Demon core from Fat Man/Little Boy

In the film Fat Man and Little Boy, there is a fictionalized account of the Demon Core which was responsible for the death of two scientists in two different accidents. John Cusack’s character is conducting experiments on a core when an accident causes it to monetarily become critical. After knocking the top half off with his hand, he instructs the others in the room to mark their positions on the floor and remove anything they are wearing that is made of metal. Why would it be a good idea to remove anything made of metal? What sort of radiation were they exposed to that would make this necessary? I would expect a high density neutron flux to cause most elements to become radioactive, not merely metal ones.

This is way outside of my specialty, but I suspect that heavier elements have nuclei with a larger neutron cross section, whereas the neutrons (mostly) pass through lighter stuff without incident. Neutron capture, if that’s the right term, can result in unstable nuclei that can break down and emit radiation after exposure.

I suspect that at that time, there wasn’t a lot of experience with the emissions of just barely critical plutonium and what it would affect. I mean, the guy was “twisting the tail” with plutonium using just a screwdriver.

It’s a movie. They’d never let facts get in the way of a good dramatic scene…

The highest cross-section for interaction in general is actually for light elements. But that’s going to be mostly elastic collision, not absorption.

It was probably neutron activation that was the reason for the removal of metal stuff.

Different metals have differences in activation potential, half-lives, and radiation after activation. Where the items were worn on the body and their orientation in relation to the source also matter. By analyzing the radiation from activated metals over time, a reconstruction can be made of what neutron spectrum and intensity they were exposed to, and thereby calculate the dose received by the person.

Leaving items in place marks their position relative to the source when they were exposed. It prevents their loss in the hospital or during transport, and allows easy access for the people who will analyze them. It also prevents irradiation of hospital personnel or family.

It was taught to me as a standard of practice for criticality accidents.

What was the purpose of the experiment? To see if the core was capable of the supercriticality required for the weapon design?

In the movie, John Cusack plays Michael Merriman. There was no real life Michael Merriman.

The real life Demon Core killed two different people in two different incidents. In the first incident, a scientist named Harry Daghlian was performing neutron reflection experiments on the core. He was placing tungsten-carbide bricks around the core and measuring how much their neutron reflectivity affected the core. The more bricks placed around the core, the greater the neutron reflectivity, increasing the core’s reactivity. Daghlian accidentally dropped a brick, causing the core to briefly go prompt critical. He quickly removed the brick, but had already received a lethal dose of radiation and died 25 days later.

In the second incident, a scientist named Louis Slotin was again testing neutron reflectivity to determine the exact point at which a sub-critical mass could me made to go critical by the use of neutron reflectors. His experiment used two half-spheres of beryllium, one under and one over the core. The top half-sphere was manually moved into place and Slotin kept them spaced apart using a screwdriver. There were other scientists who did not approve of this experiment, and Enrico Fermi reportedly told Slotin that he would be dead within a year if he kept doing it. Sadly, Fermi proved to be correct. After doing the experiment dozens of times, Slotin’s screwdriver finally slipped, allowing the top half-sphere to drop far enough that the increased neutron reflectivity caused the core to go prompt critical. Slotin immediately jerked upward on the screwdriver, flipping the beryllium half-sphere off of the core and onto the floor, but he had already received a lethal dose of radiation and died 9 days later. Another scientist, Alvin C. Graves, had been watching over Slotin’s shoulder and suffered from severe radiation poisoning, but did not receive a fatal dose of radiation. He suffered from chronic neurological and vision problems for the rest of his life, and died only 20 years later. It’s not clear if the radiation poisoning shortened his life, but many people think that it did.

Cusack’s character is a combination of both Daghlian and Slotin, and the scene seems to be mostly based around the second incident.

After the second incident, hands-on experiments like those that Slotin had performed were banned. Raemer Schreiber, one of the physicists who had been in the room with Slotin, designed several remote control devices that could be used for testing, and all personnel were required to be a minimum of a quarter mile from the test apparatus.

ETA: Wikipedia’s article on the Demon Core:

Thanks for the very thorough explanation. A question:

In both of those incidents, the victim apparently corrected his mistake within a split second of making it.

What if he hadn’t? If those guys had fainted or something instead of just having the brick or screwdriver slip, so that the brick or beryllium remained on/near the core for an extended period, could there have been a meltdown? If so, it sounds incredibly reckless of them (and their management) to have such disaster-prone procedures.

There are two possibilities. As the core went critical, heat from the reaction caused it to expand. If this expansion is enough to move the atoms out just far enough to stop the critical reaction, then the reaction would just stop. However, as the core cooled it would shrink, and would then go back into criticality and the reaction would start up again. So basically you’d end up with this pulsing and pretty much unshielded reactor that started and stopped itself over and over until someone removed the brick/half-sphere and stopped the reaction.

On the other hand, if the thermal expansion didn’t stop the reaction, then the core would melt.

One thing I don’t understand: Aren’t the plutonium cores surrounded by neutron-reflecting material all the time when they’re inside missile warheads or bombs? Isn’t that part of the weapons design? Then why don’t they have criticality stuff going on 24/7 while located inside the warhead/bomb?

Because the warheads are designed with the mass in a subcritical configuration. It requires a LOT of work to move them properly into a critical configuration.

That part I did not know, thanks.

Yeah, like a soccer ball coated with plutonium powder. Explosives jam it all together so fast it can go critical before blowing itself apart. Very hard to engineer, which is a good thing.

The thing about Plutonium is that it can critical very easily, with no outside help. Even without reflectors or moderators, enough Plutonium mass together will cause an explosion. Not a huge one, because it vaporizes or it becomes molten very quickly and blow itself apart, stopping the reaction. It’s why they couldn’t use a gun and fire two pieces together, like they did with enriched Uranium, to make a bomb. Even at high velocity, long before the two pieces reached each other, a reaction would cause the entire mass (and all the metal surrounding it) to vaporize or splatter apart as a molten mess.

An “explosion” of sorts, but not an atomic bomb type.

Well, no.

The “pit” of a Plutonium bomb is almost pure Plutonium. The explosives are arranged around it, and when they are detonated, the metal itself is compressed to a significant degree, which makes it go critical. It helps that Plutonium undergoes a phase change when compressed.

I was trying to mislead about the design of an H bomb. So are you obviously.


No, my description is correct.

Pardon my ignorance, but why was the plutonium ball in front of Slotin able to go critical simply by putting a beryllium shell (a neutron-reflector) over it, thus making it go from subcritical to critical, but the plutonium ball in a nuclear weapon is still subcritical even though it’s surrounded by neutron-reflecting material?
Isn’t the plutonium core in the same size and shape in both cases?

What I’m trying to say is, it seems to me that putting a plutonium core in a nuclear bomb, then surrounding it with neutron-reflecting material, is doing to it exactly the same thing that Daghlian and Slotin did by accident to the core, yet it doesn’t go critical.

IANANuclear physicist nor a weapons designer, so I might be entirely wrong here, but my understanding is that there is not enough plutonium in the weapon core for it to go critical, even with the neutron reflecting material that may be around it. When the conventional explosives in the warhead go off all around the core, they compress the whole thing momentarily, putting it under considerably higher pressure, which makes the lower mass of plutonium dense enough to react energetically.

Basically, a wee bit of plutonium isn’t putting out enough neutrons (protons? It’s been years since I took a high school science class) to create a chain reaction. Like a small pile of nitrate fertilizer, it’ll just sit there. Get enough of it together and it’ll fizzle. Get way too much of it in one pile and it’ll go bang. Set a smaller amount of it up in a warhead with a detonator designed to go off more easily in a specific controlled fashion, and it’ll go bang much more easily at the time and place of your choosing.

Mind you, the whole thing is designed to only work if it goes off just right. If it malfunctions, or if the vehicle transporting it is in a crash, the bomb won’t go off in a huge blast, it’ll just make a nasty hazmat mess in the vicinity of the crash but otherwise leave the area intact. The fact that the warheads are designed to fail-safe so easily is also why it used to be (and may still be) the practice to have several warheads earmarked for each target. If your only nuke that hits the Kremlin fails to go off, that’s embarrassing. If one nuke out of fifteen that you drop on the Kremlin fails to go off, it’s not like you were worried about overkill to begin with.