Abstract: Nuclear fuel rods have cladding or fuel with physical parameters that substantially change based on axial position within a rod. Parameters include inner and outer cladding and fuel diameters or widths, volume, mass, internal volume, thickness, rod width, etc. Parameters are selected and implemented based on calculated operating conditions and/or desired fuel response at an axial position across an entire rod length and/or fuel assembly position, including both fueled regions and non-fueled zones. Desired parameters can be achieved through fabrication or later alterations. Parameter variations versus axial position and fuel assembly position are intentional and achieve desired fuel properties and responses, such as optimized fuel mass, pressure drop, over-pressurization protection, etc. Fuel rods can be compatible with existing fuel types and replace conventional fuel rods therein.