Cladding liner surface effects and PCI

A soft, metallurgically bonded inner layer of zirconium with small quantities of alloying elements is used in BWR zircaloy fuel cladding as a remedy against pellet-cladding interaction (PCI) fuel failures. The inner surface of the liner becomes significantly hardened during irradiation. Local micro-hardness and elemental composition profiles close to the inner surface of the cladding liner have been measured in a fuel rod subjected to power ramp testing. In addition, small incipient cracks in the hardened zone have been detected and characterised after the ramp test. The extent of the hardening, its cause and its impact on the initiation of PCI cracks are discussed in the paper. (authors)