JAEA develops new method for recovering heavy water with high efficiency while producing hydrogen

Heavy water is used in semiconductor and optical fiber manufacturing and as a raw material for pharmaceuticals; it also contains deuterium, a fuel for nuclear fusion power generation, making it an important material. However, it accounts for only 0.015% of naturally occurring water, and enrichment requires substantial energy costs. Japan depends entirely on imports and recycling for its supply. A research team from the Japan Atomic Energy Agency (JAEA), made up of Researcher Masahiro Yano and Team Leader Satoshi Yasuda from the Advanced Science Research Center and Senior Principal Researcher Shinji Kubo from the Oarai Nuclear Engineering Institute, has developed an operating method for water electrolysis that recovers heavy water roughly 20% more efficiently than conventional methods while simultaneously producing hydrogen. Yano noted: "Simply changing the feedwater supply point to the cathode side in a proton exchange membrane (PEM) water electrolysis unit raises the heavy water concentration, contributing to supply stability at low cost. We have obtained a patent and are already in talks with companies in the relevant industry." The findings were published in Chemical Engineering Journal.

Hydrogen production is expanding rapidly, with proton exchange membrane (PEM) water electrolysis units in particular being increasingly deployed. In a PEM-type unit, water electrolysis is carried out using a structure in which a solid polymer membrane is placed between an anode and a cathode. When feedwater containing heavy water is supplied to the anode for electrolysis, hydrogen ions and deuterium ions migrate through the polymer membrane to the cathode; in the process, some of the water and heavy water is also carried along to the cathode side. At the cathode, hydrogen isotope gas is generated on the catalyst surface, while the water and heavy water transported there remain as a liquid near the catalyst.

At this point, an isotope exchange reaction occurs between the gas generated and the nearby water, with deuterium transferring back and forth until equilibrium is reached at a certain ratio. The further this reaction proceeds, the more deuterium that would otherwise escape as gas is absorbed into the water, making it easier to recover as liquid heavy water. By returning the recovered heavy water to the feedwater side and continuing electrolysis, the deuterium concentration in the feedwater gradually rises and enrichment progresses.

This equilibrium shifts depending on the heavy water concentration involved in the reaction, and the research team focused on this property. Yano explained: "Our group had been working on methods to increase deuterium concentration, but we shifted our thinking and asked whether we could also enrich heavy water. While we were organizing our understanding of heavy water enrichment, we realized that the isotope exchange reaction was the key." Because lowering the heavy water concentration near the cathode drives the reaction in a direction that compensates for the deficit, the team deliberately supplied low-concentration heavy water to the cathode side to reduce the local heavy water concentration near the cathode. This creates conditions that make it easier for deuterium in the gas phase to transfer into the water, thereby reinforcing the process of recapturing it as liquid heavy water.

Under conditions where the deuterium concentration in the feedwater was approximately 10%, the conventional method (without water supply to the cathode) shifted the heavy water concentration from 10% to 23.8%, while the new method achieved a shift from 10.5% to 27.3%, a roughly 20% improvement in recovery efficiency over the conventional approach. A similar improvement was confirmed at a feedwater deuterium concentration of approximately 90%, demonstrating that the method is effective across a wide range of concentrations, from low to high.

In hydrogen production plants, electrolysis units are connected in series, so applying this method enables the deuterium concentration to be raised in a stepwise manner. Carrying out enrichment once the concentration reaches a certain level could enable domestic production of heavy water for industrial use. Converting previously vented deuterium and discarded heavy water into recoverable resources has the potential to strengthen Japan's industrial competitiveness.

Journal Information
Publication: Chemical Engineering Journal
Title: Enhancement of deuterium enrichment efficiency in PEM water electrolysis via isotope exchange equilibrium shift
DOI: 10.1016/j.cej.2025.172423