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
> was it the physical contact of the reflector that caused the radiation to be released? How?
Not the physical contact per se, but the proximity to the core.
> reflectors speed up fission kind of how a potato cooks faster when wrapped in foil.
Sort of? The neutron reflectors increase the amount of neutrons passing through the radioactive material which increases the amount of interactions, meaning the radioactive activity increases.
> Is it really all that it takes to cause nuclear fission
Yes. The elaborate setups are to do it in a specific, controlled way. But just cramming enough radioactive material together will result in a runaway chain reaction.
> My question is, was it the physical contact of the reflector that caused the radiation to be released?
No, it was the geometry of the reflectors. With the two half-sphere reflectors slightly separated the core was sub-critical, because too many neutrons escaped for a chain reaction to take place. When the two reflectors completely encased the core they reflected enough neutrons back to cause the core to become a critical mass, and a brief but intense power excursion resulted.
After a few seconds the heating of the core caused it to expand enough to drop below the criticality threshold again. That’s why it wasn’t a bomb. To get a nuclear detonation you have to force a supercritical mass by violently compressing fissile material.
Nuclear fission is caused by neutrons breaking apart uranium and/or plutonium atoms — creating atoms of lighter elements and generating a couple more neutrons in the process.
The key to a nuclear chain reaction is convincing those two neutrons to break apart another atom each, which release another two neutrons, and on it goes.
When this happens, the mass has gone ‘critical.’
When you have a ball of subcritical plutonium, most of the neutrons just end up flying out into space. When you introduce a neutron reflector, those neutrons bounce back into the mass to cause more fission.
In a nuclear reactor fission is managed by retractable neutron-absorbing control rods that keep the reaction from racing out of control.
When Slotin’s reflector slipped and fell on the plutonium pit, the reaction raced out of control and showered the surrounding area with neutrons. If the reflector hadn’t been removed, the pit probably would have heated to the point of melting.
An atomic bomb is little more than a bunch of high explosives that crush a subcritical mass into a supercritical mass — the plutonium (or uranium) atoms are shoved so close together the nuclear chain reaction builds exponentially to the point where the energy is explosively released in a matter of microseconds.
The amazing thing about such short timescales is that weapons designers add things like heavy metal tampers designed to contain the exploding reaction for just a few millionths of a second — long enough to get one or two extra neutron generations in and amplify the explosion by several orders of magnitude.
> 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).
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