No idea why you're getting downvoted...

It's not like modern nuclear reactors can only fail catastrophically, much less a predictable failure case that engineers are actively monitoring to ensure the safe operation of said reactor.

What's terrifying are the failure cases that the engineers aren't aware of -- the unknown unknowns -- which are probably significantly fewer than the cases they are aware of and are monitoring.

Because we already know what happens when you can't insert the rods and stop the reaction.

How do we know? Are you referring to the Borax experiments or the tests on EBR-II? How are they relevant to an AGR?