On the Relationship Between Plasma and Tritium Fuel Cycle Through Matter Injection and Particle Exhaust
This work identifies an inconsistency between plasma operating scenarios and tritium fuel cycle (TFC) requirements, calling for a re-examination of the traditional reactor-led design approach. The key point is simple: in current TFC architectures, fuel puffing must contain tritium. Moscheni et al. (2026 Nucl. Fusion 66 026008) investigated fuel puffing rates in detached operation. Expanding that database, puffing is shown to exceed core fuelling by about an order of magnitude, from present-day tokamaks to next-step stellarators. Though not unknown in the plasma community, TFC models instead assumed core fuelling to dominate. The implications are severe. In recent TFC architectures, direct internal recycling (DIR) is intended to minimise tritium inventory, but assumes near-50:50 D:T composition. This assumption may become self-defeating: a substantial fraction of the puffed fuel must be tritium. Tritium inventory, doubling time, required breeding ratio, and pump sizing therefore become critical once puffing is properly accounted for. Mitigation is assessed by extending the models of Meschini et al. (2023 Nucl. Fusion 63 126005). For a notional plant, realistic TFC requirements can be met with D-rich, T-lean puffing, at the cost of about 10% lower fusion power. Alternatively, for near-50:50 D:T puffing, reduced fuel puffing with stronger impurity seeding can maintain detachment while alleviating TFC constraints, albeit with higher core contamination. Combined use of these strategies enables scenarios that minimise tritium inventory and throughput while balancing competing requirements. Ultimately, these results place renewed emphasis on the TFC as a central element of reactor design. A viable fusion reactor requires joint optimisation of core plasma, edge plasma, and TFC, implying unavoidable trade-offs across all three.