The SL-1, or Stationary Low-Power Reactor Number One, was a United States Army experimental nuclear power reactor which underwent a steam explosion and meltdown in January 1961, killing its three operators. The direct cause was the improper withdrawal of the only movable control rod. The event is the only fatal reactor accident in the United States.cite book
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The facility, located at the National Reactor Testing Station approximately forty miles (60 km) west of Idaho Falls, Idaho, was part of the Army Nuclear Power Program and was known as the Argonne Low Power Reactor (ALPR) during its design and build phase. It was 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). Operating power was 200 kW electrical and 400 kW thermal for space heating. NASA system failure studies have cited that the core power level reached nearly 20 GW in just four milliseconds, precipitating the reactor accident and steam explosion. [cite journal | journal = System Failure Case Studies | url = | publisher = NASA | title = Supercritical | author = Steve Wander (editor) | volume=1 | issue=4 |month=February | year=2007|format=PDF]

The accident

On December 21, 1960, the reactor was shut down for maintenance, calibration of the instruments, installation of auxiliary instruments, and installation of 44 flux wires to monitor the neutron flux levels in the reactor core. The wires were made of aluminum, and contained slugs of aluminum-cobalt alloy.

On January 3, 1961 the reactor was restarted after a shutdown of eleven days. Maintenance procedures commenced, which required the main central control rod to be withdrawn a few inches; at 9:01 p.m. this rod was withdrawn almost to the top of the core, causing SL-1 to go prompt critical. In four milliseconds, the heat generated by the resulting enormous power surge caused water surrounding the core to begin to explosively vaporize. The water vapor caused a pressure wave to strike the top of the reactor vessel. This propelled the control rod and the entire reactor vessel upwards, which killed the operator who had been standing on top of the vessel, leaving him pinned to the ceiling by a control rod. The other two military personnel, a supervisor and a trainee, were also killed. The victims were Army Specialists John A. Byrnes and Richard L. McKinley and Navy Electrician's Mate Richard C. Legg.

Reactor principles and events

Fission produces neutrons with a wide range of energies. In all light-water-moderated reactors (LWR), to sustain fission of the U-235 the reactor core needs to have water present to moderate (slow down) the neutrons produced by the nuclear reaction. This process is called "thermalizing" and increases the probability of the neutrons causing fission. When reactivity is inserted in the reactor core, more neutrons are available and power rises. Several factors limit the increase in power.

The first limiting factor is that, given a proper initial spectrum of neutron energies, water has a negative reactivity coefficient. Having a negative reactivity coefficient means that, as the water heats up, the molecules are farther apart (water expands and eventually changes phase) and neutrons are less likely to hit hydrogen atoms, so fewer neutrons are thermalized by collisions with the hydrogen in the water and the probability of fission decreases. This removes reactivity from the core. The lower the temperature, the closer the molecules, the greater the number of neutrons thermalized and the greater the core reactivity. It is also possible to design a reactor core that has an entirely different neutron energy spectrum such that it has conditions for which water has a POSITIVE reactivity coefficient. A graphite-moderated, water-cooled reactor like the RBMK reactors at Chernobyl may have a positive reactivity coefficient for coolant (water) temperature.

Normally there are lots of delayed neutrons in a nuclear reactor and the power changes are driven by these neutrons. In the case of an ejected control assembly or a large reactivity insertion at zero power (no delayed neutrons present) it is possible for the reactor to become critical (making as many neutrons as it is losing) on the prompt neutrons (released by the fission itself) alone. When the reactor is prompt critical, the time to double the power is in the order of 10-5 seconds. As soon as the heat from the increased power reaches the fuel, the coolant temperature begins to rise. How long it takes for temperature to turn power 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. However, a portion of the power deposited in the fuel is deposited directly by gamma rays. In the SL-1 design it was about 6 ms before steam formation started.

Note: Chernobyl was a RBMK type reactor with a Graphite Moderator. For such a reactor, Graphite [] has a positive temperature coefficient of reactivity. Thus as temperature rises, more power is transferred, raising the temperature even more.

As a small reactor, the SL-1 was designed with a main central control rod which was able to produce a very large excess reactivity if it were completely removed. The excess reactivity is a measure of how much more capacity there is to accelerate the nuclear reaction than is required to start a controlled nuclear reaction for power generation. The potential for excess reactivity is always required because the fuel becomes less reactive over time. A greater excess reactivity causes a faster increase of the rate of the nuclear reaction. In normal operation, the control rods are withdrawn only enough to cause sufficient reactivity for a sustained nuclear reaction and power generation.

In this accident, the reactivity addition was sufficient to take the reactor prompt critical and produced a period estimated at 3.6 ms. That was too fast for the heat from the core to get through the aluminum cladding and boil enough water to fully stop the power growth in all parts of the core.

Instead, the final control method happened in parts of the core: destructive vaporization and consequent conventional explosive expansion of the parts of the reactor core where the greatest amount of power was being produced most quickly. It was estimated that this core heating and vaporization process happened in about 7.5 ms, before enough steam had been formed to shut down the reaction, beating the steam shutdown by a few milliseconds.

Events after the power excursion

There were no other people at the reactor site. The ending of the nuclear reaction was caused solely by the design of the reactor and the basic physics of heated water and core elements vaporizing, separating the core elements and removing the moderator.

Heat sensors above the reactor set off an alarm at the central test site security facility. The first response crew, of firemen, arrived nine minutes later and initially noticed nothing unusual, with only a little steam rising from the building, normal for the cold (−20 °F or −30 °C) night. The control building appeared normal. On approaching the reactor building, their radiation detectors jumped sharply to above their maximum range limit, and they withdrew, unable to know whether they could safely proceed or for how long they could remain.

At 9:17 p.m., a health physicist arrived. He and a fireman, both wearing air tanks and masks with positive pressure in the mask to force out any potential contaminants, approached the reactor building stairs. Their detectors read 25 Roentgens per hour (R/hr) as they started up the stairs, and they withdrew.

Some minutes later, a health physics response team arrived with radiation meters capable of measuring gamma radiation up to 500 R/hr—and full-body protective clothing. One health physicist and two firefighters ascended the stairs and, from the top, could see damage in the reactor room. With the meter showing maximum scale readings, they withdrew rather than approach the reactor more closely and risk further exposure.

Around 10:30 p.m., the supervisor for the contractor running the site and a contractor health physicist arrived. They entered the reactor building and found two mutilated men: one clearly dead, the other moving slightly. With a one minute and one entry per person limit, a team of five men with stretchers recovered the operator who was still breathing; he did not regain consciousness and died of his head injury at about 11 p.m. Even stripped, his body was so contaminated that it was emitting about 500 R/hr. They looked for but did not find the third man. With all potential survivors now recovered, safety of rescuers took precedence and work was slowed to protect them.

On the night of 4 January, a team of six volunteers used a plan involving teams of two to recover the second body. Radioactive Gold 198Au from the man's golden watch buckle and Copper 64Cu from a screw in a cigarette lighter subsequently proved that the reactor had indeed gone supercritical.

The third man was not discovered for several days because he was pinned to the ceiling above the reactor by a control rod. On 9 January, in relays of two at a time, a team of eight men, allowed no more than 65 seconds exposure each, used a net and crane arrangement to recover his body.

The bodies of all three were buried in lead-lined caskets sealed with concrete and placed in metal vaults with a concrete cover. All had major physical injuries, including severed limbs and fragments of the fuel assembly in their wounds. Richard Leroy McKinley is buried in section 31 of Arlington National Cemetery.


One of the required maintenance procedures called for the main control rod to be manually withdrawn approximately four inches in order to attach it to its automated control mechanism, from which it had been disconnected. Post accident calculations estimate that the main control rod was actually withdrawn approximately twenty inches, causing the reactor to go prompt critical, which resulted in the steam explosion. The three most common theories proposed for this discrepancy are sabotage or suicide attempt by one of the operators, inadvertent withdrawal of the main control rod, or 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 is unlikely to ever be known. There were scratches suggesting that at the time of the accident the operator had overdrawn the main control by 16 inches, and had settled back down to the normal 4 inches after the excursion. The investigation took almost two years to complete.


The remains of the SL-1 reactor are now buried near the original site.

The accident caused this design to be abandoned and future reactors to be designed so that a single control rod removal wouldn't have the ability to produce the very large excess reactivity which was possible with this design. Today this is known as the "one stuck rod" criterion and requires complete shutdown capability even with the most reactive rod stuck in the fully withdrawn position. The reduced excess reactivity limits the possible size and speed of the power surge.

The accident also showed that in a genuine extreme accident both the vaporizing of the core and the water to steam conversion would shut down the nuclear reaction. This demonstrates in a real accident the inherent safety of the water moderated design against the possibility of a nuclear explosion.

In addition to a sudden power surge, a nuclear explosion requires sufficient force to hold the reacting nuclear components together for a short but necessary time. This is achieved in a nuclear fission weapon by surrounding the core with a carefully engineered symmetrical inward-facing conventional explosion. This element is not present in nuclear reactors. Lacking such explosive compression to hold the vaporized core components together, the components fly apart as in this accident; the reaction ends, resulting in a steam explosion and a badly damaged reactor core, but not the type of explosion as would be achieved with a nuclear weapon.

Even without an engineered containment building like those used today, the building contained most of the radioactivity, though iodine-131 levels on plants during several days of monitoring reached fifty times background levels downwind.

Radiation exposure limits prior to the accident were 100 Röntgens to save a life and 25 to save valuable property. During the response to the accident, 22 people received doses of 3 to 27 Röntgens full-body exposure and three doses above 27 R. In March 1962, the Atomic Energy Commission awarded certificates of heroism to 32 participants in the response.

The documentation and procedures required for operating nuclear reactors expanded substantially, becoming far more formal as procedures which had previously taken two pages expanded to hundreds. Radiation meters were changed to allow higher ranges for emergency response activities.

After a pause for evaluation of procedures the Army continued its use of reactors, operating the Mobile Low-Power Reactor (ML-1), which started full power operation on 28 February 1963, becoming the smallest nuclear power plant on record to do so. That design proved too advanced for the materials available and was eventually abandoned after corrosion problems. While the tests had shown that nuclear power was likely to have lower total costs, the financial pressures of the Vietnam War caused the Army to favor lower initial costs and it abandoned its reactor program in 1965.

Movie and book

SL-1 is the title of a 1983 movie, written and directed by Diane Orr and C. Larry Roberts, about the nuclear reactor explosion. Interviews with scientists, archival film, and contemporary footage, as well as slow-motion sequences, are used in the film. [ [ SL-1 (1983)] ] The events of the accident are also the subject of a book, published in 2003, "Idaho Falls: The untold story of America's first nuclear accident". [cite book |last=McKeown |first=William |title=Idaho Falls: The Untold Story of America's First Nuclear Accident |isbn=978-1550225624]

ee also

*BORAX Experiments, 1953-4, which proved that the transformation of water to steam would safely limit a boiling water reactor power excursion, similar to that in this accident.
*Idaho National Laboratory
*International Nuclear Events Scale
*List of civilian radiation accidents
*List of civilian nuclear accidents
*List of military nuclear accidents
*List of nuclear reactors
*Nuclear contamination
*Nuclear debate
*Nuclear safety
*Nuclear power
*Radioactive contamination


* [ INEEL Publication "Proving the Principle"]
** [ Chapter 15 "The SL-1 Reactor" ] 9.5 MB PDF
** [ Chapter 16 "The Aftermath"] 4.3 MB PDF
* [ US Department of Energy Idaho Freedom of Information Act request archives] , including eight documents on the SL-1 accident.
* [ "SL-1 Reactor Accident on January 3, 1961, Interim Report"] , May 1961. From the above page. 15.5 MB PDF.
* [ SL-1 Accident]
* [ Atomic Energy Insights - July 1996]
* [ Brain Candy #59 - The SL-1 Accident]
* [ ALPR] summary page for this design.
* [ Dr. George Voelz, M.D.] , one of two doctors at the site, oral history of the events.
* KPVI, a television station in eastern Idaho, has posted a two-part story about the SL-1 accident. Some of the original video footage is included in the story and can be viewed online. []
* [,8816,828698,00.html Runaway Reactor]
* [ SL-1 The Accident: Phases I and II] U.S. Atomic Energy Commission Idaho Operations Office video on YouTube (40 min)

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