It is fascinating for sure. I don't think there's anything in chemistry like it. It depends a lot on the geometry. A chemical reaction can be sped up or slowed down by the shape of something, but that's just because of exposed surface area.
In the case of Slotin, the thing he dropped onto the core was a neutron reflector so it redirected neutrons back into the core.
This is an interesting read, it's a story about a more recent near criticality that took place in 2011.. You can see a picture in the article of the dangerous configuration -- it's just a few rods of plutonium near each other. Any closer, if one tips over into the other, and they might go hot and release a huge amount of radiation.
> On August 21, 1945, less than a week after Japan notified the US that it would accept the terms of the Potsdam Declaration, physicist Harry Daghlian was performing an experiment when he accidentally dropped a piece of “tamper” material, used to reflect neutrons back into the core, and triggered a critical mass. Daghlian used his bare hands to pull the mass apart to stop the chain reaction, and absorbed a fatal dose of radiation. He died three weeks later.
> As luck had it that August day, a supervisor returned from her lunch break, noticed the dangerous configuration, and ordered a technician to move the rods apart.
> But in so doing, she violated safety rules calling for a swift evacuation of all personnel in "criticality" events, because bodies — and even hands — can reflect and slow the neutrons emitted by plutonium, increasing the likelihood of a nuclear chain reaction.
> A more senior lab official instead improperly decided that others in the room should keep working, according to a witness and an Energy Department report describing the incident.
This part is confusingly worded.
Once the dangerous configuration was noticed what was the right thing to do?
> As a result, Nichols said, the first thing to do upon noticing a near-criticality is "the opposite of what you want to do," such as reach in and separate the offending materials. Instead, he said, those in charge should get "everyone to back off" and then call for engineers to start calculating safe approaches.
If you look at the energy per unit mass, e.g., J/kg, of different materials, you'll note that curiously many fuels (hydrogen, methane, petrol) and explosives (gunpowder, TNT, ANFO), you'll find that the former are roughly ten times (or more) greater.
What makes explosives, well, explosive isn't the total energy contained within them, it's the rate at which it's released. Jet fuel contains ten times the energy per unit mass than C4, and can in fact melt (or at least significantly weaken) steel beams, but it does so by burning over time.
What explosives do is to combine oxidiser and fuel in the same package (as with gunpowder and ANFO), or contain chemically-unstable bonds with high potential in a state which can be triggered by a sharp shock (TNT, C-4/RDX). The total energy released is smaller, but the rate of release is far greater.
It may be possible to use more conventional fuels to generate explosions. This happens with hydrogen gas, particularly in a stoichiometric combination with oxygen, with petrol within an internal combustion engine (the fuel burn is explosive), and in a fuel-air bomb (a/k/a thermobaric weapon), in which a fuel is widely dispersed in the atmosphere and then ignited. The blast generated is typically far weaker than of an equivalent conventional explosive, but can still be explosive rather than a deflagration (rapid combustion not generating a shock wave). Incidentally, virtually all cinematic "explosions" are in fact deflagrations, often using either flammable gas or suspended powder. There are also relatively frequent dust explosions involving powdered foodstuffs (grain, flour, sugar, etc.) which are a hazard where large quantities of such materials are stored or processed (grain silos, processing plants).
In particular, storage combinations of potential fuels and oxidisers in close proximity can lead to explosions.
There's also the case of spontaneous combustion particularly of oil-soaked rags or compost piles which shares some characteristics with criticality incidents. That's where heat release which in smaller concentrations would be benign reaches the ignition point of the materials involved. Large heaps of freshly-mown grass in particular can spontaneously ignite. I've had the experience of moving a large pile of woodchips which had been left in sub-freezing weather and discovering that the core of that pile was literally steaming hot, and was melting snow and evaporating water which had flowed in toward it. The chips weren't charring, but they were distinctly warm.
In the case of Slotin, the thing he dropped onto the core was a neutron reflector so it redirected neutrons back into the core.
https://www.science.org/content/article/near-disaster-federa...
This is an interesting read, it's a story about a more recent near criticality that took place in 2011.. You can see a picture in the article of the dangerous configuration -- it's just a few rods of plutonium near each other. Any closer, if one tips over into the other, and they might go hot and release a huge amount of radiation.