How did the Louis Slotin criticality accident actually cause radiation release?

Question

I read a bit on radiation and criticality, so I have some familiarity with that. I was reading about the details of the criticality accident. From what I understand, he was trying to place a neutron reflector on top of the demon core, and kept it apart only with a screwdriver wedged between the two reflector halves. The screwdriver slipped, the reflector made contact with the core, and radiation was released.

My question is, was it the physical contact of the reflector that caused the radiation to be released? How? From what I understand, reflectors speed up fission kind of how a potato cooks faster when wrapped in foil. Is it really all that it takes to cause nuclear fission, and not some elaborate bomb or reactor mechanism?

In: Physics

Answer 1

> Is it really all that it takes to cause nuclear fission, and not some elaborate bomb or reactor mechanism?

What is “takes” is access to an adequate amount of fissile material. If you have a ball of pure plutonium-239 sitting around, it takes very little to make it go critical. All you need to do is increase the reaction rate over the “non-critical” to “critical” tipping point, which is all Slotin’s accident did. (An alternate way Slotin could have made it go critical would have been to drop it in a sink full of water. There are many properties that can tip such a system over a point. That is basically what a reactor is — a bunch of fissile material in water, or some other kind of neutron moderator.)

To make a large explosion, you do need an elaborate mechanism to keep the reaction going at a higher rate than what Slotin accomplished. This is why it didn’t explode. Reactors are only complicated because they are designed to keep the reaction running for a long time in a safe way, and to siphon off the heat or neutrons or whatever they are designed to do. Slotin effectively did create an unshielded reactor for a fraction of a second.

But to emphasize: the “hard part” is the fuel production. If you already have a near-critical amount of fissile material, making it go critical is not hard; there are several variables that could change that would make it do that (increasing neutron reflection is what Slotin did; increasing neutron moderation is another; changing its proximity to more fissile material is another).