How is heavy water prepared from neutral water?

Heavy water is the oxide of deuterium (D₂O) and is formed from ordinary water by exhaustive electrolysis of water containing 3% NaOH. As a result of repeated electrolysis of ordinary water, protium is liberated at the cathode in preference to deuterium. Consequently, the concentration of heavy water in ordinary water increases. Actually, electrolysis is carried out in different stages. The gases obtained from each stage are separately burnt and water thus formed is returned to the previous stage. At the end of the seventh stage, 99% D₂O is obtained. This is subjected to careful fractional distillation. 100% heavy water is obtained.