When you think of a US Nuclear accident, you probably think of Three Mile Island. However, there have been over 50 accidents of varying severity in the US, with few direct casualties. (No one died directly from the Three Mile Island incident, although there are some studies that show increased cancer rates in the area.)

Indeed, where there are fatalities, it hasn’t been really related to the reactor. Take the four people who died at the Surry Nuclear Power Plant accident: they were killed when a steam pipe burst and fatally scalded them. At Arkansas Nuclear One, a 525-ton generator was being moved, the crane failed to hold it, and one person died. That sort of thing could happen in any kind of industrial setting.

But one incident that you have probably never heard of took three lives as a direct result of the reactor. True, it was a misuse of the reactor, and it led to design changes to ensure it can’t happen again. And while the incident was nuclear-related, the radiation didn’t kill them, although it probably would have if they had survived their injuries.

Background

The large cylinder housed the SL-1 reactor. The picture is from some time before the accident (public domain).
It may be a misattribution, but it is often said that Napoleon said something like, “An army marches on its stomach.” A modern army might just as well march on electrical power. So the military has a keen interest in small nuclear reactors to both heat sites in cold climates and generate electricity when in remote locations or in, as they like to call it, denied areas.

In the mid-1950s, the Army tasked Argonne National Laboratory to prototype a small reactor. They wanted it portable, so it had to break down to relatively small pieces, if you consider something weighing 10 tons as small, and could be set up in the field.

The resulting prototype was the Stationary Low-Power Reactor Number One, known as SL-1, operated by the Army in late 1958. It could provide about 400 kW of heating or 200 kW of electricity. The reactor core was rated for 3 MW (thermal) but had been tested at 4.7 MW a few times. It would end operations due to an accident in 1961.

Design

Sketch of the reactor internals (public domain).
The reactor was a conventional boiling-water reactor design that used natural circulation of light water as both coolant and moderator. The fuel was in the form of plates of a uranium-aluminum alloy.

The reactor was inside a 48-foot-tall cylinder 38.5 feet in diameter. It was made of quarter-inch plate steel. Because the thing was in the middle of nowhere in Idaho, this was deemed sufficient. There was no containment shell like you’d find on reactors nearer to population centers.

The reactor, at the time of the accident, had five control rods, although it could accommodate nine. It could also hold 59 fuel assemblies, but only 40 were in use. Because of the reduced number of fuel plates, the reactor’s center region was more active than it would have been under full operation. The rods were eight in a circle with four dummies and a ninth one in the center. Because of the missing outer rods, the center control rod was more critical than the four others.

The Accident

In January of 1961, the reactor had been shut down for 11 days over the holiday. In preparation for restarting, workers had to reconnect the rods to their drive motors. The procedure was to pull the rod up four inches to allow the motor attachment.
Cutaway of the SL-1 and the control building (public domain).
There were three workers: Specialist Richard McKinley, Specialist John Byrnes, and a Navy Seabee Electrician First Class Richard Legg. Legg was in charge, and McKinley was a trainee.

From a post-accident investigation, they are fairly sure that Byrnes inexplicably pulled the center rod out 20 inches instead of the requisite four inches. The reactor went prompt critical, and, in roughly four milliseconds, the 3 MW core reached 20 GW. There wasn’t enough time for sufficient steam to form to trigger the safeties, which took 7.5 milliseconds.

The extreme heat melted the fuel, which explosively vaporized. The reactor couldn’t dissipate so much heat so quickly, and a pressure wave of about 10,000 pounds hit the top of the reactor vessel. The 13-ton vessel flew up at about 18 miles an hour, and plugs flew out, allowing radioactive boiling water and steam to spray the room. At about nine feet, it collided with the ceiling and a crane and fell back down. All this occurred in about two seconds.

As you might imagine, you didn’t want to be in the room, much less on top of the reactor. Two of the operators were thrown to the floor. Byrnes’ fall causes his rib to fatally pierce his heart. McKinley was also badly injured but only survived for about two hours after the accident. Legg was found dead and stuck to the ceiling, an ejected shield plug impaling him.

Why?

Actual photo of the destroyed reactor taken by a camera on the end of a crane.
You can place a lot of blame here. Of course, you probably shouldn’t have been able to pull the rod up that far, especially given that it was carrying more of the load than the other rods. The contractor that helped operate the facility wasn’t available around the clock due to “budget reasons.” There’s no way to know if that would have helped, of course.

But the real question is: why did they pull the rod up 20 inches instead of four? We may never know. There are, of course, theories. Improbably, people have tried to explain it as sabotage or murder-suicide due to some dispute between Byrnes and one of the other men. But that doesn’t seem to be the most likely explanation.

Apparently, the rods sometimes stuck due to misalignment, corrosion, or wear. During a ten-month period, for example, about 2.5% of the drop-and-scram tests failed because of this sticking: a total of 40 incidents. However, many of those causes only apply when the rods are automatically moved. Logbooks showed that manual movement of the rods had been done well over 500 times. There was no record of any sticking during manual operations. Several operators were asked, and none could recall any sticking. However, the rate of sticking was increasing right before the incident, just not from manual motion.

However, it is easy to imagine the 48-pound rod being stuck, pulling hard on it, and then having it give way. We’ve all done something like that, just not with such dire consequences.

Aftermath

First responders had a difficult time with this incident due to radiological problems. There had been false alarms before, so when six firefighters arrived on the scene, they weren’t too concerned. But when they entered the building, they saw radiation warning lights on and their radiation detectors pegged.

Even specialized responders with better equipment couldn’t determine just how much radiation was there, except for “plenty.” Air packs were fogging, limiting visibility. During the rescue of McKinley, one rescuer had to remove a defective air pack and breathe contaminated air for about three minutes. Freeing Legg’s body required ten men working in pairs, because each team could only work in the contaminated zone for 65 seconds. The rule had been that you could tolerate 100 Röntgens (about 1 Sv or 100 rem) to save a life and 25 (0.25 Sv or 25 rem) to save valuable property. Of the 32 people involved in the initial response, 22 received between 3 and 27 Röntgens exposure. Further, 790 people were exposed to harmful radiation levels during the subsequent cleanup.

The reactor building did prevent most of the radioactive material from escaping, but iodine-131 levels in some areas reached about 50 times normal levels. The remains of the site are buried nearby, and that’s the source of most residual radiation.

Lessons Learned

Unsurprisingly, the SL-1 design was abandoned. Future designs require that the reactor be safe even if one rod is entirely removed: the so-called “one stuck rod” rule. This also led to stricter operating procedures. What’s more, it is now necessary to ensure emergency responders have radiation meters with higher ranges. Regulations are often written in blood.

The Atomic Energy Commission made a film about the incident for internal use but, of course, now, you can watch it from your computer, below.

youtube.com/embed/gIBQMkd96CA?…

You might also enjoy this presentation by one of the first responders who was actually there, which you can see below. If you want a more detailed history, check out Chapters 15 and 16 of [Susan M. Stacy’s] book “Proving the Principle” that you can read online.

youtube.com/embed/gMNqPUT-yP0?…

Nuclear accidents can ruin your day. We are always surprised at how many ordinary mistakes happen at reactors like Brown’s Ferry.

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