Analysis and modelling of Inner Fuel Cycle performance using exhaust bypass and Direct Internal Recycling

Fusion power plants require robust fuel cycle (FC) architectures that minimize tritium inventories while managing impurity build-up and isotopic imbalances. This work investigates the performance of a Inner Fuel Cycle (IFC) architecture based on the Direct Internal Recycling (DIR) concept, with an additional bypass loop for recycling of exhaust gases utilized in gas puffing. Particular focus is given to fuel dilution due to impurity accumulation and deuterium–tritium (D–T) imbalance. A new Julia-based dynamic fuel cycle modeller, MINERVA (Modelling and Integration of Nuclear fusion Energy Reactor fuel cycle for Versatile Analysis), is introduces and used to evaluate the performances of the proposed architecture and for understanding the dynamics and criticalities of a DIR-based FC. Protium build-up is identified as a potential challenge, with accumulation becoming problematic at high separation efficiencies without dedicated removal systems. Two reactor case studies are analysed, EU DEMO 2018 and Gauss Fusion's GIGA reactor. Results demonstrate that the proposed architecture effectively manages impurity concentrations below 1% for protium while maintaining optimal D–T ratios through active control systems. The proposed architecture achieves significant reductions in external fuel requirements, with effective conversion ratios growing exponentially with DIR separation efficiency. The bypass loop successfully provides the majority of gas puffing requirements without causing excessive impurity accumulation. This work establishes a foundation for advanced fuel cycle optimization studies essential for the development of commercial fusion power plants.