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Enhancing Nuclear Safety


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Macroscopic Examination and Microscopic Analysis of the Degraded Phebus FPT2 Bundle at ITU

EMAS 2005, 9th European Workshop, Florence, 22 to 26 May 2005

P.D.W. Bottomley, S. Brémier, D. Papaioannou, C.T. Walker, J.P. Glatz, S. Schlutig, M. Barrachin.

Document type > *Congrès/colloque

Keywords > safety, PHEBUS Fission Products program, severe accident

Research Unit >

Authors > BARRACHIN Marc, SCHLUTIG Sandrine

Publication Date > 23/05/2005

Summary

The Phebus PF Project is an international reactor safety project supported by the European Commission and organised by the Institut de Radioprotection et Sûreté Nucléaire (IRSN). There is also the support of the European national institutes as well as 5 non-EU partners : Switzerland, Japan, Korea, Canada and USA. The severe accident project looks at the degradation of a 1 m long 20-rod bundle of irradiated fuel and the release of the fission products and their transport down a simulated primary circuit and into a containment. The first test FPT0 provided the baseline data using trace-irradiated fuel. FPT1 was a bundle of 18 irradiated fuel rods (plus 2 non irradiated and instrumented fuel rods), that was degraded under relatively oxidising (high steam flow) conditions. The most recent test was FPT2 (Oct. 2000) where, by contrast an irradiated UO2 fuel bundle was degraded under conditions of extremely low steam flow, creating very reducing conditions. ITU was responsible for cutting discs from the FPT2 bundle. ITU also examined the degradation as well as confirming the preliminary tomography data. The bundle had (as in FPT0 and FPT1) a corium pool that accumulated at one-quarter height, above which was a very extended central cavity and at the uppermost levels were severely degraded and oxidised rods. The cavity was larger than that formed under non-reducing conditions and the corium pool had a greater mass. This confirmed that reducing conditions can cause more damage than steam-rich conditions since the zircaloy cladding melted rather than oxidised and had relocated to a greater extent. ITU had extracted cores from selected sites in the discs for further microscopic examination and analysis. Under the optical microscope the rapidity of oxidation and the influence of double-sided oxidation on cladding rupture were clearly seen along with the interaction between structural materials, cladding and fuel. Considerable debris was noted at points below the corium pool. Electron microprobe analysis of the principal phases will be carried out to examine in detail the influence of the reducing conditions (steam starvation) on fuel/structure interactions, corium pool composition and fission product retention.

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