The SL-1 Nuclear Incident

Daniel Berrios
February 16, 2017

Submitted as coursework for PH241, Stanford University, Winter 2017


Fig. 1: The SL-1 Nuclear Facility prior to the accident, with the reactor held in the cylindrical building and administrative/support buildings surrounding. (Source: Wikimedia Commons)

In the heart of winter 1961, in a remote area of the desert forty miles west of Idaho Falls, an Army- commissioned nuclear project went horribly wrong when the three on-duty operators were killed by a sudden steam explosion and subsequent reactor meltdown. The reactor was formally known as the Stationary Low- Power Reactor Number One, or informally as SL-1, and was built by Argonne National Laboratory. While the residual effects of the accident were limited due to the remote location and relatively small scale of the reactor, it remains today the only such incident to ever result in immediate deaths. [1]


With the Cold War in full swing in mid-century America, the United States military had a strong need to execute a plan referred to as the "Distant Early Warning system, in which they would set up several radar stations along the Arctic Circle to watch for evidence of a Soviet invasion. These stations had the important responsibility of relaying notice of an attack to centralized command posts, which would give the broader military crucial extra hours or even just minutes for preparation. However, residing in such remote locations meant that there was a very high cost to refueling, and consequently a high demand for a low-power, long-lasting energy source. The solution that was proposed to combat this problem was construction of simple, low-power on-site nuclear reactors that could run for long periods of time with little interference and lessen the burden in resupplying the outposts. Once plans for such a facility had been designed, a preliminary prototype was constructed before wider adoption to determine the viability of the plan. This was the origin of the SL-1 reactor. [1]

Stationary Low-Power Reactor Number One

The facility, depicted in Fig. 1 before the accident, was constructed from July 1957 to July 1958, and became operational just over two years before the incident, on October 24, 1958. It was cared after by a crew of servicemen that had been specially trained in its operation and necessary safety procedures. It was located in a cylindrical steel building 38 feet in diameter with a height of 48 feet, and was not designed to contain high levels of pressure as would have been the case had the facility been located closer to a large population. [1] The reactor itself was designed for 3 Mega-Watt-thermal (MWt) input energy, While the core was designed for greater capacity, at the time of the incident it had 40 fuel elements, and was controlled by 5 cross-shaped rods, each made of 1.5mm thick cadmium, coated with 2.0mm of aluminum, and weighing 84 lbs. Each had an effective length of 32 inches. [2] The plant was designed to use 93.2% highly enriched uranium fuel, and operated with natural circulation, using light water as a coolant, which operated at 300 lbs. per square inch. Key in this design was the fact that the under-capacity fuel loading gave the central rod much more reactivity. [2]

America's First Nuclear Accident

Fig. 2: An image of the reactor core that was used to warn of the dangers of nuclear energy. (Source: Wikimedia Commons)

On December 21, 1960, the reactor was shut down for scheduled maintenance, and the primary crew of operators left for the holidays. In the meantime, a maintenance crew of three operators took over at the facility. On January 3 at 9:01pm, as the reactor was being prepared to come back online, procedures required that the central control rod be manually withdrawn by a matter of inches. Specifically, the safe limit of extension was to be reached at 4.2 inches. However, the rod was instead extended approximately 20 inches. Since the control rods regulate the rate of the fission reaction by absorbing excess neutrons released by the U-235 atoms (with 2.4 released per atom on average) and maintaining a steady rate of neutrons allowed to cause new fission events (i.e. an effective neutron multiplication factor, or k-effective, of 1), the removal of the central rod past its safe limit caused the reactor to achieve prompt criticality. [2] Consequently, only four milliseconds later, enough heat was generated in the surrounding water to cause it to vaporize. This released an extremely concentrated amount of steam up from the reactor, causing the entire housing (weighing 26,000 lbs.) to jump 9.1 feet vertically, and for control rods and various other pieces of the assembly to be propelled upwards with great enough force to become lodged into the ceiling. The blast immediately knocked Army Specialists John A. Byrnes (27) and Richard Leroy McKinley (22) to the floor, killing Byrnes (the reactor operator) and severely injuring McKinley (a trainee). The third man, Navy Seabee Construction Electrician First Class Richard C. Legg (26, and the shift supervisor), who had been standing atop the vessel, was himself impaled and pinned to the ceiling. While nearby crews were alerted to the emergency through an alarm system, and bravely exposed themselves to dangerous levels of radiation in an effort to help the operators, all three eventually passed, with McKinley found alive but later succumbing to his injuries. [2,3]


The events of that night sparked several long-lasting consequences. Besides ending the usage of the design in all further reactors, and leading to a widespread overhaul of safety procedures in all future designs to prevent the possibility of prompt criticality, the incident caught the world by surprise. Cleanup of the event exposed hundreds of people to dangerous levels of radiation despite the remote location. [3] In doing so, it took what was hailed as a revolutionary technology that was to provide a seemingly unlimited, stable power source at little cost, and turned it into an issue of public concern and skepticism that continues to plague the field to this day, as demonstrated by Fig. 2. How that skepticism plays out in the future as we continue to advance in nuclear technology remains to be seen.

© Daniel Berrios. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] S. M. Stacy, Proving the Principle: A History of the Idaho National Engineering and Environmental Laboratory, 1949-1999, U.S. Department of Energy, Idaho Field Office, DOE/ID-10799, (U.S. Government Printing Office, 2000) [220 MB].

[2] "SL-1 Reactor Accident on January 3, 1961: Interim Report," Combustion Engineering, U.S. Department of Energy, Idaho Operations Office, IDO-19300, 15 May 61.

[3] "Final Report of the SL-1 Recovery Operation," General Electric Co, Atomic Energy Commission Report, U.S. Department of Energy, Idaho Operations Office, IDO-19311, 27 Jun 62.