The Heavy-Water-Moderated Reactor

The incentive to use heavy water as moderator arises from the excellent neutron economy made possible by the low neutron cross sections of both deuterium and oxygen (0.5 and 0.27 mb, respectively). In combination with its slowing-down power of 0.175 cm-1, the low cross section of D2O leads to a moderating ratio of 6000, making heavy water, by a wide margin, the most desirable moderator on purely theoretical grounds. The fast and slow leakages from a heavy water reactor tend to be roughly equal, in contrast to the situation for a light water system, where the fast leakage is much the greater.