Development of a selective americium separation process using TPAEN as a
water-soluble stripping agent
Cécile Marie,1 Marie-Thérèse Duchesne,1 Peter Kaufholz,2 Andreas Wilden,2 Giuseppe Modolo,2
Alessandro Casnati,3 Nathalie Boubals, 1 Marie-Christine Charbonnel, 1 Manuel Miguirditchian1
1
Commissariat à l’Energie Atomique et aux Energies Alternatives, Nuclear Energy Division, RadioChemistry & Processes Department,
Separation Process Chemistry & Modeling Service, France.
2
Jülich Research Center GmbH, Institute of Energy and climate Research, IEK-6: Nuclear Management and Reactor Safety, Germany.
3
University of Parma, Chemistry Department, Italy.
Recycling americium from spent fuels is an important option considered for the future nuclear cycle
as americium is the main contributor to the long-term radiotoxicity and heat power of the ultimate
waste.
The separation of americium alone from a PUREX raffinate can be achieved by co-extracting
lanthanide (Ln) and actinide(III) cations in an organic phase containing TODGA diglycolamide
extractant, and then strip Am(III) with selectivity towards curium and lanthanides (by analogy with
the i-SANEX process for example). The water soluble ligand TPAEN (N,N,N',N'-tetrakis[(6carboxypyridin-2-yl)methyl]-ethylenediamine, Figure 1) was tested to selectively strip Am from a
loaded organic phase. Used in combination with TODGA in the organic phase, TPAEN shows a quite
high Cm/Am selectivity (SFCm/Am = 3.6 at 0.1M HNO3), which allows to selectively strip Am while Cm
and Ln remain extracted in the organic phase.1 The molecule can be used at pH 1 without addition of
nitrate salts.
Figure 1. Structure of TPAEN
Additional batch data were acquired to evaluate best conditions to develop a liquid-liquid separation
flowsheet with this promising TODGA/TPAEN separation system. It was observed that with
macroconcentrations of lanthanides, an increase of the TPAEN concentration leads to higher Cm/Am
selectivity (SFCm/Am up to 5 were measured). It was also demonstrated that TPAEN has a strong
complexing capacity allowing the back extraction of 241Am in macroconcentration (1 – 2 mM) even
with low ligand concentrations (around 1.5 equivalent of ligand is enough). TPAEN has a higher
affinity for light lanthanides and the selectivity between Ce (for example) and Am might become low
depending on the experimental conditions. Further batch extraction data were acquired to design a
process flowsheet like extraction isotherms to evaluate the influence of parameters such as
americium concentration, temperature, pH, ligand concentration, etc. Kinetics experiments in batch
1
Patent FR20100059749, Borrini et al. Solv. Extr. Ion Exch. 2015 accepted manuscript
contactors (centrifuges or mixer settlers) are also important to characterize times necessary to reach
equilibrium.
The impact of radiolysis on TPAEN stripping performances was also characterized after irradiating
some samples up to 200 kGy showing that the ligand is particularly sensitive. Further studies should
be implemented to better characterize degradation products and their impact on process
performances.
Besides these extraction data, spectroscopic studies are in progress to provide detailed knowledge
about the chemical species formed in the aqueous phase with TPAEN (UV-visible, TRLIFS, ESI-MS, and
EXAFS). These results show the formation of 1:1 species especially with Nd and Am.
The main drawback of this molecule is its poor solubility in water. Different batches of TPAEN were
synthesized at UNIPR showing that the degree of purity of the molecule has a strong impact on its
solubility in acidic medium and Cm/Am separation performances. Further work is on-going to try
synthesizing a more soluble derivative of TPAEN.
This work is the result of collaboration in the framework of the SACSESS European Project.