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),
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, 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.
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).
- All components limited to packages measuring and weighing It operated with natural circulation, using light water as a coolant (vs. heavy water) and moderator. 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 MW<sub>thermal</sub> 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".
thumb|The ALPR before the accident. The large cylindrical building holds the nuclear reactor embedded in gravel at the bottom, the main operating area or operating floor in the middle, and the condenser fan room near the top. Miscellaneous support and administration buildings surround it.
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 , An emergency exit door led to an exterior stairwell to the ground level. 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). 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. 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 instantly. 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 .
Autopsy also confirmed the timeline: Byrnes and Legg died instantly, while McKinley showed signs of diffuse bleeding within his scalp, indicating he survived about two hours before succumbing to his wounds.
Reactor principles and events
Early press reports indicated that the explosion may have been due to a chemical reaction, but that was ruled out quickly. Fast neutron activation had occurred to various materials in the room, indicating a nuclear power excursion.
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. Without sufficient moderation, cores such as SL-1 would be unable to sustain a nuclear chain reaction. When the moderator is removed from the core, the chain reaction attenuates (dies down). Water, when used as a moderator, is maintained under high pressure to ensure it remains liquid under heat absorption; steam formation in the channels around the nuclear fuel does not moderate neutrons effectively and would suppress the fission chain reaction.
Another control is the effect of the delayed neutrons on the chain reaction in the core. Most neutrons (the neutrons) are produced nearly instantaneously by the fission of U-235. But a few —approximately 0.7 percent in a U-235-fueled reactor operating at steady-state— are produced through the relatively slow radioactive decay of certain fission products. (These fission products are trapped inside the fuel plates in close proximity to the uranium-235 fuel.) This delayed production of a fraction of the neutrons enables reactor power changes to be controlled on a time scale amenable to humans and machinery.
In the case of an ejected control assembly or neutron absorber, it is possible for the reactor to become critical (i.e. prompt critical). When the reactor is prompt critical, the time to double the power is of the order of 10 microseconds. The duration necessary for temperature to follow the power level depends on the design of the reactor core. Typically, the coolant temperature lags behind the power by 3 to 5 seconds in a conventional LWR. In the SL-1 design, it was about 6 milliseconds before steam formation started. That was too fast for the heat from the fuel to permeate the aluminum cladding and boil enough water to fully stop the power growth in all parts of the core via negative moderator temperature and void feedback. They noticed nothing unusual at first, with only a little steam rising from the building, normal for the cold night. The firefighters, unable to hail anyone inside the SL-1 facility, had a security guard open the gate for them. 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.
The building appeared normal, but was unoccupied. Three mugs of warm coffee were in the break room, and three jackets were hanging nearby. <!-- It's apparent that the official narrative conflicts heavily with the testimony of the first responders, who describe multiple ascents up to the reactor floor, while the published narrative lists only a few. Referring to the non-governmental sources for the events here will provide much more insight into the happenings prior to the recovery of McKinley. --> First responders would later report several attempts to penetrate the previously unfathomable high radiation fields, which were met with confusion due to their inexperience and their limited training. No fireman training had ever covered such a high radiation field for first responders. However, they persisted in the face of extreme danger, climbing the stairs and looking briefly into the reactor room before withdrawing. Finding a higher-scale ion chamber detector, the pair reached the top of the stairs to look inside the reactor room for the three missing men.
During the movement of McKinley, two men had their Scott Air-Paks freeze up and cease to work. Duckworth evacuated due to the malfunction, while Vallario removed his mask and breathed contaminated air to complete the evacuation of McKinley. the author indicates that the rescue teams identified Byrnes as the man found still alive, believing that Legg's body was the one found next to the reactor shield and recovered the night after the accident, and that McKinley was impaled by the control rod to the ceiling directly above the reactor. The misidentification, caused by the severe blast injuries to the victims, was rectified during the autopsies conducted by Clarence Lushbaugh, but this caused confusion for some time as the autopsy was classified until the 1990s.
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.
Cause
One of the required maintenance procedures called for Rod 9 to be manually withdrawn about in order to attach it to the automated control mechanism from which it had been disconnected. Post-accident calculations, as well as examination of scratches on Rod 9, estimate that it had actually been withdrawn about , causing the reactor to go prompt critical and triggering the steam explosion.
The most common theories proposed for the withdrawal of the rod are (1) sabotage or suicide by one of the operators, (2) a murder-suicide involving an affair with the wife of one of the other operators, (3) inadvertent withdrawal of the main control rod, or (4) an intentional attempt to "exercise" the rod (to make it travel more smoothly within its sheath). The maintenance logs do not address what the technicians were attempting to do, and thus the actual cause of the accident will never be known. However, it seems unlikely that it was a suicide.
Post-accident experiments were conducted with an identically weighted mock control rod to determine whether it was possible or feasible for one or two men to have withdrawn Rod 9 by 20 inches. Experiments included a simulation of the possibility that the