Question

why graphite or heavy water used in nuclear reactors to slow down the speed of the reactions

Answer 1

Because graphite and heavy water are non metals as the can slow the reactions

Answer 2

actually,it becomes important in nuclear fission to slow down the thermal neutron so that it can get more time to get in contact with its reactant material.to form a nuclear fission we need a heavy nucleus like uranium and a thermal neutron.as we know neutron and proton resides inside the nucleus of an atom so it is very difficult to take it out after we take the neutron out of the nucleus, it shows its  energic and fast movement .with that type of nucleus we cant perform nuclear fission because of its random movement.as the random movement will not allow much time to fuse with the heavy nucleus as it will bounce back from the nucleus.so to continue the nuclear fission ,the thermal neutron is passed through heavy water and it slows down its movement by realeasing some energy.thus the neutron now can perform fission.


hope this had helped you.