What is the Purpose of using Heavy Water in Nuclear Reactor?

Heavy water is used as a moderator. It is used to slow the neutrons being directed at the fissionable material, by means of the molecules of the moderator physically impacting the incoming neutrons and absorbing some of the kinetic energy they posses, thus slowing them down.The reason that the neutrons have to be slowed is that most fissionable materials are more likely to absorb thermal neutrons (2.2km/s) than fast neutrons (14,000km/s).This means that when heavy water is used as a moderator, enough neutrons get through that even with very low levels of U-235 (even the very low levels found in natural uranium), criticality can be maintained, and power is produced.

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(Useful for UPSC General Studies, APPSC Group 1 Group 2 , Mains Material, Paper 1, Paper 4 Section 1 Section 2 Section 3, ROLE AND IMPACT OF SCIENCE AND TECHNOLOGY IN THE DEVELOPMENT OF INDIA, GENERAL AWARENESS WITH THE MODERN TRENDS IN LIFE SCIENCES, DEVELOPMENT & ENVIRONMENT PROBLEMS) 

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