Metallic fuels such as U–Zr and U–Pu–Zr
alloys
can be reprocessed by the aqueous-based PUREX process. Zr which is
present in large quantities (60 kg/ton of fuel) is transferred to
high level waste (HLW) solutions during the aqueous reprocessing of
metal fuel. It is important to recover and reuse Zr from the HLW solutions.
The recovered Zr can be recycled for the production of fresh metallic
fuel. In this context, batch solvent extraction studies were carried
out with tri-isoamyl phosphate (TiAP) and tri-n-butyl
phosphate (TBP) in cross-current mode to evaluate the number of stages
required for the quantitative extraction and stripping of Zr. Based
on batch solvent extraction studies, continuous counter-current mixer-settler
runs were demonstrated for the recovery of zirconium with 1.47 M TBP/n-dodecane (n-DD) from 6 M HNO3 using an ejector mixer-settler facility. The extraction mechanism
of Zr by TiAP and TBP was evaluated using density functional theory
(DFT) calculations in order to gain insight into the nature of interactions
operating between Zr(IV) and the extractants. Complexation energies
of Zr complexes with TiAP and TBP were computed using DFT and found
to be in good agreement with the experimentally measured distribution
ratios. The coordination behavior of zirconium nitrate with TiAP and
TBP was investigated by using extended X-ray absorption fine structure
(EXAFS) studies. The coordination number and bond distances of Zr(NO3)4 complexes with TiAP and TBP were obtained from
EXAFS analysis. These studies demonstrate the quantitative extraction
and stripping of Zr from nitric acid and organic solutions, respectively.