Polyoxometallate Materials for Actinide Retreatment – POMAR

The work program is divided into four tasks that will be conducted over a period of 60 months. The overall coordination of the project can be found in task one. A scientific meeting between all participants to the different phases of the proposal is regularly done. The three other tasks are devoted to the scientific realization of the project. They turn around the synthesis and solid-state characterization of the materials built upon the association between polyoxometalates and tetravalent actinides (task 2), solution stability studies on the formed materials (task 3) and finally the application of the developed strategy for the separation of tetravalent cations in AnIV/LnIII mixtures, combined with solid-state and solution characterizations of the obtained phases (task 4). To fulfill the goals of the project, several researchers with complementary skills have been gathered in the laboratory. They have expertise in line with the different aspects of the project, particularly around coordination behavior and crystallochemistry of actinides elements and, structural characterizations of the envisioned materials. A Ph.D. student is working on the project.

Until now, the project mainly focuses on the first and second point of the project with the characterization of several molecular species containing tetravalent radionuclides (uranium and thorium) or tetravalent surrogate like the cerium cation. We concentrated our work using two approaches:
1- The use of trivacant polyanionic moieties to act as a ligand.
2- The use of cryptant molecules derived from trivacant polyanion able to bind several mononuclear cationic elements.

The first part has given several interesting results. A group of POM compounds incorporating tetravalent actinides or surrogates were characterized in solution and the stability of some compounds was also investigated by the use of solution techniques like 183W, 29 Si NMR, or electronic spectroscopies.
More explanations are available on the scientific report.

The cryptant molecule described in the scientific report seems to be an interesting precursor as its architecture is maintained during the complexation of the actinides. This observation allows us to think that it could be a “clever” choice compound for the third part of the project dealing with a mixture of AnIV/LnIII. We hope to be able to study the selectivity of the molecule for the tetravalent cation towards the trivalent one and eventually obtain if there is no selectivity, the capture of both oxidation states to form mixed-valence compounds. This kind of system could have interesting properties in luminescence of magnetism and could nevertheless be interesting for transposition with trivalent highly radioactive actinides.
We are also pursuing the work by the use of trivacant or polyvacant POM moieties.

At this stage, four publication in peer reviewed journals about this work have been accepted :

S. Duval, S. Béghin, C. Falaise, X. Trivelli, P. Rabu, T. Loiseau, Inorg. Chem., 2015, 54, 8271-8280.
S. Duval, S. Sobanska, P. Roussel, T. Loiseau, Dalton trans., 2015, 44, 19772-19776.
S. Duval, X. Trivelli, P. Roussel, T. Loiseau, Eur. J. Inorg. Chem., 2016, 5373-5379.
S. Duval, P. Roussel, T. Loiseau, Inorg. Chem. Commun., 2017, 52-54.

The increasing energy requirement needed by our societies is constantly answered by industries and scientists trying to find more efficient processes. This ascertainment suited well with the nuclear industry for which efforts are invested to enhance the retreatment processes of spent nuclear fuels issued from power plants. The PUREX process currently used by AREVA and taking place in the industrial unit of la Hague has its own limits regarding the absence of methods available for the recycling of minor actinides and lanthanides in view of potential strategic material economy. Another drawback of this process is the high quantity of effluent created during the extraction and purification operations.
Relatively recently, several new processes started to appear. For instance, the most studied DIAMEX-SANEX process could take place directly with the refined solutions issues from the PUREX processes and have been shown to help in the separation of the minor actinides from solutions for better treatment/recycling of the fuels. This process is based on the coordination behavior of nitro- or oxo-donors organic ligands to bind specifically lanthanides or actinides. However, the quantity of effluent is still important in these processes. As a part of this new process (after the first DIAMEX step), polyoxometallates thanks to their properties to stabilize tetravalent cations were considered in the experimental SESAME process for the separation, by selective oxidation, of the americium and the curium. More work has still to be done on these processes and heteropolyoxometalates could effectively play an important role due to their strongly coordinative properties and selectivity towards the tetravalent oxidation states of actinides cations. Despite the potential interest in this area, no recent papers have been published on the subject.
The available literature information on the topic concerns only the use of monovacant polyoxometallic species derived from the [P2W18O62]6- Dawson archetype. Herein, we propose here the use of poly-vacant species as chelate for the stabilization and, possible extraction of tetravalent actinides cations. The envisioned deliverables for this project are compounds containing stable tetravalent actinides isolated from aqueous solutions and, information regarding a possible transposition for AnIV/AnIII mixtures and best POMs precursors to fulfill this role. The first step will be focused on the reactivity of the POMs species with actinides and the unambiguous characterization of the obtained species in solid state and in solution. It will then allow the study of more complex solutions of surrogate AnIV/LnIII mixtures to accumulate data regarding the mixture of actinide cations.
Alongside with the potential industrial applications of this project in the nuclear area, some of the materials expected here could have an impact on the luminescent properties of these kinds of compounds. Actually, little is known in this domain about these compounds, which could possibly possess mixed valence clusters of actinides and lanthanides cations.