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Stationary Low-Power Reactor Number One, also known as SL-1, initially the Argonne Low Power Reactor (ALPR), was a United States Army experimental nuclear reactor at the National Reactor Testing Station (NRTS) in Idaho about forty miles (65 km) west of Idaho Falls, now the Idaho National Laboratory.

It operated from 1958 to 1961, when an accidental explosion killed three plant operators, leading to changes in reactor design. This is the only U.S. reactor accident to have caused 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 had been performed in the months before the accident. On January 3, 1961, at 9:01 pm MST, an operator fully withdrew the central control rod, a component designed to absorb neutrons in the reactor's core. This caused the reactor to go from shut down to prompt critical. Within four milliseconds, the core power level reached nearly 20 GW. The intense heat from the nuclear reaction expanded the water inside the core, producing extreme water hammer and causing water, steam, reactor components, debris, and fuel to vent from the top of the reactor. As the water struck the top of the reactor vessel, it propelled the vessel to the ceiling of the reactor room. A supervisor who had been on top of the reactor lid was impaled by an expelled control rod shield plug and pinned to the ceiling. The accident released about 1,100 curies (41 TBq) of fission products into the atmosphere, including the isotopes of xenon, isotopes of krypton, strontium-91, and yttrium-91 detected in the tiny town of Atomic City, Idaho. This was not considered significant, due to the reactor's location in the remote high desert of Eastern Idaho. A memorial plaque for the three men was erected in 2022 at the Experimental Breeder Reactor site.

The Army Reactors Branch had written guidelines for the project and hired Argonne National Laboratory (ANL) to design, build, and test a prototype reactor plant to be called the Argonne Low Power Reactor (ALPR).

In a thermal-neutron reactor such as SL-1, neutrons are moderated (slowed down) to control the nuclear fission process and increase the likelihood of fission with U-235 fuel.

They donned their Scott Air-Paks, and arrived at the Support Facilities Building (SL-1 602, a 1-story building that connects to the reactor building and has the control room) to investigate. Coming from nearby Idaho Falls, the lead SL-1 health physicist, Ed Vallario, and Paul Duckworth, the SL-1 Operations Supervisor, arrived at SL-1 around 10:30 pm. The seven rescuers who carried McKinley and received Carnegie Hero awards from the Carnegie Hero Fund in 1962 were: Edward Vallario, SL-1 Health Physicist; Paul Duckworth, the SL-1 Operations Supervisor; Sidney Cohen, the SL-1 Test supervisor; William Rausch, SL-1 Assistant Operations Supervisor; William Gammill, the on-duty AEC Site Survey Chief; Lovell Callister, health physicist, and Delos Richards, health physics technician.

The equipment is the same as that on SL-1 except for the control rod, which is simulated by a weight to give a total movable load of 84 lb, the net weight of the SL-1 movable assembly in water. At SL-1, control rods would sometimes get stuck in the control rod channel.

Because the SL-1 accident killed all three of the military operators on site, Tucker calls it "the deadliest nuclear reactor incident in U.S. history."

"SL-1 Reactor Accident on January 3, 1961, Interim Report", May 1961. "IDO Report on the Nuclear Incident at the SL-1 Reactor on January 3, 1961, at the National Reactor Testing Station, January 1962.

Quick Facts

• Coming from nearby Idaho Falls, the lead SL-1 health physicist, Ed Vallario, and Paul Duckworth, the SL-1 Operations Supervisor, arrived at SL-1 around 10:30 pm.
• 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.
• As the water struck the top of the reactor vessel, it propelled the vessel to the ceiling of the reactor room.
• The intense heat from the nuclear reaction expanded the water inside the core, producing extreme water hammer and causing water, steam, reactor components, debris, and fuel to vent from the top of the reactor.
• On January 3, 1961, at 9:01 pm MST, an operator fully withdrew the central control rod, a component designed to absorb neutrons in the reactor's core.

Source material: Wikipedia - "SL-1". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.