MATERIAL EFFECT IN THE NUCLEAR FUEL - COOLANT INTERACTION: ANALYSES OF PROTOTYPIC MELT FRAGMENTATION AND SOLIDIFICATION IN THE KROTOS FACILITY

摘要

The molten Fuel - Coolant Interaction (FCI) presents still an important issue of the nuclear reactor severe accident research and developments that lacks of rigorous understanding. Presented paper is focused on the so-called material effect in FCI. In the frame of OECD/Serena 2 project a set of experiments was performed in the KROTOS facility using prototypic melts with various initial composition. Debris from three tests was characterized by various chemical and metallographic techniques (scanning electron microscopy, energy dispersive X-ray spectrometry, X-ray powder diffraction, inductively coupled plasma mass spectroscopy) in order to obtain information about the debris morphology, composition and structure. The morphology of particles participating to the fine (thermal) fragmentation was described. Moreover, a method was developed allowing quantification of the melt part participating to fine fragmentation based on the image analyses of debris micrographs. The results show that the melt drops participating to the steam explosion have significant porosity (i.e. 20 %) in the form of small voids and channels. The melt solidification path was described as well. The initial U_(1-x)Z_xO_(2+y) molten mixture solidifies into face-centered cubic crystalline phase, whatever is the FCI progression (steam explosion or not) for all KROTOS tests.