When the third fire alarm rings out at a reactor testing station on a freezing January night,
firefighters race to respond. Expectations of another false alarm are soon dashed…instead,
a nuclear disaster unfolds in front of them.
Inside "Fire Station Number 1" at the Army’s National Reactor Testing Station in
Idaho, United States, the siren sounds at 9:01 pm. A false alarm had been raised twice before
in the furnace room of the support building - which houses the stationary low-power nuclear
reactor. Each alarm calls for attention in a place like this, where nuclear research is conducted.
The sound sends the six firefighters to put their jackets on, board the truck,
and head toward the site. It's January 3, 1961, and the night is freezing.
On arrival, the first place they check is the furnace room, believing another false
alarm drew them out of their warm station. To their surprise, the alarm in the room is
silent. It is coming from somewhere else. They better check the main building where
the reactor crew is housed. Over there, the firefighters find a warm but empty room. All
the lights are on, and three mugs of freshly brewed coffee are still steaming on the table.
There's no one in the reactor control room either…only a threatening red indicator blinking
on the panel: a "HIGH RADIATION" alert. All of a sudden, the firefighters realize
what is going on. The alarm is coming from the central building where the nuclear reactor is.
Definitely not a false alarm. The firefighters quickly come to their senses
and rush out of the building. They run toward the truck to get their air masks and oxygen tanks.
With these on, they head back into the main building and climb the stairs toward
the area where the nuclear reactor is - a tall, three-stage cylindrical building.
At the bottom level is the reactor; above it is the operating platform with the control equipment,
and on top is the ventilation fan. The three-man reactor crew is operating on the platform,
where the firefighters are hoping to find them. From the outside, there’s no trace of an accident.
However, the portable radiation detector goes wild as the firefighters climb the stairs. The
needle shows a maximum of 200 roentgen per hour as the beeping sound turns into a steady buzzing.
Believing that the detector might be wrong, Assistant Chief Ken Dearden sends
one of the firefighters to bring another detector from the truck. Unfortunately,
there's nothing wrong with it. The other one also shows the highest radiation level.
On the operating platform level, the firefighters peer through a window in the door to figure out
what is happening. The air inside is clear, and the reactor seems to be in its place. But,
next to it lies the bodies of two crewmen, army specialists Richard Leroy McKinley and John A.
Byrnes, soaking wet and covered with blood. Around them are scattered rocks and metal
debris. The situation calls for an all-out alarm. Within only 17 minutes, a health physicist,
accompanied by assistant chief Moshberger, entered the facility with a higher-scale ion
chamber detector. As they approached the operating platform, the detector showed
a radiation level of 500 roentgen per hour, above the lethal dose for prolonged exposure.
At 10:30 pm, Chief Health Physicist Ed Vallario and the reactor Operations Supervisor Paul
Duckworth arrived from nearby Idaho Falls. Joined by three other health physicists,
they put Scott Air Packs on and rushed inside the reactor building.
Inside, one of the crew members, Richard Leroy McKinley, was moaning and still showing signs
of life, even though he was covered in blood and in pretty bad shape. Vallario
and Duckworth decided to remove him from the building as quickly as possible to get him
medical care. With the emergency exit blocked, the two carried McKinley all the way through
the main building. Unfortunately, during the extraction, Vallario and Duckworth’s air masks
stopped working, forcing Vallario to take off his masks and breathe contaminated air.
The entire rescue operation lasted just three minutes. Sadly,
McKinley never managed to reach the hospital. He died of his wounds in the ambulance. His other
colleague John A. Byrnes died on the platform. Mystery surrounded the third crewman, Navy Seabee
Construction Electrician First Class Richard C. Legg. No one could find him, dead or alive.
Was his disappearance related to what happened at the reactor?
The firefighters and health physicists on the scene quickly established that an exploded
reactor had caused the death of crewmen. Meanwhile, they learned that the explosion
had nothing to do with the fact that Richard Legg was missing… in fact, he wasn't missing at all.
Legg had also died in the accident, but no one could find his body since a piece of
the reactor impaled him to the ceiling of the building. His body hung from above unnoticed.
However, what puzzled the experts was what caused such a powerful explosion.
Contrary to popular belief, nuclear reactors are not dangerous contraptions. They can't
explode on their own. There's a strict procedure prescribed for their operation,
and an accident can only happen if one doesn't follow it. So, what went wrong with the reactor
at the Idaho National Reactor Testing Station? The nuclear reactor in question was experimental,
known as Stationary Low-Power Reactor Number One or simply SL-1. It was designed as a mid-scale
reactor with an output of 3 megawatts of thermal energy. SL-1 was part of the United States Army
program to equip the radar stations in remote regions of the Arctic to supply
them with electricity and space heating. The contract to design, build and test
the reactor prototype was given to the Argonne National Laboratory,
which specialized in non-weapon-related nuclear physics. The prototype was constructed at the
National Reactor Testing Station in Idaho and became operational on October 24, 1958.
It used 93.20% highly enriched uranium fuel for its 3-megawatt boiling water reactor.
It operated with natural circulation, using light water as a coolant and moderator. Following
extensive testing, the plant was turned over to the Army to train its personnel.
The reactor and most operating equipment were housed in a large cylindrical silo-like
building made of 1/4 inch (6 mm) steel plates. Unlike reactors positioned near urban areas,
SL-1 had no pressure-type containment shell. Nevertheless, the building was able to contain
most of the radioactive particles released by the eventual explosion. SL-1 was designed to function
and last for decades, even in harsh conditions like the Arctic circle. Still, it exploded and
took the lives of three young servicemen. The cause for the disaster lay inside the reactor.
Each nuclear reactor consists of fuel and control rods inside a sealed steel vessel filled with
water. It uses a chain reaction of nuclear fissions to produce heat. Once the reaction
becomes self-sustaining, the reactor reaches the so-called "critical state." In the process,
controlling the nuclear chain reaction is vital. That’s what control rods are used
for. They limit the nuclear fission inside the reactor. If the control rods are fully
inserted in the reactor, they stop fission. However, if they are entirely extracted,
the nuclear chain reaction will go out of control and cause a reactor meltdown.
The SL-1 nuclear reactor had a capacity of 59 fuel assemblies and nine control rods. However,
it operated with only 40 fuel elements and one control rod. Experts estimated
that such an arrangement would produce more energy in the central area of the reactor.
SL-1's fuel element rods consisted of 52 mils (1.3 mm) of uranium-aluminum alloy "meat" covered
by aluminum cladding. In total, the reactor contained 31 pounds (14 kg) of uranium-235 - a
uranium isotope used to build the Little Boy atomic bomb dropped on Hiroshima in 1945.
The cruciform control rod was made of 60 mils (1.5 mm) thick cadmium clad with aluminum. Being
the only active control rod in the reactor, it was essential for the reactor operation...
That turned out to be the core of the problem and the leading cause of the accident.
Since all three crewmen involved in the accident were dead,
it was difficult to reconstruct the events that took place in the reactor building that day.
The initial investigation determined that the reactor vessel was detached but still
in its bed. However, four shield plugs designed to hold control rods were ejected.
And the inspection of the control rod showed it was extracted from its bed. That gave
the investigators an idea of what caused the accident - it was the result of one wrong move.
On the night, three crewmen underwent a routine maintenance procedure of reconnecting the control
rod. Due to the Christmas holidays, the reactor was shut down for eleven days,
and the operators were going to put it back into operation. The procedure required the control
rod to be manually withdrawn from the reactor, connected to its drive mechanism, and lowered
back into the reactor. It was a step-by-step process in which the crew members strictly
followed the operating manual instructions. First, one of the crew members, probably John
Byrnes, extracted the control rod three inches from the reactor. Next, the other crewmen,
presumably Richard Legg, put the c-clamp on the rod to hold it in place while the drive mechanism
was attached. After completing the reconnection, Byrnes was supposed to manually lift the rod for
another two inches for Legg to take off the c-clamp and lower it to the reactor. However,
instead of lifting the rod two inches, Byrnes lifted it twenty inches. A seemingly minor detail,
but in the case of a nuclear reactor, it was the one that caused a terrible accident.
As is familiar, the control rod's role in the reactor is to limit the number
of nuclear fissions in order not to cause an uncontrolled chain reaction. Unfortunately,
it was precisely what happened with the control rod extracted too far. The nuclear
reaction could not be contained, creating way more fissions than the reactor could withstand.
That is what nuclear scientists call "prompt criticality." In just four milliseconds,
the reactor designed to produce 3 megawatts of energy produced a power output of 20 megawatts.
The power excursion was so big that the fuel assemblies melted down. The fuel vaporized and
produced extreme pressure inside the reactor vessel, pushing the water upward. The water
hit the vessel's roof with a pressure of 10,000 pounds per square inch, sending the entire vessel
up 9 feet (2.74 m) in one-third of a second. The water broke through the shield plugs, launching
them into the air at 57 mph (93 km/h) and pinning Legg to the ceiling. Contaminated water and steam
broke out of the vessel and sprayed the whole operating platform, killing Byrnes and Legg.
McKinley, who was standing aside, was severely wounded. He died on his way to the hospital.
The theory was backed up by traces of neutron-activated elements on the crewmen's
clothes and belongings. Also, an experiment with dummy control rods proved feasible for one man
to extract the control rod twenty inches in a single pull. But the crewmen were familiar with
the reactor and the dangers of extracting the control rod too far - so why would they do it?
The answer lies in a misfunction of the control rod. Official reports mention
that the center control rod performed more reliably than other rods. However,
there is no mention that it still got stuck seven times in the month leading up to the accident.
What’s more, the frequency of control rod problems increased over time. But,
the operating contractor, Combustion Engineering Incorporated, did nothing about it.
The SL-1 Project Manager, W. B. Allred, later defended himself during the congressional hearings
about the accident, saying that he was unaware of the sharp increase in rod jamming. According
to him, if he knew, he "would have shut the plant down for more detailed examination."
And so, the reason for the disaster on January 3, 1961, became clear: When Byrnes tried to lift the
control rod two inches, it got stuck. In order to release the jammed rod, he used excessive force
and accidentally lifted it by twenty inches. The government downplayed the facts to avoid
undermining public trust in the nuclear industry. The accident was portrayed only as
a minor one that had no serious repercussions The Atomic Energy Commission reported that
no fission products were detected within the SL-1 perimeter fence other than 0.1 curies of
strontium-90 and 0.5 cesium-137. In their report, the AEC not only left out many other radionuclides
that were emitted but also underestimated the amount of fission produced that night.
Even with the low-balled curie releases, the SL-1 accident was serious. When the vessel was
examined afterward, roughly 30 percent of the core's fuel inventory had evaporated.
The SL-1 nuclear reactor accident resulted in three lives lost, a destroyed nuclear
reactor facility, and an abandoned development program of the SL-1 reactor. In February 1963,
the Army replaced it with the Mobile Low-Power Reactor - ML-1. It was a valuable lesson,
though. As a result, all the future reactors were designed by applying a "one stuck rod" criterion
to ensure that a single control rod removal would not result in excessive reactivity.
As for the SL-1 nuclear reactor, it was completely dismantled. Teams of workers
removed the bulk of the contaminated debris using powerful vacuum cleaners and overhead cranes.
Twenty-six thousand pounds (12,000 kg) of radioactive debris,
including the reactor vessel, were buried in the proximity of the reactor's site.
The depth of three pits dug for the purpose ranged from eight to fourteen feet (2.4 to 4.3 m). The
excavations were covered with at least 2 feet (0.6 meters) of clean backfill. In September 1962,
shallow mounds of soil were added over the pits; in 2000, a riprap layer was added.
In front of this awkward burial ground, fenced with wire, lies an even more awkward tombstone
warning unsuspecting passers-by of the radioactive danger still lurking from below the ground.
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