TEM observations on fission gas bubbles at high burn up in irradiated UO₂ fuel.

Mathieu Angleraud1

C. Sabathier1, C. Onofri1, D. Reyes Vasquez1, D. Drouan1, A. Chartier2, B. Warot-Fonrose3

1CEA, DES, IRESNE, DEC, Cadarache 13108 Saint-Paul-Lez-Durance, France;
2CEA, DES, ISAS, DPC, Saclay 91191 Gif-sur-Yvette, France;
3CNRS, CEMES, 31055 Toulouse, France.


During its use in a nuclear reactor, the uranium dioxide fuel undergoes significant structural and physico-chemical changes under the effect of irradiation. Indeed, the nuclear fission reaction leads to the formation of fission products involving the creation of defects and new chemical elements in the matrix. Fission products in the gaseous state need a particular attention. These gaseous fission products (FG), such as Xenon and Krypton, have a very low solubility in the UO2 matrix and therefore precipitate into bubbles that will grow from nano to micrometric-scale. Acquiring high-fidelity FG bubbles characteristics at the nano-scale should help improving predictivity of PWR fuel performance codes.
To complete existing literature and update fuel behaviour codes, the present work proposes to study the characteristics (size and density) of the bubble population in UO2 fuel irradiated in a power reactor. Thanks to advanced characterization technologies, the ultimate goal is to obtain a more comprehensive database of bubble characteristics as a function of burn up and irradiation temperature. To achieve this goal, Transmission Electronic Microscopy (TEM) characterizations were done in the LECA-STAR hot laboratory at the CEA Cadarache in France. The sample studied comes from a rod irradiated five annual cycles up to an average burn up of 56.7 GWd/tU in the Gravelines 5 reactor between August 1988 and August 1995. To analyse the microstructure from the center to the edge of the pellet, TEM samples were collected at different radial positions using a FIB-SEM.
Thanks to TEM observations coupled with EDS mapping and the determination of the local thickness of the sample using EELS technique, this study allows us to obtain a fine characterization of fission gas bubbles, as well as their local environment such as dislocations, metallic precipitates or grain boundaries, for different radial positions.

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Thursday – 16th September 2021
Mathieu Angleraud