The effects of irradiation condition and microstructural change on lattice parameter, crystal lattice strain and crystallite size in high burnup UO2 pellet

Pellet samples (average burnup: 37–62 GWd/t) were prepared from three kinds of fuel rods which were irradiated in the Halden Heavy Water Reactor in Norway, and microstructural changes in the pellet samples were investigated by means of optical microscopy, SEM/EPMA and micro-X-ray diffractometry. The measured lattice parameters tended to be smaller than the values reported previously, and it is likely that the measured lattice parameters were affected by the temperature conditions during the irradiation tests and the microstructural changes which occurred in the samples. Considering the burnup dependence of uniform and non-uniform strains, the following things are suggested: the interstitial atoms which cause uniform strain begin firstly to form dislocations as a recovery process of irradiation defect and the dislocation density increases. With increasing burnup, the accumulation of dislocations in the crystallite saturates and the migration of dislocations becomes dominant as a recovery process of irradiation defects in the crystallite.