Safety studies for Pressurized Water Reactors (PWRs) require demonstrating the absence of fuel melting by maintaining its temperature below the melting threshold in all operating conditions. In this context, the melting behavior of the fuel and the solidus/liquidus temperatures are critical input data. However, the available experimental data, which are limited and outdated, show significant variability, particularly for the solidus temperature, demanding substantial safety margins. As a consequence, the current safety approach ensures the absence of fuel melting by accounting for sufficient power margins, but research aimed at improving the understanding of the underlying mechanisms could pave the way for optimization and operational margin gains.

This PhD thesis has two main objectives: clarifying the impact of fission products on solidus/liquidus temperatures; to finely characterize the microstructural evolution during this transition. To achieve this, the Ph.D. student will develop a high-temperature measurement method (>2700°C) with a thermal gradient on the CEA's laser platforms, in collaboration with the Fresnel Institute. The method must ensure sufficient precision for the unambiguous characterization of solid-liquid phase transition temperatures. Tests will use SIMFUEL samples (fresh UO2 doped to simulate irradiated fuel). A detailed characterization of the samples before and after testing (optical microscopy, SEM, EDX) will provide clear indicators of microstructural evolution. The predesign and interpretation of the experiments will rely on thermochemical calculations to consolidate modeling in support of the understanding the melting behavior of irradiated UO2.

This project, to be carried out at CEA Cadarache within the Institute for Research on Nuclear Systems for Low-Carbon Energy Production (IRESNE) in close collaboration with the Fresnel Institute in Marseille, will provide an opportunity to develop strong expertise in the behavior of composite materials, thermochemistry, as well as high-temperature instrumentation and optics, opening up a wide range of career opportunities in the energy and materials sectors.
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2/ V. D’Ambrosi et al., Nuclear Technology, 210, 285–307, 2024. DOI : 10.1080/00295450.2023.2253660.