SL-1 reactor accident
The SL-1 reactor accident, which occurred on January 3, 1961, at the Idaho National Engineering Laboratory, is considered the first fatal nuclear reactor incident in the United States. Designed to provide power for military operations, the small Stationary Low-Power Reactor Number One (SL-1) was operated by the U.S. Army. During a routine procedure to reconnect the control rod mechanism, the crew mistakenly withdrew the control rod further than intended, leading to an unprecedented power surge and a steam explosion that killed all three operators present.
The reactor's design lacked a containment structure, a significant factor that influenced the incident's outcome. Although some radioactive iodine was released, the remote location of the reactor meant that no external population was affected. The SL-1 accident was classified as a level 4 incident on the International Nuclear Event Scale, denoting its local consequences. Investigations into the accident highlighted deficiencies in the reactor's design and prompted the establishment of stricter safety criteria for reactor operations. Despite this severe event, the incident also demonstrated the inherent safety of water-moderated reactor designs, as the reactor successfully ceased operation following the explosion, averting a more significant nuclear meltdown.
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SL-1 reactor accident
THE EVENT: Destruction of the SL-1 nuclear reactor, a test reactor at the Idaho National Engineering Laboratory
DATE: January 3, 1961
The accident involving the SL-1 reactor was the first nuclear reactor accident in the United States that resulted in fatalities. As a result of this incident, new design criteria were developed to ensure that a complete reactor shutdown could be accomplished safely.
In February, 1954, the U.S. secretary of defense authorized the U.S. Army to develop small nuclear reactors to provide electrical power and heat for military facilities in remote locations, such as Distant Early Warning (DEW) Line radar sites in Alaska and Greenland. A prototype, the Stationary Low-Power Reactor Number One (SL-1), began operation on August 11, 1958, at the National Reactor Testing Station in the desert west of Idaho Falls, Idaho. This small reactor was designed to operate at a maximum power of 3 megawatts, to produce electricity and steam heat for the crews and equipment at DEW Line radar sites or other military installations.
Combustion Engineering, the contractor that operated SL-1 for the Army, shut the reactor down on December 23, 1960, for the Christmas holiday. As part of the shutdown procedure, the control rod, which determined the rate of reaction in the core, was fully inserted and disconnected from its drive mechanism. A three-man crew arrived at the reactor site on the evening of January 3, 1961, to reconnect the drive mechanism for the main control rod in preparation for restarting the reactor. This required them to move the control rod out about 10 centimeters (4 inches). For reasons that were never determined, the crew instead moved the rod out about 51 centimeters (20 inches), resulting in an extreme power surge, with the reactor producing an estimated 20,000 megawatts. Within a few milliseconds the core overheated and the resulting steam explosion propelled the control rod upward and disrupted the reactor core. All three of the crew, the only people near the reactor, were killed.
Because the Army was intending to operate reactors like SL-1 in remote locations, the reactor had been built to be small and lightweight; it had no containment structure like the ones on commercial power reactors. The reactor was housed in a steel cylinder with walls 0.63 centimeter (0.25 inch) thick, which trapped most of the released in the explosion. Some radioactive iodine escaped, and downwind indicated that its radioactivity reached fifty times background levels near the plant. Because of the remote location of the SL-1 reactor no humans outside the test site were adversely affected. According to the International Nuclear Event Scale, a categorization system used by the International Atomic Energy Agency (IAEA) since 1990, the SL-1 reactor accident was classified as a level 4 event on a scale of 0 to 7. A level 4 event is said to be an “accident with local consequences.”
Radioactivity inside the reactor structure was so high that each member of the rescue team, wearing whole-body protective clothing, was permitted to enter the site only once for a period of one minute to assess the situation and recover the victims. Because of its high radioactivity the reactor was buried on-site.
An investigation of the accident found that the direct cause was the improper withdrawal of the main control rod, but the investigators also concluded that the design of the SL-1 reactor was poor, because a single rod controlled 80 percent of the activity in the core. As a result, new design criteria were put in place requiring that a complete reactor shutdown could be accomplished with the most reactive control rod in its full out position. Nonetheless, the SL-1 accident demonstrated the general safety of the water-moderated reactor design, since even this severe accident resulted in dispersal of the nuclear core, shutting down the reaction, rather than a more serious nuclear meltdown.
Bibliography
"International Nuclear and Radiological Event Scale (INES)." International Atomic Energy Agency, 2024, www.iaea.org/resources/databases/international-nuclear-and-radiological-event-scale. Accessed 23 July 2024.
McKeown, William. Idaho Falls: The Untold Story of America’s First Nuclear Accident. Toronto: ECW Press, 2003.
Stacy, Susan M. Proving the Principle: A History of the Idaho National Engineering and Environmental Laboratory, 1949-1999. Washington, D.C.: Government Printing Office, 2000.
Touran, Nick. " The SL-1 Disaster." What Is Nuclear, 29 Sept. 2022, whatisnuclear.com/safety-minutes/sl1-disaster.html. Accessed 23 July 2024.
Tucker, Todd. Atomic America: How a Deadly Explosion and a Feared Admiral Changed the Course of Nuclear History. New York: Free Press, 2009.