I'm already getting downvoted to oblivion for my gp comment, but whatever: what I was trying to convey based on my limited exposure is that a single rod will have a zone restricted shadow or local effect and multiple uniformly distributed rod banks are therefore needed for a an effective control or emergency shutoff. Additionally, on the flip side, I feel a single rod capable of shutoff will insert excessive positive reactivity in case of an ejection (could be a local steam overpressure, idk, don't hunt me down with hounds).