fuelcladding

Fuel cladding is the outermost sheath of a nuclear fuel rod, encasing the fuel pellets within a reactor. Its primary function is to contain radioactive fission products, maintain the geometry of the fuel pellet stack, and act as the first barrier to the release of fission material. Cladding must conduct heat from the fuel to the coolant and retain its integrity under irradiation, high temperature, and corrosive reactor conditions.

The most common cladding material in light-water reactors is a zirconium alloy, historically known as Zircaloy, with modern variants such as ZIRLO and M5. Zirconium alloys offer low neutron absorption, good corrosion resistance in hot water, and adequate mechanical strength under irradiation. While zirconium alloys dominate, other materials have been studied for niche applications, including steel cladding in some fast reactors. Cladding is fabricated as thin-walled tubes, typically made by rolling and welding processes, and capped at each end to seal the rod.

Inside a rod, the fuel pellets are packed with a small plenum gap to accommodate thermal expansion and fission-gas buildup. The cladding must tolerate fuel pellet swelling, mechanical contact, and fretting, while transferring heat efficiently to the coolant and allowing gas release through designed pathways.

Degradation mechanisms include oxidation and corrosion in reactor coolant, hydrogen pickup that forms zirconium hydrides and can embrittle the material, and mechanical phenomena such as pellet–clad interaction and stress-corrosion cracking. Normal operation and accident scenarios require cladding to remain intact to prevent rapid release of fission products.

Ongoing research seeks improved alloys and coatings, and alternative materials such as iron-chromium-aluminum alloys or silicon carbide composites, aiming to enhance accident tolerance and allow higher burnup while maintaining safe performance.