Within the EUROPART collaborative project, a new class of N-donor polyazines was designed and synthesized to better conform to the require­ments of the SANEX process development: the bis-triazinyl-bipyridines (BTBPs, Fig. 11.12, Foreman et al., 2005, 2006). The extraction properties of these tetradentate ligands were investigated and they appeared to differ from those of BTP ligands, in that M : L complexes were identified instead of M : L3 complexes (Drew et al., 2005, Nilsson et al., 2006a, b, Retegan et al., 2007a, b). The selectivity of alkyl-BTBP ligands toward An(III) and their kinetics of extraction are similar to those of alkyl-BTP ligands: the bulkier the alkyl groups are, the slower the mass transfer is, thus requiring a phase transfer catalyst, such as a diamide (Geist et al., 2006).

In the particular case of the bis-annulated-triazine-bipyridines, CyMe4- BTBP appeared less selective toward An(III) (SFAm/Eu > 100) than CyMe4- BTP (SFAm/Eu > 1000), certainly due to the formation of M : L2 complexes as opposed to the rigidified M : L3 complexes observed with CyMe4-BTP. In the M : L2 complexes, two tetradentate CyMe4-BTBP molecules coordinate with the extracted trivalent f cations, hence leaving an additional free access to their inner coordination spheres for an extra ligand (water molecule?). This difference in the mass action law of complex formation is somehow beneficial for the development of partitioning processes based on CyMe4- BTBP, in that the impact of the solvent hydrolytic/radiolytic degradation on the extraction performances is reduced: the apparent decrease of extraction efficiency, resulting from the destruction of the extractant, will be smaller for CyMe4-BTBP than for CyMe4-BTP (because DM <x [CyMe4-BTBP]2).

The formulation of the CyMe4-BTBP solvent was optimized, based on the iPr-BTP solvent formulation (i. e., CyMe4-BTBP and DMDOHEMA, respectively dissolved at 0.015 and 0.25 mol. L-1 in n-octanol), in order to elaborate a SANEX partitioning flowsheet (Geist et al., 2006). Although slow, the kinetics of extraction and stripping allowed a counter-current hot test to be performed in laboratory centrifuges at the ITU (Karlsruhe, Germany), implementing the highly active ‘An(III)+Ln(III)’ product coming from the TODGA hot test (see Fig. 11.15). Excellent feed decon­tamination factors for Am (7000) and Cm (1000) were obtained and the recoveries of these elements were higher than 99.9%. More than 99.9% of the lanthanides were directed to the raffinate except Gd for which 0.32% was recovered in the product (Fig. 11.16, Magnusson et al., 2009c). Nevertheless, the radiolytic stability of CyMe4-BTBP is still weaker than those of TBP, CMPO or diamides, and the possibility of recycling CyMe4- BTBP solvents has not been demonstrated yet.

11.5 Conclusions

In order to ensure the closure of future nuclear fuel cycles and minimize the long-term radiotoxicity of discarded nuclear waste, the separation of all the valuable and/or hazardous radionuclides, such as long-lived fission prod­ucts and minor actinides contained in the spent nuclear fuels will undeni­ably be necessary. The recycling of the minor actinides will probably be managed by applying a partitioning and transmutation policy. Furthermore, separating out the hazardous unrecyclable radionuclides will simplify their future route. The attractiveness of hydrometallurgical partitioning proc­esses to treat spent nuclear fuels or nuclear waste continuously, with high recovery and purification yields but only low energy inputs, largely benefits from the successes of industrial implementations such as the PUREX process.

However, the development of an efficient partitioning process is based on the design of a highly selective hydrophilic ligand or lipophilic extract­ant, that must fulfil specific requirements of solvent extraction chemistry, such as: a high affinity and selectivity toward the target element(s) to be separated, fast mass transfer kinetics, high chemical resistance, etc.

Thousands of creative ideas have emerged from radiochemists’ imagina­tions, but very few compounds have been developed up to optimized solvent formulations and tested on genuine spent fuel dissolver solutions.

Calix[4]arenes-crown-6 are pleasing examples of macrocyclic extractants, designed by functionalizing a calix[4]arene platform with a crown ether: they are perfectly suited for the selective extraction of caesium from acidic as well as basic nuclear waste. Bis-triazinyl-(bi)pyridines, although not very stable chemically, are another good example of linear nitrogen-donor ligands that are highly selective toward An(III), thanks to their particular mode of polydentate complexation. There are, however, single-step proc­esses currently in developement throughout the world that allow the sepa­ration of trivalent minor actinides directly from PUREX raffinates. They are based on the selective stripping of An(III) by hydrophilic ligands, such polyaminocarboxylic acids, in buffered solutions.