Companion piece · 2026-10-12
At 9:01 p.m. on January 3, 1961, an automatic alarm began transmitting from a small nuclear reactor facility in the Idaho desert.
Three servicemen had been working inside, preparing the reactor for its scheduled restart the following morning.
When emergency personnel arrived, they encountered radiation levels high enough to complicate their entry. Inside, the reactor had been severely damaged by a sudden nuclear power excursion and steam explosion.
Two men had died immediately. The third survived for approximately two hours.
The accident occurred at Stationary Low-Power Reactor Number One, commonly known as SL-1.
It remains the only fatal nuclear reactor accident in United States history.
An Experimental Reactor for the Military
SL-1 was located at the National Reactor Testing Station, approximately 40 miles west of Idaho Falls.
Developed under the U.S. Army's nuclear power programme, the reactor was intended to demonstrate a compact power system suitable for remote military installations.
It began operating in August 1958.
SL-1 was a boiling-water reactor rated at approximately three megawatts of thermal power. Its relatively simple design was intended to make operation and maintenance practical for trained military personnel.
Five control rods regulated the nuclear chain reaction by absorbing neutrons. Inserting them reduced reactor power; withdrawing them increased it.
The central rod was particularly important because of its substantial effect on the reactor's reactivity.
The facility operated under the authority of the Atomic Energy Commission, with Combustion Engineering serving as the operating contractor.
Military personnel performed routine operations and maintenance.
By late 1960, the reactor had accumulated more than two years of operating experience. Records also documented recurring difficulties with its control rod mechanisms, including occasions when rods did not move freely.
These problems would receive considerable attention after the accident.
January 3, 1961: Preparing for Restart
SL-1 had been shut down on December 23, 1960, for routine maintenance and modifications.
During the shutdown, workers installed instruments intended to measure neutron distribution within the reactor core.
The work required partial disassembly of the control rod drive mechanisms.
On January 3, the daytime crew completed the instrumentation work. The evening shift was assigned to restore the reactor to operating condition.
Three men were on duty:
- Army Specialist John A. Byrnes, 22, the reactor operator. - Navy Construction Electrician First Class Richard C. Legg, 26, the shift supervisor. - Army Specialist Richard Leroy McKinley, 27, an operator in training.
Their assignment included reconnecting the control rods to their mechanical drive systems.
The procedure required an operator to lift each rod manually by a short distance.
Instructions limited this movement to approximately four inches.
There was, however, no physical stop preventing an operator from lifting a rod considerably farther.
At approximately 9:01 p.m., the central control rod was withdrawn well beyond the prescribed limit.
The Power Excursion
Subsequent analysis established that the central rod had been withdrawn approximately twenty inches when the reactor core was destroyed.
The withdrawal introduced sufficient positive reactivity to initiate a prompt-critical nuclear excursion.
Reactor power increased extremely rapidly, generating substantial heat in a fraction of a second.
Fuel elements were damaged, and water inside the reactor underwent sudden heating and steam formation.
The resulting pressure surge produced a violent hydraulic impact, commonly described as a water hammer.
Investigators estimated pressures reaching approximately 10,000 pounds per square inch.
The reactor pressure vessel was driven upward approximately nine feet before falling back.
A heavy shielding component was expelled upward, striking Legg and pinning him against the ceiling.
Byrnes sustained fatal traumatic injuries during the explosion.
McKinley suffered severe head injuries and was subsequently found alive but unconscious.
All three men received potentially lethal radiation exposure. Their immediate deaths, however, resulted from physical injuries associated with the explosion.
The nuclear chain reaction had already ended by the time emergency personnel entered the building.
The damaged reactor remained radioactive.
The Recovery Operation
Emergency services received the automatic alarm at 9:01 p.m.
Responders initially faced uncertainty about the reactor's condition and the possibility of further nuclear activity.
Radiation measurements inside the operating area reached hundreds of roentgens per hour, with some readings approaching 1,000.
Rescue teams were required to limit their time inside.
McKinley was removed shortly before 11 p.m. but died from his injuries. Another victim was recovered during the early response.
Legg's body remained suspended above the reactor.
Recovering him required specialized equipment, remote observation, and carefully timed entries into the contaminated building.
The final body was removed during the early morning of January 9.
Twenty-two personnel involved in emergency response and recovery received significant radiation exposures, reported in the range of approximately three to twenty-seven roentgens.
Radioactive material escaped from the damaged reactor core, but most contamination remained within the facility.
Environmental monitoring found limited radioactive releases beyond the building, without significant off-site radiation exposure according to official assessments.
Investigating the Cause
The Atomic Energy Commission established an investigation into the accident.
Physical examination of the damaged reactor supported the conclusion that excessive withdrawal of the central control rod initiated the power excursion.
Determining why the rod was withdrawn so far proved more difficult.
Operating records revealed a history of sticking and sluggish control rod movement. During November and December 1960, numerous instances of unsatisfactory control rod performance had been documented.
Investigators also identified weaknesses in maintenance procedures and safety oversight.
Although operators had been instructed not to exceed the four-inch withdrawal limit, the procedure depended on individual judgment rather than a mechanical restraint.
The precise circumstances of the withdrawal could not be reconstructed because all three witnesses had died.
Rumours subsequently suggested that the accident might have been deliberately caused by an operator experiencing personal difficulties.
No credible evidence established this explanation. Idaho National Laboratory has explicitly stated that the evidence does not support the murder-suicide theory.
The established finding remains excessive control rod withdrawal. The reason for that movement remains uncertain.
Dismantling SL-1
SL-1 was never restarted.
During 1961, recovery personnel stabilized the facility and prepared the damaged reactor for examination.
The pressure vessel and core were transported to a specialized hot-cell facility, where investigators could study highly radioactive components using remote equipment.
General Electric carried out the principal dismantling and recovery programme between May 1961 and July 1962.
The reactor building was demolished, and contaminated materials were placed in designated burial areas.
Environmental remediation continued decades afterward.
During the early 2000s, more than 50,000 cubic yards of residual contaminated soil were removed from the SL-1 area and placed in an engineered disposal facility.
The site remains subject to periodic environmental review.
The Lasting Consequences
The SL-1 accident influenced subsequent reactor safety practices, particularly the design of control systems, maintenance procedures, and organizational oversight.
It demonstrated the risks of depending on administrative instructions when a single mechanical action could produce a rapid and dangerous increase in reactor power.
The accident also exposed weaknesses in equipment reliability reporting and the supervision of experimental reactors.
Later safety improvements placed greater emphasis on engineered safeguards, control rod reliability, independent review, and formal operating procedures.
SL-1 had been developed to demonstrate that small nuclear reactors could operate with relatively simple systems and limited staffing.
Its destruction established that simplicity required safeguards of its own.
The reactor operated for approximately two and a half years.
The accident that ended its service took only seconds.
This has been Nuclear Shadows. I'm Morgan Miller.