The optimum angles of inclination for maximum separation, maximum production rate and minimum column length in a flat-plate thermal-diffusion column for enrichment of heavy water have been determined. Essential improvement in performance is obtainable if the column is tilted at the optimum angle, instead of being placed vertically, 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, considerable saving in. particularly for lower production rate operations, or for smaller degree fixed and operating charges can be achieved. of separation. (C) 2002 Elsevier Science B.V. All rights reserved.