>>12394813the rods in a pressurized water reactor are good for controlling a situation where they were initially responsible in the first place (aka reactivity from the rods). Pretty much everything else (loss of pressure control, water flow, etc) requires more than just rod movement to correct for.
If for example a primary system rupture did occur. Scramming (sending all rods to the bottom -> adding lots of negative reactivity) wouldn't be enough because you'd still be generating decay heat. Ideally you'd want to isolate the leak (so shut some valves). And you'd wanna maintain pressure (water covering the core maintains convection heat transfer from the core helping prevent the melt down) so some form of water injection (reactor fill) would be desired as well.
