The SL-1 Reactor Accident

This is a pretty narrow-cast post. I figure frequent commenter Jeff Gauch and one or two other nucs read the blog, and might find it of interest.

In the late 1950s, the Army was looking to small nuclear reactors to provide power to certain remote locations, such as DEW Line sights, where resupply of fuel for conventional generators was problematical at best.

And so the Atomic Energy Commission commissioned the design and construction of a prototype small powerplant know as the Argonne Low Power Reactor. Later known as the SL-1, the reactor was built in Idaho, and training of Army personnel to operate it began under the supervision of the operating contractor, Combustion Engineering.

On January 3rd, 1961, the reactor was shut down for routine maintenance. Three Army personnel were conducting that maintenance. An error in manually withdrawing one of the control blades too far allowed a “prompt critical” incident. For us non-nucs, basically a the reactor not only achieved a criticality, it reacted at a far, far greater rate than normal power generation. Not an explosion, per se, but the criticality formed a steam bubble instantly inside the pressure vessel. The steam formed so rapidly that rather than rising to the top of the vessel, it instead pushed the cold water above it up like a slug, displacing the vessel itself, and blasting the control rod mechanisms free from their fittings at the top of the vessel. Two of the three Army personnel were killed instantly. The third was found alive by first responders, but died during transport to medical facilities.

The reactor pressure vessel was breached, and considerable nuclear materiel and other radioactive contamination was released into the reactor building, with small amounts released to the outside environment. It would take eighteen months to dismantle the reactor building, decontaminate the site, and determine the cause of the accident.

This was the first, and to date, worst, reactor accident in US history.

Jeff and the other nucs have almost certainly heard of the incident. They may appreciate this AEC report on it.

The latter parts of the film show the actual reactor, fuel plates and control blades and associated fittings.



11 responses to “The SL-1 Reactor Accident

  1. Jeff Gauch

    Thanks, that was fascinating. I especially enjoyed the soundtrack. My current job is what the film describes as a health physicist, and seeing the procedures from the stone knife era of nuclear power was really interesting.


    • Yeah, not sure you can pay me enough to go into a 500Roentgens environment for 30 seconds to take pics.


    • Jeff Gauch

      It’s less than half a REM, about what you would pick up from background radiation.

      The worst part would be working in double coveralls, plastic over-garment, and respirator in the desert summer.


    • We blew past my knowledge of nuclear engineering very early in the show. I understood the Boiling Water Reactor/PWR part. And the control rods/fuel stacks. And I kinda get the diff between the neutron flux and the radioactive decay contamination. But after that, Og say fire bad!


    • Jeff Gauch

      Well, let’s just say that the state of the art has evolved quite a bit since those days. You’d never see anything like that in response to a US nuclear accident.

      One part that really raised my eyebrows was when they said the gravel shielding wasn’t heavily contaminated since a handful(!) only read 3mR/hour. You need a mucking shedload of contamination to get those kind of radiation levels.


  2. When I was in Electrical Engineering back in the early 80s, I took a Nuke Engineering course the quarter before I got hurt and forced out of school for 7 years. The instructor told us about that incident and he had a few choice words for the guy pulling the control rod.

    3 mR/hour is a bit on the high side. Nothing minor about that. But, then, much of the reactor is lying about in pieces. I’d hate to see what the entire gravel shield was emitting.

    The basic idea of a small reactor plant to power remote locations is a good one. I’d probably use Thorium as the fuel, these days, rather than U235 or PU239. Thorium reactors are far more forgiving than those fueled with other fissiles. Too bad the Army didn’t stay on track, rather than throwing in the towel on Nuke power.


    • Look up the ML-1 portable reactor. Used compressed nitrogen in a closed gas turbine. Instead of shielding, they just said you had to be 500′ away during operations.


  3. This incident is why I don’t claim that no one has ever died from a nuclear power plant accident in this country.

    For the smart alecks who do know about it, I reply that no one has ever died from an accident in a commercial, non-experimental power plant.

    After that I go for “Ted Kennedy 1, Three Mile Island 0”


    • Jeff Gauch

      Nuclear power will kill you quite dead if you don’t respect it. In that sense it’s identical to every other technology since – and including – fire. Compared to the number of casualties steam power had even a hundred years after it became practical, nuclear power is harmless.

      And Three Mile Island should be trumpeted by the nuclear industry. Everything that could have gone wrong, poor design, bad maintenance practices, poor paperwork, confusing indications, poorly trained operators who didn’t understand plant conditions and took exactly the wrong actions, etc. did go wrong and nobody was killed. No member of the public even received significant radiation exposure. We know how to build safe reactors. People who say otherwise are either ignorant or pushing an agenda.


  4. Philip Ngai

    I see people talk about thermal nuclear rockets like NERVA now and then. Given the power levels of a rocket, how close would you say it operates to “prompt critical”?


    • I presume you’re talking about the use of plutonium to power some satellites and space probes?

      It simply can’t go critical at all, either prompt or delayed. The plutonium is naturally radioactively decaying into other isotopes, and as a product of that, heat is generated, which is converted to power. There is no chain reaction involved.

      You could do pretty much any implosion you wish with the plutonium on board and absolutely no sort criticality would be reached.

      The concern about breaching a plutonium power cell on a failed launch isn’t even so much about the risk of radioactive contamination, but rather the extreme chemical toxicity of plutonium.