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

Abstract

110mAg contributes significantly to the plate-out activity in the coolant circuit of high-temperature gas-cooled reactors. The 110mAg diffusion coefficient in graphitic matrices is needed to estimate the 110mAg core release rate. The diffusion of 110mAg in original, oxidized and fast neutron irradiated graphitic matrix A3-3 and in original A3-27 was studied in vacuum by measuring the 110mAg release kinetics from cylindrical specimens. The latter were previously doped with carrier-free 110mAg at 1000°C. Concentration profiles were measured after the release of a part of the 110mAg present initially. The release kinetics and the concentration profiles satisfied Fick's second law. In the temperature range of 800–1300°C at Ag concentrations < 4 × 10 −4 at% the diffusion coefficient data are given by D = 6.80 × 10 5 exp( − 2.71 kT ) cm 2s −1 for original A3-3, D = 1.61 × 10 4 exp( − 2.67 kT ) cm 2s −1 for irradiated A3-3, D = 1.32 × 10 4 exp( − 2.55 kT ) cm 2 s −1 for original A3-27, where k = 8.618 × 10 −5 eV/ K and T is in K. Oxidation of A3-3 accelerated the Ag migration. 110mAg was found to be strongly enriched in the binder carbon which links the graphite grains in the matrix.