Key parameters for the High Burnup Structure formation thresholds in oxide fuels

摘要

Since the first observations of the High Burnup Structures (HBS) on PWR fuels at the beginning of the 1980's, many experimental data have been accumulated, coming as well from the experimental feedback of the irradiated rods in commercial reactors as from separate effect irradiations in research reactors. This microstructure, characterized by the development of a high micrometric porosity, a loss of detection with EPMA and a grain subdivision, was first called "rim" structure, because it was observed at pellet periphery in UO_2 fuel, as a result of the highest burnup induced by the Pu build-up. But it is also observed in the Pu rich agglomerates of the MOX MIMAS fuel, and it was also observed earlier in external parts of the FBR fuels, but without in depth investigation. Finally, this transformation is considered as characteristic of the high burnup zones irradiated in relatively cold conditions. With the current increase in the nuclear fuel discharge burnup and the introduction of MOX fuel in France and worldwide, it became necessary to understand the mechanisms leading to this microstructure change and to determine the major influencing parameters. Indeed, the progression of the HBS zones can have consequences on fuel rod behavior under operating and accidental conditions. The purpose of this paper is to carefully analyze the experimental data from different sources in order to identify the major parameters which govern the formation and the development of restructured zones. The most important considered database is issued from the post irradiation examinations which have been performed in the frame of surveillance programs on commercial fuel rods (UO_2 and MOX) in France. But an overview of experimental data available in the open literature is also realized in order to extend the field of investigation with different parameters (fuel type, enrichment, reactor conditions, commercial or research reactors). Furthermore, in addition to UNR fuels (UO_2 and MOX), FBR fuels are included, for which a consistent database is available in France, issued in major part from the irradiations performed in the Phenix reactor. From this analysis, it is concluded that the influencing parameters on the HBS formation thresholds are the instantaneous fission rate, the temperature, the Pu content and the constraint. The dominant parameter is the instantaneous fission rate, while the other parameters appear as catalyst or inhibitory parameters. The concept of a burnup threshold, which integrates the instantaneous fission rate, appears as a resultant of all these parameters, and can vary significantly, according to the other parameters. Nevertheless, even if the influencing parameters have been identified, the basic mechanisms are not totally understood for some of them and further investigations are necessary in this field.