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-1 test was realised in 1996 and had to main objective to watch degradation of non-irradiated fuel and the behaviour of the fission products under water vapor with irradiated fuel.
This test took place in 1996 under the same conditions as test FPT-0; that is, low pressure (0.2 MPa) and an atmosphere rich in water vapour (oxidising conditions). The degradation of fuel, this time irradiated at 23 GWd/tU (a level equivalent to two cycles of irradiation in a PWR), and the behaviour of the fission products, in particular of iodine under these conditions, were studied here too in the presence of a control rod made of a silver, indium and cadmium alloy. The primary circuit was set at 700°C in the hot leg and 150°C in the cold leg, and the containment vessel had painted surfaces allowing the interactions of iodine with paints to be studied.
Observations and results
Most of the fission products were transported in the hot leg of the primary circuit in the form of aerosols, with the exception of iodine, cadmium and to some extent caesium. The first two were deposited in a significant quantity at the inlet of the steam generator, and caesium was deposited in an equal quantity in each of the two areas due to its lower volatility.
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, and that in the vertical line above the fuel bundle is underestimated.