The u[formula omitted]si[formula omitted]-H system

Abstract

U3Si2 is of interest to the nuclear industry as a candidate fuel material due to its high uranium density and high thermal conductivity. However, it has been observed to react with hydrogen, resulting in material decrepitation. As a result, it is important to understand the thermodynamics of the U3Si2-H system. In this study, the thermodynamics of the hydrogen absorption reaction of U3Si2 were determined experimentally using Sievert’s gas absorption and related to crystallographic evolution with hydrogen content using X-ray diffraction. Experimentally-determined thermodynamic parameters were compared with results from density functional theory modeling. Results from this study were also compared with those determined in previous work. Sievert’s gas absorption results were used to develop the pressure-composition-temperature (PCT) curves of the U3Si2-H system. It was found that the hydride phase exhibited a maximum stoichiometry between U3Si2H1.8 and U3Si2H2. The two-phase region for hydride formation from U3Si2 exhibited a miscibility gap with a critical temperature between 623 and 673 K, as calculated from the PCT curves. Analysis of the PCT curves also showed that both the enthalpy and entropy of the hydrogen absorption reaction increased with hydrogen content but were lower than the values for uranium trihydride formation from uranium metal. The enthalpy of reaction for hydrogen absorption was calculated to range between -86.9 and -94.8 kJ mol−1, while the entropy of reaction was calculated to range between 101.9 and 138.8 J mol−1 K−1. DFT modeling of the thermoydnamic stability of the U3Si2 hydride phases yielded a decomposition temperature of U3Si2H2 of approximately 600 K, which was consistent with the experimental results. Similarly, the DFT-calculated enthalpy and entropy of reaction to form U3Si2H1.5 were determined to be -106.5kJ mol−1 and 121.8J mol−1 K−1, respectively, which were both in close agreement with the experimentally-determined values.