Heavy water (deuterium oxide, D20) is present to the extent of 0.016%o in ordi­nary water. Heavy water may be separated from ordinary water by various processes, which is an expensive business requiring a very large plant. Never­theless, heavy water has considerable merit as a reactor coolant; it has a much lower thermal neutron absorption cross section than light water, which enables reactors using heavy water as a coolant to be operated without enrichment of 235U in the fuel. The most common example of such a reactor is the Canadian C^NDU, which was described in Chapter 2.

With the exception of neutron absorption, heavy water has practically the same physical properties and therefore the same disadvantages as light water.

Since heavy water is a very valuable material, losses and contamination with light water must be minimized. This demands a high-integrity primary circuit, particularly in the steam generators, where light and heavy water are separated only by the heat transfer surface. In practice, an annual loss of about 2% of the heavy-water inventory seems unavoidable, probably mainly in the form of vapor escaping through leaks.

Another problem with heavy water is that in a neutron flux the component f deuterium is converted, to a small but significant extent, to tritium (hydrogen-3),

f; which is radioactive and decays to helium-3 with the emission of a р-particle. Be­

cause tritium has a relatively long half-life (12 years), tritium contamination of the environment by coolant leaks from the reactor is a problem that must be taken into account in the design.