Why Heavy water is used as moderators in nuclear reactions? And, can ordinary water act as moderators instead heavy water or not? If yes please or if no how and why?

Heavy water is basically used as a moderator in nuclear reactors to slow down the neutrons so that they are captured and become effective to bring about the fission reaction. The main reason why heavy water is used as a moderator is because it captures less neutrons than the normal water. Hence it can be used for the fuel which is not too enriched in U-235 (the fissile material). Also, the chain reaction can continue for longer period of time. Ordinary water is also used in nuclear reactors. The nuclear reactors which use ordinary water are known as light water reactors. But as ordinary water captures more neutrons than heavy water, therefore the neutrons that get through the fissile material are less. As a result of this, the chain reaction cannot continue for too long. Also, when ordinary water is used as a moderator, then the fuel should be enriched in the fissile material. 

Thus with heavy water, we can use fuel not too enriched in U-235, but with light water, the nuclear fuel has to be rich in U-235 to main criticality. You will learn more about criticality in higher grades. 

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