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So, in Wikipedia it says the following:

"Most of the reactor control rods are inserted from above; 24 shortened rods are inserted from below and are used to augment the axial power distribution control of the core.

But how do they do this? Why does it matter whether they're inserted from above or below?

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  • $\begingroup$ It doesn't really matter, in terms of their control function. Conceptually their presence or not determines the energy produced, by controlling the amount of neutrons absorbed, which otherwise would be causing fission reactions. Maybe for specific designs it is desirable on case or the other, but in any case this has nothing to do with their function in the reactor, but with the mechanical design or other considerations from the engineering point of view. $\endgroup$ – rmhleo Feb 15 '16 at 14:21
  • $\begingroup$ Ok I will make it into an answer with maybe some extra reference or something more. $\endgroup$ – rmhleo Feb 15 '16 at 14:30
  • $\begingroup$ Comment to the post (v1): Consider to spell out acronyms. $\endgroup$ – Qmechanic Mar 27 '16 at 15:14
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The function of the reactor's control rods is to regulate the amount of fission occurring in the fuel. They are made of Boron and Cadmium and other elements, to absorb most of the neutrons. So the higher portion of them inserted into the fuel, the more absorption and the less fission reactions occur. And removing them has the opposite effect, allowing more neutrons to scatter around, thermalize, and cause fission.

So the principle has nothing to do with their shape or from which side they are introduced. You could have control layers in a sandwich-like reactor, which go in and out to control fission rate in a similar way.

However, the specific way in which they are built, geometry and shape, might be important for mechanical and engineering reasons. If a layer needs to be very thin, mechanically would be undesirable because of its deformability at high temperatures, which could lead to accident scenarios more likely.

Summing up, the direction and shape of the controls is not directly related to the concept of the reactor, but rather to the feasibility and engineering constraints.

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  • $\begingroup$ Sure! Welcome to PSE :) $\endgroup$ – rmhleo Feb 15 '16 at 15:02
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The power distribution in a nuclear reactor is never uniform. Peripheral leakages causes the neutronic flux to be lower in the peripheral areas than in the center of the core. In a theoritical cylindrical homogeneous reactor, the axial distribution of power is as a cosinus(z).

The insertion of control rods from the top of the reactor shifts the neutronic flux and power distributions to the bottom areas of the core.

Controlling the axial distribution of power is a major concern in most of large reactors as there are subject to instabilities known as "xenon oscillations". More, for RBMK's, there is as stronger source of axial instabilities due to the inhomogeneous distribution of void in the coolant (boiling water).

A few short control rods, inserted upwards from the bottom of the core, allow the operator to even the axial distribution of power.

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