During irradiation in a nuclear reactor, the fuel pellet experiences non-uniform fission rate and strong thermo-mechanical loads, which result in alterations and restructuring occurring non-homogeneously in the fuel volume. The irradiated fuel (see Fig. 1) is characterized by a highly heterogeneous structure, with a correspondingly heterogeneous distribution of phases and properties.
It is a big technical challenge for a hot cell facility to be able to cover effectively a broad variety of fuel types and compounds, characterized by different compositions, physico-chemical properties, geometries and configurations. The first challenge is to provide testing environments and procedures which do not alter or degrade the original irradiated fuel properties and are technically suitable for hot cell or shielded facility operation. For instance, non-oxide fuel compounds require protective atmosphere to avoid sample oxidation during storage, preparation and testing. Moreover, suitable adaptations of tools and measuring devices must be available to characterize fuel configurations which differ from the conventional cylindrical pellet-cladding geometries (e.g. spherical fuel elements for pebble bed HTR and TRISO coated fuel particles). Finally, adequate scientific methods of investigation must be available to measure properties and study the behaviour of highly heterogeneous irradiated fuel. In addition to basic techniques for non-destructive and destructive examination of nuclear fuel systems (e.g. covering fuel and cladding or fuel and coatings, or other configurations), "in-depth" methods are applied to determine physical, thermo-mechanical and micro-analytical properties of specific phases/features present in irradiated fuel.
This paper presents relevant trends and examples from fuel safety studies performed at JRC-Karlsruhe and in other laboratories which highlight the "added value" obtainable by applying combined techniques, high resolution tools or 3D methods of analysis for the experimental "multiscale" characterization of irradiated fuel.