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-4 test was realised in 1999 and has to purpose to study the kinetics of release of low-volatility fission products and actinides from a “debris bed”.
This fourth PHEBUS FP test, which took place in 1999, aimed to study the kinetics of release of low-volatility fission products and actinides from a “debris bed” (the result of fuel rod degradation during a core meltdown accident) composed of solid particles heated until a bath of melted corium was obtained. These conditions could occur during failure of an attempted reflood. The test section was composed of a crucible filled with 2 to 5 mm fragments of fuel from a reactor and fragments of oxidised Zircaloy cladding.
Observations and results
This test allowed fuel degradation to be observed. Analysis of the results of destructive tests of the fuel shows that it was oxidised and that solid-state interactions could have led to early partial liquefaction (at 2350°C). One part of the corium was formed by separate melting of UO2 and ZrO2. Analysis of the composition of the melted phases showed that temperatures greater than 2700°C were reached. The models of uranium release were revised on the basis of these results.