The steady-state theory of fuel pin swelling in which the majority of fuel pin diametral increase is ascribed to the effects of solid and gaseous fission product swelling of the fuel is in apparent conflict with a body of data relating to oxide fuel pins. The non steady-state or ratchetting theory holds that fuel-cladding mechanical interaction occurs by thermal expansion interaction of the fuel and cladding during reactor temperature or power changes. The primary rate determining mechanism is considered to be something other than the steady-fuel swelling rate. Examples of non-steady-state mechanisms, which experimental evidence suggests may be important, include fuel clad interference occurring during the first startup of the fuel pin, the buildup of porosity in the fuel due to a cracking and healing process, mass transport of fuel from the inside to the outside of the fuel column, and fuel clad interference occurring during reactor shutdown and subsequent removal of the pin from the reactor.