At 9:01pm, alarms sounded at the fire stations and security headquarters of the U.S. National Reactor Testing Station where the reactor was located. Investigation found two operators dead (third died later), and detected high radiation levels in the building. 

A careful examination of the remains of the core and the vessel concluded that the control rod was manually withdrawn by about 50 centimeters (40 centimeters would have been enough to make the reactor critical), largely increasing the reactivity. The resulting power surge caused the reactor power to reach 20,000MW in about .01 seconds, causing the plate-type fuel to melt. The molten fuel interacted with the water in the vessel, producing an explosive formation of steam that caused the water above the core to rise with such force that when it hit the lid of the pressure vessel, the vessel itself rose 3 meters in the air before dropping back down.

The SL-1 accident was the first fatal nuclear accident in the United States.  The men killed in the incident were two Army Specialists, John Byrnes, age 25 and Richard McKinley, age 22, and Richard Legg, a 25 year old Navy Electricians Mate.  Richard McKinley was interred in Arlington National Cemetery.  John Byrnes and Richard Legg were buried in their hometowns in New York and Michigan.
 

See the entire SL-1 Reactor Accident documentary!  Over 90 minutes of video on DVD! These two films from the 1960s include actual footage of the recovery effort, views from both inside and outside the reactor building, close up reactor images and rare views inside the former NRTS facilities.  Click here to visit our store for this DVD and more!

As quoted by Arlington National Cemetary Records:

"3 January 1961: A reactor explosion (attributed by a Nuclear Regulatory Commission source to sabotage) at the National Reactor Testing Station in Idaho Falls, Idaho, killed one navy technician and two army technicians, and released radioactivity "largely confined" (words of John A. McCone, Director of the Atomic Energy Commission) to the reactor building. The three men were killed as they moved fuel rods in a "routine" preparation for the reactor start-up. One technician was blown to the ceiling of the containment dome and impaled on a control rod. His body remained there until it was taken down six days later. The men were so heavily exposed to radiation that their hands had to be buried separately with other radioactive waste, and their bodies were interred in lead coffins."

From the Arlington National Cemetary Records:

HEADQUARTERS 
MILITARY DISTRICT OF WASHINGTON 
WASHINGTON 28, D.C. 
In Reply Refer To 
AMHRC 31 January 1961 

SUBJECT: Internment of Radioactive Remains 

TO: Superintendent 
Arlington National Cemetery 
Arlington 11, Virginia 

1. Radioactive remains of SP4 Richard L. McKinley were interred at Arlington National Cemetery on 25 January 1961. 

2. It is desired that the following remark be placed onthe permanent record, DA Form 2122, Record ofInternment: 

"Victim of nuclear accident. Body is contaminated with long-life radio-active isotopes. Under no circumstances will the body be moved from this location without prior approval of the Atomic Energy Commission in consultation with this headquarters." 

FOR THE COMMANDER: 

Leon S. Monroe, II 
2d Lt. AGC
Assistant Adjutant General

See also: 

Idaho National Engineering Laboratory