Control rods are often classified according to their function. Thus it is possible to distinguish:

1. Regulating rods used for automatic control.

2. Shim rods used for compensating slow reactivity changes (e. g. burn-up).

3. Safety rods used for scram.

In many cases it is not possible to distinguish the control-rod functions because the same rods are used for the functions described in points 2 and 3. This is done in order to limit the total number of control rods, in all cases in which it is not explicitly required that some safety rod remain always out of the reactor, even in the shut-down cold condition.

According to their absorbing properties control rods can be classified as black and grey. A black rod absorbs practically all neutrons impinging on its surface; if this is not the case the rod is grey. This definition applies normally to thermal neutrons: no rod is completely black for neutrons of very high energy. Grey rods can be used when highly absorbing rods would disturb too much the neutron flux distribution. This is especially important for the rods compensating the excess reactivity, because they are most of the time inserted in the core.

The use of grey rods increases the total number of rods needed to meet the given requirement, and leads therefore to higher costs and mechanical complications. Grey rods are needed in reactors with very strict temperature limitations, but cannot be considered as absolutely necessary in HTRs.

The control-rod blackness is dependent on the composition and thickness of the absorbing material. Grey rods can be simply made of steel, while black rods consist normally of an absorbing part of B4C or borated steel canned in steel or other alloys.

The control-rod effectiveness can be increased by making them black also at higher energy. This can be obtained with a very high,0B concentration, or using additional epithermal and fast absorbers like Eu, Dy, Hf, Gd or In. The amount of absorber contained in a control rod must be sufficient to ensure sufficient absorbing properties for the required time of operation.

13.2. Control-rod configuration

It is important to program the control-rod movement in such a way as not to distort too much the power and temperature distribution. Because of this reason it is better to avoid the bank movement of the control rods, which would strongly disturb the axial flux distribution. The best solution is to have only the regulating rods half inserted, while the other rods are either fully inserted or extracted. As soon as the regulating rods have moved too far from the core centre some other rod would be moved in order to bring again the regulating rods to their position of maximum effectiveness.

It is very important to know the curve of control-rod worth as function of its insertion depth (so-called “S-curve” because of its S-shape). This curve depends on the axial flux and material distribution of the reactor and on the control-rod configuration. Very important is the number of rods being moved at the same time. A bank movement of many rods tends to shift the power towards the other end of the reactor shifting then also the S-curve. Figure 13.1 shows the S-curve for one rod, once moving the rod alone and once moving it as a bank with other rods.

The shifting of the S-curve is also important to determine the effective delay of the counteraction to accidents. If a high effectiveness is reached only at deep insertion, this results in a bigger delay.

The control-rod effectiveness is highly influenced by their relative configuration. If rods are too near to each other the flux depression due to one rod reduces the effectiveness of the other (mutual shadowing). The control rods should be distributed as homogeneously as possible in the core. Local concentration of control rods in one part of the core tends to shift the power toward other reactor zones, thus producing hot spots and reducing the control-rod effectiveness. Especially with grey rods it is possible to use them for power flattening depressing the power in large reactor zones, or simply in the surrounding of freshly loaded fuel elements.