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

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 (16)O(2) or (18)O(2), 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 UO(2) after ablation in 1 atm of pure O(2) and a 1% O(2)/Ar mixture, respectively. Energy dispersive X-ray spectroscopy (EDS) of particulates formed in pure O(2) suggest an O/U ratio of ~ 3.7, consistent with the vibrational spectroscopy analysis. Both am-UO(x) and UO(2) particulates convert to α-U(3)O(8) when heated. Lastly, experiments performed in (18)O(2) environments show the formation of (18)O-substituted uranium oxides; vibrational frequencies for am-U(18)O(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.