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I can't seem to find anything addressing this. We know that while the control rods were fully removed, the graphite extensions were fully inserted. Therefore, the reactor must have been more active in the bottom of the core to begin with, before the rods were lowered. Otherwise the lowering of the rods would have zero impact since the water was already displaced with graphite in the center.

So my question is why? I've heard that the amount of steam or voids was greater near the bottom, but I'm not sure why that would be.

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  • $\begingroup$ I happened across an explanation the other day Why Chernobyl Exploded - The Real Physics Behind The Reactor $\endgroup$
    – mmesser314
    Commented Jan 19, 2022 at 5:23
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    $\begingroup$ @mmesser314 Youtube videos tend to be hit-and-miss, but that one is a hit for sure. Great find. $\endgroup$
    – rob
    Commented Jan 19, 2022 at 5:47
  • $\begingroup$ @mmesser314 I've seen the video, upon re-watching I'm still not certain though about how it explains the uneven power distribution. If the void coefficient was responsible for it, shouldn't the reactivity still have been higher in the center where the rods completely displaced the water? Let alone replacing the space with a moderator. $\endgroup$
    – ShaneC
    Commented Jan 19, 2022 at 5:59

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The vertical distribution of the power in the corevwas initially double humped, with the power concentrated towards the top and bottom at 01:23 when the accident started. In fact the maximum was near the top. From IAEA INSAG-7

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I suppose before the accidental loss of power at 00:28, the middle of the reactor was indeed operating at higher power. But then the reactor poisoning, which was gretest where the reaction had been the most intense, reduced the reactivity there, creating the situation mentioned above.

But your statement that if the power maximum had not been at the base to begin with, the positive scram would not occur at all, is incorrect. At best it is an exaggeration, since even a small amount of power, if present, could be multiplied, even if the resulting power spike is not of such a high magnitude.

And the claim you have heard that there was more steam near the bottom is probably false. The higher power at the top us evidence of that

If you are confused as to why the power surge was mostly at the bottom, that was because

  1. The positive scram happened at the bottom

  2. The absorber section of the rods entered the top

  3. The poison in the middle blocked the neutrons rising from the bottom

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