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

Abstract

In order to model the fate and transport of particles following a nuclear explosion, there must first be an understanding of individual physical and chemical processes that affect particle formation. One interaction pertinent to fireball chemistry and resultant debris formation is that between uranium and oxygen. In this study, we use laser ablation of uranium metal in different concentrations of oxygen gas, either 16O2 or 18O2, to determine the influence of oxygen on rapidly cooling uranium. Analysis of recovered particulates using infrared absorption and Raman spectroscopies indicate that the micrometer-sized particulates are predominantly amorphous UOx (am-UOx, where 3 ≤ x ≤ 4) and UO2 after ablation in 1 atm of pure O2 and a 1% O2/Ar mixture, respectively. Energy dispersive X-ray spectroscopy (EDS) of particulates formed in pure O2 suggest an O/U ratio of ~ 3.7, consistent with the vibrational spectroscopy analysis. Both am-UOx and UO2 particulates convert to α-U3O8 when heated. Lastly, experiments performed in 18O2 environments show the formation of 18O-substituted uranium oxides; vibrational frequencies for am-U18Ox are reported for the first time. When compared to literature, this work shows that cooling timescales can affect the structural composition of uranium oxides (i.e., crystalline vs. amorphous). This indicator can be used in current models of nuclear explosions to improve our predicative capabilities of chemical speciation.

A comparative study of cerium oxides formed by pulsed laser ablation and continuous wave laser heating

“…Either a polished NaCl or silicon substrate was placed approximately 3 cm in front of the cerium surface to collect the particulates in similar fashion to Pereira et al 4 The dwell time for CW heating was 1 min on a single spot. The lasers were focused through a polished sapphire window into a sealed vacuum chamber where the cerium target was clipped to a linear translator (see Burton et al 25 for a schematic of the chamber set-up). Cerium foil (1 mm thick, 99.9% purity, Sigma-Aldrich) was mechanically polished with diamond polish followed by 0.1 µm silica bead colloidal solution.…”

“…The dwell time for CW heating was 1 min on a single spot. The lasers were focused through a polished sapphire window into a sealed vacuum chamber where the cerium target was clipped to a linear translator (see Burton et al 25 . for a schematic of the chamber set‐up).…”

Experimental and computational investigation into the hydrodynamics and chemical dynamics of laser ablation aluminum plasmas

“…3 Furthermore, LA plasmas have also been used as physicochemical surrogates for nuclear fireballs to develop reaction mechanisms and condensation pathways for nuclear fallout particle formation and debris distributions. [4][5][6] More generally, laser ablation plasmas are used as excitation sources with spectroscopic methods such as laser-induced breakdown spectroscopy (LIBS), where they have been implemented with spectrochemical material analyses across the scientific community including in archeology, 7,8 biology and medicine, 9,10 the geosciences, 11,12 forensic sciences, 13,14 nuclear nonproliferation, [15][16][17] and space exploration. 18,19 These applications exploit the physics of laser ablation, where a high-powered pulsed laser is used to vaporize (i.e., ablate) a target material to produce a luminous micro-plasma.…”

“…[21][22][23][24] Depending on thermodynamic and atmospheric conditions, molecular formation may follow different pathways and kinetics, further complicating the spectroscopic analysis. 4,25 In an attempt to mitigate or eliminate chemical reactions, measurements are performed in inert atmospheres in a controlled-laboratory setting. 26 However, this form of environmental control is not always realistic in practice, such as for in-field measurements of hazardous materials at standoff distances 27 or the chemical characterization of Martian rocks.…”

New insights into the structural transition from UO2+x to U3O7 by quantitative Raman spectroscopy

“…temperature, gaseous environment and moisture. 1,2 Oxidation of the fuel matrix, UO 2 , is assumed to proceed by first forming either U 4 O 9 or U 3 O 7 , phases that are denser than the initial UO 2 as a consequence of a net contraction. 3 Further oxidation to U 3 O 8 implies a density decrease of about 23% 4 and 36% DV/V o swelling compared to original UO 2 .…”