calculations based on a typical metal-dibutylphosphate complex eg zirconium dibutylphosphate predict the effective displacement of zirconium from DBP (10-4 M) using oxalic acid (10-2 M) in a 3 M nitric acid medium. (author)