A study of computational/analytical neutronics and heat transfer has been carried out for different types of gas-cooled fuel bundle lattices that could be used for the sub-critical fertile/fissionable blanket of a cylindrical-geometry hybrid fusion-fission reactor (HFFR) with thorium-based fuels. The HFFR concept envisioned is one with a simple cylindrical geometry, using an anticipated variant of a magnetic mirror to confine a deuterium-tritium (DT) fusion plasma. The annular-cylindrical blanket is approximately 10 meters long and 2 meters thick, and is a repeating lattice of pressure tubes filled with 0.5-meter fuel bundles that are made of (233U,Th)O2, and refuelled continuously on-line, sharing technological features with pressure-tube heavy water reactors (PT-HWR) and the Advanced Gas-Cooled Reactor (AGR) in the U.K.. With a 2-meter thick blanket, the average fissile content in the blanket needs to be at least 2.5 wt% in order for the HFFR system to be self-sustaining in power.