Dependence of Fissile Mass Production on Fertile Mass Loading in D-T Fusion Blankets

A recent study finds non-linearity in fissile production rate as a function of fertile mass loading in Li₂BeF₄ (FLiBe) ARC-class D–T fusion blankets [J. L. Ball et al., Nucl. Fusion 65, 036038 (2024)]. This contrasts with prior work by Glaser and Goldston [A. Glaser and R. J. Goldston, Nucl. Fusion 52, 043004 (2012)], which showed a linear response in Pb–Li blankets. We examine this difference by employing a simple model of fissile material production in FLiBe and Pb–Li blankets using OpenMC. We find that energy self-shielding at fertile resonance peaks saturates fissile material production rates as a function of fertile mass at low neutron energies. The result is an offset-linear response, where saturation in the resonance region at low fertile mass is added to an essentially linear response with fertile mass at higher neutron energies. This effect is more pronounced in FLiBe than in pebble beds, while Pb–Li remains nearly linear over a wide range of fertile mass due to minimal neutron moderation and therefore near-zero fissile material production in the resonance region.