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-3 test was realised in 2004 and has observed the type of control rod material, boron carbide in place of the silver, indium and cadmium alloy initially used.
Test FPT-3, the fifth and last PHEBUS FP test, was held in 2004. It was distinguished from the preceding tests by the type of control rod material, boron carbide in place of the silver, indium and cadmium alloy initially used. This material is used in French 1300 MWe and 1450 MWe reactors as well in boiling water reactors and in the Russian VVERs. It can have an impact on fuel degradation as well on the chemistry of fission products.
This experiment was also an opportunity to test and compare different catalytic recombiners intended to limit the hydrogen risk inside the containment vessel in the event of a core meltdown accident in a PWR.
Observations and results
During the test, release of methane, carbon monoxide and carbon dioxide could be observed during the fuel cladding oxidation phase. These emissions, observed for the first time, were related to the presence of carbon in the control rod.
Much less extensive, but earlier, degradation occurred in the assembly during the cladding oxidation phase than during the FPT-2 test. The experiments showed that projections of a melted boron carbide-steel mixture onto the cladding of the fuel rods adjacent to the control rod could occur and so accelerate degradation of the cladding.