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 UO_x (am-UO_x, 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-UO_x 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-U¹⁸O_x 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.