In situ Characterization of UO 2 Microstructure Changes During an Annealing Test in an Environmental Scanning Electron M
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1215-V16-44
In situ Characterization of UO2 Microstructure Changes During an Annealing Test in an Environmental Scanning Electron Microscope M. Marcet1, Y. Pontillon1, L. Desgranges1, D. Simeone2, I. Aubrun1, I. Felines1, L. Brunaud1 1 2
Commissariat à l’Energie Atomique DEN/CAD/DEC, F-13108 Saint-Paul-lez-Durance, France Commissariat à l’Energie Atomique DEN/DANS/DMN, F-91191 Gif sur Yvette, France CNRS Ecole Centrale Paris SPMS MFE, F-92290 Châtenay-Malabry, France
ABSTRACT A 1 µg High Burn Up Structure (HBS) fragment was extracted from a UO2 fuel pellet irradiated for 7 cycles in a EDF Pressurised Water Reactor (PWR). In situ examinations were performed with an Environmental Scanning Electron Microscope (ESEM) in order to characterize UO2 microstructure evolution during a temperature ramp up to 1,600K. The results are compared to previously published data on HBS annealing tests performed in a Knudsen cell where observed burst releases are explained as sample cracking during the experimental sequence.
INTRODUCTION After irradiation in normal operating conditions, used UO2 fuel exhibits different microstructure as a function of the radial position in the fuel pellet. These microstructures reveal the thermal gradient existing in the fuel pellet during operation, which is due to homogeneous production of heat by fission in the pellet. More specifically the High Burn-up Structure (HBS) appears at the outer rim of the pellet, its coldest part, when the local burn-up reaches a 60-70 GWd/tM threshold [1-4]. This structure is characteristic with the subdivision of initial grains into small grains and the presence of important porosities containing pressurised gas bubble. The fission gases behaviour during a temperature transient is a key issue regarding the PWR’s nuclear fuel licensing [5]. This behaviour is generally studied in hot cells by measuring the fission gas release out of a slice of used nuclear fuel as a function of a predetermined thermal history [6-7]. Recently the fission gas release of the HBS was studied in a Knudsen cell [8]. These releases are continuous processes except the main steps at 1,000K and 1,500K, which are characterized by explosive gas release evidenced from the pressure transients observed by the vacuum gauges. These pressure spikes are explained by burst releases of the gas contained in closed pores. Other annealing test performed in our team on a whole fuel pellet [9] confirmed that the HBS released gases at approximately the same temperature. Besides some mechanical degradation of the HBS was observed after the thermal sequence. In this study we used Environmental Scanning Electron Microscope coupled with an heating stage in order to characterize the change of the UO2 microstructure during temperature
ramps in relation to the burst release. The gas release mechanism will be discussed as a function of the cracks observed both at the surface and in the bulk.
EXPERIMENTAL SECTION 1. Fuel samples A slice taken out of a UO2 fuel rod irradiated up to 77 GWd/t (7 cycles) in a PWR operated
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