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: A scheme of control rod lowering

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): Neutron flux in space and time

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.

  • $\begingroup$ But the star line is for the moment when AZ5 was engaged (0 s). Therefore the rods are still fully extracted and only start to move. $\endgroup$ – OON Jun 13 at 17:14
  • $\begingroup$ @OON Indeed. Prior to lowering of the rods (line ✱), the reactivity on the bottom was lower than in the center, so it's not clear to me why it would surpass reactivity in the center after partial lowering of the rods (lines ○, □ and △). I understand why it would increase, but not why it would surpass (at >0s and ~6m) the maximum local reactivity at 0s (at ~2m). Note that this plot is in relative units, in absolute ones, the spike at ~6m is much taller, according to one source. $\endgroup$ – squirrel Jun 13 at 17:33

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