Application de techniques de microanalyse pour la caractérisation de combustibles céramiques nucléaires irradiés

S. Brémier, P. Poeml, F. Laheurte and R. Hasnaoui
European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, DE-76125 Karlsruhe, Germany

  
Microbeam analysis is widely used in the nuclear power industry.  It is used for routine post-irradiation examination and for research into the mechanisms affecting safe operation of the nuclear fuel. The techniques most commonly used are wave-length dispersive electron probe microanalysis (WDS-EPMA), scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS). Other microbeam analysis techniques that have been successfully applied to irradiated nuclear fuel are transmission and replica electron microscopy (TEM and REM), micro X-ray fluorescence (micro-XRF) and micro X-ray diffraction (micro-XRD).  SEM, TEM and REM have been mainly used to study the evolution of fission gas bubbles, which cause the fuel to swell during irradiation [1-3], and micro-XRD has been used to investigate the change in lattice parameter caused by irradiation damage and the build-up of fission products during irradiation and to assess their influence on the transformation of the fuel microstructure after prolonged irradiation [4].

WDS-EPMA is the microbeam analysis technique most widely available in nuclear research centres around the world. From the perspective of investigating irradiated nuclear fuel, this technique has a number of limitations. The main drawback is that it does not measure isotopes.  A further shortcoming is the detection limit, which for a radioactive sample is at best 200 ppm [5].  In addition, the fission gas krypton cannot be measured because the second order M1 U X-ray line coincides with the Kr L1 line and the Kr K X-ray lines have a high critical excitation energy of 14.3 keV. Finally, owing to the shallow electron penetration in nuclear fuel, the fission gas trapped in pores and bubbles larger than about 0.1 m cannot be detected [6,7].  These deficiencies have been overcome by combining EPMA with micro-XRF [8] and SIMS [9].

This presentation will recall some background information about nuclear fuel rods and their irradiation, followed by a description of the features that set apart the microbeam analysis of irradiated nuclear fuel from standard practice on "cold" materials. Finally, specific examples illustrating the past and present use of microbeam analysis in nuclear research are discussed, with emphasis on most valuable and unique sets of results.

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