Side-by-side high-temperature accident performance of ATF and conventional claddings in the CODEX-ATF experiment

The CODEX-ATF integral bundle test was conducted within the framework of the IAEA Testing and Simulation for Advanced Technology and Accident Tolerant Fuels (ATF-TS) project at the CODEX (COre Degradation Experiment) facility in Hungary. The electrically heated seven-rod bundle consisted of four Cr-coated and three uncoated Zr alloy cladding tubes, enabling a direct comparison of their behavior under high-temperature accident conditions. The experiment primarily aimed to investigate fuel failure and degradation mechanisms. During the test, several rods exhibited ballooning and burst phenomena. The maximum temperature exceeded 1600 °C. The transient was terminated by a bottom-up water quench. The total hydrogen generation was approximately 3 g, indicating substantial oxidation of the zirconium-based components. Intensive Zr-Cr eutectic interaction was observed in the hottest region of the bundle on the Cr-coated claddings. Post-test examinations revealed pronounced deformation and failure in both coated and uncoated claddings. • The CODEX-ATF test was performed with electrically heated bundle composed of three uncoated and four Cr-coated optZIRLO™ cladding tubes. • Under high-temperature (>1600 °C) accident conditions in steam atmosphere, both Cr-coated and uncoated fuel rods would lose their integrity. • In those parts of the core where the temperature remains below 1400 °C, oxidation would dominate the degradation process. • At temperatures above 1400 °C, the Cr layer may be lost and the formation of a Cr-Zr eutectic can cause localized melting.