Eli5: What causes a nuclear core to go super critical?
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A radioactive atom emits radiation by, basically, carving off a chunk of itself and shooting it off in a random direction. If that chunk (often a neutron) hits another radioactive atom it might cause it to also split off a chunk and emit it.
Left to its own devices, any given amount of radioactive material does this slowly enough that we can’t get any useful energy out of it. In order to build a nuclear reactor we need some way to ensure the process is *self-sustaining*: every neutron fired off by a radioactive atom causes at least one other atom to also fire off a neutron. We do this by inserting control rods that bounce the neutrons back into the radioactive material until they finally collide with something.
When this is all working properly the radioactive material produces a predictable amount of heat and we can use that to boil water and turn turbines to produce electricity. But if it starts happening too quickly then it can produce more heat than the system can handle. Things start overheating, steam pressure builds up, things start cracking and exploding.
Usually this is because something went wrong with the control rods: they were inserted too far or left in place too long or some such. There are generally failsafes for this built into the design and procedures of nuclear reactors so it’s pretty rare. When it does happen it’s usually because multiple things have gone wrong at the same time.
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