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 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 of iodine-131, which was not considered significant, due to its location in the remote high desert of Eastern Idaho. About of fission products were released into the atmosphere.
Design and operations
From 1954 to 1955, the U.S. Army evaluated their need for nuclear reactor plants that would be operable in remote regions of the Arctic. The reactors were to replace diesel generators and boilers that provided electricity and space heating for the Army's radar stations. The Army Reactors Branch wrote the 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). Some of the more important criteria included: All components able to be transported by air All components limited to packages measuring and weighing Use of standard components Minimal on-site construction Simplicity and reliability Adaptable to the Arctic permafrost region 3-year fuel operating lifetime per core loading
The prototype was constructed at the National Reactor Testing Station west of Idaho Falls from July 1957 to July 1958. It went critical on August 11, 1958, became operational on October 24, and was formally dedicated on December 2, 1958. The 3 MW (thermal) boiling water reactor (BWR) used 93.20% highly enriched uranium fuel. It operated with natural circulation, using light water as a coolant (vs. heavy water) and moderator. ANL used its experience from the BORAX experiments to design the reactor. The circulating water system operated at flowing through fuel plates of uranium-aluminum alloy. The plant was turned over to the Army for training and operating experience in December 1958 after extensive testing, with Combustion Engineering Incorporated (CEI) acting as the lead contractor beginning February 5, 1959.
Trainees in the Army Reactor Training Program included members of the Army, called cadre, who were the primary plant operators. Many maritime civilians also trained along with a few Air Force and Navy personnel. While plant operation was generally done by the cadre in two-man crews, development of the reactor was supervised directly by CEI staff. CEI decided to perform development work on the reactor as recent as the latter half of 1960 in which the reactor was to be operated at 4.7 MWthermal for a "PL-1 condenser test." As the reactor core aged and boron neutron poison strips corroded and flaked off, CEI calculated that about 18% of the boron in the core had been lost. On November 11, 1960, CEI installed cadmium sheets (also a poison) "to several tee slot positions to increase reactor shutdown margin."
Most of the plant equipment was in a cylindrical steel reactor building known as ARA-602. It was in diameter with an overall height of , and was made of plate steel, most of which had a thickness of . Access to the building was provided by an ordinary door through an enclosed exterior stairwell from ARA-603, the Support Facilities Building. An emergency exit door led to an exterior stairwell to the ground level. The reactor building was not a pressure-type containment shell as would have been used for reactors located in populated areas. Nevertheless, the building was able to contain most of the radioactive particles released by the eventual explosion.