Cecil Kelley was a 38-year-old chemical operator with 11 years of experience, with more than half of those hours at the Los Alamos lab, where one of his duties was to operate a large (1000-liter capacity) stainless-steel mixing tank. The tank contained residual plutonium-239 remaining from other experiments and applications, along with various organic solvents and acids in an aqueous solution for the purpose of recovering it for reuse. In pure form and under normal temperature and pressure conditions, plutonium – a mostly man-made element existing in trace amounts in nature – is a solid silvery metal. It tarnishes quickly when exposed to air and readily dissolves in concentrated hydrochloric, hydroiodic, and perchloric acids, as well as others. On the day of the accident the mixing tank was supposed to contain what nuclear chemists call a "lean" concentration of dissolved plutonium (≤0.1 g of plutonium per liter of solution) in a bath of highly corrosive nitric acid and a caustic stabilized aqueous organic emulsion. However, as a result of at least two "improper transfers" of plutonium waste to the tank (the sources of which were never determined, or at least never publicly disclosed, and about which Kelley had neither reason to suspect nor ability to observe), the concentration of plutonium in the mixing tank on this particular occasion was nearly 200 times higher. Worse, it was also distributed unevenly: the upper layer of solution had especially high concentrations and contained a total of over 3 kg of plutonium, which was already close to criticality before Kelley acted. When Kelley switched on the mixer, a vortex began to form. The denser aqueous layer within the tank immediately pushed outward and upward forming a "bowl", and the less dense plutonium-rich layer swirled toward the vessel's center.