Alternative Syntheses for innovaTive nUclear fabricaTion routEs – ASTUTE

Several chemical routes are envisaged for oxides synthesis:
-Oxalic precipitation is studied first because it benefits from industrial feedback. An optimized synthesis route is already under study and consists of the association of oxalate ligands and additives in order to modify actinides solubility, compounds nucleation, and crystal-growth kinetic and thus to finally access to new precursor microstructures and morphology. Precipitation will be extended to other ligands such as organic ligands.
-The heterogeneous polymerization applied to these systems allows access to the agglomerates and homogeneous powdered compounds with high reactivity. Such compounds will be useable to industrially decrease the sintering temperature.
After feedback on surrogate systems through several studies of conversion, shaping and densification with associated characterizations developed synthesis processes will be extended to actinide-based systems, for the most promising cases. The compatibility of scientific choices with the technical feasibility of an industrial application will be evaluated in each case.

During our investigations on precipitation by organic ligands, we paid special attention to the study of four particular organics ligands allowing massive precipitation of the uranyl ion. Two of them are derivatives of the hydroxybenzoquinone family, corresponding to the chloranilic acid or tetrahydroxybenzoquinone. The two other molecules of significant interest are nitrogen-bearing organic molecules. It concerns the use of dipicolinic acid, which is widely studied in the literature. The second one is the adamantane acetamide, which can be considered as a derived species of N-cyclohexyl-2-pyrrolidone.
Concerning oxalic precipitation, the morphology of tetravalent actinides oxalates is very robust and difficult to modulate but the use of organic compounds as additives during the precipitation of thorium and plutonium oxalates is able to produce several effects on the morphology. Depending upon the additives, the modulation of morphology originates from the formation of a secondary oxalate phase, the complexation of the metal which impacts the nucleation and the crystal growth, the formation of a mixed ligand complex at the nucleation stage or the modulation of agglomeration. The transposition from the thorium to the plutonium (IV) oxalate system reveals that the main trends seem to be preserved from a system to another but that there are some limitations in simulating the plutonium behavior by using thorium.

Extension of preliminary results on surrogate systems (thorium or cerium) to actinide-based systems.

Considering that this project is strongly correlated with its potential industrial applications, a special attention is devoted to patents and intellectual property. Then several patents are under progress.

The economic context and the current and future energy needs lead France today to increase its energy independence. For this purpose, the French energy policy is oriented toward a closed cycle of nuclear fuel that is characterized by the recycling of plutonium from spent fuel and its integration into a mixed oxide (U, Pu) O2 called MOX (Mixed Oxide Fuel), fuel for the 2nd and 3rd reactor generation . Thus, in addition to the preservation of natural resources and energy independence, recycling reduces the long-term radiotoxicity of ultimate radioactive wastes.
Currently, the fabrication of MOX fuel presents several disadvantages. Indeed, the use of simple oxides powder co-grinding during the manufacture process, led to the release of fine particles in fabrication units, increasing the risk of contamination or nuclear matter retention. In addition, this method based on solid state reaction and reactive sintering generates, in the final MOX, an inhomogeneous plutonium distribution which causes hot spots during irradiation in the reactor, but also the formation of insoluble phases limiting the dissolution step linked to the spent fuel reprocessing.
In order to overcome these technological challenges, (U, Pu)O2 alternative synthesis routes were internationally studied (thermal denitration ammonia precipitation, oxalic ...). Nevertheless, processes developed require additional binding steps (pre-concentration of nitric streams, valence adjustment of actinides in solution, reduction step, grinding) before precipitation and / or before forming oxide precursors.
To go beyond these limitations, this project proposes to explore new synthesis routes based on a strong expertise in the field of solution chemistry and conversion through a long-standing collaboration between CEA Marcoule and UCCS Lille. The selected wet chemistry routes will ensure better homogeneity of the oxide formed with a limited number of steps and will lead to innovative processes suited to industrial application. These methods also present the advantage of being appropriate for manufacturing based minor actinide targets for transmutation intended to be irradiated in the 4th generation RNR such as ASTRID.
Three synthetic routes will be assessed:
- The reference oxalic route will be extended to other valence states of the actinides and the addition of suitable additives will control the morphology of oxalates before thermal conversion;
- The impregnation of ion exchange resins will be studied in order to obtain microspherical precursors adapted by their flowability and their mechanical properties to the shaping step;
- The polyacrylamide gel method (external gelation) applied to these systems should allow access to the agglomerates and homogeneous powdered compounds with a high reactivity. Such compounds will be useable to industrially decrease the sintering temperature.
After a feedback on surrogate systems through several studies of conversion, shaping and densification with associated characterizations developed synthesis processes will be extended to actinide-based systems, for the most promising cases. The compatibility of scientific choices with the technical feasibility for an industrial application will be evaluated in each case.