The thorium (Th) fuel cycle is attracting attention again because of its potential merits over the uranium (U) fuel cycle, but the associated back-end technology remains insufficiently mature compared with that of the U fuel cycle. In the chemical reprocessing of spent Th oxide fuels, effective separation of UO22+ from Th4+ is required and a quite high dose rate is expected in reprocessing because of the strong radiation by 208Tl generated from 232U, an associated product of 233U. Therefore, gel/liquid extraction was applied to U/Th separation．Introducing ligands chemically into the hydrogels (via the C–C bonds) can minimize ligand leaching, and the ligands are expected to be more resistant to radiation. Some of the N,O-hybrid donor ligands that have polymerizable functional groups – Bis(di-N-allyl-N-tollyl) phenanthlorine diamide (Allyl-Tol-PTDA) and N,N,N’,N’-tetraallylpyridine-2,6-dicarboxamine (Tet-Allyl-PDA) for gel adsorbent synthesis and N,N,N’,N’-tetrahexylpyridine-2,6-dicarboxamine (Tet-Hex-PDA) – were synthesized for solvent extraction because Allyl-Tol-PTDA has low solubility in n-dodecane. Allyl-Tol-PDA and Tet-Allyl-PDA were then polymerized with N-isopropylacrylamide (NIPA) and N,N’-methylenbisacrylamide (BIS) to give thermosensitive hydrogels, namely PTDA-gel and PDA-gel. The general extraction behaviors of Allyl-Tol-PTDA and Tet-Hex-PDA instead of Allyl-Tol-PDA and the adsorption behaviors of PTDA-gel and PDA-gel were compared with those of UO22+ and Th4+ in HNO3. Consequently, differing extraction/adsorption behavior between solvent extraction and gel adsorption was determined. The PTDA gel showed no strong dependency on acidity and temperature with UO22+, but Th4+ showed a local maximum with varying acidity. By contrast, the PDA-gel showed temperature dependency: UO22+ adsorbed more at higher temperature and Th4+ adsorbed more at lower temperature. Therefore, U/Th separation is possible with (i) PTDA-gel at higher temperature and acidity, and (ii) PDA-gel at higher temperature and lower acidity.
https://doi.org/10.1080/01496395.2019.1575881
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