Evaluation of air oxidation and internal stresses induced by quenching of partially Cr-coated and uncoated optimized ZIRLO part I: Materials characterization

It is critical to develop technologies that minimize risk of nuclear reactor failure. Coated claddings present an opportunity to preserve fuel rod integrity in the case of a loss of coolant accident. In this work, the material evolution of quenched Optimized ZIRLO (OPZ) rings and Cr-coated OPZ rings is studied at temperatures up to 1000 °C, until full oxidation in air is uncovered through a variety of microstructural characterization techniques, including optical analysis and X-ray diffraction. A number of microstructural reorienting and complex, multi-stage oxidation mechanisms are found to play a role in the structural and material changes. At 1000 °C, the primary failure mode of uncoated ZIRLO is breakaway oxidation; however, the introduction of a single-sided Cr coating protects ZIRLO from oxygen penetration through the exterior surface. The material is seen to undergo several microstructural reorientations from 315° to 900°C while remaining in the α-Zr phase. At 900 °C, Cr-coated OPZ begins the α to β phase transition, and the chromium diffuses into the substrate layer. When both events are present, the Cr phase change can lead to the formation of a Cr-β-Zr eutectoid, and subsequent eutectic temperature at 1332 °C. For uncoated samples, a new phenomenon of iron-rich oxide (rust) development along the ring center in air at 1000 °C is unveiled and explained as an extension of spinodal decomposition. This study is Part I of a two-part series regarding the behavior of Cr-coated and uncoated OPZ at high temperatures. Part II investigates the internal stresses and other mechanical behaviors induced by metallic restructuring and oxide development.