A separation theory for the enrichment of heavy water in the Frazier scheme of concentric-tube thermal diffusion columns with a tight fitting wire spiral, having a diameter equal to the annular spacing, wrapped on the entire inner tube, has been developed. Equations for the best wire angle of inclination for maximum separation, maximum production rate and minimum column height have been derived. Considerable improvement in performance is obtainable if the wire spiral is employed in the column at the optimum angle, instead of the absence of wire spiral, so that the convection strength can be properly reduced and controlled, resulting in suppression of the undesirable remixing effect while still preserving the desirable cascading effect.