Early phase fuel degradation in Phébus FP: Initiating phenomena of degradation in fuel bundle tests

► Phébus FP studied fuel degradation during severe accidents in light water reactors. ► Main early phase phenomena included control rod failure and Zr-oxidation runaway. ► The phenomena seen in the early phase were consistent with existing knowledge. ► Follow-on research on B4C reactions has led to detailed code model improvements.