Towards an understanding of the expansive behavior of certain cement-based evaporator concentrates: experimental approach and simplified chemistry-transport-mechanics coupled modeling

In the nuclear industry, evaporation is a commonly used process to reduce the volume of low- or intermediate-level radioactive waste before its conditioning. This results in evaporator concentrates, high-salinity solutions that can contain a wide range of ionic species. These concentrates are then stabilized and solidified in a cement-based matrix, a material with many intrinsic qualities (low cost, availability, ease of implementation, good mechanical resistance, stability under irradiation, etc.). However, the acceptance of cemented waste packages in a repository depends on meeting a number of specifications. For instance, it is necessary to demonstrate the absence of expansion that could damage the matrix when stored in a humid environment.
The thesis will aim to understand the mechanisms governing the volumetric changes of cement matrices when stored underwater. The study will be conducted on synthetic waste, simulated by dissolving salts in water at the desired concentrations. It will begin with an experimental phase that will provide the input data for the building of a simplified physico-chemical model of the cement wasteforms to estimate their macroscopic mechanical behaviour as well as the main leached fluxes.
This research project is aimed at a PhD candidate wishing to develop skills in materials science and open new perspectives for the conditioning of radioactive waste. It will be carried out in collaboration with ONDRAF, the Belgian National Agency for Radioactive Waste Management, and will rely on the expertise of two CEA laboratories: the Laboratory of Formulation and Characterization of Mineral Materials (CEA Marcoule) and the Laboratory for the Study of the Behaviour of Concrete and Clays (CEA Saclay).

Multiphysical modeling of a dual-frequency induction-heated metallothermic reactor

The recycling of uranium extracted from spent fuel (reprocessed uranium or URT) is of major strategic interest as regards both closure and economics of the cycle as well as for national sovereignty. France has initiated the development of a reprocessing route for this URT, involving an entire production chain relying on SILVA laser enrichment technology.
In this context, the CEA is in charge of developing all the processes in this chain, in particular the steps involved in the conversion of uranium oxide into uranium metal required for laser enrichment. For this purpose, the “Laboratoire d'étude des technologies Numériques et des Procédés Avancés” (LNPA) is studying the transposition of the historical metallothermy process to a cold crucible type reactor. This dual-frequency inductive furnace is designed to melt a two-phase charge consisting of a fluorinated slag and a metal produced in situ by the metallothermic reaction.
Alongside a multi-year technology development program on reduced-scale inactive pilot plants, numerical modeling studies of the reactor are undertaken in order to consolidate the change in working scale and enable system parameters to be optimized before deployment of the technology in active operation on depleted uranium for validation tests. The aim of the proposed thesis work is to develop the magneto-thermo-hydraulic (MTH) multiphysical model of the cold crucible metallothermic furnace.

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