Nuclear fuel material changes arise over the whole fuel life-cycle, during operation, after unloading and during storage. For a good characterization and assessment of the fuel comprehensive post irradiation examination (PIE) is essential. PIE has to take the complementarities of various methods into account and the accessibility of sample material, i.e. whether an examination can be done in non-destructive manner, or whether destructive preparation steps are needed. Besides materials questions also fuel system issues need to be addressed, e.g. the interaction between pellet and cladding. Further, a PIE campaign has to be composed in a sequential and cost effective way.
Today, for a complete irradiated fuel characterization a suite of elemental and isotopic analyses is needed, together with chemical speciation and structure determination. This requires the use of advanced spectroscopic techniques allowing a resolution giving access to burn-up related questions. Thus, besides typical hot-laboratory instrumentation also synchrotron radiation is used more and more. Further, differentiation of the analytical methods between those with a more passive character, using the activity of the fuel itself, or with a more interactive character using excitation e.g. by micro-beam is useful. In practice, this study presents specific examples comparing analyses of non-irradiated with irradiated fuel in the fuel pellet center and the periphery:
• Fuel morphology and porosity.
• Intermetallics particles as temperature indicator.
• Cesium volatility, precipitation and distribution in irradiated fuel.
• Fission gas ratio and density in aggregates.
• Challenges in actinide speciation and reactivity in irradiated fuel.
The discussion focuses on the potential of the techniques and their limitation (interference, detection limit, applicability) for fuel analysis. It also discusses and compares pore, fission gas aggregate and sub-grain densities as well as other phase occurrence.
It may be concluded that in dependency on the scientific question a specific combination of isotopic, chemical, physical or structure related methods is necessary, composed in a sequential and incremental way. In future amendments are needed for targeted sample preparation, micro-tomography and testing on a microscopic level.
