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Old 06-19-2019, 09:17 AM
Frankenstein Monster is offline
Join Date: Apr 2004
Location: Europe
Posts: 706
Not a reactor expert either but I think that picture is not of the reactivity of the reactor but the neutron flux.

The higher the neutron flux, the higher the increase in neutron flux in response to a small increase in reactivity.

A large contributing factor to the accident was the large axial difference in power distribution in the core at the time of the accident. Spatial power differences in a large core like RBMK were already "difficult" to control. (That is, it worked well during normal operation but was very much amplified during the conditions prior to the accident.) The accident happened when a small amount of extra reactivity was added to a region that already had an extra large power distribution and extra large positive void coefficient.

Remember that the "problem" of the extra reactivity due to the graphite displacers was known but not thought to be very important, since the reactor was not supposed to be anywhere near the state it was in during the accident.

IAEA report