On January 3, 1961, the reactor was being prepared for restart after a shutdown of eleven days over the holidays. Maintenance procedures were in progress, which required the main central control rod to be manually withdrawn a few inches to reconnect it to its drive mechanism; at 9:01 p.m. this rod was suddenly withdrawn too far, causing SL-1 to go prompt critical instantly. In four milliseconds, the heat generated by the resulting enormous power surge caused water surrounding the core to begin to explosively vaporize. The water vapor caused a pressure wave to strike the top of the reactor vessel, causing water and steam to spray from the top of the vessel. This extreme form of water hammer propelled control rods, shield plugs, and the entire reactor vessel upwards. A later investigation concluded that the 26,000 pounds (12,000 kg) vessel had jumped 9 feet 1 inch (2.77 m) and the upper control rod drive mechanisms had struck the ceiling of the reactor building.[7][8] The spray of water and steam knocked two of the men onto the floor, killing one and severely injuring another. One of the shield plugs on top of the reactor vessel impaled the third man through his groin and exited his shoulder, pinning him to the ceiling.[7] The victims were Army Specialists John A. Byrnes (age 27) and Richard Leroy McKinley (age 22), and Navy Electrician's Mate Richard C. Legg (age 26).[9] It was later established that Byrnes had lifted the rod and caused the excursion, Legg was standing on top of the reactor vessel and was impaled and pinned to the ceiling, and McKinley, who stood nearby, was later found alive by rescuers.[7] All three men succumbed to injuries from physical trauma (not the radiation).[7]