Optimizing the Use of Enriched Lithium in Fusion Reactor Blankets: A Neutronics-Based Economic Analysis

Abstract

Deuterium-tritium (D-T) fusion is the principal reaction of interest for near-term fusion energy development. Because tritium, the third isotope of hydrogen, occurs naturally only in trace amounts, it must be produced onsite to sustain the fuel cycle of any fusion reactor utilizing the D-T pathway. This is typically achieved through neutron capture reactions with lithium in the tritium breeding blanket (TBB). In this regard, it is deuterium and lithium, not tritium, that constitute the primary resources required for D-T fusion. Lithium-6 (Li-6) more readily breeds tritium across most neutron energies than lithium-7 (Li-7); however, its natural abundance is only ~7.5%. This has motivated TBB designs that require various levels of Li-6 enrichment. However, the supply chains and industrial capacity needed to produce enriched Li-6 at scale do not currently exist. To address this critical barrier to fusion deployment, this thesis evaluates the neutronics performance, shielding implications, and economic tradeoffs of lithium-based breeder materials for use in TBBs. A particular focus is placed on the role of Li-6 enrichment. A comprehensive modeling workflow was developed to assess five candidate breeder materials: 2LiF-BeF2 (“FLiBe”), the eutectic Li17Pb83 (“PbLi”), pure lithium metal, lithium titanate (Li2TiO3), and lithium orthosilicate (Li4SiO4). A simplified cylindrical blanket model was constructed to enable straightforward comparative analysis across a broad parameter space of enrichment and breeder thickness, with geometry loosely approximating the proposed ARC reactor, designed at the Massachusetts Institute of Technology. Neutronics simulations were conducted using OpenMC to calculate the tritium breeding ratio (TBR) for over 800 blanket configurations. A one-dimensional analytical model was then applied to estimate the minimum shielding thickness required to protect the toroidal field (TF) coils for each blanket configuration, based on neutron leakage fraction tallies and conservative fluence limits for REBCO high-temperature superconductor (HTS) tape. These results were integrated into a materials-based economic analysis to quantify tradeoffs between tritium production, Li-6 requirements, and system costs. Key findings include the identification of material-specific scaling behaviors and diminishing returns in TBR performance with increasing enrichment and thickness. FLiBe demonstrated robust performance at minimal enrichment with compact radial builds. In contrast, PbLi offered higher peak TBRs while requiring less lithium by mass, but at the cost of greater enrichment and shielding. Lithium metal exhibited flexible performance across enrichment levels, albeit with significant performance sensitivity and greater technical uncertainty under practical constraints. Across all materials, economically optimal configurations were found to strike a balance between tritium self-sufficiency and Li-6 minimization, while maintaining compact geometry. These insights provide actionable guidance for fusion reactor blanket design under existing Li-6 supply constraints.