Trace element migration during UF4 bomb reduction: Implications to metal fuel production, worker health and safety, and nuclear forensics

Understanding the migration of trace contaminants during the production of U metal is vital for fabricating advanced nuclear fuels, for managing worker health and safety at foundry and processing facilities, and for advancing nuclear forensic science. A common method used to produce actinide metals is the bomb reduction of suitable U precursors. Here, we report the results of a series of experiments designed to quantitatively track the trace contaminants Th, Ca, and Mg through a bomb reduction of UF4 using a vacuum induction furnace. In this series, UF4 charges were doped with elemental Th at 0 (a blank), 1, 10, 100, and 1000 ppm Th/U. Following reduction, the metal ingot products and the associated slag and crucibles were individually digested and analyzed using inductively coupled plasma-mass spectrometry. The results show that Th fractionation occurred at all concentrations but was most significant, and near quantitative, in samples starting with Th concentrations below 100 ppm. Thorium was found to incorporate into the slag and crucible in roughly equal proportions during reduction. A significant amount of U and Ca migrated into the crucible walls, each correlating positively with the quantities of Mg migrating from the MgO crucible to the U metal product.