I have been trying to understand a particular aspect of the Chernobyl accident - the role of the graphite displacers on the end of the control rods. The basic story is that the graphite displacers caused the reactivity of the core to increase when the rods moved down, a non-intuitive effect that the operators were not expecting/sufficiently aware of.
All literature I can find tells basically the same story, but I shall refer here to this PDF.
On page 17 of that PDF it describes the graphite displacers and shows a schematic of a control rod in the raised position and in the "initial insertion" position.
The text indicates that:
On moving down of absorbers into the core, their displacers displace water columns from the lower part of the core. Thus, inserting of absorbers from their extreme top position introduces a positive reactivity into the core because graphite absorbs neutrons much less than water
The graphic shows a control rod "extreme top" and "initial insertion" positions. In the second case the graphite has moved in the channel, displacing water there and increasing reactivity in the lower part of the core. The image has a "+" symbol showing this.
Also, there is reduced reactivity above the displacer as a result of water replacing the graphite in that position, and reduced reactivity at the very top of the core due to the presence of the boron control rod. There are "-" symbols showing these effects.
My question is, if the graphite displacer was already in the water column, how could moving it downwards change the overall reactivity of the core?
Obviously the reactivity distribution changes, but it seems to me looking at the diagram that the (+) and (-) from the graphite movement would cancel out overall, leaving the (-) from the boron rod: an overall reduction in reactivity.
Clearly I'm misunderstanding something.