A two-dimensional mathematical model was developed theoretically to predict the degree of separation of heavy water in concentric circular thermal-diffusion columns with total fixed expense and with plate spacing variations. An orthogonal expansion technique for solving separation problems of heavy water enrichment to estimate optimal plate spacing for maximum separation has been developed analytically. Considerable improvements in device performance were obtained when the thermal-diffusion column with optimal plate spacing were used. The effects of plate spacing and mass flow rate and on the degree of separation have been investigated with a consideration of the fixed total expense. Further improvement can be achieved if the feed position and flow-rate fractions of top and bottom products are suitably adjusted.