[en] Single grain Pt-free and Pt doped Y-Ba-Cu-O (YBCO) superconductors with starting compositions YBa₂Cu₃O_y+30 mol% Y₂O₃ and containing 0.1 wt% enriched U₃O₈ or 0.1 wt% depleted UO₂ have been fabricated using a top seeded melt growth technique. The U²³⁵ 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 nm and Y₂BaCuO₅ phase particles are embedded in the YBa₂Cu₃O_y 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 Y₂Ba₄CuUO_y. Single grain bulk superconductors of starting composition YBa₂Cu₃O_y+20 mol% Y₂O₃+10 mol% Y₂Ba₄CuUO_y 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 Y₂Ba₄CuUO_y 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. (orig.)