One of the most important fuel performance phenomena is pellet-clad interaction (PCI), which can cause fuel cladding failure even in the absence of manufacturing defects.
During irradiation, the UO₂ fuel pellet swells due to the accumulation of fission products and radiation-induced restructuring. Simultaneously, the pellet develops radial cracks due to steep thermal gradients (the centre of the pellet may be at 1200—1700 °C while the outer surface is at 400—500 °C). These cracked pellet fragments can press against the inner surface of the cladding, particularly during power ramps (rapid increases in reactor power). If the local stress exceeds the cladding’s yield strength, and if aggressive fission products — especially iodine and caesium — are present at the pellet-clad interface, stress corrosion cracking (SCC) of the Zircaloy cladding can occur.
Modern fuel designs mitigate PCI through several approaches: chamfered pellet edges (reducing stress concentrations), dished pellet ends (providing space for thermal expansion), graphite coatings on the pellet surface (providing lubrication), and operational restrictions on power ramp rates. Some advanced fuel designs incorporate a thin zirconium liner on the inner surface of the cladding (barrier fuel), which is more resistant to iodine-induced SCC.