High fuel duty, eighteen-month and longer cycle lengths, and higher burnups have put increased demands on pressurized water reactor (PWR) fuel during recent years. As a result of these demanding operating conditions, the fretting-wear performance of PWR fuel rods against structural grid supports is being challenged. To understand the in-reactor fretting-wear performance of PWR fuel, Westinghouse Electric Company (W) and Atomic Energy of Canada Limited (AECL) have performed investigations to study the fretting-wear behavior of zirconium alloy nuclear fuel rods and grid supports. Experimental work was carried out at the AECL Chalk River Laboratories (Canada) in autoclave impact fretting-wear test machines and at the W Columbia, South Carolina (USA) plant in the Westinghouse — Vibration Investigation Pressure-Drop Experimental Research (VIPER) hydraulic test loop.
Autoclave impact fretting-wear tests were conducted for several grid-support/fuel-rod designs using either ZIRLO™, Zircaloy-4, and/or OPTIN™ materials. Rod-to-grid relative displacements and contact forces were measured throughout each test. The wear volumes of each fret mark on the fuel rod specimens were measured via surface profilometry at the end of each test. From these wear volumes, wear rates and wear coefficients for the fuel rod in contact with either springs or dimples were determined.
The VIPER loop is used for full-scale nuclear fuel assembly vibration and wear testing. Endurance tests were performed with severe flow conditions to induce fuel fretting-wear for different grid support and assembly designs using either all ZIRLO™ or all Zircaloy-4 materials. The wear volumes of the most severe fret marks on the fuel rods were measured via surface profilometry at the end of each test. From the profilometry of each fret mark, and using the fretting-wear coefficient derived from autoclave wear tests, the available energy (i.e., work-rate) to cause the fretting-wear damage was determined.
Fretting-wear scars on ZIRLO™, Zircaloy-4, and/or OPTIN™ fuel rods from the autoclave impact fretting-wear tests and the VIPER hydraulic tests were examined using scanning electron microscopy (SEM) to compare the fretting-wear mechanisms for different grid-support designs. Fretting-wear mechanisms from both types of tests were shown to be comparable and related to the grid-support design. For a given material (i.e., ZIRLO™, Zircaloy-4, or OPTIN™) under similar test conditions, there was no difference in the fretting-wear performance.