Fuel breeding in water-cooled reactors has been pursued for decades, mostly through using tight lattice fuel to reduce water moderation and thorium-uranium cycle with higher neutron multiplication. More recently, attention shifted to the U-Pu fuel cycle. At CANES a heterogenous assembly (CONFU) with U fuel and inert-hosted PuO2 fuel was shown to break even in transuranic actinides. Also, the Hitachi proposed Resource-renewable Boiling Water Reactor (RBWR) feasibility was assessed, in collaboration with UC-Berkeley and U. of Michigan. Neutronic, thermal-hydraulic, and fuel performance aspects of the axially heterogeneous design were evaluated. New correlations for predicting the void fraction and critical heat flux in tight fuel lattices were produced. Furthermore, using nitride fuel in place of the oxide has been assessed. The higher density nitride fuel hardens the neutron energy spectrum and results in increasing the breeding ratio from 1.04 to 1.14. Another BWR nitride fuel assembly design increased pitch-to-diameter ratio from 1.13 to 1.20, which maintained the 1.04 breeding ratio but increased the minimum critical power ratio from 1.22 to 1.36. A high-porosity (15%) nitride fuel is recommended for high burnup conditions.