Use of a micromechanical approach to investigate transient fuel fragmentation mechanisms
Résumé
Under LOCA conditions, the temperature gradient evolution within the fuel pellets combined with a reduction of the cladding confinement can lead to fuel fragmentation. This phenomenon provides additional fission gas releases, inducing a higher rod internal pressure and possibly an additional driving force to disperse the smallest fuel fragments out of the cladding when the cladding balloons and bursts. Experiments show the pellets are fractured in many fragments, with size ranges varying from few millimetres to few microns. Experimental fuel fragmentation thresholds have been defined as a function of pellet Burnup and temperature. Nevertheless, despite of a good agreement between these empirical thresholds and integral LOCA tests results, these thresholds do not included neither fracture mechanical theories nor microstructure considerations. The aim of this study is to define a fragmentation threshold based on a micro mechanical approach to complement the former experimental observations. Usually the hypothesis used to explain fuel pellet fragmentation during transient, is grain cleavage induced by over pressurized fission gas bubbles, located at the grain boundary. In this paper we will then present the first steps of the fuel microstructure modelling including pressurized bubbles in order to have a better understanding of the fuel micro mechanical behaviour and establish a fragmentation threshold: (1) model the fuel micro structure at the beginning of the transient, (2) simulate its micro mechanical behaviour using a 3D Finite Elements approach, (3) develop a macroscopic criterion based on the 3D local results. To characterize the fission gases bubbles, the initial conditions of the Studsvik LOCA test specimens have been calculated with the fuel performance code ALCYONE V1.4 of the PLEIADES software environment co-developed by CEA, EDF. These results are useful to set up a Representative Volume Element (RVE) for each type of microstructure considered in the pellet. Then a LOCA transient is applied to the 3D finite element model of the RVE, considering a local behaviour, based on a damage law developed by CEA. Simulation results will be then be used to better understand fuel behaviour under LOCA conditions with a view to predict fuel pellet fragments size distribution.
