Compatibility of tritium-lean operation with Direct Internal Recycling (DIR)

Determine whether fusion power plants operating with tritium-lean deuterium–tritium fuel ratios enabled by asymmetric deuterium–tritium transport (quantified by the core D–T particle diffusivity ratio Θ_DT = D_D/D_T) and differential divertor pumping (quantified by the D–T pumping speed ratio Σ_DT = S_D/S_T) can be compatible with Direct Internal Recycling operations, which require equality between the tritium injection fraction F_T^in and the separatrix tritium fraction F_T^sep and typically operate with tritium enrichment H_T ≈ 1.0, especially in regimes where Σ_DT significantly differs from 1 and the divertor composition departs from the fueling composition.

Background

The paper analyzes strategies to increase tritium burn efficiency by operating in tritium-lean regimes and leveraging asymmetric deuterium–tritium transport (Θ_DT) and differential pumping (Σ_DT). These strategies generally lead to divertor exhaust streams whose D–T ratios deviate substantially from the fueling ratios, complicating direct recycling assumptions.

Direct Internal Recycling (DIR) aims to rapidly return exhaust species to fueling with minimal processing time, benefiting from similar D–T ratios at injection and exhaust. The authors note that when Σ_DT ≈ 1 and F_T^in ≈ F_T^div, DIR operates with tritium enrichment H_T ≈ 1. However, strong asymmetry in pumping (Σ_DT ≠ 1) and tritium-lean operation can break this alignment, raising uncertainty about whether DIR can function effectively under these conditions.