Pertechnetate/perrhenate-capped Zr/Hf-Dihydroxide Dimers: Elucidating Zr-TcO 4 Co-Mobility in the Nuclear Fuel Cycle

Spent nuclear fuel contains heavy element fission products that must be separated for effective reprocessing for a safe and sustainable nuclear fuel cycle. Zr and Tc are high-yield fission products that co-transport in liquid-liquid extraction processes. Here we seek atomic-level information of this co-extraction process, as well as fundamental knowledge about Zr (and Hf ) aqueous speciation in the presence of topology-directing ligands such as pertechnetate (TcO ) and non-radioactive surrogate perrhenate (ReO ). In this context, we show that the flat tetrameric oxyhydroxyl-cluster [M (OH) (H O) ] (and related polymers) is dissociated by perrhenate/pertechnetate to yield isostructural dimers, M (OH) (XO ) (H O) ⋅ 3H O (M=Zr/Hf ; X=Re/Tc ), elucidated by single-crystal X-ray diffraction. We used these model compounds to understand the pervasive Zr- Tc coextraction with further speciation studies in water, nitric acid, and tetrabutylphosphate (TBP) -kerosene; where the latter two media are relevant to nuclear fuel reprocessing. SAXS (small angle X-ray scattering), compositional evaluation, and where experimentally feasible, ESI-MS (electrospray ionization mass spectrometry) showed that perrhenate/pertechnetate influence Zr/Hf -speciation in water. In Zr-XO solvent extraction studies to simulate fuel reprocessing, we provide evidence that TcO enhances extraction of Zr , and compositional analysis of the extracted metal-complexes (Zr-ReO study) is consistent with the crystallized Zr (OH) (Re O ) (H O) ⋅dimer.