Single grain Pt-free and Pt doped Y-Ba-Cu-O (YBCO)superconductors with starting compositions YBa2Cu3Oy+ 30 mol % Y2O3 and containing 0.1wt% enriched U3O8 or 0.1wt% depleted UO2 have been fabricated using a top seeded melt growth technique. The U-235 content of the samples has been varied between 0.213% and 3.003% by adding enriched U3O8 to the precursor powder. Microstructural studies show that approximately spherical, uranium-containing phase particles of diameter 300-500 run and Y2BaCuO5 phase particles are embedded in the YBa2Cu3Oy matrix in all samples. The microstructure of YBCO doped with enriched U3O8 is compared with samples doped with depleted UO2. The chemical composition of the uranium-phase particles in Pt-free YBCO samples containing depleted uranium oxide has been identified as Y2Ba4CuUOy. Single grain bulk superconductors of starting composition YBa2Cu3Oy + 20mol% Y2O3 + 10 mol% Y2Ba4CuUOy have subsequently been fabricated. Significantly, the size of the U-phase particles in this sample is the same as in that doped with UO2. The magnetic properties Of Y2Ba4CuUOy powder have been measured. Finally, critical current density and trapped field measurements performed on various single grain samples indicate improved superconducting properties in single grain YBCO doped with both depleted UO2 and enriched U3O8 compared to U-free material.