So trying to reduce it to something your grandmother could understand, recognizing there are 1000 exceptions and caveats
A nculear power reactor operates like a stagecoach going over a mountain.
When things start up, you have to hook up the horses to get it going . the horses have to work to climb up the mountain and are eating their hay. When the stagecoach reaches the top of the mountain and the stagecoach is now able move along by itself, it has “gone critical ” and isn’t going to roll backwards down the hill.
So now the stagecoach goes down the hill. To keep things running slow and safe, you keep a really tight grip on the brakes, and don’t let the thing go faster than 2 MPH. If your brakes suddenly fail, you apply a second set of brakes that immediately stop the stagecoach, and you don’t move again until the stagecoach repair man comes out. But keeping the last ditch brakes on requires you to sit in the stagecoach holding the brake in place. If you let go of fall asleep, the stagecoach might start rolling down hill again, depending on if you stopped on a flat place on the hill.
If you hop out of the stagecoach and it starts rolling again, and there is nobody controlling the brakes, the thing will start going down hill on its own, going faster and faster until the wheels are going so fast, they break about into a million pieces and the thing crashes, and we’ve had a “criticality event”.
Remember the objective is to make it so your grandmother can understand it, not accuracy :) Feel free to craft one of your own.
