Kärnbränslepelleten

Kärnbränslepelleten is the Swedish term for the small cylindrical unit of fissile material that forms the active element of a nuclear fuel rod. In most reactors, the pellet is made of uranium dioxide (UO2) and is enriched in U-235 to around 3–5 percent to sustain a controlled chain reaction. MOX pellets, containing uranium oxide mixed with plutonium oxide, are used in some reactors and fuel cycles.

Pellets are produced by converting uranium oxide powder into a dense ceramic via pressing and high-temperature sintering. The resulting pellets are ground and finished to tight tolerances, then loaded into corrosion-resistant cladding tubes—usually zirconium alloy—to form fuel rods. Several dozen to over a hundred pellets are stacked in each rod, depending on rod length and pellet dimension, with a small plenum gap at the ends to accommodate thermal expansion and fission-gas release.

During reactor operation, heat is generated by fission within the fuel. The pellet's high density, thermal conductivity, and microstructure are optimized to withstand high temperatures and radiation. UO2 melts at about 2,900°C, well above normal reactor operating temperatures, helping to retain fission products. Fission gas, pellet-clad interaction, and burnup-induced swelling are monitored by design and quality controls.

Spent fuel contains the used pellets, which can be stored on-site and later recycled or disposed of as high-level waste.