Characterisation of high temperature refractory ceramics for nuclear applications.

Abstract

The ternary oxide ceramic system UO2-ZRO2-FeO is a refractory system that is of great relevance to the nuclear industry as it represents one of the main systems resulting from the interaction of the Zircaloy cladding, the UO2 fuel and the structural elements of a nuclear reactor. It is particularly the high temperature properties that require investigation; that is, when substantial overheating of the nuclear core occurs and interactions can lead to its degradation and melting. There has been much work on the UO2-ZrO2 system and also on the ternary system with FeO but there is still a need to examine 2 further aspects; firstly the effect of sub-oxidised systems, the UO2-Zr and FeO-Zr systems, and secondly the effect of Fe/Zr or Fe/U ratios on the melting point of the U-Zr-Fe oxide system. Samples of UO2-Zr and UO2-ZrO2-FeO were fabricated at ITU and then characterised by electron and optical microscopy (OM & SEM-EDS) and X-ray diffraction to determine the ceramic's structure and verify the composition. Thereafter the samples were melted by laser flash heating and their liquidus and solidus temperatures determined by pyrometry. The frozen samples were then sectioned, polished and the molten zone micro-analytically examined by OM & SEM-EDS in order to determine its structure and composition and to comparte it with the existing phase diagrams. Finally, a reacted Zr-FeO thermite mixture was examined, which had been used to generate high temperatures during tests of reactor melt-concrete interactions. The aim was to assess the reaction and estimate the heat generation from this novel technique. These results allow verification or improvement of the phase diagram, and are of primary importance as input to models used to predict materials interactions in a severe accident.