Gas Phase Chemical Evolution of Uranium, Aluminum, and Iron Oxides

Abstract

We use a recently developed plasma-flow reactor to experimentally investigate the formation of oxide nanoparticles from gas phase metal atoms during oxidation, homogeneous nucleation, condensation, and agglomeration processes. Gas phase uranium, aluminum, and iron atoms were cooled from 5000 K to 1000 K over short-time scales (∆t < 30 ms) at atmospheric pressures in the presence of excess oxygen. In-situ emission spectroscopy is used to measure the variation in monoxide/atomic emission intensity ratios as a function of temperature and oxygen fugacity. Condensed oxide nanoparticles are collected inside the reactor for ex-situ analyses using scanning and transmission electron microscopy (SEM, TEM) to determine their structural compositions and sizes. A chemical kinetics model is also developed to describe the gas phase reactions of iron and aluminum metals. The resulting sizes and forms of the crystalline nanoparticles (FeO-wustite, eta-Al2O3, UO2, and alpha-UO3) depend on the thermodynamic properties, kinetically-limited gas phase chemical reactions, and local redox conditions. This work shows the nucleation and growth of metal oxide particles in rapidly-cooling gas is closely coupled to the kinetically-controlled chemical pathways for vapor-phase oxide formation.

Description of nucleation, growth and coagulation processes in the modeling of debris formation after a nuclear burst

“…In addition to nucleation, condensation, and agglomeration, it is necessary to have a good representation of the chemistry of the fireball, especially in the gas phase. At early time, chemical kinetics can be expected to be fast because of the high temperatures, but there is still substantial uncertainty about the reaction rates and species involved (Koroglu et al [2018], Finko et al [2017]). In the past, oxides were assumed to be the dominant species (Miller [1960]), but recently researchers have suggested the possibility of reducing conditions (Giuli et al [2010], Cassata et al [2014]).…”

The effect of oxygen concentration on the speciation of laser ablated uranium

“…Following a nuclear event, fallout particulates are formed from and affected by gas-phase chemical reactions and micro-physical processes involving hot nuclear material and the environment 1 , 2 . Understanding reactions between device materials, such as uranium, and the surrounding environment under these conditions is essential to improve nuclear debris formation models developed in the early days of nuclear testing 3 – 5 .…”

“…The interaction of uranium and oxygen is one system pertinent to fireball chemistry and resultant debris formation 1 , 8 – 10 . The uranium–oxygen system is complicated to study as there are numerous stable stoichiometric phases (with O/U ranging from 2 to 4) with multiple polymorphs for a given stoichiometry (e.g., amorphous vs crystalline UO 3 ), the possibility of sub- and super stoichiometric phases, and the ability for phases to interconvert depending on the conditions (e.g., temperature, hydration, and gaseous environment) 11 – 14 .…”

“…In addition to temperature, it is also important to understand how uranium reacts in atmospheres with both abundant and scarce amounts of oxygen as interactions with the environment (e.g., from shockwaves) are likely to affect local oxygen availability and distribution for a nuclear fireball. Recent studies by Koroglu et al 1 , 23 have used an inductively coupled plasma (ICP) to investigate the evolution of uranium oxidation as a function of temperature from 5000 to 1000 K. These authors found gas-phase UO forms shortly after the torch but was no longer detected below 2000 K, which was attributed to the formation of higher-oxides. Particulates recovered at 1000 K were determined to be fcc-UO 2 or α-UO 3 , depending on the input oxygen concentration, demonstrating that oxygen availability will influence the speciation, and thus chemical volatility, of uranium in such environments.…”

“…As this approach can be performed inside a sealed vacuum chamber, isotopic reagents (e.g., 18 O 2 ) can be more readily used. Furthermore, although the ICP apparatus used by Koroglu et al 1 , 23 can study vapor phase cooling over milliseconds, this timescale is still orders of magnitude away from the cooling of a nuclear fireball. As it is difficult to achieve high temperature cooling over seconds, data collected at microseconds (LA) and milliseconds (ICP) could be used to extrapolate to longer cooling timescales.…”