Why is heavy water (D2O) used in moderators of Nuclear Power Plants insted of normal water (H2O)?

Question

In moderators,Deuterium Oxide (D2O) is used instead of normal water (H2O).

Answer 1

The chain reaction in a reactor is sustained by the free neutrons coming from one fission being absorbed by a Uranium nucleus, which then fissions itself. The probability of this absorbtion is increased if the neutrons are moving slowly. The purpose of the moderator is to slow the neutrons down.

Lighter nuclei make better moderators than heavy nuclei. If a neutron were to hit an infinitely heavy nucleus, it would bounce off in a different direction with the same velocity. When a neutron hits a nucleus of atomic mass one head on, it comes to a complete stop.

So you want to use a light nucleus. The reason water (H20) is unsuitable is that it is likely to absorb the neutrons instead of slowing them down. The probability of water absorbing a neutron is more than 200 times higher than the probability of deuterium (which already has a neutron) doing so.

Answer 2

It's used in nuclear fission plants because the deuterium will give up its extra neutron when bombarded. That means you have extra neutron generation sustaining the chain reaction. Which means you don't have to enrich the uranium.

Answer 3

Heavy water helps to "absorb" and slow neutrons. This prevents neutrons from causing a runaway nuclear reaction.

Answer 4

Like any "Nuclear Moderator" it is better at absorbing Neutrons....thus it is a good Moderator ;-)