Communication Dans Un Congrès Année : 2024

Enigma of the "U$_8$Si$_8$O" phase

Résumé

Using U3Si2 as a nuclear fuel for research and test reactors requires a thorough knowledge of the U-Si phase diagram to predict reliably the in-use fuel behavior. U3Si2 seems to be a perfectly stoichiometric compound. Industrial manufacturers use slightly over-stoichiometric in silicon compositions, to avoid the presence of residual -U or U3Si that affect the irradiation behavior of this fuel. Consequently, a secondary phase with atomic ratio close to 50%U-50%Si is present in the industrial fuel plates. From the binary phase diagram [1], the thermodynamically stable compound is U34Si34.5. Nevertheless, our investigations on both laboratory and industrial samples by electron backscattered diffraction (EBSD) revealed the presence of the tetragonal U34Si34.5 as well as the orthorhombic “U8Si8O” phases in arc-melted samples [2]. The latter was first described as USi [3], but later studies suggested that its stability was achieved only by adding some oxygen in the structure [4]. The presence of light elements (mainly carbon) in metallic uranium may justify the formation of this pseudo-binary phase during the synthesis of Si-rich U3Si2. So why does this “U8Si8X” (X = C, O) phase formation depend on the cooling rate after melting? Why does it decompose upon annealing? To answer these important questions and to understand the phase equilibria in the U-Si system, experimental and theoretical studies were undertaken. On the one hand, phonon calculations predict instability of the binary “USi” at 0 K, but its stabilization if a small amount of C is added. On the other hand, the quantity of “USi” increases with Si-content in rapidly-cooled arc-melted samples, from 41 to 46 at.% Si, and then drops giving way to U34Si34.5, the latter being the only visible phase (aside U3Si2) from 48 at.% Si. It should be noted that U5Si4 was not been observed, except when C was intentionally added (U20Si16C3). A summary of results obtained from various samples we synthesized will be presented, including ab-initio phonon calculations, X-ray diffraction and EBSD analyses, and scanning and transmission electron microscopies. In particular, the hypothetical stabilization origin (kinetic vs. thermodynamic phases, epitaxy…) of the “USi” phase will be discussed, highlighting the open questions that remain around that topic.
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Dates et versions

cea-04911635 , version 1 (24-01-2025)

Identifiants

  • HAL Id : cea-04911635 , version 1

Citer

Julien Havette, Valérie Demange, Bruno Fontaine, Jean-François Halet, Ladislav Havela, et al.. Enigma of the "U$_8$Si$_8$O" phase. JDA 2024 - 53èmes Journées des Actinides, Apr 2024, Lille, France. ⟨cea-04911635⟩
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