An OpenMC model of the SPARC tokamak for the diagnostic scoping studies

This paper presents an OpenMC model for Monte Carlo neutronics simulations supporting the design scoping studies for various diagnostics for the SPARC tokamak. This constructive solid geometry (CSG) model uses realistic SPARC dimensions. Key components are modeled as a collection of homogenized cells of similar shapes with material composition preserved. A midplane port with a shielded opening, which allows a high flux of direct plasma neutrons reaching the tokamak hall and diagnostic hall neutron detectors, is modeled in greater detail for higher fidelity of neutron diagnostics simulations. The OpenMC model is verified with a CAD-based MCNP model for SPARC built by Commonwealth Fusion Systems, and it is found that the two models produce consistent tokamak hall neutron flux spectra. Compared to the CAD-based MCNP model, the CSG-based OpenMC model is easier to modify for parametric analyses to support rapid design iterations needed in a project like SPARC, which demands speedy engineering and physics design convergence. Multiple neutron diagnostics components are conceptualized and scoped using this model including the fast neutron collimators for the neutron camera and magnetic proton recoil neutron spectrometer, moderation and shielding for neutron flux monitors, and irradiation ends for the activation foil system. The uncollided neutron fluxes at detectors with collimated fields of view are verified using the optical code ToFu. The 14.1 MeV neutron peaks behind the collimators, which are planned to be 1–3 cm in diameter and 280 cm in length, are dominated by uncollided DT fusion neutrons. Activation foils have the best signal strength and uncollided/total neutron ratio at the plasma end of the foil channel in the port shielding.