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AbstractIn this work, laser heating is used to experimentally investigate the high-temperature behavior of the U-Fe-Zr-O system using arc-melted samples with various nominal compositions. Three-phase transitions are observed in the vicinity of ~1100, ~1700, and ~2200 K. Principal component analysis of the phase transition temperatures in the course of laser-heating thermal cycling indicates that the phase transition around ~1100 K is driven by the interaction of stainless steel (SS) with metallic U, the phase transition around ~1700 K by the melting of stainless steel, and the phase transition above ~2000 K by the eutectic melting of UO2. The results also reveal two hitherto overlooked interactions in the U-Fe-Zr-O system, which could have severe consequences for the containment of corium inside the reactor pressure vessel (RPV). First, the phase transition temperatures of the samples varied extensively as a result of the laser-driven rapid thermal cycling. Variations of up to 390 K were observed in the phase transition temperatures, suggesting that depending on the initial conditions of corium formation, the corium-driven ablation of the RPV wall could commence significantly earlier than the current state-of-the-art severe accident codes would predict. Additionally, evidence of a large exothermic reaction between zirconium and molten steel was observed upon SS melting. Such phenomenon may also be driven by material segregation during fast heating and cooling. If such a mechanism is activated during a severe nuclear accident, it can have an important impact on the overall thermal balance of the RPV.