Doctorant (M/F) - Influence des produits de fission sur la stabilité des phases silicatées et oxydes au sein de coriums

Job Description

Thesis Subject

Effect of Fission Products on the Stability of Silicate and Oxide Phases in Coriums

Scientific Background
During a severe accident in a nuclear reactor, the loss of cooling systems can cause the UO₂ fuel and surrounding structural materials to overheat significantly. At high temperatures, the fuel interacts with the zirconium alloy cladding and then with the metal structures of the reactor vessel, leading to the formation of a complex mixture of solid and liquid phases known as in-vessel corium.
Subsequently, in the event of a reactor vessel breach, the corium may interact with the concrete foundation (SiO2, CaO, Al2O3, …). This interaction profoundly alters its chemical composition through the incorporation of elements such as silicon, calcium, and aluminum, leading to the formation of ex-vessel corium. Among the phases likely to form is the mixed silicate (U,Zr)SiO4, known as tchernobylite, which was identified in the lava formed during the Chernobyl accident.
Understanding the phases likely to form under these conditions is essential for improving the modeling of severe accidents. Simulation codes used in the nuclear field rely on thermodynamic calculations fed by databases describing the properties of the materials present in coriums. Despite recent progress regarding the thermodynamic description of the UO2–ZrO2–SiO2 and U-Zr-O systems, many uncertainties remain regarding the role played by fission products in the stability of the various phases.

Objectives of the PhD work
The main objective of this thesis is to study the influence of fission products on the formation and stability of phases present in nuclear coriums. The work will focus specifically on the synthesis and characterization of solid solutions of the following types:
• (Zr,U)SiO4 : PFs ;
• (U,Zr)O2 : PFs ;
where PF represents various fission products (lanthanides, barium, strontium, and cesium). This study will help fill the gap in thermodynamic data regarding the systems UO2–SiO2–ZrO2–Ln2O3 (Ln = Ce, Nd, Gd, Yb); U–Zr–Ln–O; UO2–SiO2–ZrO2–BaO (or SrO) and U–Zr–Ba (or Sr)–O. The data produced will ultimately feed into the thermodynamic databases used in severe accident simulation tools.

Research Program
The PhD student will develop synthesis protocols tailored to the various materials under study:
• Hydrothermal syntheses for silicate phases;
• Hydroxide precipitation followed by heat treatments for oxides;
• Optimization of preparation and purification conditions for the resulting solid solutions.
The synthesized materials will then be characterized using a wide range of physicochemical and structural techniques. Experiments on large-scale instruments will notably be conducted on the ROBL beamline at the ESRF (Grenoble), enabling the implementation of advanced techniques such as:
• High-resolution X-ray diffraction (HR-XRD);
• EXAFS and XANES spectroscopies;
• HERFD-XANES.
The thermal stability of the materials will be studied using controlled annealing and thermogravimetric analysis (TGA). In parallel, the thermodynamic stability of the solid solutions will be determined based on:
• solubility experiments to determine the free energies of formation;
• potential dissolution calorimetry measurements conducted in collaboration with Washington State University.
All of this work will establish a coherent set of fundamental thermodynamic data (ΔfH°, ΔfS°, and ΔfG°) for the phases under study.

Desired Profile
The candidate must hold (or be in the process of obtaining) a Master's degree or an engineering degree with a specialization in:
• Nuclear materials science;
• Separation chemistry and recycling;
• Radiochemistry;
Knowledge of thermodynamics, materials characterization, and X-ray diffraction is desirable. As the project involves experiments in a regulated laboratory, knowledge of radiation protection is desirable. The candidate must demonstrate a strong interest in experimental work (especially in glove boxes).

Your Work Environment

This thesis is part of the PEPR SCIAM (Upstream Sciences for Nuclear Fission) program, specifically WP2 – Basic Data Related to Coriums. It will provide the doctoral student with multidisciplinary training at the interface between solid-state chemistry, thermodynamics of nuclear materials, and advanced characterization. It will be conducted within a research context closely linked to nuclear safety issues and will benefit from national and international collaborations, notably with the ESRF and Washington State University. This work will be mainly performed at ICSM – Marcoule (UMR 5257 CNRS, CEA, Univ Montpellier, ENSCM). This latter is located in the city of Chusclan (Gard).

Constraints and risks

Since the project involves experiments with radioactive uranium-bearing materials, all work will be conducted in a regulated laboratory. Knowledge of radiation protection is strongly appreciated.
Training in Risk Prevention Level 1 will be provided to obtain authorization to work in a regulated area.
Candidates must demonstrate a strong interest in experimental work, particularly in a glove box.

Compensation and benefits

Compensation

2300 € gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

CNRS

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