Nuke stuff is always an interesting read because the numbers are so mind-boggling.
The SL-1, or Stationary Low-Power Reactor Number One, was a United States Army experimental nuclear power reactor in the United States that underwent a steam explosion and meltdown on January 3, 1961, killing its three operators. The direct cause was the improper withdrawal of the central control rod, responsible for absorbing neutrons in the reactor core. The event is the only reactor accident in the U.S. that resulted in immediate fatalities.
The design power was 3 megawatts (thermal), but some 4.7 megawatts tests were performed in the months prior to the accident. Operating power was 200 kilowatts electrical and 400 kilowatts thermal for space heating. During the accident the core power level reached nearly 20 GIGAWATTS (20,000 megawatts) in just four milliseconds, precipitating the steam explosion.
The spray of water and steam knocked two operators onto the floor, killing one and severely injuring another. The No. 7 shield plug from the top of the reactor vessel impaled the third man through his groin and exited his shoulder, pinning him to the ceiling.