Model development and simulation of coupling between plume migration and chemical perturbations

The fate of chemicals in the environment is of importance in fields, including fuel-cycle or radioecology. Migration models describe the behaviour of radionuclides and relationships with properties: e.g. electrical charge, redox state. In addition, the retention on mineral surfaces strongly delays migration. This later mechanism may be described using various approaches and levels of complexity:
- Non-reactive approach, considering retention (Kd) without species chemistry,
- Reactive-transport, considering speciation in solution and on surfaces,
- Multi-component approaches, specifying diffusion for each compounds, e.g. NO3, EDTA, Th(IV), etc.
- Multi-species approaches, specifying the behaviour of each species for a given compound, e.g. [UO2] 2+, [CaUO2(CO3)3]2-, [Ca2UO2(CO3)3]0.
The work will focus on the development of multi-component and multi-species migration models. Models will be applied to assess more accurately the spread of chemical perturbations in natural barriers (soils, sediments). To this aim, available experimental data will be used as input data. A main objective is to quantify the differences between approaches and potential implications for radionuclide migration & corresponding mitigation strategies.

Transverse dispersion of pollutants in a river with riffle-pool systems : example of the middle Durance River

When releasing a pollutant into a river, it is important to know the downstream distance from which the pollutant will be homogeneously distributed throughout the cross-section, in order to be able to delimit the mixing zone. To estimate this, the usual method is to apply an advection-diffusion model based on the estimation of a transverse mixing coefficient. While numerous formulas exist for estimating this coefficient, most of them have only been validated in certain river configurations.
In a previous study, Lorris Gond [2022] divided a part of the Durance into successive reaches, according to the hydromorphological facies encountered, and determined the transverse mixing coefficient in each section by releasing dye. These results confirm the hypothesis of a transverse mixing process specific to the riffle-pool sequences encountered in the Middle Durance. The aim of the proposed thesis is to verify this hypothesis in rivers with riffle-pool type structures. The aim is then to determine a methodology for computing a global mixing coefficient for the structure based on a priori knowledge of the facies, so as to avoid the need for new in situ measurements each time the bed undergoes morphogenic changes. To this end, the thesis will involve field characterization of the geometry of a section of the middle Durance, dye release to quantify the transverse mixing coefficient in the river, and laboratory experiments on a small-scale riffle-pool structure.
A master internship is proposed by the team in addition to the thesis.

Kinetics and mechanisms of corium leaching

During a severe nuclear accident, significant quantities of "volatile" fission products are released during core degradation. A further significant proportion of radionuclides (RN) from degraded nuclear fuels are incorporated into solidified materials (corium and fuel debris), and their medium- to long-term stability is of major importance for accident site safety. Depending on the progress of the accident and subsequent interactions with different materials (cladding, internal structures, vessel materials and concrete), corium may have different structures and compositions. Generally speaking, it is a highly heterogeneous, multiphase material within which the distribution of radionuclides is poorly understood. In this context, the aim of this thesis is to improve our understanding of the mechanisms of corium leaching and radionuclide release into water, through parametric studies on model materials. This work, carried out with a view to increasing the complexity of the materials studied, will enable us to prioritize the influence of different parameters (temperature, solution composition, presence of oxidizing radiolytic species) on the transfer of RNs into solution. Taken together, these results will improve our understanding of the evolution of the RN inventory contained in a corium in an underwater cooling scenario, and our knowledge of the chemical durability of the different phases it contains.

Candidate profile:
Master II or engineering degree with a specialization in nuclear cycle chemistry or materials chemistry.

Professional value for the candidate:
At the end of this thesis work, the candidate will be able to enhance several technical skills (i) speciation calculations using geochemical calculation software (ii) analysis of solutions by various techniques (ICP-OES, ICP-MS, ion chromatography, UV-vis spectroscopy), (iii) structural and morphological characterizations of materials (DRX, IR and Raman spectroscopies, optical and electron microscopies, X-ray absorption). In addition to the knowledge acquired in the field of materials leaching, this work will provide the candidate with skills in scientific project management (managing different tasks involving several partners, meeting deadlines, production of deliverables, etc.) and scientific communication (oral and written). This thesis work will also enable the candidate to build up a professional network in the nuclear field, and make a name for himself/herself within the CEA and the national and international scientific community.

Predictive modeling of the alteration of nuclear waste containment glass.

In France, vitrified waste from nuclear fuel processing is to be disposed deep underground in clay geological strata. In this confined, low-porosity environment, the chemical interaction between the glass, the corrosion products of the metal containers and the site clay should control the weathering of the glass once the medium is again saturated with water. Predicting the long-term weathering of vitrified packages requires a thorough understanding of the many interrelated reaction mechanisms involved. In practice, these mechanisms are classified according to their importance on weathering kinetics, and then integrated within models adapted to the relevant time and space scales.
This thesis focuses on the geochemical modeling of glass weathering, which is carried out using a reactive transport code (CHESS/HYTEC), a thermodynamic database and a kinetic law for glasses (GRAAL2 model, derived from the GRAAL model (Frugier et al. 2018)). This model takes into account the role of the composition of the glass weathering layer on its protective character, enabling GRAAL2 to simulate variations in weathering rate as a function of glass composition and environmental conditions.
We are looking for a student with a master's degree related to modeling or geochemistry, strong Python programming skills, and an interest in understanding processes through models. At the end of these three years, the student will have gained a strong understanding of geochemical modeling (using the CHESS code), mass transfer modeling (using the Hytec code), glass alteration modeling (with the GRAAL2 model), as well as skills in numerical programming in Python. The student will also develop a deep understanding of concepts related to deviation, error, and uncertainty. He will become familiar with issues related to the transfer of pollutants in the environment and nuclear waste management. The knowledge and skills acquired are important in various fields related to material sustainability, the environment, and modeling, including the rigorous methodologies that underlie these approaches.