I’ve read the Wikipedia article and watched a couple videos I’m still a bit confused
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So, the important things about plutonium and other fission fuels (thorium, uranium-235 etc) are
– exposing them to free neutrons makes them unstable
– and when they decay they *produce* free neutrons
So they’re the nuclear equivalent of being on fire, very slowly smoldering unless you do one of two things:
– add neutrons from an external source
– do things that help the neutrons stay inside the fuel
Scientists were experimenting with the second technique, called “criticality.” They made a piece of plutonium that was very close to the critical point. Normally, enough neutrons (barely enough) were able to escape into the environment (hopefully into a shield) but a nearby neutron reflector would allow the reaction to grow.
It grows slowly at first but keeps accelerating (exponential growth) and after that it still keeps accelerating, and keeps accelerating until something changes and causes neutrons to escape.
This is similar to a fire with a *lot* of available fuel. Like a fire in a lumber yard. It can smoulder for a while but when it starts to become big, it becomes BIG shockingly fast.
Both accidents involved putting pieces of reflector material near the core, by hand and fumbling it somehow. The flare-up (a “power excursion”) released a lot of heat and radiation.
Fun fact: experiments to find the critical point are *still* very common. That’s how you start up a power reactor – gently find the critical point. You can guess pretty well by using the data you have, but the exact point depends on the exact condition of the fuel in the reactor.
The difference is that there are fast and automatic shutdown systems, the intended startup rate is pretty slow, all the controls are remote, and of course there’s a *lot more shielding.* Nobody starts chain reactions by hand anymore. Especially not while wearing cowboy boots.
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