Study of an innovative route for the co-conversion of actinides for the back end of the nuclear fuel cycle (M/F)

Job Description

Thesis Subject

Technological Context
In France, the main source of electricity comes from nuclear power, accounting for nearly 70% of the national production. The French nuclear fleet currently consists of 56 Pressurized Water Reactors (PWRs), which allow the use of energy generated by the fission of heavy atoms when impacted by a flux of thermal neutrons. Nuclear reactors use fissile material in the form of UOx (Uranium Oxide) or MOx (Mixed Oxide of plutonium and uranium) pellets stacked in zirconium alloy cladding. Since the 1990s, France has included the processing of spent nuclear fuel in its energy policy. MOx fuel makes it possible to make use of plutonium, limiting the consumption of natural uranium while reducing the amount of waste to be stored. The Orano La Hague processing plant carries out the first stage of recycling spent fuel from nuclear reactors. The Orano Melox recycling plant handles the production of MOx fuel assemblies for pressurized water reactors (PWRs). Currently, MOx pellets are made using powder metallurgy, meaning they mix uranium oxide and plutonium oxide, press them, and then sinter them. The uniformity of the pellets is a major industrial concern. Indeed, there's a direct link between pellet uniformity and how easily they dissolve during spent fuel reprocessing. With the goal of increasing the plutonium content in fuels for future fast neutron reactors, controlling the uniformity of the oxide mixture is a big challenge.

State of the art:
Wet processes are of interest for achieving better uniformity in MOx fuels. These processes involve dissolving the actinide precursors and then carrying out precipitation, gelation, or denitration steps, which allow precise control over the composition and morphology of the powders. Among this group, only precipitation and denitration have been developed industrially (AuPuC, ADUPu, Granat, microwave denitration processes). Coprecipitation in solution involves simultaneously precipitating uranium and plutonium as oxalates, carbonates, or hydroxides from nitrate solutions, followed by calcination to obtain UO2-PuO2. Careful control of pH and temperature is crucial to avoid the formation of unwanted phases. Thermal denitration (TDN) is a continuous process where uranium and plutonium nitrates are directly converted into oxides through heat treatment. This process, used in Japan, is still at the R&D stage in France. It generates little liquid waste but produces gaseous effluents (NOₓ) that require specific treatment. TDN is suitable for large-scale production but requires strict control of the atmosphere to avoid the formation of non-stoichiometric phases, like PuO2-x. Recent innovations, like using microwaves to speed up calcination or recycling NOₓ, could improve the efficiency and sustainability of this process.

Study description
This thesis will focus more specifically on thermal denitration. Experiments will be done on model compounds based on uranium and lanthanides (Nd or Ce), used as substitutes for plutonium, which requires handling in specialized facilities. The goal will be to characterize the uniformity of the oxide mixtures formed and the impact of this uniformity on the properties of the pellets.

Your Work Environment

The research activities will be carried out at the Catalysis and Solid-State Chemistry Unit in Lille (UCCS UMR CNRS 8181) in partnership with Orano. It will draw on the expertise of the CIMEND team, which has extensive experience in the co-conversion processes of actinides and lanthanides using wet methods, gained through about ten theses done in collaboration with Orano and the CEA Marcoule.

Constraints and risks

Experimental work involving the handling of uranium
Management of uranium materials and the wastes produced during synthesis.
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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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