Depth profiles of damage accumulation in UO2 and (U,Gd)O2 pellets irradiated with 100 MeV iodine ions

To study the initial defect formation and accumulation process during fission events, sliced pellet specimens of UO2 and (U,Gd)O2 were irradiated at ambient temperatures below 200 C, with 100 MeV iodine ions over a fluence range of 1.0 x 1018-2.0 x 1019 ions/m2. The surface of the specimens was analyzed by scanning electron microscopy (SEM) and X-ray diffractometry (XRD), and then the depth profiles of incident iodine ions and defect clusters were measured by secondary ion mass spectrometry (SIMS) and transmission electron microscopy (TEM), respectively. Lattice parameter change, which is associated with point defect accumulation, increased with ion fluence. Defect clusters of dislocations and dislocation loops were recognized, and their depth profiles were in good agreement with the calculated damage profile. These profiles of iodine ions and dislocation loops in both UO2 and (U,Gd)O2 were discussed in terms of inelastic and elastic collisions. (orig.)