[19] Some of the more important criteria included: A classified 1956 preliminary design study, using BORAX-III as a basis, calculated the total construction cost for the prototype reactor to be $228,789.

[20] The plant was turned over to the Army for training and operating experience in December 1958 after extensive testing, with Combustion Engineering Incorporated (CEI) acting as the lead contractor beginning February 5, 1959.

On November 11, 1960, CEI installed cadmium sheets (also a poison) "to several tee slot positions to increase reactor shutdown margin.

In four milliseconds, the heat generated by the resulting enormous power excursion caused fuel inside the core to melt and to explosively vaporize.

[21] With six holes on the top of the reactor vessel, high-pressure water and steam sprayed the entire room with radioactive debris from the damaged core.

A later investigation concluded that the 26,000-pound (12,000 kg) (or thirteen short tons) vessel had jumped 9 feet 1 inch (2.77 m), parts of it striking the ceiling of the reactor building before settling back into its original location,[12][26][21] and depositing insulation and gravel on the operating floor.

[12] This was consistent with the analysis of the SL-1 Board of Investigation[30] and with the results of the autopsies, which suggested that Byrnes and Legg died instantly, while McKinley showed signs of diffuse bleeding within his scalp, indicating he survived about two hours before succumbing to his wounds.

[27] The autopsy identified each man by name,[27] concluded that all three men died of physical trauma,[12][27] and, due to their peculiar injuries, established the most likely locations of each when the reactor exploded.

Fast neutron activation had occurred to various materials in the room, indicating a nuclear power excursion unlike a properly operating reactor.

The delayed production of a fraction of the neutrons enables reactor power changes to be controlled on a time scale amenable to humans and machinery.

[33] That was too fast for the heat from the fuel to permeate the aluminum cladding and boil enough water to fully stop the power growth in all parts of the core via negative moderator temperature and void feedback.

The response team of six firemen (Ken Dearden, assistant chief; Mel Hess, lieutenant; Bob Archer; Carl Johnson; Egon Lamprecht; Gerald Stuart; Vern Conlon) arrived nine minutes later, expecting another false alarm.

[35] Finding a higher-scale ion chamber detector, the pair reached the top of the stairs to look inside the reactor room for the three missing men.

Coming from nearby Idaho Falls, the lead SL-1 health physicist, Ed Vallario, and Paul Duckworth, the SL-1 Operations Supervisor, arrived at SL-1 around 10:30 pm.

[35] On January 9, in relays of two at a time, a team of ten men, allowed no more than 65 seconds exposure each, used sharp hooks on the end of long poles to pull Legg's body free of the No.

[12][26][35] Radioactive copper 64Cu from a cigarette lighter screw on McKinley and a brass watch band buckle from Byrnes both proved that the reactor had indeed gone prompt critical.

In an unusual finding for an autopsy, hair samples taken from the head and pubis of the victims were analyzed to suggest their relative positions during the reactor excursion and to estimate the number of fissions using Phosphorus-32 activity.

[27] Before these discoveries of neutron-activated elements in the men's belongings and in their hair, scientists had doubted that a nuclear excursion had occurred, believing the reactor was inherently safe.

[40][41] One of the required maintenance procedures called for Rod 9 to be manually withdrawn about four inches (10 cm) in order to attach it to the automated control mechanism from which it had been disconnected.

Post-accident calculations, as well as examination of scratches on Rod 9, estimate that it had actually been withdrawn about twenty inches (51 cm), causing the reactor to go prompt critical and triggering the steam explosion.

[12] When testing the theory that Rod 9 was rapidly withdrawn manually, three men took part in timed trials and their efforts were compared to the energy of the nuclear excursion that had occurred.

During congressional hearings in June 1961, the SL-1 Project Manager, W. B. Allred, admitted that the lack of supervision by CEI of SL-1 plant operation on an "around-the-clock basis" was because the Atomic Energy Commission (AEC) had rejected the idea "for budget reasons.

The fuel plates showed signs of catastrophic destruction leaving voids, but "no appreciable amount of glazed molten material was recovered or observed."

[21] The manual overhead crane above the operating floor was used to move numerous heavy objects weighing up to 19,600 pounds (8,900 kg) for them to be dumped out onto the ground outside.

With the operating room floor relatively clean and radiation fields manageable, the manual overhead crane was employed to do a trial lift of the reactor vessel.

The reactor core minus the fuel, along with the other components sent to the Hot Shop for study, was eventually disposed of at the Radioactive Waste Management Complex.

During a survey of surface soil in June 1994, "hot spots," areas of higher radioactivity, were found within the burial ground with activities ranging from 0.1 to 50 milliroentgen (mR)/hour.

Prompt Critical is the title of a 2012 short film, viewable on YouTube, written and directed by James Lawrence Sicard, dramatizing the events surrounding the SL-1 accident.

[56] Another author, Todd Tucker, studied the accident and published a book detailing the historical aspects of nuclear reactor programs of the U.S. military branches.

Tucker used the Freedom of Information Act to obtain reports, including autopsies of the victims, writing in detail how each person died and how parts of their bodies were severed, analyzed, and buried as radioactive waste.