Selective recovery of uranium from THOREX feed by hollow fibre supported liquid membrane technique containing di(2-ethylhexyl) isobutyramide (D2EHIBA) as the carrier

Thorium as an abundant source of nuclear energy and challenges in separation science

Today about 440 nuclear power plants, with total installed capacity of 390 GW(e) are in operation worldwide generating around 10% of global electricity which are largely fuelled by enriched uranium oxide [Nuclear Power in the World Today. https://world-nuclear.org/information-library/current-and-future-generation/nuclear-power-in-the-world-today.aspx]. Thorium is 3–4 times more abundant than uranium and needs to be exploited by countries with limited stock of uranium. Historically, there have been several attempts to develop Th based reactors, but none has reached commercial scale. In recent years, High Temperature Reactor based on thorium has gained prominence for production of hydrogen with long term goal of complete carbon neutrality. However, unlike natural uranium, which contains ∼0.7% fissile 235U isotope, natural thorium does not contain any ‘fissile’ material and is made up exclusively of the ‘fertile’ 232Th which can be converted to ‘fissile’ 233U, thereby enlarging the fissile material resources. However, there is a need to develop robust closed fuel cycle to address to the challenges of high gamma dose due to the presence of decay products of 232U. It is necessary to gain more experience with promising THOREX process to achieve the desired recovery and D.F. of 233U from the irradiated 232Th. There is also scope to have a close look at some of the alternative extractants and emerging separation techniques for the reprocessing of spent Th based fuels. In view of the distinct advantages of non aqueous reprocessing over aqueous reprocessing, there is need to intensify the efforts to develop the former on a commercial scale. Molten Salt Breeder Reactor (MSBR) is particularly promising in this context. There is a need to investigate Th as energy amplifier under Accelerator Driven Sub-critical System (ADSS) which has potential to burn long lived radio nuclides, considered as a threat to environment over million of years.

Polymers in separation processes

Application of polymer materials as membranes and ion-exchange resins was presented with a focus on their use for the recovery of metal ions from aqueous solutions. Several membrane techniques were described including reverse osmosis, nanofiltration, ultrafiltration, diffusion and Donnan dialysis, electrodialysis and membrane extraction system (polymer inclusion and supported membranes). Moreover, the examples of using ion-exchange resins in metal recovery were presented. The possibility of modification of the resin was discussed, including hybrid system with metal cation or metal oxide immobilized on polymer matrices or solvent impregnated resin.

A ‘cold’ actinide partitioning run at 20 L scale with hollow fibre supported liquid membrane using diglycolamide extractants

‘Actinide partitioning’ studies were attempted by hollow fibre supported liquid membrane (HFSLM) technique using pressurized heavy water reactor simulated high level waste (PHWR-SHLW) as the feed. Two diglycolamide extractants for actinide partitioning, viz. 0.1 M TODGA (N,N,N´,N´-tetraoctyl diglycolamide) + 0.5 M DHOA (N,N-dihexyl octanamide) and 0.2 M T2EHDGA (N,N,N´,N´-tetra-2-ethylhexyl diglycolamide) + 30% iso-decanol in n-dodecane were used as the carrier solvent. Quantitative recovery of all trivalent actinides and lanthanides from PHWR-SHLW was achieved with both the carriers within 30 min when the feed volume was 0.5 L. On the other hand, about 18 h were necessary for a similar study carried out using a feed volume of 20 L. None of the other elements present in the PHWR-SHLW were transported, except small quantities of Sr and Mo. The product could be concentrated to two and four times by maintaining the feed to receiver phase volume ratio of 2:1 and 4:1, respectively. The transport behaviour of trivalent actinides and lanthanides by the two diglycolamide extractants were remarkably similar. The present studies revealed that diglycolamide-HFSLM system offers a promising alternative approach for ‘actinide partitioning’, where the use of organic solvent inventory could be drastically reduced. A mathematical model was developed and there was good agreement between the predicted and experimentally obtained data.