N,N-Dialkylamide-Based Solvent Systems for Actinide Separation: A Replacement Strategy for TBP in the CHALMEX Process

Licentiate thesis, 2026

In Sweden, rising electricity demand has renewed interest in nuclear expansion, and the government is reviewing options to increase capacity. However, expanding nuclear power also increases the need for improved management of spent nuclear fuel.

Used nuclear fuel contains long-lived radionuclides, including Pu and minor actinides (Np, Am, Cm). These nuclides mainly contribute to long-term radiotoxicity and decay heat. Advanced recycling strategies are needed within a closed nuclear fuel cycle. In these, recovered actinides can be integrated into fuels for Generation IV fast reactors. This improves resource use and reduces the long-term burden of the final waste.

A range of recycling processes is under development, including the Grouped Actinide EXtraction (GANEX) process. The Chalmers variation, CHALMEX, is a solvent extraction process. It is designed to recover all actinides together as a group, using a single solvent that combines an O-donor extractant with the N-donor ligand CyMe4-BTBP in the fluorinated diluent FS-13. This enables extraction across relevant actinide oxidation states in a compact flowsheet with a limited number of stages.

In the CHALMEX solvent, tri-n-butyl phosphate (TBP) is used as the O-donor extractant. Its degradation products, formed under acidic and radiolytic conditions, complicate solvent management and impair extraction performance. This work evaluates a phosphorus-free replacement, in which TBP is replaced by the monoamide N,N-dibutyl octanamide (DBOA), with degradation products expected to be more manageable during process operation.

Extractant performance was investigated using a simulated PUREX raffinate containing fission and corrosion products in the presence of masking agents. Solvent stability was assessed by hydrolytic ageing and γ-irradiation up to 300 kGy.  The results show that TBP can be replaced by DBOA without compromising key CHALMEX separation functions while providing better radiolytic stability.  In addition to higher Pu extraction with the DBOA-based solvent, this is a potential advantage for Pu-rich feed streams, such as those expected in a GANEX raffinate.

Finally, batch flowsheet tests (extraction–scrub–strip) supported optimization of the CHALMEX system for scale-up and showed that stripping from the DBOA-based solvent is feasible. Building on these findings, further work is needed to link the recycling step to fuel fabrication and to better quantify kinetics under realistic contacting conditions to support scale-up.