2018 | ANNUAL REPORT 109 better sample preparation compared to the past [10] and preserving both the JOG and the fission products accumu- lating in the fuel-cladding chemical interaction (FCCI) layer (Figure 5). From optical microscopy images, engineering-scale measurements of the fuel microstructure features can be obtained. For instance, central void dimension and its evolution along the pin axis are reported in Figure 6. This type of quantitative information is fundamental to validate models of pore migration used in fuel perfor- mance codes [11]. Accomplishments PIE results highlighted very good and comparable performance among the pins under similar irradiation conditions. Cesium redistribution has occurred in two of the pins, as shown in Figure 3. Cesium migra- tion is a known phenomenon in SFR MOX fuels, induced by radial and axial temperature gradients. Cesium becomes volatile in the hottest parts of the fuel and migrates by successive evaporation and condensation to the colder regions, where lower tempera- tures allow formation of Cs compounds with low Cs vapor pressure [12]. Despite increased cesium concentration at specific axial locations, no enhanced cladding strain was measured, as shown in Figure 1, suggesting no detrimental impact on the cladding following cesium accumulation, at least at intermediate burnup. Fuel microstructure evolution occurs both radially and axially, depending on the local temperatures and burnup levels.At peak-power axial-node positions (i.e., at intermediate rela- tive axial positions), formation of columnar grains occurs where temperatures exceed 1800–1900°C [4], as shown in Figure 4a.An enlargement of the as-fabricated annulus occurs due to pore migra- tion, driven by steep temperature gradients (Figure 6).At the beginning of the fuel column, where power and burnup levels remain lower, no columnar grain formation occurs (Figure 4b), but the central part of the pellet shows a different texture, probably related to the enhanced diffusion-driven precipitation of fission products on grain boundaries. Analysis of the optical microscopy mounts at different axial locations showed that the gap is still open. Partially, the gap is filled with fission Figure 6. Evolution of the central void size after irradiation in pin K06. Increase of the void size is a consequence of pore migration induced by thermal gradients. Decrease of the size is due to possible material relocation or migration towards the coldest part of the pin.