A microstructure-based wear model for grid-to-rod fretting of clad nuclear fuel rods

Coolant flow-induced, grid-to-rod-fretting (GTRF) can occur in pressurized water nuclear reactor cores leading to cladding wear-through, fuel material leakage, and plant shutdowns. The goal of this effort was to derive a microstructure-based interpretation of the fretting wear factor (W-f) for use in an existing, multi-stage GTRF wear model. Physically, the magnitude of W-f (expressed in units of mm(3)/N-m) reflects the effectiveness of converting frictional work into material removal during fretting between Zircaloy-4 clad nuclear fuel rods and their supporting grids of a similar or dissimilar material. A tribosystem analysis of this complex situation not only involves mechanical influences on cladding wear, but also the effects of temperature and tribo-corrosion. A previously published GTRF wear model underlies this work. It assumes that the cladding wear rate can change over time and takes place in stages depending on the evolving nature of the materials in the interface. A materials science-based derivation of wear factor introduces four concepts: a wear process volume (WPV) from which debris particles originate, a population of wear debris embryos (DEms) of a critical size within the WPV, primary debris origins from the subsurface of the cladding alloy, and secondary debris detachment from third-body layers. The probability of nucleating loose particles from DEms and the coalescence of cracks in highly-worked Zircaloy-4 to form primary debris is estimated from wear test data obtained with a high-temperature, pressurized-water GTRF wear simulator at Oak Ridge National Laboratory (USA). The limitations and challenges for quantifying certain microstructure-based parameters that comprise the wear factor derivation are discussed.