The SL-1, or Stationary Low-Power Reactor Number One, was a United States Army experimental nuclear power reactor. It was a small test reactor at the Idaho National Engineering Laboratory, Idaho, USA.
The National Reactor Testing Station (NRTS) was approximately 40 miles (64 km) west of Idaho Falls, Idaho and was part of the American's Army Nuclear Power Program and was known as the 'Argonne Low Power Reactor' or (ALPR) during its design phase and construction.
The 1961 Accident
The SL-1 Experimental Power Station accident is also known as the 1961 Idaho National Engineering Laboratory accident and 1961 Idaho Falls atomic accident.
In 1961, the reactor underwent a major steam explosion and meltdown in January 1961, killing its 3 operators. The direct cause was the improper withdrawal by about 50 cm by a maintenance team of a single reactor control rod, the rods being responsible for absorbing neutrons in the reactor core to a meltdown. It was manually withdrawn by about 50 cm, when 40 cm would have been enough to make the reactor go critical.
The freed reactivity sky-rocket and thee resulting power surge caused the reactor power to reach 20,000MW in about 0.01 seconds, causing the plate-type atomic fuel to meltdown and mix with the water in the pressure vessel.
When it reacted with the water, the water boiled, causing a steam build-up that was so great it hit core against it hit the lid of the pressure vessel and the vessel itself jump up 3 meters.
About 1,100 curies of toxic fission products were released into the atmosphere. A few miles of land around it was quarantined for several decades afterwards.
The SL-1 Experimental Power Station accident is the only reported directly fatal reactor accident in the United States.
It originally did not officially have a INES rateing, but probably was a Level 4 INES Scale event and was accredited so in the mid 1960s.