if the cooling system fails, the fission stops by itself with no active involvement required.
At that point i already stopped reading since it emphazises that you got no clue.
Yes the fission stops. But the fuel still have to be actively cooled for month or years, because it still produces a lot of heat:
https://en.wikipedia.org/wiki/Decay_heat
Guess what happened in Fukushima. The reactors shutted down successfully, but the power supply for the cooling failed due to the flooding.
Future citizens will live in a total shithole, but at least they are less indebted.