Migration behavior of palladium in UO2

Palladium (Pd) is one of the interesting fission products in both HTGR and LWR fuel technologies. It is easily released from UO2 kernels in HTGR coated fuel particles and reacts with SiC coating layer. Also, Pd is one of the metal fission products composing the metallic precipitate in irradiated UO2, which is undissolvable residue in the reprocessing of LWR fuel. The present research is concerning the Pd migration in UO2 which was studied as follows. UO2 containing Pd and Pd+Mo was prepared, and heat-treated at 1800degC, followed by an electron probe microanalysis (EPMA) and a ceramography in order to observe growth of the Pd precipitates. Furthermore, Pd diffusion in UO2 was studied in the method that Pd was physically vapour-deposited on the surface of UO2, and the diffusion couples of UO2 were annealed at 1400degC, followed by observation by EPMA for determination of Pd diffusion coefficients. It was concluded in this study that number density of the Pd precipitate in UO2 decreased with heating time, whereas mean diameter of the precipitate increased. Influence of Mo addition on growth of the precipitate was not recognized. Also, the composition of Pd and Mo measured in the precipitate was different from the equilibrium one of the Pd-Mo phase diagram. In the diffusion experiment Pd was detected at the point of 400 μm depth from the surface. The diffusion coefficient obtained in this study was about 10-10 cm2/s. (author)