The Hydriding Kinetics of Activated Uranium Powder under Low (near Equilibrium) Hydrogen Pressure.

Abstract

Abstract The hydriding kinetics of activated uranium powder was studied under low (near equilibrium) hydrogen pressures and between 290 and 370 °C in order to elucidate the hydriding mechanism. Activation of the samples was preformed by repeated cycles of hydriding and decomposition, which led to apparently unchanging kinetics after six cycles at 370 °C and under 470 Torr hydrogen. Scanning electron micrographs show that the activated sample has a complex structure of very thin (between 0.01 and 1 μm thick) thread-like extensions. The hydriding kinetics of the activated uranium samples consist of three stages in general: an initial accelerating stage, growing shorter as the hydrogen pressure increases, associated with hydride phase nucleation and growth. A linear reaction stage follows the initial accelerating step. This stage is associated with the advance of the hydride layer within the powder particles. Finally, a decelerating stage is observed, which may involve the main part of the reaction. It was found that the pressure dependence of the linear kinetic constant, k L , can be described by the relation k L ( P , T )= k L0 ( T )( P / P 0 −1) 2 , where P 0 is the equilibrium pressure for hydrogen absorption. An explanation is suggested for the unusual parabolic pressure dependence based on a nucleation and growth mechanism. The activation energy for the pressure-independent rate constant, k L0 ( T ), is estimated to be 5.4 kcal/g-at. H (0.24 eV), which is in fair agreement with values measured for bulk uranium samples.