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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.

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  • $\begingroup$ 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
    Commented Jun 13, 2019 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
    Commented Jun 13, 2019 at 17:33

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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.]

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    $\begingroup$ as you are mentioning rapidly increasing voids in the lower part of the reactor, this reads as though the explosion was unavoidable regardless of the design of the rods $\endgroup$
    – squirrel
    Commented Jan 11, 2020 at 1:12
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    $\begingroup$ The configuration and thermodynamics of the core and coolant inside the reactor #4 in the minutes leading to the 'event' were mindbogglingly unstable. At the moment of AZ5 activation the explosion likely was already unavoidable. When T-G #8 valves opened for steam to power the reactor circ pumps (for the "electrical test") the pressure began to drop, steam voids formed in the now 'super-heated' coolant throughout the core. Virtually all of the control rods were fully withdrawn (and slow to insert). Once steam began to form throughout the core... too late regardless of the design of the rods. $\endgroup$
    – Doug
    Commented Jan 11, 2020 at 4:08
  • $\begingroup$ The coolant and fuel parameters (Tin, Tout, Tsat, Tf) at the initiation of the "electrical test" set up a perfect storm for the positive void coefficient to dominate... everything! How they came to bring the reactor to that condition is a tragedy of failure to understand what they didn't know and being pushed by those that thought they knew everything. None of them were ever told, or didn't appreciate how unstable the RBMK (in its original configuration) could be at low power with 'inadequate' shut-down margin and major Xenon going on. $\endgroup$
    – Doug
    Commented Jan 11, 2020 at 5:22
  • $\begingroup$ You should note that the increase in total power began after AZ $\endgroup$
    – Abdullah is not an Amalekite
    Commented May 15, 2022 at 10:34
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Contrary to the current answer, the power surge commenced after, and as a result of, the AZ5 pushing. This myth has been debunked more than three decades ago. See IAEA INSAG-7 Annex I Report by a Commission of the USSR State Committee for Supervision of Safety in Industry and Nuclear Power.

As for the answer to your question, there are a number of factors.

  1. The positive scram only occurred in the lower 223cm if a 7m reactor

  2. The absorber rods were entering the top

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