SOLID FIXATION AND AUTO-CONDITIONING OF ACTINIDE ELEMENTS COMING FROM CONTAMINATED LIQUID OUTFLOWS – AUTOMACT

Four main tasks will be carried out in this project:
1) Synthesis of functionalized silicas and choice of the most efficient ligand on the basis of studies on the sorption of Th, U elements. Following a bibliographic study, six ligands were selected: 1,2-Hydroxy Pyridinone (1,2- HOPO), Acetamide Phosphonate (Ac-Phos), Propionamide Phosphonate (Prop-Phos), Tributyl Phosphate (TBP), 2-Thenoyl Acetone (TA), Carbamoyl Methylphosphine Oxide (CMPO).
2) Sorption of a short-lived actinide (238Pu) on the mesoporous silica grafted with the best ligand chosen from task 1, in order to study its encapsulation in the material for obtaining a conditioning matrix.
3) Study of the behavior under irradiation the ligand used for the sorption (radiolysis) and the silica mesoporous material (collapse and closure of the porosity). External (ion, electron, gamma beams) and internal irradiations (238Pu doped material) will be used to reach this goal.
4) Closure of mesoporosity by different constraints (radiative, mechanical, thermal, chemical) and study of the alteration by water of the wasteform material obtained.

1) Synthesis and characterization
Based on preliminary sorption tests, SBA-15 silica was selected compared to MCM41 for the project. The SBA15 materials functionalized with the Ac-Phos, Pro-Phos, HOPO, TBP ligands were developed in sufficient quantities (of the order of one gram) to carry out sorption tests as well as radiolysis studies.
2) Sorption tests
These tests were carried out using solutions containing actinides (U, Th), or cesium (fission product).
The sorption capacity of the SBA15-diamino-NP hexaferrocyanide material is approximately 0.5 mmol Cs / g. These materials were compacted at a pressure of around 250 MPa to close their porosity and encapsulate the cesium. Preliminary cesium release tests highlight the effectiveness of this treatment.
The sorption capacity of SBA15 Ac-Phos and Pro-Phos materials towards uranium is greater than 10% by mass. This result shows that the content envisaged for carrying out the test using a short-lived actinide 15% by mass of 238Pu is entirely realistic.
3) Behavior under irradiation
Results showing the decrease in the specific surface of a mesoporous SBA15 or MCM41 silica for a dose varying between 100MGy - 2GGy by 2 MeV electrons were obtained.

1) Synthesis and characterization
The development of the synthesis of TA and CMPO ligands is in progress.
2) Sorption tests
The influence of pH on sorption is being studied.
In addition, the study of the influence of radiolysis of the cesium selective graft on the sorption of this element will also be undertaken.,
Finally, this work will lead to comparing the effectiveness of the different materials produced on the sorption of U and Th as a function of pH, in order to select the group used for carrying out the 238Pu test.
3) Behavior under irradiation
The mechanisms behind the decrease in porosity of materials irradiated with 2 MeV electrons must be clarified. Tests will be carried out this year to assess the influence of a decrease in the energy of the electrons, ie a modification of the ballistic / electronic ratio.
The study of the evolution of mesoporous silicas by (RXX, IR, TEM) following irradiation with high energy ions (GANIL) continued. An adaptation of the thermal tip model to the porous medium is underway in order to provide additional means for the interpretation of the results obtained.
4) Closure of mesoporosity
Initial tests have been carried out on the closure of mesoporosity by the application of mechanical stress on silicas having sorbed cesium. Under these conditions, the release process of cesium by leaching is greatly slowed down. Additional work is underway to more quantitatively assess the release rate of this element.

In progress

The objective of this basic research project is to develop a new strategy for the treatment of radioactive effluents based on the use of a porous functionalized support. This support would allow at the same time the separation of the RadioNucleide (RN) using a selective organic function, and their encapsulation after collapse of the porosity by a "soft" way (sol-gel, heating under stress, irradiation effect). This new concept would result in obtaining a primary wasteform matrix. Mesoporous silicas will be used as model support materials, because the nanometric size of their pores allows easy closure. Furthermore, the silica has a chemical composition close to high-level nuclear waste packaging materials (glass).

This new so-called separation / conditioning strategy would constitute a significant simplification of the number of step, compared to "traditional" processes for the treatment of radioactive effluents. Such traditionnal processes usually require a concentration step of radioactivity (evaporation, precipitation, etc.), followed by of a embedding step. It could be adapted to any type of liquid effluents, aqueous or organic, containing radionuclides emitter alpha, beta, gamma. This process could be interesting for the treatment of effluents produced in nuclear installations (STEL ...), but also for the treatment of effluents from dismantling sites because of its compactness.

In this project we will focus our study on the treatment of effluents containing actinides, which have a significant radiotoxicity linked to the alpha decays induced . This mode of disintegration could be beneficial for the collapse of the mesoporous structure , leading directly to a "primary wasteform matrix". The closure of the porosity under self-irradiation will therefore be particularly studied, with the realization of materials doped with short-life actinides (244Cm, 238Pu). Another innovative aspect of the AUTOMACT project will be the search and grafting of selective actinide ligands. For that, tributyl phosphate, which is used in the Purex process for the separation of uranium and plutonium, is a potential candidate.

The purpose of this project is therefore to propose a new all-in-one RN separation / conditioning route using specific materials allowing both decontamination operations and their simple evolution towards a primary containment matrix.