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. Useful power output was 200 kW electrical and 400 kW for space heating. 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. Other materials struck the two other operators, mortally injuring them as well. 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. It also released about 80 curies (3.0 TBq) of iodine-131. 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.
Design and operations From 1954 to 1955, the U.S. Army had been evaluating 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 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). Some of the more important criteria included:
All components able to be transported by air All components limited to packages measuring 7.5 by 9 by 20 feet (2.3 m × 2.7 m × 6.1 m) and weighing 20,000 pounds (9,100 kg) 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 A classified 1956 preliminary design study, using BORAX-III as a basis, calculated the total construction cost for the prototype reactor to be $228,789. This estimate for just the reactor and its components did not include any of the buildings or the rest of the reactor plant. The prototype was constructed at the National Reactor Testing Station west of Idaho Falls from July 1957 to July 1958. It went critical for the first time 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. The circulating water system operated at 300 pounds per square inch (2,100 kPa) 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.
CEI was responsible for the actual operation of the SL-1 reactor, for the routine training of military personnel and for developmental research programs. The Contractor provided at the site a Project Manager, Operations Supervisor, a Test Supervisor, and a technical staff of approximately six personnel. In recent months, the Project Manager spent approximately half time at the site and half time at the contractor's office in Connecticut. In his absence, either the Operations Supervisor or the Test Supervisor was assigned as the Project Manager. ... It was understood, as indicated by testimony before the Board, that CEI would provide supervision on any shifts when non-routine work was carried out.
... the AEC's Idaho Office and the Army Reactors Office clearly believed that the addition of night supervisors when only routine work was involved would defeat a part of the purpose of operating the reactor under the existing arrangement, i.e., to obtain plant operating experience with only military personnel. 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 absorber 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 neutron absorber) "to several tee slot positions to increase reactor shutdown margin".
… excerpt ends here. Continue reading the full article.
