Design Study of a Multicolumn Cryogenic Distillation Isotope Separation System for JA DEMO

In deuterium–tritium (D–T) fusion reactors, the isotope separation system (ISS) plays a central role in the tritium fuel cycle, as only a small fraction of the injected fuel is burned in the plasma and the remaining hydrogen isotopes must be recovered, purified, and recycled. In JA DEMO [1.5 GW(thermal)], cryogenic distillation has been selected as the technology for the ISS owing to its technical maturity, large throughputs, and extensive operational experience. ISS handles liquid hydrogen isotopes (Q2: H2, HD, HT, D2, DT, and T2) and enriches D and T to fuel-grade purity exceeding 90%, resulting in nonnegligible T inventories and significant thermal loads. Consequently, careful ISS design is essential from both safety and fuel cycle–optimization perspectives.The latest ISS conceptual design for JA DEMO consists of four cryogenic distillation columns: an H/DT separation column, an H enrichment column, a D enrichment column, and a T enrichment column. While this conceptual configuration defines the overall system structure, it does not provide sufficient quantitative information to support subsequent detailed design activities. Therefore, further development of the conceptual design is required.In this study, a design study of the ISS is performed based on the existing conceptual framework and adapted to the conditions of the JA DEMO fuel cycle. The ISS design is advanced from the conceptual level by explicitly defining the input and output conditions and system requirements. Then it presents key design parameters, including feeding stages, withdrawing stages to equilibrate equipment, operating pressures, flow rates, column geometries, heat loads, and T and Q inventories for each distillation column. These parameters enable a quantitative evaluation of system performance and a direct comparison with conventional ISS design concepts.The design results indicate that the proposed ISS configuration can substantially reduce the T and Q inventories, as well as the heat loads, of individual distillation columns compared with conventional design concepts. These improvements are achieved mainly through the direct internal recycling loop, the gas-puffing line, and optimization of liquid holdup at the reboiler.At the same time, this study identifies several technical limitations that strongly constrain the ISS’s design freedom, thereby offering valuable insights for system-level optimization of the fusion fuel cycle in JA DEMO. Quantitative assessment of the impact of these constraints on ISS performance and safety margins remains an essential topic for future research work.