The transport behaviour of metallic fission products in pyrocarbon-coated fuel particles is investigated by means of postirradiation annealing at 1400°C as a function of the fast neutron fluence. The release of barium and strontium are both increased by fast neutrons, and that even, if the kernels contain alumina additives, which improve the fission product retention by forming aluminates (e.g. BaAl 2O4). The release of these two fission products is controlled by the effective diffusion coefficient in the kernel, which is in the case of Sr 90 in the range of 5,7·10−15cm2s−1 (nonirradiated) and 5,3·10−13 cm2s−1 (fluence 2· 1022 cm−2 at E> 0,1 MeV). The most significant increase is observed in the low fluence region. Ruthenium and ceriüm are released from the kernels by recoil only, not by diffusion. The caesium release is controlled by diffusion in the outer coating layer and depends on the microstructure of the pyrocarbon: The diffusion coefficient is decreasedby fast neutrons, if the amount of fiber component in the pyrocarbon is high. At low amounts of fiber component the diffusion coefficient is increased by fast neutrons
