Characterization of slag and metal from uranium bomb reduction: Morphology, speciation, and the search for thorium

Metallic uranium is an important material for many applications, especially nuclear energy for low enriched metallic fuel forms and new reactor concepts. Due to the its high electropositivity, production pathways to the metallic form are limited. One of the historically most common synthesis routes involves the heating of a uranium (IV) halide, in this study UF₄, with a highly electropositive metal like calcium. This synthesis is referred to as “bomb reduction” due to the temperatures and pressures released by the resulting exothermic reaction. This synthesis route is important for production, but it has also been shown to separate a decay product, thorium, from the parent uranium. This fractionation could be important for purification, but also presents the opportunity for radiochronometric dating of the reduction date. Unfortunately, little characterization has been performed on the products of this reaction. The present study performed bomb reduction on ∼10 g of thorium-doped (2000 ppm) UF₄ followed by scanning electron microscopy and energy dispersive spectroscopy. This characterization revealed morphological features of the metal product and slag, the latter of which displayed a wide range of features that indicates a complex reaction in which many variables are involved. Initial characterizations also identified thorium-rich particles, which were extracted and analyzed via transmission electron microscopy and atom probe tomography. These characterizations identified a new thorium-bearing phase, Al_9-xFe_7+xTh₂Si_<1, and also indicated thorium fractionation from uranium via at least four mechanistic pathways.