The different roles of Pu-oxide overlayers in the hydrogenation of Pu-metal: An ab initio molecular dynamics study based on van der Waals density functional (vdW-DF)+U

Abstract

Based on the van der Waals density functional theory (vdW-DFT)+U scheme, we carry out the ab initio molecular dynamics (AIMD) study of the interaction dynamics for H 2 impingement against the stoichiometric PuO 2 (111), the reduced PuO 2 (111), and the stoichiometric α-Pu 2 O 3 (111) surfaces. The hydrogen molecular physisorption states, which can not be captured by pure DFT+U method, are obtained by employing the vdW-DFT+U scheme. We show that except for the weak physisorption, PuO 2 (111) surfaces are so difficult of access that almost all of the H 2 molecules will bounce back to the vacuum when their initial kinetic energies are not sufficient. Although the dissociative adsorption of H 2 on PuO 2 (111) surfaces is found to be very exothermic, the collision-induced dissociation barriers of H 2 are calculated to be as high as 3.2 eV and 2.0 eV for stoichiometric and reduced PuO 2 surfaces, respectively. Unlike PuO 2 , our AIMD study directly reveals that the hydrogen molecules can penetrate into α-Pu 2 O 3 (111) surface and diffuse easily due to the 25% native O vacancies located along the 111 diagonals of α-Pu 2 O 3 matrix. By examining the temperature effect and the internal vibrational excitations of H 2 , we provide a detailed insight into the interaction dynamics of H 2 in α-Pu 2 O 3 . The optimum pathways for hydrogen penetration and diffusion, the corresponding energy barriers (1.0 eV and 0.53 eV, respectively) and rate constants are systematically calculated. Overall, our study fairly reveals the different interaction mechanisms between H 2 and Pu-oxide surfaces, which have strong implications to the interpretation of experimental observations.

Plutonium Oxidation States in Complex Molecular Solids

“…A series of calculation tests on PuO 2 show that Pu OS qm is largely independent of U values (3–5 eV), pseudopotentials (LDA, PBE, PW91, or hybrid DFT), spin–orbit coupling interaction, relaxation scheme, magnetic order, and much more, implying the insensitivity of calculation parameters on OS qm . The conclusion holds for the calculation test on van der Waals dispersion correction despite that the weak interaction might play a role in stabilizing Pu molecular solids …”

Phase Diagram and Bonding States of Pu–H Binary Compounds at High Pressures

“…Plutonium (Pu) is a key material in the nuclear industry for both energy production and nuclear explosives. − During the extended storage, transport, and safe handling of Pu metal, the reactive Pu sample inevitably suffers environment-dependent chemical corrosions in experimental setups, among which the surface hydrogenation is problematic due to the Pu-hydride product being able to catalyze violently exothermic oxidation reactions with Pu and even induce pyrophoricity, , which is significantly associated with the structure particularity of plutonium hydrides. However, it is very difficult to directly characterize the crystal structure in experiments since Pu-hydride products are extremely reactive and potentially more dangerous than metallic Pu .…”

A First-Principles Study of Hydrogen Diffusivity and Dissociation on δ-Pu (100) and (111) Surfaces

“…Surface hydrogenation in particular is problematic, where the Pu-hydride product can catalyze violently exothermic oxidation reactions and even induce pyrophoricity. 5 Despite these pressing issues, relatively little is known about the chemical mechanism for corrosion, and experimental studies would greatly benefit from detailed atomistic knowledge of the chemical reactivity of hydrogen on δ-Pu surfaces.…”