PhD Defense | Filip Kolesar | Development of a new extraction system for selective americium separation from spent nuclear fuels

Development of a new extraction system for selective americium separation from spent nuclear fuels

Although nuclear energy is widely used and seen as a source of green energy, a major drawback is the generation of long-lived spent nuclear fuel. The only practical solution to this spent fuel is its disposal in deep underground repositories, where the waste is isolated from the environment. However, the amount of spent fuel that can be stored is limited by its long-term heat generation, and without any reprocessing of the waste, the repositories need to be able to contain the waste over time frames of hundreds of thousands of years before its radiotoxicity reaches the level of natural uranium. Both the heat generation and the decay times can be minimized by performing partitioning and transmutation, a strategy wherein the long-lived actinides are separated from the nuclear waste, and “burnt” in nuclear reactors, either as repurposed fuel or as targets. One such partitioning process which is already performed on an industrial scale is the PUREX process, which separates uranium and plutonium from spent nuclear fuel, eliminating a majority of the long-term activity. After uranium and plutonium, the long-term heat load and radiotoxicity are dominated by the minor actinides (neptunium, americium, and curium), but so far no scalable extraction process has been developed yet to eliminate these. 

The goal of minor actinide partitioning processes is primarily a separation of the actinides from the fission products present in spent nuclear fuel, and from the lanthanides in particular. Secondly, recent strategies have also focused on separating americium from curium, as the latter is present in smaller quantities, has lower half-lives, and exhibits difficult to shield neutron radiation, complicating all downstream processes. However, a great degree of chemical similarity is found between the minor actinides and the lanthanides, as well as between americium and curium, making the design of separation processes challenging. 

This work continues on the previously developed AmSel process. This is a two-step process: first a co-extraction of Ln(III), Am(III), and Cm(III) from the PUREX HAR is performed with N,N,N’,N’-tetraoctyldiglycolamide (TODGA) dissolved in an aliphatic diluent (5 vol% 1-octanol in Exxsol D80 diluent). In a second step, Am(III) is selectively stripped with a fresh aqueous phase containing 3,3’,3”,3”’-([2,2’-bipyridine] 6,6’-diylbis(1,2,4-triazine-3,5,6-triyl))tetrabenzenesulfonate (SO₃-Ph-BTBP) dissolved in a nitric acid solution. This work aims to reexamine all aspects of the process, and improve upon two drawbacks in particular. First, this work aims to eliminate the presence of sulfur from the system, creating an extraction process that is compliant with the CHON-principle, i.e. containing only carbon, hydrogen, oxygen, or nitrogen atoms. This is to avoid contamination of the transmutation targets with elements such as sulfur or phosphorus. Secondly, this work aims to improve the separation between americium and curium. 

In a first stage, this work attempted to replace the diluent of the AmSel system with the ionic liquid Aliquat-336 nitrate, but the high viscosity resulted in slow kinetics making the original diluent preferable. Secondly, this work aimed to replace TODGA with a novel unsymmetrical diglycolamide. Initially, N,N-diisopropyl-N’,N’-didodecyldiglycolamide (iPDdDGA) was tested, but this extractant showed excessive extraction strength making recovery of americium difficult. The screening of 5 new unsymmetrical diglycolamides revealed N,N-dipentyl-N’,N’-didodecyldiglycolamide to be the most promising candidate for further development. Finally, this new extractant was combined with two CHON-compliant hydrophilic complexants: PrOH-BPTD and BTrzPhen-tetraol. Both complexants, when combined with PnDdDGA, yielded viable extraction systems, containing only CHON elements, and having a separation factor comparable to the original AmSel process. Furthermore, demonstration on a simulated highly active raffinate revealed no major interference from other fission products. This work therefore represents an important step forward in the implementation of a sustainable nuclear fuel cycle. 

Promoters:

• Koen Binnemans (KULEUVEN)
• Thomas Cardinaels (KULEUVEN)

SCK CEN mentors:

• Karen Van Hecke
• Ken Verguts