If the rod internal pressure becomes larger than the reactor system pres­sure, the fuel cladding may start to creep outwards (Fig. 5.5) (Strasser

et al., 2010a). If the fuel cladding outward creep rate exceeds the fuel swelling rate (due to fission product production during irradiation), the pellet-cladding gap may increase. This phenomenon is denoted cladding liftoff. Since this gap constitutes a significant heat flux barrier, such a gap increase may result in increased fuel pellet temperature. This higher tem­perature will in turn increase the gaseous fission product release rate, fur­ther increasing the fuel rod overpressure and leading to an even higher outward cladding creep rate. Such a thermal feedback condition may lead to fuel failure.

A larger fuel rod free volume, lower FGR rate and increased clad creep strength increases the margins towards liftoff (i. e. a larger rod internal pres­sure can be accepted without getting liftoff) (Strasser et al, 2010a). Free volume refers to the void volume bounded by the inner surfaces of the clad­ding and end plugs and the outer pellet surface minus the volume of ple­num springs and other internal hardware. Note that closed pellet porosity is within the pellet volume, while open porosity, dishes, chips and other surface irregularities with finite, open volume are in the free volume.