A moderator is a material used in a nuclear reactor to slow down the neutrons produced from fission. By slowing the neutrons down the probability of a neutron interacting with Uranium-235 nuclei is greatly increased thereby maintaining the chain reaction. Heavy water (D2O) is used as a moderator in nuclear reactor. It containing significantly more than the natural proportions (one in 6,500) of heavy hydrogen (deuterium, D) atoms to ordinary hydrogen atoms. Heavy water is used as a moderator in some reactors because it slows down neutrons effectively and also has a low probability of absorption of neutrons.

Bohr Atomic Model. Bohr Atomic Model : In 1913 Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus.