Eli5: What causes a nuclear core to go super critical?

Question

Eli5: What causes a nuclear core to go super critical?

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Answer 1

“Critical” is when the reaction is balanced, so on average 100 fissioning atoms lead to 100 more fissioning atoms.

When you want more power from the reactor, you want it to be “supercritical” for a while. So pull out the control rods, and watch the activity in the core increase. This is fine if it’s under control, and doesn’t exceed the safe maximum level.