A criticality accident occurred 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 occur outside of a nuclear reactor, though it was the third such event to take place in 1958 after events 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 killed one man, Cecil Kelley, a chemical operator, by severe radiation poisoning, within 35 hours.