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.