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 in the western United States at the National Reactor Testing Station (NRTS) in Idaho about forty miles (65 km) west of Idaho Falls, now the Idaho National Laboratory. On January 3, 1961, a steam explosion killed all three of its young military operators, pinning one of them to the ceiling with a reactor vessel plug. It remains the only U.S. reactor accident to cause immediate deaths.

Part of the Army Nuclear Power Program, SL-1 was a prototype for reactors 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 before 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, causing the explosion. The direct cause was the over-withdrawal of the central control rod that absorbed neutrons in the reactor's core. 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.