[CGB_Spender]

Today i red that Gadolinium can be used as a rod of safety in a nuclear reactor. Thats because Gd, has the bigger microscopic active cut for the absorbsion of thermic neutrons. (49000 barns) However in nuclear reactors the use of Graphite is very-very frequent. Especially in Heavy-water reactors. I want to ask why. Is it because of the price or is something else. Thanks.

[pantone159]

Graphite is used for the completely opposite reason from Gd, it is to make the neutrons MORE effective, not less.

Graphite has a very low neutron cross section (when very pure!), and since it has a small atomic mass, neutrons lose a lot of kinetic energy when deflecting off C nuclei, and so are slowed down to become 'thermal'.  When slow, neutrons are absorbed much more readily by U nuclei, so interactions are more likely to result in fissions.

[CGB_Spender]

Ok, i see. I have see a diagramm of a heavy water reactor,that uses rods of graphite as control rods. If - as you say, they use graphite for the creation of thermal-more effective neutrons, then they must have and rods for stop the reaction. I didnt see rods of Gd or something the same effective as it. If you have rods for the creation of the effective thermal neutrons, then you must have something that allows you to stop this, correct? So what they use for stop the reaction? How they do this, without the use of the proper rods? Thanks for your time.   

[Borek]

So what they use for stop the reaction? How they do this, without the use of the proper rods?

No idea, however, you are overlooking a very important factor - if you remove carbon rods number of thermal neutrons falls down which should slow reaction down - and that's what you wanted.

[Will]

They use control rods to cool the reaction in case of emergency, or to stop the reaction continuing. See here for info on control rods. I knew that hafnium was used for control rods, but they use Gd, as you said and more commonly "silver, indium and cadmium."

Other elements that can be used include boron, cobalt, hafnium, gadolinium, and europium.

In many water moderated reactors it is common to add a small amount of boric acid to the coolant to act as a chemical shim  this has the effect of lowering the reactivity of the core in the same way as a solid control could. This chemical shim  along with the use of burnable neutron poisons are used to assist regulate the long term reactivity of the core^[1], while the control rods are used for rapid changes to the reactor power (eg shutdown and start up).

There are probably other elements/compounds which could be used, which aren't mentioned here.
I hope that answers part of your question.