As both industry and government move toward meeting their decarbonization goals, nuclear power is increasingly being considered due to its legacy of providing clean and reliable power. Pebble bed (PB) high-temperature gas-cooled reactors (HTGRs) are a generation-IV reactor design being considered because of their past operational experience, intrinsic safety features, and online refueling capabilities. PB-HTGRs are conventionally moderated with graphite, but in the high-temperature and high-fluence environment of PB-HTGRs, graphite undergoes several undesirable property changes that limit the lifetime of the graphite components in the reactor.

Composite moderators have been proposed as an alternative to graphite as a moderating material in HTGRs because of their desirable properties in the high-temperature and high-fluence environment of HTGRs. Two-phase composite moderators are comprised of a highly moderating phase that is entrained in a radiation-stable matrix phase.

This work seeks to optimize a micro-modular PB-HTGR design for the use of the composite moderator MgO-ZrH, through adjustments to the core height. The optimization makes use of the novel A-ZEM (axial zone equilibrium modeling) method. The best-performing PB-HTGR had a core height of 285.5 cm, achieving a discharge burnup of 131.460 GWd/tonne heavy metal, an improvement of 24.34% compared to the reference graphite moderated model and 16.51% compared to a prismatic HTGR moderated with MgO-ZrH.

An additional goal of this work was to determine the applicability of the linear reactivity model (LRM), a fuel management model, as a scoping tool for PB-HTGRs. Comparing the theoretical discharge burnup calculated by the LRM to the equilibrium discharge burnup, differences did not exceed 10%. Therefore, it was concluded that the LRM can be an effective scoping tool for PB-HTGRs to allow for understanding the key equilibrium metric of discharge burnup without requiring the generation of an equilibrium model.