What is the purpose of heavy water?

Answer 1

This is a complicated question to answer, but I'll do my best.

Basicaly heavy water is used as a moderator in a nuclear reactor. It is used to slow the neutrons being directed at the fissionable material, by means of the molecules of the moderator physicaly impacting the incoming neutrons and absorbing some of the kenetic energy they posses, thus slowing them down, in the same way that two billiard balls impacting each other would slow down the incoming one (or both if they were both moving). 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).

Light water (the name usually used for regular H2O when talking about nuclear reactors), is the most common type of moderator, because it is cheap, very available, and is more effecient at slowing the incoming neutrons, due to the fact that the hydorgen atoms in the water posses only one proton and one electron, and thus are almost exactly the same mass as the incoming neutrons (the hydrogen atom weighs only as much as one electron more than the neutrons, and electrons are very light when compard to protons and neutrons, which are equal in mass). The problem with using light water as a moderator, however, is that the hydrogen atoms may absorb some of the neutrons, thus preventing them from getting through to the fissionable material. Thus, once the percentage of U-235 (the fissionable isotope of uranium) is too low (such as in natural uranium, where the percentage of U-235 is about 0.72%), then the amount of neutrons getting through the moderator without being abosorbed is not high enough to maintain criticality (the point at which the amount of neutrons being produced is equal to the amount escaping the system or being absorbed but not resulting in fission), and the chain reaction can no longer continue, and the reactor can no longer produce power.

Heavy water, however, is deuterium oxide. Deuterium is an isotope of hydrogen with one proton and one neutron. Thus the hydrogen atom already has one extra neutron, and is much less likely to absorb another. 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. So even though the efficiency of the D2O (heavy water) molecules at slowing the neutrons is slightly less than that of regualr H2O (water, or light water) molecules, the use of heavy water as a moderator allows natural uranium to be used as a fuel with little, if any, enrichment (which is a costly process, and controversial, as enriched uranium can be used to make nuclear weapons).

This is why CANDU (Canadian Deuterium-Uranium) reactors can use natural uranium, or even the waste uranium from conventional light water reactors as fuel.

Answer 2

This is the CANDU reactor, developed in Canada. They have been successful and produce a large amount of power in Canada and other countries where they have been built.

CANDU stands for CANadian DeUterium. The heavy water (D2O) is a better moderator than ordinary (light) water, and allows a reactor to be built that runs on unenriched uranium (u-238) as opposed to slightly enriched uranium (u-235, about 4%). The heavy water slows the fast neutrons down more, allowing better absorbtion, and the subsequent use of u-238.

Answer 3

The important applications are:
- moderator in nuclear reactors
- cooling fluid in nuclear reactors
- neutrino detector
- solvent for NMR
- preparation of deuterated compounds

Answer 4

In the PWR and BWR designs, normal water is used to act both as a moderator (to slow fast neutrons down) and as the reactor coolant which transfers heat to an external circuit.