HIGH-RESOLUTION TEM OF HIGH BURNUP UO2 FUEL

High-resolution TEM (HRTEM) observations and nano-area EDX analyses we re carried out on small intragranular bubbles in the outer region of h igh burnup UO2 pellets. Sample was prepared from the outer region of U O2 pellet, which had been irradiated to the pellet average burnups of 49 GWd/t in a BWR. HRTEM observations and element analyses were made w ith a 200 KV cold-type field emission TEM (Hitachi FE-2000) having an ultra-thin window EDX (Noran Voyager). Lattice image and nano-area EDX results indicate the presence of 4-8 nm size Xe-Kr bubbles along with fission products of five metal particles, Mo-Tc-Ru-Rh-Pd. Nano-diffra ction patterns from bubbles show two different new patterns besides ma trix UO2. From the Xe/U proportion obtained by nano-area EDX peak and nano-diffraction patterns from bubbles, it was concluded that Xe in th e small bubbles was present in a solid or near solid state at very hig h pressure. Furthermore, from the results of high resolution images an d diffractions obtained from recrystallized grains in rim structure re gion, neighboring recrystallized grains were clarified to be present w ith high angle grain boundaries. (C) 1997 Elsevier Science B.V.