[en] As part of the United States Nuclear Regulatory Commission's Reactor Safety Research Program, irradiated and unirradiated fuel rods are being forced into film boiling operation under controlled test conditions. The tests are being conducted on heavily instrumented zircaloy clad UO₂ fuel rods in the Power Burst Facility at the Idaho National Engineering Laboratory. The tests to date have been conducted at rod powers of 51 to 80 kW/m and have resulted in film boiling periods of a few seconds to more than ten minutes. Cladding surface temperatures experienced have been as high as 1400 C. Fuel temperatures have exceeded the UO₂ melting temperature. Extensive oxidation of the zircaloy cladding surface has resulted from metal-water reaction. Following cladding collapse onto the fuel pellets in the areas of high temperature, an internal reaction between the UO₂ and the zircaloy has contributed to additional cladding embrittlement. The combined effects of internal and external cladding attack have resulted in failure of the fuel rods in several of the tests following reactor shutdown. The highly embrittled cladding fails a few minutes following shutdown. None of the test fuel rods have failed at power in spite of the presence of molten fuel and severe mechanical interaction. Fuel rod failure propagation or severe fuel-coolant interactions are considered unlikely during an abnormal event in a power reactor during conditions similar to those of tests conducted to date