The equations which may be employed to predict the optimum wire angles of inclination for enrichment of heavy water in a concentric-tube thermal-diffusion column with a tight fitting wire spiral, having a diameter equal to the annular spacing, wrapped on the entire inner tube, operating with transverse sampling streams, have been derived. Considerable improvement in performance is obtainable if 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 suppressing the undesirable remixing effect while still preserving the desirable cascading effect. Accordingly, essential savings in fixed charges and operating expense can be achieved, particularly for lower production rate operations or for smaller degree of separation.