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        Ordinary pure water and Heavy water are used to moderate the speed of neutrons that are generated during  the fission process.  They are used in Pressured water nuclear reactor and Boiled water nuclear reactors.

     Uranium isotopes are fissioned in a nuclear reactors.  The two emitted neutrons have an energy of 2 Mev and very very high temperature. The neutrons have to collide with other uranium atoms to sustain nuclear fission chain reaction.  For this the speed of the neutrons have to be brought under control, to increase the probability of absorption/collision with uranium atom. 

      The efficiency of moderation of speed of neutrons is better when neutrons collide elastically with atoms of similar radius/size.   Thus hydrogen atoms provide good moderators.  This is the reason to use water H2O and Deuterium (D2O).

      Moderator efficiency is higher when mass is less, scattering cross section is higher, and absorption cross section is lower.

Light (ordinary) water is less efficient as compared to heavy water.  However, water reactors are cheaper.

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