Enthalpies of formation and phase stability relations of USi, U3Si5 and U3Si2

Helium-induced damage in U3Si5 by first-principles studies

“…U 3 Si 5 , though having a uranium density (7.5 g U cm À3 ) lower than both traditional UO 2 (9.7 g U cm À3 ) and U 3 Si 2 (11.3 g U cm À3 ), 17 and presenting a high melting point (2043 K) compared with those of UO 2 (3130 K) and U 3 Si 2 (1983 K), 18 possesses a higher thermal conductivity than UO 2 in a broad temperature range from $573 K to at least 1773 K. 17 In addition, it may have better resistance toward oxidation than U 3 Si 2 by both experimental and theoretical studies. 19,20 Moreover, the UN-U 3 Si 5 composite fuel using U 3 Si 5 as the second phase has received much attention as a potential nuclear fuel material for ATF 4,21 due to its neutronic similarity to UO 2 and improved oxidation resistance compared to U 3 Si 2 . In our previous theoretical studies, 22 the silicon vacancies of b-USi 2 are determined to be more prone to form among the different types of point defects studied and U 3 Si 5 can be treated as a structural derivation of hexagonal b-USi 2 with silicon vacancy defects.…”

From arc-melted ingot to MTR fuel plate: A SEM/EBSD microstructural study of U3Si2

“…2c). This secondary phase is often reported in literature; it seems clear that this phase is stabilized by impurities, even if it is sometimes labelled USi [36,37]. As the presence of oxygen in this minor secondary phase was not definitely assessed neither in the present work nor in other publications [36,37], and knowing that this phase was also observed in the U-Si-N ternary system [38] this phase will be referred to as U 8 Si 8 X in the following of the present study, with X corresponding to light impurities (mainly O and/or other light elements).…”

Instability of U3Si2 in pressurized water media at elevated temperatures

“…This disparity suggests that the U-bearing phase interacting with the aqueous solution is not UO 2 , and is a phase characterized by significantly higher reactivity. As previously mentioned, U 3 Si 2 (as well as other U silicides) is not stable in contact with water 8,9 , and, thus, cannot be considered as a potential candidate. However, the interaction of U 3 Si 2 with water not only exposes U to oxidation but also Si; it is, therefore, logical to predict the formation of a phase involving both oxidized U and Si.…”

“…This has been indirectly confirmed by SEM images of post-experimental pellets, which suggest high porosity in the layers of U oxides formed 4 . In order to provide efficient protection of the bulk material, the formed layer must be highly dense and impermeable: high-temperature drop-calorimetric measurements of the standard enthalpy of formation of U 3 Si 2 yielded a value of −33.2 ± 3.1 kJ/mol•at.% 8,9 , which suggests that even at room temperature, any contact with water will result in the immediate oxidation of U 3 Si 2 to U oxides. Second, the hydriding of U 3 Si 2 does not explain the high stability of this material at temperatures ≤300°C and near-immediate decomposition at temperatures exceeding this limit.…”

“…) plane (2.66 Å) of this phase is similar to that of the (210) plane (2.64 Å) of USi, the (321) plane d-spacing (1.81 Å) is quite different from those of other d-spacings of USi; thus, the occurrence of USi is highly unlikely. Moreover, the formation of USi cannot be supported from the viewpoint of phase stability relations: in fact, USi is as unstable with water as U 3 Si 29,26 . It should be noted that the observed orientation of the USiO 4 /UO 2 interface (USiO 4 (112) || UO 2 (020) and USiO 4 (321) closely parallel to UO 2 (200)) may not be representative throughout the whole interface.…”