SL-1 Reactor Explosion, United States (1961)

The SL-1 reactor explosion killed all three operators instantly, making it the only fatal U.S. nuclear reactor accident, with a 100% lethality rate far higher than the Tokaimura accident's 2 deaths out of 667 exposed. On January 3, 1961, a manual control rod withdrawal at a small Army reactor in Idaho caused an instantaneous criticality excursion that released 2.6 megacuries of fission products, impaling one operator on the ceiling by a 2,000-pound shield plug. Unlike Goiânia's open acknowledgment, SL-1 was initially shrouded in Cold War secrecy. Despite its small scale—the reactor produced only 3 megawatts of thermal power—the explosion was 50% more violent than Windscale's fire per unit of energy, halting operations forever and serving as a stark lesson in human error.