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Phebus FPT3 : Overview of main results concerning core degradation and fission product behaviour



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International Conference on Advances in Nuclear Power Plants 2011 / 2-5 mai 2011, Nice (France)

Type de document > *Congrès/colloque

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Unité de recherche > IRSN/DPAM/SEREM/LE2M

Auteurs > CLEMENT Bernard, BIARD Bruno, MANENC Christelle, PAYOT Frédéric, MARCH Philippe, SIMONDI-TEISSEIRE Béatrice, ZEYEN Roland

Date de publication > 02/05/2011


FPT3 was the last of the five in-pile integral experiments in the Phébus FP programme, whose overall purpose was to investigate fuel rod degradation and behaviour of Fission Products (FPs) released via the primary coolant circuit into the containment building. The results contribute to validation of models and computer codes used for calculation of the source term in case of a severe accident with core meltdown in a Light Water Reactor (LWR). Unlike the previous tests, FPT3 used boron carbide as absorber material in the pre-irradiated (24.5 GWd/tU) fuel bundle, while featuring a steam-poor period as in the previous test FPT2, that used silver-indium-cadmium absorber material. This paper summarises the main experimental results of FPT3, highlighting the effect of the boron carbide on degradation of the fuel rod bundle, transport and deposition of fission products and structural materials in the hot leg part of the circuit up to the steam generator entrance. In the bundle, a strong effect of the boron carbide in promoting bundle degradation and material relocation was seen; unlike the previous tests no remnant molten pool was observed, but instead substantial refrozen low melting point material below the bottom of the heated section. Carbonaceous gas production (CO, CO2) accounted for ~77% of the carbon in the absorber material, while the hydrogen production was consistent with ~73% oxidation of the Zircaloy present. Fission product and structural material releases were broadly similar to those in previous Phébus FP tests, taking account of the different control rod materials, with high releases of the volatile FPs such as Cs, I, Te, Cd and Ag with the noble gases Xe and Kr, medium release of Mo, and low releases of Rh, Ba, Pa, Tc, Ru, Ce, Sr, Y, Eu and Nb. In the circuit hot leg (700°C), iodine was mainly present in gaseous/vapour form, whereas it was mainly in vapour form in FPT2. In contrast to FPT2, Cs was mainly observed in aerosol form. Strong depositions of fission products were observed in the hot leg, as well as in the upper part of the bundle, and the aerosols transmitted were characterized in terms of morphology and size. Material behaviour downstream (steam generator, cold leg, and containment) and comparison of results amongst the Phébus FP experiments will form the subject of future publications.