Mechanism of surface uranium hydride formation during corrosion of uranium

Abstract

Uranium is widely used in the nuclear industry and it is well known that uranium hydride, UH 3 , forms when uranium is exposed to air. The associated volume change during this transition can cause the surface region to crack, compromising structural integrity. Here, hydriding regions beneath hydride surface craters are studied by secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS). Our results indicate that strain transition regions exist, which are induced by hydrogen with a certain thickness between hydride craters and the uranium bulk. The SIMS and XPS results suggest that hydrogen exists covalently with uranium and oxygen in these transition regions. A micro-scale induction period model based on the transition region and previous hydriding models is developed. Within the so-called micro-scale induction period, hydrogen diffuses and accumulates at particular sites before reaching the critical concentration required to form stoichiometric UH 3 in the transition regions.

Environmental Stability of Actinide Oxides Mapped by Integrated and Accurate First-Principles Calculations

“…It is the spontaneously formed oxide films that serve as the native corrosion barriers for the metal substrates because of the reduced surface adsorption and blocked defect permeation. , Thus, in various chemical and mechanical treatment methods (e.g., alloying and ultrasonic surface rolling) that can obviously improve the corrosion resistance of actinide metals, , the key mechanisms are always associated with the optimized microscopic states of the native oxide films. The environmental water molecules may aggravate the breakdown of oxide films through accelerating the surface electrochemical reactions on these native outer-layer oxide films and then forming loose and pyrophoric hydrides at the oxide/metal interfaces. ,,− Apart from serving as the protective films on actinide metals, actinide oxides themselves are the major chemical forms for nuclear fission fuels. The traditional fuels mainly consist of UO 2 , plus some possible oxidization byproducts (e.g., U 4 O 9 , U 3 O 7 , U 3 O 8 , and UO 3 ). ,,,− Mixed-oxide fuels consisting of UO 2 and PuO 2 powders have also been developed for the new-generation fast breeder reactors, making it possible to dispose the industrial and military Pu in a more sustainable recycling approach. , The thermodynamic oxidations of UO 2 and PuO 2 under an environment, if not precisely predicted and appropriately controlled, will bring unexpected volume expansion and stress accumulation.…”

Elucidating the Initial Steps in α-Uranium Hydriding Using First-Principles Calculations

“…These values would appear to indicate that the hydriding processes exhibit significant barriers under ambient conditions, in stark contrast to the usual spontaneous process. Previous experiments have largely investigated bulk properties as well as hydrogen-attacking regions beneath the hydride craters and have not directly probed surface effects. Such studies could overlook the potential importance of trap sites and the prevalence of hydriding at the surface and near the subsurface.…”

What is the Role of Nb on Preferential Hydriding of Double-Phased Uranium, Stabilizing γ-U, or Avoiding Hydrogen Aggregation?

“…Among all metal elements, uranium is the heaviest naturally occurring one. The corrosion of metallic uranium by hydrogen has attracted special interest due to the present emphasis on enhancing nuclear security through safe stockpiles and safe recovery of uranium. − Normally, hydrides are formed during hydriding corrosion of uranium metal, and physical disintegration happens − afterward. With a continuous supply of hydrogen in the environment, the hydriding reaction occurs in a self-propagating way. , Thermodynamically, the hydriding reaction is exothermic and the formed hydrides are pyrophoric in the atmosphere. − Due to these deleterious consequences, it is technologically important to search for possible methods to improve corrosion resistances against hydriding …”