The [RR] rod of CSS is represented on Pic. 16 consist of two sections: section of an absorber of neutrons made of Boron carbide, having length almost equal to height of an active zone of the reactor (~7m) and ousting section made of graphite (~4,5 m). These sections connected one with another by telescopic draw. Rods move in channels of CSS (similar to technological channels in which take places Heat Generating Assemblies HGA) and are cooled by water.
When the rod is in extreme top position (Pic. 16a) its graphite part located in the reactor core. Graphite is a moderator, almost not absorbing neutrons, unlike water which is moderator too, but absorbs neutrons. If the rod is in extreme bottom position (Pic.16d) then the strong absorber Boron carbide is located in the reactor core.
Thereby moving of a rod from extreme top in extreme bottom position inserts in the reactor the big negative reactivity, capable to shat down the reactor at any emergency (if, of course, it thus does not collapse).
Let us look however, how this negative reactivity is brought during the time. At rod moving (Pic.16 b), there is brought negative reactivity in the top part of reactor core, at the expense of immersing in the core of a strong absorber (Boron carbide). During the same time the water in the channel of CSS at the bottom part of reactor core is supplanted by graphite and it brings positive reactivity as graphite absorbs neutrons, much less than water. It proceeds until all water column in the bottom part of reactor core will be ousted, then only negative reactivity is brought (Pic.16 b,c).
If the negative reactivity brought in the top part of reactor core appears less than positive one, brought in the bottom part, it will turn out that the rods at some stage of plunging into the reactor core, make the reactor runaway instead of shut it down.
The reactivity value which brings a rod moving in the core depends on value of a neutron flux in that place where this reactivity is brought. If the density of a neutron flux is in regular intervals distributed on height of the core (as on Pic. 16a), i.e. it is identical above and below, then of course, there is much bigger (approximately in 2 times) negative reactivity brought above, than positive reactivity brought below, and summary brought reactivity is negative. If the neutron flux below is much more than above the situation is opposite, and the summary brought reactivity is positive.
The value of a neutron flux in the given local place depends in turn on presence or absence in this place of an absorber. I.e. spatial distribution of a neutron flux (a neutron field) varies by moving of rods, in one place it caves in, and but in other place bulge out.
If rods in a reactor core are in any casual positions, at simultaneous movement of all rods downwards (as occurs at SCRAM) these changes of a neutron stream are local and also are casual so as a whole (in distribution of neutrons) on the reactor varies nothing. There is a normal input of negative reactivity by constant speed of movement of rods. If almost all rods are in extreme top position, the distribution of neutrons across height of a core will become strong deformed at their simultaneous movement. So it was at that time in the Chernobyl accident , and so it is shown on the Pic. 17 a), b) and c).
And in the reactor by the SCRAM button were entered for some seconds by control rods a positive reactivity while the water columns were superseded.