Measurements of fuel stack and cladding length changes by means of differential transformer elongation detectors on more than 50 fuel rods irradiated in the Halden Boiling Heavy Water Reactor (HBWR) have resulted in the following general conclusions regarding in-pile dimensional behavior of fuel rods. When a fuel rod is brought to power for the first time, the clearance between pellet stack and cladding will decrease with power due to the larger thermal expansion of the fuel. The radial temperature distribution through the fuel will be parabolic and the originally flat-ended cylindrical fuel pellets will therefore change to hourglass-shaped bodies with convex ends. The pellet stack will expand freely inside the cladding until at some power level the “corners” of the hourglass-shaped, hottest fuel pellets will first touch and then make force contact with the cladding. The force system set up between the fuel stack and cladding will increase with power and affect their dimensional changes so that these are no longer decided by individual thermal expansion alone. The mechanical interaction will increase with power and the cladding will finally be permanently deformed to fit the external dimensions of the pellet stack, and the fuel rod will at this point look like a bamboo rod. Mechanical interaction will later take place only if the power is increased beyond its previous level, as long as fuel swelling has not affected the fuel outside dimensions. Knowing that the cladding material loses ductility because of irradiation damage and chemical attack, the conclusion is reached, therefore, that fuel should be operated hard when new so that the bamboo ridges and other forms of strain concentrations caused by interaction are formed when the cladding is ductile. To avoid further interaction, the power should preferably be tapered off gradually with burnup so that part of the volume used to accommodate thermal expansion can be made available for the volume increase because of fuel swelling.