The fuel-self-sustaining RBWR-Th core concept and parametric studies - 14278

This study searches for the optimal fuel assembly design for the RBWR-Th core - a reduced-moderation BWR which is fuel-self-sustaining. Except for the initial fuel loading, it is charged with only thoria and discharges only fission products, recycling all actinides (with the exception of actinide impurities that end up in the waste streams). The design 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 ABWR pressure vessel. The RBWR-Th shares these characteristics but replaces depleted urania with thoria as the primary fertile fuel, eliminates the internal blanket while axially elongating the seed region, and eliminates absorbers from the axial reflectors. The simulation is performed using MCNP6.1 for neutron transport, ORIGEN2.2 for transmutation, and a single-channel heat balance and void fraction correlation for a self-consistent neutronics thermal hydraulic solution. These three computational modules are iteratively executed by the MocDown driver code developed to automatically search for the equilibrium core composition and cycle length. The design variables of the parametric studies include the length of the seed and blanket zones, fuel rod diameter, lattice pitch, and concentration distribution of the recycled trans-fertile (TRF) isotopes in the seed. MocDown searches for the cycle length that will result in an end-of-cycle fissile inventory ratio of 1.0 along with estimated five batch core end-of-cycle k_eff value of 1.0. It was found that using thoria for the makeup fuel it is not possible to meet the shutdown margin constraint. Mixing in 25 to 30% depleted urania into the thoria makeup for the seed allows reduction of the magnitude of the negative void coefficient of reactivity so that adequate shutdown margin could be achieved along with adequate stability against two-phase coolant density oscillations. Promising designs arrived at so far are described. The performance of the RBWR-Th core is highly sensitive to modeling assumptions. Using the assumptions and correlations Hitachi used for the design of their RBWR-AC, the RBWR-Th average discharge burnup is 61 GWd/t versus 45 GWd/t of the depleted uranium fueled RBWR-AC. (authors)