Stationary Low-Power Reactor Number One, also known as SL-1 or the Argonne Low Power Reactor (ALPR), was a United States Army experimental nuclear reactor located at the National Reactor Testing Station (NRTS) at Idaho Falls, Idaho, United States. At 9:01pm, on the night of January 3, 1961, SL-1 underwent a steam explosion and meltdown, killing its three operators. The direct cause was the improper withdrawal of the central control rod, responsible for absorbing neutrons in the reactor's core. The event is the only reactor accident in U.S. history to have resulted in immediate fatalities. The accident released about 80 curies (3.0 TBq) of iodine-131, which was not considered significant due to its location in the remote high desert of eastern Idaho. About 1,100 curies (41 TBq) of fission products were released into the atmosphere.
The facility housing SL-1, located approximately 40 miles (65 km) west of Idaho Falls, was part of the Army Nuclear Power Program. The reactor was intended to provide electrical power and heat for small, remote military facilities, such as radar sites near the Arctic Circle, and those in the DEW Line. The design power was 3 MW (thermal), but some 4.7 MW tests were performed in the months prior to the accident. Operating power was 200 kW electrical and 400 kW thermal for space heating. During the accident, the core power level reached nearly 20 GW in just four milliseconds, precipitating the steam explosion.