## 2 Materials and Methods
### Full Countercurrent Test and Radiochemical Analyses  
The ALSEP process demonstration was run in the 1 cm annular miniature centrifugal contact setup, installed in the laboratories of Forschungszentrum Julich, Germany. The tested flowsheet is shown in Figure 7. Because only 16 contactors were available in the testing rig, the test had to be split into two parts and run on consecutive days. On the first day, the extraction and scrubbing stages were performed and the loaded solvent was collected and stored overnight. On the next day, the collected loaded solvent was used as the organic feed (as shown in Figure 7), and the Ln re-extraction, MA stripping, and Ln stripping sections of the flowsheet were run. On both days the experiments were run until steady state was reached. During the first day, the test was continued \begin{table}
\begin{tabular}{l c c} \hline \multicolumn{3}{c}{**Recovery in the Loaded Solvent after Scrub 2**} \\ \hline  & **Steady State** & **Average** \\ \hline Sm & \textgreater{}99.99\% & \textgreater{}99.99\% \\ Eu & \textgreater{}99.99\% & \textgreater{}99.99\% \\ Cd & \textgreater{}99.99\% & \textgreater{}99.98\% \\ Mo & \textless{}0.01\% & \textless{}0.01\% \\ Fe & 0.64\% & 0.16\% \\ Zr & \textless{}0.01\% & 0.01\% \\ Ru & \textless{}0.01\% & \textless{}0.01\% \\ Rb & \textless{}0.01\% & \textless{}0.01\% \\ Sr & \textless{}0.01\% & \textless{}0.01\% \\ Cs & \textless{}0.01\% & \textless{}0.01\% \\ Y & 47.00\% & 78.00\% \\ Rh & \textless{}0.01\% & \textless{}0.01\% \\ Pd & \textless{}0.01\% & \textless{}0.01\% \\ Sn & \textless{}0.01\% & \textless{}0.01\% \\ Te & \textless{}0.01\% & \textless{}0.01\% \\ \hline \end{tabular}
\end{table} 
Table 3: _Cont._  
Figure 7: Flowsheet of the Actinide Lanthanide Separation Process (ALSEP) process demonstration. until enough loaded solvent for the second day was collected. The outlets of the centrifugal contactor battery (raffinate, loaded solvent, An product, Ln product, and spent solvent) were monitored by sampling and quick gamma measurements of \({}^{241}\)Am and \({}^{152}\)Eu. These initial gamma spectroscopy results were used to assess the approach to steady state during the test. After stopping the tests (stopping pumps and contactors), the contents of the mixing chambers were quickly transferred to test tubes and centrifuged to achieve quantitative phase separation. Both phases were samples and analyzed by gamma and alpha spectroscopy and ICP-MS analysis, and the pH was measured or the aqueous phase was titrated, depending on the HNO\({}_{3}\) concentration. The stage profiles for Am and Eu, as well as Am and Cm are shown in Figure 8 and Figure S1, respectively. Americium and Cm showed comparable profiles. Figure S2 shows the corresponding distribution ratios. The results for several elements analyzed by different analytical techniques were compared to each other and found to be in very good agreement (e.g., Am profiles were determined by gamma, alpha, and ICP-MS measurements). The Ln stripping with TEDGA worked very well and four stages were sufficient for a good back-extraction from the solvent. The only contaminants found in the spent solvent were Fe (12.1%), Sn (0.8%), and Pd (0.1%). As discussed below, the Fe content found in the spent solvent probably stems from a slight corrosion of the stainless-steel contactor housing, as some Cr (typical stainless-steel additive) was also found in those samples (Cr was not added with the feed). Evaluation of the ICP-MS data show that the non-Ln fission products were mostly routed to the raffinate. Palladium and Zr masking in the feed with CDTA was effective, and the Mo scrubbing also worked well, as predicted by AMUSE calculations. The Mo and Zr stage profiles are shown in Figure 10. The heavier lanthanides (Pr, Nd, Sm, Eu, and Gd) and Y were mainly routed to the Ln product, whereas the lighter lanthanides (La, Ce) were also partly found in the raffinate, as expected based on previous batch distribution measurements and flowsheet calculations.