Heavy water (D2O) is the most feasible moderator and coolant in nuclear-fis
sion reactors while deuterium (D) will be the nuclear fuel for thermonuclea
r fusion in the distant future. The enrichment of D2O and HDO from the H2O-
HDO-D2O system by thermal diffusion has been investigated. The degrees of s
eparation for each component of water-isotope mixtures in thermal diffusion
columns are predicted and discussed.