Neutronics impacts of limiters in a Tokamak reactor*

A fusion power plant requires a robust first wall to withstand high heat and particle fluxes under neutron irradiation conditions. In exploring the design

space for fusion power plant design, a high-fidelity engineering modeling and simulation framework to evaluate first wall protection limiters has been developed. The limiters absorb main-chamber plasma fluxes before they reach the more fragile first wall components. This work explores the impact of adding limiters to the neutronics performance of the tokamak, namely the tritium breeding ratio (TBR). The analysis is done by developing automated parametric geometry generation with various limiter and tokamak configurations, coupled with a neutronics workflow that performs particle transport calculations using the generated CAD geometry.  Results show that the impact of adding limiters highly depends on the configuration of the tokamak, such as lithium enrichment of the blanket material and neutron  multiplier layer thickness. It is postulated that adding limiters can reduce the total volume of tungsten on the first wall, which increases tritium breeding ratio for  blankets with low lithium enrichment because the flux is less attenuated by the first wall. Plausible main chamber limiter configurations can increase TBR up to ∼ 1.4%  for natural lithium configurations with reduction of tungsten volume, but can cause a TBR reduction of ∼ 5% for 90% enriched configurations. A sensitivity study  of limiter dimensions and numbers shows that increase in limiter volume increases TBR for 90% enriched configurations but decreases TBR for non-enriched blanket  configurations. This work is currently being expanded to couple other analyses such as plasma physics and computational fluid dynamics to quantify the potential design space of limiters and its impact on tokamak engineering metrics.