Lattice design and coolant selection for A 333 MWTH PWR Civil marine propulsion core using thorium-based checkerboard micro-heterogeneous fuel

Civilian marine reactors face a unique set of de-sign challenges in addition to the usual irradiation and thermal-hydraulic limits affecting all reactors. These include requirements for a small core size, long core lifetime, a 20% cap on fissile loading, and lim- itations on using soluble boron. One way to achieve higher burnup/long core life is to alter the neutron spectrum by changing the hydrogen-to-heavy-metal ra-tio, thus increasing the conversion of fertile isotopes in the fuel. In this reactor physics study, we optimize the lattice geometry of a 333 MWth soluble-boron- free marine PWR for 15% U-235 enriched UO₂ and 18% enriched micro-heterogeneous ThO₂-UO₂duplex fuel arranged in a simple checkerboard configuration. Comparisons are made between the homogeneously mixed 15% U-235 enriched all-UO₂ case and the checkerboard configuration. We have considered two types of coolant: H₂O and mixed 80% D₂O + 20% H₂O. The main objective of this study is to observe in which spectrum maximum discharge burnup is ob-tained in order to improve uranium utilization while satisfying the moderator temperature coeficient con- straint. Considering the entire range of moderation options, it was observed that higher discharge burnup for the candidate fuels is achievable by using either a wetter lattice or a much drier lattice than normal while while epithermal lattices are distinctly inferior performers. Checkerboard fuel exhibits higher dis-charge burnup potential than the all-UO₂ fuel for all moderation regimes for both coolants. Candidate fu-els exhibit higher initial reactivity and discharge bur- nup with the mixed D₂O-H₂O coolant than the H₂O coolant in the under-moderated regime, whereas these values are lower for the D₂O-H₂O coolant in the over- moderated regime.