[edit] - as an nuclear engineer - that study you cited reminds me of perturbation theory. You make a small tweak to the system and develop a model to fit how such small changes affect the larger system. But perturbation theory conclusions don't scale accurately up to "large" perturbations. If you pull the control rods a tiny bit in a subcritical reactor - you get higher neutron-multiplication factor for your existing/present neutron sources. So you might think that if you pull the rods 10 times as far, you get 10 times the multiplication - but you'd be wrong. If you pull them 10 times as far you go super critical and then you get like 1,000,000,000 times as much multiplication.