Fuel performance studies at the Halden reactor have given valuable information on how various design parameters affect the mechanical interaction between fuel and cladding. The experiments have also indicated how the interaction is dependent on burnup and on the actual power history of the fuel rod. This information was obtained by means of differential transformer type of detector, measuring the changes in length and diameter of fuel rods while operating at power in the reactor. Based on this experience, a simple graphical model has been proposed for the prediction of interaction between fuel and cladding as a function of power history and bumup. This concept, referred to as “iso-gap curves,” clearly demonstrates the importance of avoiding an increase in power at high burnup and could be useful when planning reactor operations with respect to fuel management schemes, i.e., power changes, control rod movement, fuel shuffling, and loading.