The PHEBUS FP (Fission Products) had to purpose to improve the understanding of the phenomena occurring during a core meltdown accident in a light water reactor and to validate the computational software used to represent these phenomena in reactor safety evaluations. Five test have been realised. The FPT-0 test was realised in 1993 and had to main objective to watch degradation of non-irradiated fuel and the behaviour of the fission products under water vapor.
This first test was conducted in 1993 under conditions of low pressure (0.2 MPa) and an atmosphere rich in water vapour (oxidising conditions). The main objective was to study the degradation of non-irradiated fuel and the behaviour of the fission products, in particular iodine, under these conditions, and in the presence of a control rod made of a silver, indium and cadmium alloy. These conditions were those of a 900 mWe PWR. The primary circuit was set at 700°C in the hot leg and 150°C in the cold leg. Part of the walls of the containment vessel was covered with paint to allow study of the interactions of iodine with paints.
Observations and results
In the event of a core meltdown accident in a reactor, fuel degradation begins with oxidation of the cladding, with a rapid temperature increase. During test FPT-0, this temperature increase was greater than that calculated in preparing the test, with a high level of hydrogen and an increase in fuel rod temperatures above 2400°C. The models used in the computational software to simulate these phenomena were modified to allow proper calculation of the oxidation of cladding and the associated hydrogen production for the following tests.
In general, the calculations predict the behaviour of vapours and aerosols of fission products in the primary circuit satisfactorily. The value of overall retention in the steam generator is however overestimated by the models, and that in the vertical line above the fuel bundle is underestimated.