How exactly did lowering of the graphite parts of the control rods on Chernobyl's RBMK reactor result in reactivity surge?

Question

There are many sources giving different data on the initial position and the dimensions of the rods, but most suggest that the absorber was located just outside the core when AZ-5 was pressed, and the displacer graphite was located roughly in the middle of the core. Then the control rod begun slowly lowering, as can be seen here:

What happens next is puzzling, and I couldn't find a good explanation of it. According to the illustration, there are two reactivity decreasing zones ⊖, and only one reactivity increasing zone ⊕, the latter stemming from the graphite absorbing fewer neutrons than water while still lowering their speed. But this zone looks roughly equivalent to the second ⊖ zone, which is the inverse of it.

So in order for this to lead to a positive reactivity surge, aka positive scram effect, aka end-rods effect, there should be a lot more of reactivity going on in the bottom of the core, but it seems that the opposite was the case; in fact, more neutron flux was recorded on the top (see the ✱ line):

So why did the surge occur? Could it be that a lot of xenon-135 accumulated in the middle section of the reactor, and the bottom was relatively free of it, allowing for an unchecked reactivity increase in the presence of the graphite?

Illustration source: INSAG-7, pp. 123, 122.

Doug · Answer

At the moment of AZ5 initiation(*): The power excursion has already commenced. Driven primarily by the rapidly increasing formation of steam bubbles (positive void coefficient) along the entire length of the fuel (because the entering water was not subcooled). The steam bubbles collect and concentrate in the upward flowing coolant; i.e. more voids and reactivity at the top. At AZ5 initiation, the upper section of the core is accelerating faster than the lower section due to the greater void cross-section as the steam voids concentrate toward the top of the fuel channel.

Then, at 4 seconds and later, the profiles indicate that the 'active' control rod insertion is reversing the power excursion at the top of the core as the boron section enters from the top... but the lower part of the core is still increasing and accelerating, responding to the rapidly increasing voids AND a positive reactivity kick from the graphite displacer section of the control rods, entering the lower core. [The brakes were working near the top of the core, but the throttle went beyond wide-open near the bottom.]

How exactly did lowering of the graphite parts of the control rods on Chernobyl's RBMK reactor result in reactivity surge?

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