Experimental Investigation of Uranium Volatility during Vapor Condensation

Abstract

The predictive models that describe the fate and transport of radioactive materials in the atmosphere following a nuclear incident (explosion or reactor accident) assume that uranium-bearing particulates would attain chemical equilibrium during vapor condensation. In this study, we show that kinetically driven processes in a system of rapidly decreasing temperature can result in substantial deviations from chemical equilibrium. This can cause uranium to condense out in oxidation states (e.g., UO3 vs UO2) that have different vapor pressures, significantly affecting uranium transport. To demonstrate this, we synthesized uranium oxide nanoparticles using a flow reactor under controlled conditions of temperature, pressure, and oxygen concentration. The atomized chemical reactants passing through an inductively coupled plasma cool from ∼5000 to 1000 K within milliseconds and form nanoparticles inside a flow reactor. The ex situ analysis of particulates by transmission electron microscopy revealed 2–10 nm crystallites of fcc-UO2 or α-UO3 depending on the amount of oxygen in the system. α-UO3 is the least thermodynamically preferred polymorph of UO3. The absence of stable uranium oxides with intermediate stoichiometries (e.g., U3O8) and sensitivity of the uranium oxidation states to local redox conditions highlight the importance of in situ measurements at high temperatures. Therefore, we developed a laser-based diagnostic to detect uranium oxide particles as they are formed inside the flow reactor. Our in situ measurements allowed us to quantify the changes in the number densities of the uranium oxide nanoparticles (e.g., UO3) as a function of oxygen gas concentration. Our results indicate that uranium can prefer to be in metastable crystal forms (i.e., α-UO3) that have higher vapor pressures than the refractory form (i.e., UO2) depending on the oxygen abundance in the surrounding environment. This demonstrates that the equilibrium processes may not dominate during rapid condensation processes, and thus kinetic models are required to fully describe uranium transport subsequent to nuclear incidents.

Characterization of Uranyl Coordinated by Equatorial Oxygen: Oxo in UO3 versus Oxyl in UO3+

“…1 Uranium trioxide is a vapor species under high temperature conditions, making the gas-phase reactivity of UO3 particularly relevant to nuclear material processing and mishaps. 12,13 The T-shaped structure of UO3 has been confirmed by IR spectroscopy in inert matrices, validating the description as bent UO2 2+ coordinated by equatorial O 2-. [14][15][16][17][18] The C2v structure of UO3 contrasts with typical D3h transition metal trioxides with three equivalent oxo groups.…”

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

“…Furthermore, plume dynamic modeling of laser ablated uranium suggests higher oxides (e.g., UO 3 ) will form at the outer edge of the plume-atmosphere interface 50 , in agreement with the observed major products here. As previously mentioned, Koroglu et al 23 conclude α-UO 3 is produced when uranium nitrate and O 2 are passed through an ICP and allowed to cool. In that study, the millisecond timescale of reactions may allow for UO 3 to arrange in a crystalline fashion whereas the microsecond timescale of a laser ablation process may only allow amorphous material to form.…”

“…One approach to emulate the temperature conditions relevant to study nuclear fireball chemistry in the laboratory is with plasma discharges or high-power lasers 18 – 23 . Such studies, for instance, have shown the formation of gas-phase UO after laser ablation (LA) of uranium 24 – 29 .…”

“…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.…”