This study searches for the optimal fuel assembly design for the RBWR-Th core, which is a reduced-moderation boiling water reactor that is fuel-self-sustaining. Except for the initial fuel loading, it is charged with only fertile fuel and discharges only fission products, recycling all actinides. The RBWR-Th is a variant of the RBWR-AC core proposed by Hitachi, which arranges its fuel in a hexagonal tight lattice, has a high outlet void fraction, axially segregates seed and blanket regions, and fits within the advanced boiling water reactor (ABWR) pressure vessel. The RBWR-Th shares these characteristics but replaces depleted uranium (DU) with thoria as the primary fertile fuel, eliminates the internal blanket while elongating the seed region, and eliminates absorbers from the axial reflectors.

The sensitivity of important RBWR-Th core performance parameters to change in each one of a dozen design variables was established. These sensitivities provide useful insight and guidance to search for the optimal core design. The design variables of the sensitivity studies include the length of the seed and blanket zones, fuel rod diameter, lattice pitch, number of pins per assembly, concentration distribution of the recycled transfertile (transuranium + transthorium) isotopes in the seed, amount of DU in the seed makeup, coolant mass flow rate, and simulated depletion cycle length. The performance of the RBWR-Th core was found to be highly sensitive to the pitch-to-diameter ratio and to modeling assumptions. Using the conservative modeling assumptions, it was not possible to get the full ABWR power level without exceeding the pressure drop constraint.