Even if lithium enrichment isn’t a problem, its no solution.
The contour plots below are based on data from 1,029 OpenMC simulations, each tracking 150,000 neutrons at 14.1 MeV, emitted from a point source at the center of a lithium sphere. All simulations were identical except for two variables: the sphere’s radius (i.e., blanket thickness) and the lithium-6 enrichment level, which I varied in 5% increments. For reference, a horizontal line marks Li-6 at 7.5%—the natural, unenriched baseline.
Notice (above), for blankets <= 50 cm thick, the contour lines slant up to the right. That means going straight up (increasing the proportion of Li6) lowers the TBR. To reach TBRs > 1, blankets need to be > 70 cm thick. In all cases, extreme enrichment (near 100% Li6) results in lower TBR. For example, consider a 100 cm blanket. Its TBR drops from 1.20 (natural 7.5% Li6) to 0.95 when fully enriched (100% Li6).
The contours in the plot above correspond to the proportion of 14.1 MeV neutron energy from the initial fusion release that is deposited in the blanket. To calculate the average energy per neutron, multiply the contour values by 14.1 MeV. Note that for blankets less than 75 cm thick, a significant amount of energy is wasted—deposited outside the blanket. The simulation only considered prompt reactions as contributors to blanket heating. Delayed reactions, such as Li‑8 → Be‑8* (t₁/₂ ≈ 0.838 s) → 2α + e⁻ + ν̄ₑ, which follows the prompt Li-7 + n → Li‑8 + ~2 MeV gamma neutron capture reaction, were not included. However, the effect of including delayed reactions would be imperceptible, since fewer than 3% of neutrons contribute to delayed reactions, and most of beta decay energy escapes the blanket, carried away by high-energy neutrinos.
Leakage refers to the fraction of neutrons that escape from the tritium-producing blanket. These high-energy neutrons pose serious risks: they can damage surrounding structural materials and potentially cause biological harm, including mutations. In an ideal design, all neutrons would be absorbed within the blanket. However, when the blanket is less than 1 meter thick, even maximum lithium enrichment cannot prevent more than 15% of the neutrons from leaking out.
All the data point the same way: thicker blankets work best. A compact blanket—a design that achieves the necessary breeding thickness within a minimized radial footprint, reducing both structural bulk and material costs—remains the most cost‑efficient solution for thicknesses beyond 2.5 meters.
The plot (above) shows how blanket volume scales with distance from the center. For a 2.25‑meter‑thick lithium blanket, placing it 5 m from the core requires ~1075 m³ of lithium, while a compact design at 0 m achieves the same thickness with only ~50 m³. This illustrates the dramatic material savings (over 20×) that can be achieved by compact blanket geometries.