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Cecil Kelley criticality accident
The Cecil Kelley criticality accident was a criticality accident that took place on December 30, 1958, at the Los Alamos National Laboratory in Los Alamos, New Mexico, in the United States. It is one of only ten such events to ever occur outside of a nuclear reactor, though it was the third such event to take place in 1958 (the others having taken place on 16 June at the Y-12 Plant in Oak Ridge, Tennessee, and on 15 October at the Vinča Nuclear Institute in Vinča, Yugoslavia). The accident involved plutonium compounds dissolved in liquid chemical reagents, and it caused the death of one man – Cecil Kelley, a chemical operator – from severe radiation poisoning within 35 hours.
==Context of the accident==
Cecil Kelley was a 38-year-old chemical operator with 11 years of experience, more than half of those while at the Los Alamos lab where one of his duties was to operate a large (1,000 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. However, 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 (≤0.1g 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 what were later determined to be at least two "improper transfers" of plutonium waste to the tank (the sources for which were never determined or at least never publicly disclosed and about which Kelley had no 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 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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