Neutronics modeling of activation diagnostics in SPARC

The SPARC tokamak, now under construction in Devens, Massachusetts, is slated to operate with a

maximum of 140 MW of DT fusion power and thus a total neutron rate of approximately 5 × 10^19 n/s

(Creely et al. 2020). Activation diagnostics have been used with success on both TFTR and JET during DT

operation to measure time-integrated fusion power in combination with other diagnostics (Jarvis et al. 1991)

(Nieschmidt, 1986). SPARC plans to use activation diagnostics in combination with fission-chamber based

flux monitors to measure fusion energy during both DD and DT operation. The Monte-Carlo neutronics

code OpenMC is used to model the neutron flux in various locations in the device in order to assess possible

activation foil locations as well as predict the induced activity. A realistic SPARC geometry is used with

several different neutron source configurations in order to asses the sensitivity of the system to source geom-

etry and energy spectrum. A target activity of 100 μCuries is used to design the activation foil geometry.

Various possible activation foil materials are then analyzed including Silicon-28, Zirconium-90, and Iron-56.