Measurements on crystal lattice strain and crystallite size in irradiated UO pellet by X-ray diffractometry

Amaya, Masaki  ; Nakamura, Jinichi ; Fuketa, Toyoshi

Lattice parameters, crystallite sizes and non-uniform strains of high burnup UO fuel samples were measured using micro-X-ray diffractometry in order to investigate the so-called rim structure formation, in UO pellets. The pellet-averaged burnups of the samples were in the range 41 to 61GWd/t. While the lattice parameters of the samples increased linearly in the burnup region up to approximately 70GWd/t, the lattice parameters quickly decreased and tended to level off above 70 GWd/t. Measured crystallite sizes were in the range 100 to 200 nm and these were nearly the same size as the "recrystalized grains" in the rim structure. Elastic strain energy densities, which were evaluated from the lattice parameters and non-uniform strains, tended to increase with burnup and show two plateaus at burnups of approximately 50 and 70 GWd/t, respectively.