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 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.LA-3611 A Review of Criticality Accidents , William R. Stratton, Los Alamos Scientific Laboratory, 1967LA-13638 A Review of Criticality Accidents (2000 Revision) , Thomas P. McLaughlin, et al., Los Alamos National Laboratory, 2000.
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 of fission products into the atmosphere,Horan, J. R., and J. B. Braun, 1993, Occupational Radiation Exposure History of Idaho Field Office Operations at the INEL, EGG-CS-11143, EG&G Idaho, Inc., October, Idaho Falls, Idaho. 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 of iodine-131.The Nuclear Power Deception Table 7: Some Reactor Accidents 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 airANL-5566, ALPR Preliminary Design Study (Argonne Low Power Reactor) Phase I , Reactor Engineering Division, Argonne National Laboratory, Lemont, Illinois, April 20, 1956. Reactors - Special Features of Military Package Power Reactors, M-3679 (18th edition), M. Treshow, E. Hamer, H. Pearlman, D. Rossin, D. Shaftman 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 loadingSEC-00219, Petition Evaluation Report, Idaho National Laboratory (INL), Revision 2 , NIOSH/ORAU, Idaho National Laboratory, March 2017
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. IDO-19311 Final Report of SL-1 Recovery Operation, Idaho Test Station, General Electric Corporation, July 27, 1962. It operated with natural circulation, using light water as a coolant (vs. heavy water) and moderator. 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.IDO-19012, CEND-82, SL-1 Annual Operating Report, Feb. 1959 – Feb 1960 , Canfield, Vallario, Crudele, Young, Rausch, Combustion Engineering Nuclear Division, May 1, 1960. 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".Radiation Safety and Regulation Hearings, Joint Committee on Atomic Energy, US Congress, June 12–15, 1961, including SL-1 Accident Atomic Energy Commission Investigation Board Report, Joint Committee on Atomic Energy Congress of the United States, First Session on Radiation Safety and Regulation, Washington, DC. 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".IDO-19024 SL-1 Annual Operating Report, February 1960 – January 3, 1961 Combustion Engineering Nuclear Division, CEND-1009, W. B. Allred, June 15, 1961.
Most of the plant equipment was in a cylindrical steel reactor building known as ARA-603 and SL-1 603. 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-602, the Support Facilities Building.HAER ID-33-D-136, ARA-II Plot plan as it appeared in 1980, when interior modifications were being prepared to remodel electrical apparatus in ARA-602 in connection with use as a research and development joining laboratory. EG&G, Idaho, Inc. 1570-ARA-II-100-1. April 1980. INEEL index code 070-0199-00-220-159749. Idaho National Engineering Laboratory, Army Reactors Experimental Area, Scoville, Butte County, ID, Photos from Survey HAER ID-33-D, Historic American Engineering Record (Library of Congress), Larry Reynolds 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.
The reactor core structure was built to hold 59 fuel assemblies, one startup neutron source assembly, and nine control rods. The actual core in use had 40 fuel elements and five active control rods. The five active rods were in the shape of a plus symbol (+) in cross section: one in the center (Rod Number 9), and four on the periphery of the active core (Rods 1, 3, 5, and 7). The control rods were made of thick cadmium, clad with of aluminum. They had an overall span of and an effective length of . The 40 fuel assemblies were composed of nine fuel plates each. The plates were thick, consisting of of uranium–aluminum alloy "meat" covered by of X-8001 aluminum cladding. The meat was long and wide. The water gap between fuel plates was . Water channels within the control rod shrouds was . The initial loading of the 40-assembly core contained of highly enriched (93.2%) U-235.
With 2/3 the maximum fuel assemblies but only 55 percent the number of control rods, the reactor was more active than in its maximum configuration. The centre is the most active region and this configuration so accentuated this that the four outer control rods were not even used in the smaller core after tests concluded they were not necessary.Report on the SL-1 Incident, January 3, 1961, The General Manager's Board of Investigation, For Release in Newspapers dated Sunday, Curtis A. Nelson, Clifford Beck, Peter Morris, Donald Walker, Forrest Western, June 11, 1961. In the operating SL-1 core, Rods 2, 4, 6, and 8 were dummy rods, had newly installed cadmium shims, or were filled with test sensors, and were shaped like the capital letter T. The effort to minimize the size of the core gave an abnormally-large reactivity worth to Rod 9, the center control rod, meaning that any irregular handling might result in sudden extreme changes in the reactor's total flux and heat generation.
Accident and response On Tuesday, January 3, 1961, SL-1 was being prepared for restart after a shutdown of 11 days over the holidays. The crew performing the restart procedure consisted of Army Specialists John A. Byrnes (reactor operator, age 22) and Navy Seabee Construction Electrician First Class (CE1) Richard C. Legg (age 26; shift supervisor),Spokane Daily Chronicle - Jan 4, 1961 . Byrnes was a Spec/5 from Utica, New York, McKinley was a Spec/4 from Kenton, Ohio, and Legg was a Seabee and "Navy electrician L.C." from Roscommon, Michigan, with Army Specialist Richard Leroy McKinley (age 27) as an observing reactor operator in training.
Maintenance procedures required that rods be manually withdrawn a few inches to reconnect each one to its drive mechanism. As it was later established, at 9:01 pm MST, Byrnes withdrew Rod 9 too far, causing SL-1 to go prompt critical. In four milliseconds, the heat generated by the resulting enormous power excursion caused fuel inside the core to melt and to explosively vaporize along with some of the reactor's water. The expanding fuel plates produced an extreme pressure wave that blasted steam and the remaining water upward, striking the top of the reactor vessel with a peak pressure of . The slug of water was propelled at with average pressure of around . This extreme water hammer propelled the entire reactor vessel upward at , while the shield plugs were ejected at . With six holes on the top of the reactor vessel, high-pressure water and steam sprayed the entire room with radioactive debris from the damaged core. A later investigation concluded that the (or thirteen short tons) vessel had jumped , parts of it striking the ceiling of the reactor building before settling back into its original location, Chapter 15. and depositing insulation and gravel on the operating floor. If the vessel's #5 seal housing had not hit the overhead crane, it would have risen about . The excursion, steam explosion, and vessel movement took two to four seconds.