Fission products at the fuel-cladding interface of PWR fuel rods: morphological and chemical study

During irradiation in light water reactors, numerous fission products are produced by the fission reactions. At the periphery of the fuel, these elements implant by recoil in the cladding inner oxide layer (zirconia) located at the fuel-cladding interface. In the present study, two irradiated UO2 fuel rods with different burnups (37.3 and 61.4 GWd/tU) have been analyzed. The coupling of FIB-SEM tomography and S/TEM-EDS chemical analyses allowed 3D analysis of the distribution, sizes, shape and chemical composition of the bubbles and precipitates of fission products in zirconia. The fission product structure, observed in the two samples was quite similar. A complex morphology of the fission product agglomerates have been highlighted. They are organized in three parts: the bubble (Xe), the deposit (Cs, I) and the metallic precipitates (Mo, Tc, Ru, Rh, Pd, Sn). Tellurium is partly found with the deposit, but also with the metallic precipitates. Different kind of bubbles have been observed, classified by size, potentially linked to different steps of growth. In particular, Xe form bubbles arranged in three populations: sub and/or nanobubbles within the grains and along the grain boundaries (< 1 nm), larger bubbles (a few nanometers to a few tens of nanometers) anchored to the metallic precipitates, and finally, much larger bubbles (several tens to several hundreds of nanometers), which are part of more complex systems, and are also stuck to metallic precipitates.