Measurements of Crystal Lattice Strain and Crystallite Size in Irradiated UO2 Pellet by X-ray Diffractometry

Abstract

Lattice parameters, crystallite sizes, and nonuniform strains of high-burnup UO2 fuel samples were measured using micro-X-ray diffractometry in order to investigate the effects of grain subdivision and strain distribution between crystallites on the microstructural changes, so-called rim structure formation, in UO2 pellets. The samples were prepared from fuel rods irradiated in commercial reactors, and pelletaveraged burnups of the samples were in the range from 41 to 61GWd/t. While the lattice parameters of the samples increased linearly in the burnup region up to approximately 70 GWd/t, the lattice parameters slightly decreased and tended to level off above 70GWd/t. The measured crystallite sizes were in the range from 100 to 200 nm and these were nearly the same as those of the “recrystalized grains” in the rim structure. The elastic strain energy densities, which were evaluated from the lattice parameters and nonuniform strains, tended to increase with burnup and show two plateaus at burnups of approximately 50 and 70GWd/t, respectively. The saturation of the measured strain energy density at approximately 50GWd/t can be attributed to the tangle of accumulated dislocations, and the increase in the strain energy density above 60GWd/t can be explained by the strain caused by crystallite rotation under the restraint conditions between crystallites.