Late phase fuel degradation in the Phébus FP tests

The aim of the experimental Phébus FP program was to study the degradation phenomena and the behaviour of the fission products during the progression of a severe accident. The present paper focuses on the late phase fuel degradation in these tests and more particularly on some phenomena that happened during this phase, which could explain the transition processes from the fuel bundle geometry to a debris bed or molten pool. One of the main results produced by the program is the evidence of the loss of the fuel rod geometry systematically in the same range of temperature, (2200 ± 200 °C), in spite of the different test conditions. The severe degradations at these temperatures appear linked to important chemical interactions between the fuel and structural materials, principally the Zircaloy cladding of the fuel rods and possibly with stainless steel oxides from the control rod guide tube (and with boron oxide in FPT3). The oxidation of fuel itself could lead to a lowering of the fuel rod relocation temperature. The irradiation effect was not clearly identified as important in these degradations.