A thimble-sized ceramic cylinder (approximately 3/8-inch in diameter and 5/8-inch in length), consisting of uranium (typically uranium oxide, UO₂), which has been enriched to increase the concentration of uranium-235 (U-235) to fuel a nuclear reactor. Modern reactor cores in pressurized-water reactors (PWRs) and boiling-water reactors (BWRs) may contain up to 10 million pellets, stacked in the fuel rods that form fuel assemblies.