NOVELTY - Electrolytic enrichment method for heavy water, involves enriching heavy water by electrolysis using an alkaline water electrolysis cell (1) consisting of an anode chamber (2) that holds an anode, a cathode chamber (3) that holds a cathode, and a diaphragm (4) that divides between the anode chamber and the cathode chamber. An electrolyte is circularly supplied to both electrolysis chambers including the anode chamber and the cathode chamber from a circulation tank (5) containing the electrolyte.    USE - The electrolytic enrichment method is useful for heavy water (claimed).    ADVANTAGE - The method: directly decomposes distilled water (i.e. dissolution, neutralization, and removal of an electrolyte, which are essential for an electrolytic method of an alkaline aqueous solution, are not required, and a volume reduction factor of sample water is basically unlimited); does not cover an electrode surface with bubbles, and thus electrolysis can be performed with a large current, hence shortening the electrolysis time; and generates hydrogen gas and oxygen gas separately on both sides of the membrane, thus the gases can be easily treated, and hence the method is safer than a usual method including handling explosive mixed gas. The electrolytic enrichment method is capable of electrolytic enrichment and fractionation of raw material water containing a large amount of heavy water by an alkaline water electrolytic method, and capable of producing high-purity hydrogen gas and/or high-purity oxygen gas.    DETAILED DESCRIPTION - Electrolytic enrichment method for heavy water, involves enriching heavy water by electrolysis using an alkaline water electrolysis cell (1) consisting of an anode chamber (2) that holds an anode, a cathode chamber (3) that holds a cathode, and a diaphragm (4) that divides between the anode chamber and the cathode chamber. An electrolyte prepared by adding high-concentration alkaline water to raw material water consisting of heavy water containing tritium is circularly supplied to both electrolysis chambers including the anode chamber and the cathode chamber from a circulation tank (5) containing the electrolyte. An anode-side gas-liquid separator and an anode-side water-seal device are connected to the anode chamber, and a cathode-side gas-liquid separator and a cathode-side water-seal device are connected to the cathode chamber. Electrolysis is continued while the alkali concentration in the electrolyte supplied to the both electrolysis chambers is maintained at a constant concentration by circularly supplying, to the circulation tank, the electrolyte from which the gas generated from each of the anode-side gas-liquid separator and the cathode-side gas-liquid separator is separated, so that heavy water in the electrolyte is enriched and, at the same time, the hydrogen gas is recovered or discharged from the cathode-side gas-liquid separator and the oxygen gas is recovered or discharged from the anode-side gas-liquid separator.    DESCRIPTION OF DRAWING(S) - The figure shows a flow diagram of an electrolytic enrichment method for heavy water.    Alkaline water electrolysis cell (1)    Anode chamber (2)    Cathode chamber (3)    Diaphragm (4)    Circulation tank. (5)