2018 | AFC ACCOMPLISHMENTS 101 Accomplishments: The PIEs took place in the Hot Fuel Examination Facility (HFEF) at INL. A broad spectrum of PIE has been employed to evaluate fuel integrity, fission gas release, fission product distribution, burnup, fuel swelling and cladding strain. Performed PIE included neutron radiography, gamma spectrometry and tomography, dimensional inspection, fission gas release and chemical burnup analysis. Microscopy samples were examined using the new high resolution optical microscope (OM) installed in HFEF. Analysis of neutron radiography images (Figure 1) and low magnifica- tion OM images (Figure 2) show a limited number of cracks in the fuel matrix, as direct consequence of the reduced thermal stresses. Figure 3 shows two examples of the irradiated U3Si2 microstructure, one acquired on the pellet periphery (Figure 3a) and the other at the center (Figure 3b). As in the as-fabricated microstructure, secondary light and dark phases are present in the main matrix, corresponding to Si-rich and UO2 impurities present from fabrication. Porosity is co-localized with the UO2 phase.Towards the pellet center (Figure 3b), additional small spherical features appear, which are likely to be fission gas bubbles. Image analysis algorithms were developed to perform segmentation of the secondary phases and fission gas bubbles.The value of the porosity associated with the UO2 precipitates remains as in the as-fabricated samples, excluding densification. Fission gas bubbles development is consistently observed within 60% of the pellet radius for both irradiated rodlets.The formation of bubbles in the central part of the pellet is consistent with the local higher irradiation temperatures. It is not to be excluded that smaller bubbles not resolvable with the optical micros- copy exist in the pellet periphery. However, the overall swelling of the pellets remains limited and fission gas release remains very low (0.05% of the produced inventory). The microstructure investigation and the overall results of the PIE campaign suggest a good performance of this ATF candidate at low burnup and under operating conditions.