Фазовая диаграмма системы H-U

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H-U (Hydrogen-Uranium) P. Chiotti The assessed phase diagram for the U-H system is well established and is based on [Hansen]. It represents the equilibrium phases and compositions under the equilibrium dissociation pressure of the hydride produced by the reaction: UH3- x = U (solid solution) + (3 -xH2 )/2. The hydrogen pressure increases rapidly with temperature. [67Che1] and [67Che2] indicate that the boundaries for the two-phase (gU) - UH3 - x region merge to a critical point between 800 and 830 C. [75Lak] reported a definite plateau in the pressure isotherm at 1050 C, indicating the existence of the U-rich and UH3 - x phases at this temperature. The isotherms corresponding to the a <259> b and b <259> g transitions for U occur at the same temperatures. The same is true for the eutectic temperature of 1120 C. The increase in hydrogen content due to higher hydrogen pressures would be expected to lower the isotherms to some extent. However, as shown in the assessed diagram, the decrease in temperature with increase in hydrogen content is quite small; that is, the phase boundaries are nearly horizontal. Consequently, there would be only a small and perhaps insignificant decrease. The solubility of H in (U) as estimated from the pressure isotherms indicate a H/U ratio of 0.256 to 0.051 (0.108 to 0.022 wt.% H) in the temperature range 846 to 673 C, respectively [67Che1]. There are two polymorphs of UH3, both of which are cubic. The a form is produced by reacting hydrogen with finely divided uranium powder at 25 C or lower temperatures. However, the a polymorph is metastable and transforms to bUH3 on heating. 67Che1: J. Chevallier, P. Desr‚, and J. Spitz, J. Nucl. Mater., 23, 289-301 ( 1967). 67Che2: J. Chevallier, J. Spitz, and P.L. Blum, C.R. Hebd. S‚ances Acad. Sci., 264, 93-96 (1967). 75Lak: J.F. Lakner, Lawrence Livermore Laboratory, Berkeley, CA, Report UCRL 51865 (1975). Published in Bull. Alloy Phase Diagrams, 1(2), Jun 1980. Complete evaluation contains 4 figures, 4 tables, and 24 references. 1