SL1

SL-1, short for Stationary Low-Power Reactor Number 1, was an experimental nuclear reactor operated by the United States Army at the National Reactor Testing Station in Idaho Falls, Idaho (now part of the Idaho National Laboratory). It was a small, water-cooled, water-moderated, pool-type reactor used to study reactor dynamics, heat transfer, and to train operators. The unit operated at roughly 200 kilowatts of thermal power.

On January 3, 1961, during a routine maintenance procedure, a control rod was manually withdrawn too far, provoking a prompt-critical excursion. A steam explosion ruptured the reactor vessel and destroyed portions of the facility, including the top of the building. Three technicians on site were killed, and the reactor and surrounding structures were destroyed. The site was subsequently decommissioned and the surrounding facility was rebuilt or repurposed. Investigations by the Atomic Energy Commission and the Army examined causes and contributing factors, including operator procedure and safety system design, and produced recommendations to strengthen reactor safety.

SL-1 is notable as the only fatal nuclear reactor accident in the United States. The incident highlighted risks associated with manual control of reactor cores and prompt criticality, and it contributed to improvements in safety systems, interlocks, instrumentation, training, and operating procedures that influenced later reactor design and operations at national facilities. The original facility was dismantled, and the site became part of ongoing activities at Idaho National Laboratory.