THE DIFFUSION OF SILVER IN THE GRAPHITIC MATRICES A3-3 AND A3-27

Ag-110m contributes significantly to the plate-out activity in the coo lant circuit of high-temperature gas-cooled reactors. The Ag-110m diff usion coefficient in graphitic matrices is needed to estimate the Ag-1 10m core release rate. The diffusion of Ag-110m in original, oxidized and fast neutron irradiated graphitic matrix A3-3 and in original A3-2 7 was studied in vacuum by measuring the Ag-110m release kinetics from cylindrical specimens. The latter were previously doped with carrier- free Ag-110m at 1000-degrees-C. Concentration profiles were measured a fter the release of a part of the Ag-110m present initially. The relea se kinetics and the concentration profiles satisfied Fick's second law . In the temperature range of 800-1300-degrees-C at Ag concentrations < 4 X 10(-4) at% the diffusion coefficient data are given by D = 6.80 x 10(5) exp(- 2.71/kT) cm2 s-1 for original A3-3, D = 1.61 X 10(4) exp (- 2.67/kT) cm2 s-1 for irradiated A3-3, D = 1.32 x 10(4) exp(-2.55/kT ) cm2 s-1 for original A3-27, where k = 8.618 x 10(-5) eV/K and T is i n K 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.