that's the thing though, the exponential chain reaction isn't possible.

The problem is that when fuel breaks the strictly controlled fuel rod environment, it stops being cooled properly, and regulating it becomes more interesting (not impossible, there are some clever solutions out there, look at metal cooled reactors for example) and as a result, the spicy particle generation tends to break containment, which is why we have things like PCVs, which contain the corium long enough to at least prevent the elephants foot troll, which is then contained by the secondary containment (the building around it) which is also contained by the rest of the building, surrounding the containment building.

It's pretty hard to fuck up a reactor. Even harder when the idle state of the reactor is safe, as is with metal cooled reactors. Those are some of the most promising designs, because you can literally just do nothing with them, and nothing bad happens.