Méghan ALIBERT1, Camille-Marie ALOIN1, Patrice SIGNORET1, Damien PRIEUR2, Nicolas CLAVIER1,4
1CEA; 2Univ Montpellier; 3HZDR; 4ICSM.
In the context of Generation IV Sodium-cooled Fast Reactors (SFR), U1-yPuyO2-x mixed oxide fuels (MOX, with 19 ≤ y ≤ 40%) are currently considered as the most suited among all fuel types. In order to ensure both safety and efficiency requirements of new generation power plants, these materials have to meet precise physico-chemical specifications, such as structural integrity, chemical homogeneity and stability, hypostoichiometry (1.94<Oxygen/Metal<1.99), thermal conductivity, etc. However, all these parameters are known to change drastically during irradiation, notably due to the formation of Fission Products (FPs) and hence their interaction with the (U,Pu)O2 matrix. The oxidation states of these FPs is smaller than the father nuclide, thus an increase of the oxygen potential inside the fuel, and hence of the fuel O/M ratio over time, is observed . Furthermore, some of the FPs are accommodated as a solid solution in the fluorite structure while other form different precipitates and volatile phases. Investigating the behaviour of the FPs inside the MOX fuel is thus of the utmost importance for the understanding of the evolution of the fuel properties during irradiation. However, considering its high radiotoxicity, the literature on spent fuel characterization is quite scarce, thus limiting our understanding of FPs behaviour. In order to overcome this issue, model materials called SIMfuel have been developed. They are obtained by manufacturing fresh nuclear fuel doped with stable FP isotopes. Traditionally, due to their minimal radiological risk, only UO2 based SIMfuel have been studied [2–5]. Due to the peculiar behaviour of the U-Pu-O system, Pu-bearing SIMfuel (Pu-SIMfuel) are necessary in order to correctly mimic the behaviour of irradiated MOX fuel. In this work, we extend the methodology of SIMfuel fabrication and characterization to Pu-SIMfuel. For the first time, we synthesized U0.74Pu0.26O2-x samples doped with several fission products theoretically soluble in the fuel matrix: Ce, La, Nd, Sr, Y, and Zr. The targeted final compositions is representative of a burnup of about 13 at.%. Afterwards, different thermal annealing treatments have been employed to investigate the impact of oxygen potential on the fission products behaviour in the specimens.
The samples have been characterized by Raman microscopy, X-Ray diffraction, EPMA, and SEM-BSD in order to study crystallographic phases, chemical homogeneity, grain size, and species segregation. Thanks to these techniques, a thorough analysis has been carried out, and deep insights have been obtained on the FPs-MOX interaction in different oxygen potentials conditions.
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 C. Le Gall, These de doctorat, Université de Grenoble, 2019.
 E. Geiger, These de doctorat, Université Paris-Saclay, 2016.
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Tuesday – 14th September 2021