The Onset and Propagation of GTRF Wear.

Grid-to-rod fretting (GTRF) wear in pressurized water reactors (PWR) is caused by the vibration of fuel rods against the spacer grid that supports them. GTRF problems ac-count for over 70% of fuel failures of PWR worldwide. Due to its complex dependence on factors such as geometry design of the spacer grid, creep deformation, gap formation and coolant flow, the root mechanism of the GTRF problems is still not well understood. The purpose of this dissertation is to provide a fundamental understanding of how the wear initiates from the con-tact edges and propagates over the entire contact interface, as well as how the key factors such as gap size and excitation frequency affect the wear growth. The geometry design of the spacer grid is a critical factor determining the GTRF wear. Understanding the contact mechanics in the onset of partial-slip and the propagation of wear scar is the basis for geometry optimization. A new wear modeling method in which the geometry is updated by assigning fictitious eigenstrains to a set of surface elements is developed. It is shown that the sensitivity of slip to the geometrical details of a corner is resolved when a finite interfacial shear strength is incorporated with a Coulomb friction law. The wear scar will propagate continuously to the entire contact interface when either an interfacial shear strength or a yielding strength is introduced. As creep plays a similar role as wear in stress relaxation and gap formation, an algorithm is developed to couple wear and creep. The dependence of wear growth rate on the gap and excitation frequency has been also systematically analyzed. It is found that the fuel rod may have three types of vibrational responses: subharmonic, period doubling and chaotic. It is found that there is a critical excitation frequency that leads to the maximum wear growth rate. A concept of system natural frequency is introduced. It is shown that the system natural frequency is determined by the gap size and closely related to the maximum wear rate.PhDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133283/1/zupanhu_1.pd