진학프로석·박사 채용 접수 플랫폼

Scientific Background
During a severe nuclear reactor accident, the progressive degradation of the core can lead to the formation of a complex material called corium, resulting from interactions between the nuclear fuel, structural materials, and, in certain scenarios, the concrete of the reactor base. These interactions lead to the formation of a multiphase mixture whose composition depends heavily on the accident's thermochemical history. Among the phases likely to form are solid solutions of uranium- and zirconium-based oxides, such as (U,Zr) O2, solid solutions and complex oxide compounds incorporating elements from the fuel, structural materials, concrete, and fission products, as well as complex silicates such as chernobylite (U,Zr)SiO4, identified in materials from the Chernobyl accident. Knowledge of these phases, their stability, and their evolution upon contact with water is essential for evaluating the short- and long-term behavior of the corium.
Furthermore, regardless of the severe accident scenario considered, water injection or spraying systems are generally deployed to cool the corium, limit its spread, and preserve the integrity of the containment vessel. These actions lead to significant interactions between the corium—whether in a liquid state or in the process of solidification—and water. In this context, understanding the behavior of corium under leaching conditions is a major challenge for assessing its long-term stability as well as for predicting potential releases of radioactive elements.
Recent advances in the study of solid/solution interfaces have demonstrated the value of approaches that combine macroscopic dissolution measurements (release kinetics, dissolution congruence) with operando monitoring of evolving interfaces using advanced characterization techniques. Despite these advances, the mechanisms controlling the alteration of coriums in contact with water remain largely unknown. The phenomena of dissolution, reprecipitation, and the formation of secondary phases remain insufficiently described due to the lack of reliable kinetic and thermodynamic data. The acquisition of such data is therefore essential to improve our understanding of the mechanisms governing corium leaching, assess the stability of existing or newly formed phases, and enhance the predictive capability of models used to analyze severe accidents and their long-term consequences.

Objectives of the Research
The objective of this postdoctoral research is to improve our understanding of the behavior of nuclear coriums, from their formation during a severe accident through to their long-term evolution in contact with water. In the first part, the study aims to determine the influence of fission products on the formation, stability, and thermodynamic properties of the main phases constituting coriums, notably solid oxide solutions of the (U,Zr)O2 type and silicates of the (Zr,U)SiO4 type. Particular attention will be paid to the effects of aliovalent elements representative of fission products (lanthanides, barium, strontium, cesium), in order to fill existing gaps in the thermodynamic description of complex systems containing uranium, zirconium, silicon, and these elements. The acquired data will contribute to the improvement of thermodynamic databases used in severe accident simulation tools, particularly for the systems UO2–SiO2–ZrO2–Ln2O3 (Ln = Ce, Nd, Gd, Yb); U–Zr–Ln–O; UO2–SiO2–ZrO2–BaO(SrO); and U–Zr–Ba(Sr)–O.
In a second phase, the project will focus on studying the behavior of these phases under leaching conditions in environmental settings, in order to identify the mechanisms controlling their alteration and long-term stability. To this end, a multi-scale approach will be implemented, combining the study of dissolution kinetics at the macroscopic scale with in situ monitoring of solid/solution interfaces at the microscopic scale. This approach will enable the acquisition of kinetic and thermodynamic data regarding the dissolution of the phases constituting the coriums, the possible formation of secondary phases, and their influence on elemental releases. The impact of certain radiolytic species on these alteration processes may also be studied to assess their role in the evolution of interfaces and the stability of newly formed phases. All of this work aims to provide the knowledge necessary for the development of more robust predictive models for evaluating the behavior of coriums under accident and post-accident conditions.

The candidate will participate in a research project studying the long-term behavior of nuclear coriums, as well as the influence of fission products on the stability and weathering of their primary phases. The candidate's work will focus on two complementary areas.

First, the candidate will synthesize and characterize model materials representing the phases present in coriums. These materials will include (U,Zr)O2 -type oxide solid solutions and (U,Zr)SiO4 -type silicates that incorporate various fission products, such as lanthanides, barium, strontium, and cesium. The candidate will then characterize these materials structurally, microstructurally, and physicochemically to study the mechanisms of fission product incorporation, the stability regions of the formed phases, and their thermodynamic behavior.

Second, the candidate will study the behavior of these materials in the presence of water by conducting leaching tests under various physicochemical conditions. The candidate will participate in acquiring kinetic data on the dissolution of phases, determining the source terms associated with the studied materials, and evaluating their solubility. The candidate will implement a multi-scale approach, combining macroscopic measurements with operando monitoring of solid/solution interfaces. This will be done using advanced characterization techniques such as environmental scanning electron microscopy (ESEM), atomic force microscopy (AFM), Raman spectroscopy, and grazing-incidence X-ray diffraction.

Particular attention will be paid to identifying and characterizing secondary phases that are likely to form during weathering processes, as well as their influence on dissolution and reprecipitation mechanisms.

Finally, the candidate will participate in analyzing and interpreting experimental results and comparing them with existing thermodynamic models.

This work is part of the PEPR SCIAM (Scientific Basis Data for Nuclear Fission) program, specifically WP2 – Basic Data Related to Coriums. This work will be conducted within a research framework closely linked to nuclear safety issues and will benefit from national and international collaborations, notably with the ESRF and Washington State University. The host laboratory will be the ICSM in Marcoule (UMR 5257 CNRS, CEA, University of Montpellier, ENSCM). This latter is located in the city of Chusclan (Gard).