Mechanism of surface uranium hydride formation during corrosion of uranium

Hefei Ji,Peng Shi,Xiaolin Wang,Xianglin Chen,Qifa Pan,Xiandong Meng,Haoxi Wu,Dingzhou Cai

doi:10.1038/s41529-019-0073-6

Abstract

Uranium is widely used in the nuclear industry and it is well known that uranium hydride, UH3, 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 UH3 in the transition regions.

Highlights

Uranium hydride (UH3), a radioactive intermediate-level waste material with pyrophoricity when exposed to air, is an unwanted corrosion product of metallic uranium (U)
The corresponding results after approximately 5 μm are shown in Fig. 1b, c, in which the black features are hydride craters that were not fully abraded
It can be seen that, each black crater is surrounded by a large number of line-type or platelike gray transition regions, which lie between the black craters

Summary

Introduction

Uranium hydride (UH3), a radioactive intermediate-level waste material with pyrophoricity when exposed to air, is an unwanted corrosion product of metallic uranium (U). Because of the difference in density between the metallic U (19.1 g/cm3) and hydride (10.95 g/cm3), the regional attack of hydrogen results in robust volume expansion underneath the oxide at the metal-oxide interface. The reaction between U and H are widely accepted to be governed by four stages: the induction period, nucleation and growth period, bulk reaction, and termination period.[1,2] The first two are thought to be the key periods, during which H aggregates and hydride sites nucleate and grow. Hydrogen penetrates the surface passivation layer of oxide gathers at the oxide-metal interface. For the nucleation and growth period, once the hydride nucleated and subsequently grew, because of the volume expansion the surface oxide layer will be broken through, hydrogen keeps on penetrating into and reacting with the bulk material. The hydride sites always grow along the surface and into the bulk with the progression of the reaction front to form discrete hydride craters

Methods

Results

Conclusion