The diffusion of Ag in the graphitic matrices A3-3 and A3-27

Ag-110m contributes significantly to the plate-out activity in the coolant circuit of pebble bed reactors. The diffusion coefficient in graphitic matrices is needed to estimate the core release rate. The diffusion of Ag-110m in original, oxidized and fast neutron irradiated graphitic Matrix A3-3 and in original A3-27 was studied in vacuum by measuring the release kinetics from cylindrical specimens. The latter were previously doped with carrier-free Ag-110m at 1000 C. Concentration profiles were measured after the release of a part of the Ag-110m present initially. The release kinetics and the concentration profiles satisfied Ficks second law. In the temperature range of 800-1300 C at Ag concentrations < 4 appm the diffusion coefficient data are given. Oxidation of A3-3 accelerated the Ag migration. Ag-110m was found to be strongly enriched in the binder carbon which links the graphite grains in the matrix. (orig./HP)